EPA-542-R-05-028 

March 2006 

Off-Gas Treatment Technologies for 

Soil Vapor Extraction Systems: 

State of the Practice 

March 2006 

Prepared by: 

Office of Superfund Remediation and Technology Innovation 

Office Solid Waste and Emergency Response 

U.S. Environmental Protection Agency 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

March 2006 i

CONTENTS 

Section Page

ACRONYMS AND ABBREVIATIONS.......................................................................................................vi 

NOTICE AND DISCLAIMER ....................................................................................................................viii 

ACKNOWLEDGMENT ...............................................................................................................................ix 

EXECUTIVE SUMMARY ...................................................................................................................... ES-1 

1.0 INTRODUCTION......................................................................................................................... 1-1

1.1 Purpose........................................................................................................................... 1-1

1.2 Document Structure........................................................................................................ 1-2

1.3 Sources of Information ................................................................................................... 1-2

1.4 Types of Environmental Remediation Systems ............................................................ 1-3

1.5 Summary of Off-Gas Treatment Technologies Evaluated............................................ 1-3

1.5.1 Thermal Treatment............................................................................................ 1-3

1.5.2 Adsorption.......................................................................................................... 1-4

1.5.3 Biofiltration ......................................................................................................... 1-4

1.5.4 Emerging Technologies..................................................................................... 1-4 

1.5.4.1 Non-Thermal Plasma.......................................................................... 1-5

1.5.4.2 Photolytic and Photocatalytic Technologies ...................................... 1-5

1.5.4.3 Membrane Separation ........................................................................ 1-5

1.5.4.4 Other Technologies............................................................................. 1-5

1.6 Evaluation Criteria .......................................................................................................... 1-6 

2.0 BACKGROUND .......................................................................................................................... 2-1

2.1 State of Practice for Off-Gas Treatment........................................................................ 2-1 

2.2 History of Soil Vapor Extraction ..................................................................................... 2-1 

2.3 Soil Vapor Extraction and Multi-Phase Extraction Treatment Systems....................... 2-2 

2.3.1 Treatment System Components....................................................................... 2-2 

2.3.2 Treatment System Operations.......................................................................... 2-4 

2.4 Contaminant Sources and Off-Gas Characteristics...................................................... 2-5 

2.5 Regulatory Requirements .............................................................................................. 2-7 

3.0 THERMAL TREATMENT TECHNOLOGIES ............................................................................ 3-1

3.1 Thermal Oxidation .......................................................................................................... 3-1 

3.1.1 Technology Description..................................................................................... 3-2 

3.1.1.1 Direct-Flame Thermal Oxidizers......................................................... 3-3 

3.1.1.2 Flameless Thermal Oxidizers............................................................. 3-3 

3.1.1.3 Catalytic Oxidizers .............................................................................. 3-5 

3.1.1.4 Hybrid Thermal/Catalytic Oxidizer Systems ...................................... 3-5 

3.1.1.5 Heat Recovery..................................................................................... 3-6 

3.1.2 Applicability........................................................................................................ 3-8 

3.1.3 Limitations........................................................................................................ 3-10 

3.1.4 Performance .................................................................................................... 3-11 

3.1.5 Engineering Considerations............................................................................ 3-14 

3.1.5.1 Design Issues.................................................................................... 3-14 

3.1.5.2 Operating Issues ............................................................................... 3-15 

3.1.6 Residuals Management .................................................................................. 3-15 

3.1.6.1 Products of Complete Combustion .................................................. 3-15 

3.1.6.2 Products of Incomplete Combustion ................................................ 3-16 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

CONTENTS (Continued) 

Section Page

 March 2006 ii

3.1.7 Cost and Economics ....................................................................................... 3-18 

3.1.7.1 Capital Cost....................................................................................... 3-18 

3.1.7.2 Operating Cost .................................................................................. 3-19 

3.1.8 Developmental Status ..................................................................................... 3-21 

3.2 Internal Combustion Engines....................................................................................... 3-21 

3.2.1 Technology Description................................................................................... 3-22 

3.2.2 Applicability...................................................................................................... 3-22 

3.2.3 Limitations........................................................................................................ 3-23 

3.2.4 Performance .................................................................................................... 3-23 

3.2.5 Engineering Considerations............................................................................ 3-24 

3.2.6 Residuals Management .................................................................................. 3-24 

3.2.7 Cost and Economics ....................................................................................... 3-25 

3.2.8 Developmental Status ..................................................................................... 3-25 

4.0 ADSORPTION TECHNOLOGIES.............................................................................................. 4-1

4.1 Activated Carbon Adsorption ......................................................................................... 4-1 

4.1.1 Technology Description..................................................................................... 4-2 

4.1.2 Applicability........................................................................................................ 4-4 

4.1.3 Limitations.......................................................................................................... 4-4 

4.1.4 Performance ...................................................................................................... 4-6 

4.1.5 Engineering Considerations.............................................................................. 4-6 

4.1.6 Residuals Management .................................................................................... 4-8 

4.1.7 Cost and Economics ......................................................................................... 4-8 

4.1.8 Developmental Status ....................................................................................... 4-9 

4.2 Zeolite Adsorption........................................................................................................... 4-9 

4.2.1 Technology Description..................................................................................... 4-9 

4.2.2 Applicability...................................................................................................... 4-10 

4.2.3 Limitations........................................................................................................ 4-12 

4.2.4 Performance .................................................................................................... 4-12 

4.2.5 Engineering Considerations............................................................................ 4-14 

4.2.6 Residuals Management .................................................................................. 4-15 

4.2.7 Cost and Economics ....................................................................................... 4-16 

4.2.8 Developmental Status ..................................................................................... 4-16 

4.3 Synthetic Polymer Adsorption...................................................................................... 4-17 

4.3.1 Technology Description................................................................................... 4-17 

4.3.2 Applicability...................................................................................................... 4-18 

4.3.3 Limitations........................................................................................................ 4-19 

4.3.4 Performance .................................................................................................... 4-19 

4.3.5 Engineering Considerations............................................................................ 4-20 

4.3.6 Residuals Management .................................................................................. 4-20 

4.3.7 Cost and Economics ....................................................................................... 4-20 

4.3.8 Developmental Status ..................................................................................... 4-21 

5.0 BIOFILTRATION TECHNOLOGIES.......................................................................................... 5-1

5.1 Technology Description.................................................................................................. 5-1

5.2 Applicability ..................................................................................................................... 5-4 

5.3 Limitations....................................................................................................................... 5-5 

5.4 Performance ................................................................................................................... 5-6 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

CONTENTS (Continued) 

Section Page

 March 2006 iii

5.5 Engineering Considerations........................................................................................... 5-9 

5.6 Residuals Management................................................................................................ 5-13 

5.7 Cost and Economics..................................................................................................... 5-14

5.8 Developmental Status .................................................................................................. 5-15 

6.0 EMERGING TECHNOLOGIES .................................................................................................. 6-1

6.1 Non-Thermal Plasma Technologies .............................................................................. 6-1 

6.1.1 Technology Description and Applicability......................................................... 6-1 

6.1.1.1 Silent Discharge Plasma..................................................................... 6-3 

6.1.2.1 Gas-Phase Corona Reactor ............................................................... 6-3 

6.1.2 Developmental Status ....................................................................................... 6-5 

6.1.3 Performance ...................................................................................................... 6-6 

6.1.4 Engineering Considerations, Limitations, and Cost ......................................... 6-8 

6.2 Photolytic and Photocatalytic Technologies................................................................ 6-10 

6.2.1 Technology Description and Applicability....................................................... 6-10 

6.2.2 Developmental Status ..................................................................................... 6-11 

6.2.2.1 Photolytic Treatment ......................................................................... 6-11 

6.2.2.2 Photocatalytic Treatment.................................................................. 6-12 

6.2.3 Performance .................................................................................................... 6-13 

6.2.4 Engineering Considerations, Limitations, and Cost ....................................... 6-15 

6.3 Membrane Separation Technologies........................................................................... 6-16 

6.3.1 Technology Description and Applicability....................................................... 6-17 

6.3.2 Developmental Status ..................................................................................... 6-18 

6.3.3 Performance .................................................................................................... 6-19 

6.3.4 Engineering Considerations, Limitations, and Costs ..................................... 6-19 

6.4 Other Technologies ...................................................................................................... 6-20 

6.4.1 Gas Absorption Technologies......................................................................... 6-20 

6.4.2 Vapor Condensation Technologies ................................................................ 6-22 

7.0 SUMMARY .................................................................................................................................. 7-1 

8-0 REFERENCES ............................................................................................................................ 8-1 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

 March 2006 iv

LIST OF TABLES 

Section Page

3-1 Lower Explosive Limits for Selected Hydrocarbons................................................................... 3-9 

3-2 Required Oxidation Temperatures to Achieve 99% Destruction and Removal Efficiencies 

 for Select Compounds...............................................................................................................3-13

4-1 Estimated Capital Cost Range for Vapor-Phase Activated Carbon Treatment Units 

(Including Carbon)....................................................................................................................... 4-9 

6-1 Gas-Phase Corona Plasma Reactor Technology Results ........................................................ 6-7

6-2 Representative Destruction and Removal Efficiencies for Photocatalytic Treatment of 

 Volatile Organic Compound-Contaminated Air .......................................................................6-13

6-3 Destruction and Removal Efficiencies for Photolytic Treatment of Soil Vapor Extraction 

Contaminants from McClellan Air Force Base .........................................................................6-14

7-1 Generalized Volatile Organic Compound Influent Concentration Ranges for Commercially 

Available Technologies ............................................................................................................... 7-2

7-2 Evaluation Factors for Thermal Oxidation and Carbon Adsorption Technology Selection...... 7-3
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

 March 2006 v

LIST OF FIGURES 

Section Page

2-1 Typical Soil Vapor Extraction System......................................................................................... 2-3

2-2 Typical Multi-Phase Extraction System ......................................................................................2-4

3-1 Generalized Flow Diagram of Typical Thermal Oxidation System............................................ 3-2

3-2 Flameless Thermal Oxidizer System.......................................................................................... 3-4

3-3 Hybrid Thermal/Catalytic Oxidizer System................................................................................. 3-6

3-4 Recuperative Thermal Oxidizer .................................................................................................. 3-7

3-5 Regenerative Thermal Oxidizer .................................................................................................. 3-8

3-6 Internal Combustion Engine Remediation System ..................................................................3-22

4-1 Typical Regenerable Activated Carbon System ........................................................................ 4-3

4-2 Zeolite Adsorption System ........................................................................................................4-11

4-3 Comparison of Inlet Concentration Effects on Adsorption Capacity of Adsorbents ...............4-13

4-4 Comparison of Humidity Effects on Adsorption Capacity of Adsorbents................................4-14

4-5 Polymer Adsorption System......................................................................................................4-18

5-1 Typical Biofiltration System ......................................................................................................... 5-3

6-1 Packed Bed Corona Reactor ...................................................................................................... 6-4

6-2 Typical Two-Stage Membrane Separation System .................................................................6-18
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

March 2006 vi

ACRONYMS AND ABBREVIATIONS 

Ǻ Angstrom 

AC Alternating current 

AFCEE Air Force Center for Environmental Excellence 

AFB Air Force Base 

AST Aboveground storage tank 

ATSDR Agency for Toxic Substances and Disease Registry 

BACT Best available control technology 

Battelle Battelle Pacific Northwest Laboratories 

BTEX Benzene, toluene, ethylbenzene, and xylene 

BTU British thermal unit 

BTU/cubic ft British thermal unit per cubic foot 

CAA Clean Air Act 

Cat-Ox Catalytic oxidizer 

CDC Centers for Disease Control 

cfm Cubic foot per minute 

CO Carbon monoxide 

DCA Dichloroethane 

DCE Dichloroethene 

DFTO Direct flame thermal oxidizer 

DNAPL Dense nonaqueous-phase liquid 

DOE U.S. Department of Energy 

DPE Dual-phase extraction 

DRE Destruction and removal efficiency 

EBCT Empty bed contact time 

fpm Feet per minute 

FRTR Federal Remediation Technologies Roundtable 

FTO Flameless thermal oxidizer 

g/m3

/hr Gram per cubic meter per hour 

GAC Granular activated carbon 

HAP Hazardous air pollutant 

ICAC Institute of Clean Air Companies 

ICE Internal combustion engine 

IP Ionization potential 

kV Kilovolt 

LANL Los Alamos National Laboratory 

lb/hr Pound per hour 

LEL Lower explosive limit 

LNAPL Light nonaqueous-phase liquid 

MACT Maximum achievable control technology 

MCP Massachusetts Contingency Plan 

MEC Maximum elimination capacity 

MEK Methyl ethyl ketone 

MIBK Methyl isobutyl ketone 

mm Hg Millimeter of mercury 

MPE Multi-phase extraction 

MTBE Methyl tert-butyl ether 

NAPL Nonaqueous-phase liquid 

NAS Naval Air Station 

NCER National Center for Environmental Research 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

March 2006 vii

ACRONYMS AND ABBREVIATIONS (CONTINUED) 

NESHAP National Emission Standard for Hazardous Air Pollutant 

NIOSH National Institute for Occupational Safety and Health 

nm Nanometer 

NOx Nitrogen oxides 

O&M Operation and maintenance 

PCB Polychlorinated biphenyl 

PCC Product of complete combustion 

PCDD Polychlorinated dibenzo-p-dioxin 

PCDD/F Polychlorinated dibenzo-p-dioxins and dibenzofurans 

PCDF Polychlorinated dibenzofuran 

PCE Tetrachloroethene 

PIC Product of incomplete combustion 

ppm Part per million 

ppmv Part per million by volume 

RCRA Resource Conservation and Recovery Act 

RTN Remediation Technologies Network 

RTO Regenerative thermal oxidizer 

scfm Standard cubic foot per minute 

SITE Superfund Innovative Technology Evaluation 

SOx Sulfur oxides 

SVE Soil vapor extraction 

SVOC Semivolatile organic compound 

TCA Trichloroethane 

TCE Trichloroethene 

TiO2 Titanium dioxide 

TPH Total petroleum hydrocarbon 

UEL Upper explosive limit 

USACE U.S. Army Corps of Engineers 

U.S. EPA U.S. Environmental Protection Agency 

UST Underground storage tank 

UV Ultraviolet 

VOC Volatile organic compound 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

March 2006 viii

NOTICE AND DISCLAIMER 

This report has been prepared for the U.S. Environmental Protection Agency (U.S. EPA) Office of 

Superfund Remediation and Technology Innovation (OSRTI) under Contract Number 68-W-02-034. 

Information in this report is derived from numerous sources, including personal communications with 

experts in the field. Some of the source documents have been peer-reviewed. This report has 

undergone U.S. EPA and external review by subject matter experts. Mention of trade names or 

commercial products does not constitute endorsement or recommendation for use. 

If you have questions about this report, please contact Kelly Madalinski, U.S. EPA OSRTI, at (703) 

603-9901 or madalinski.kelly@epa.gov. 

A portable document format (pdf) version of “Off-Gas Treatment Technologies for Soil Vapor 

Extraction Systems: State of the Practice” may be viewed or downloaded at the Hazardous Waste 

Cleanup Information (CLU-IN) system Web site at http://clu-in.org/542R05028. A limited number of 

printed copies are available free of charge and may be ordered via the Web site, by mail, or by fax 

from: 

U.S. EPA/National Service Center for Environmental Publications 

P.O. Box 42419 

Cincinnati, OH 45242-2419 

Telephone: (513) 489-8190 or (800) 490-9198 

Fax: (513) 489-8695 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

March 2006 ix

ACKNOWLEDGMENT 

This document has been prepared for the U.S. Environmental Protection Agency (U.S. EPA) Office 

of Superfund Remediation and Technology Innovation (OSRTI) under Contract Number 68-W-02-

034. Special acknowledgment is given to the remediation professionals for their review and 

thoughtful suggestions to support the preparation of this document, including the following federal 

staff: Harold Ball (U.S. EPA), Ray Cody (U.S. EPA), JoAnn Eskelsen (U.S. EPA), Michael Gill (U.S. 

EPA), Bernard Schorle (U.S. EPA), Michelle Simon (U.S. EPA), Cynthia Wetmore (U.S. EPA), Dave 

Becker (U.S. Army Corps of Engineers), and Edward Mead (U.S. Army Corps of Engineers). 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 ES-1

EXECUTIVE SUMMARY 

This document provides state-of-the-practice information on off-gas treatment technologies for soil 

vapor extraction (SVE) systems currently being used to clean up hazardous waste sites. It provides 

information on a wide variety of processes, including common practices as well as innovative emerging 

alternatives to illustrate the state of the practice. This information can help SVE project managers and 

practitioners with the following: 

• Identifying available technologies for off-gas treatment and understanding their applicability 

• Understanding principles of the various off-gas treatment technologies, their strengths, and 

their limitations 

• Screening technologies based on site-specific off-gas attributes, treatment goals, and costs 

• Finding detailed information about conventional technologies and new emerging alternatives 

Treatment of remediation system off-gases is necessary because the volatile organic compounds 

(VOC) typically present in SVE off-gases are hazardous because of their toxicity (acute or long-term 

carcinogenicity), ignitability, or other reasons. Often, the direct discharge of off-gases without 

treatment is unacceptable because of health, safety, or public concerns. The goal of off-gas treatment 

is to improve the off-gas quality for release to the atmosphere, with minimal impact to human health or 

the environment. 

SVE is one of the most efficient and cost-effective methods of removing VOCs from contaminated soil. 

In the United States, SVE is an accepted technology that has been used at landfill sites and leaking 

underground storage tank (UST) sites since the 1970s. In 1993, the U.S. Environmental Protection 

Agency (U.S. EPA) approved SVE as a presumptive remedy for treating soils contaminated with 

VOCs. Multi-phase extraction (MPE) is a related technology, and the types of off-gas treatment 

technologies used for MPE are often very similar to those used for SVE. The nature of the off-gas 

streams produced by SVE and MPE systems affects the selection of the vapor treatment technology 

and engineering requirements for the treatment system. 

Depending on the site and specific remediation goals, a typical SVE or MPE system operates for six 

months to several years. Sites with the most stringent soil clean-up criteria or with very large 

contaminant masses may require the use of SVE systems for much longer. Off-gas streams 

generated from these remediation systems are generally low- to medium-flows containing dilute 

concentrations of VOCs. The vapor flow rates encountered by most SVE systems generally range 

from 100 to 1,500 standard cubic feet per minute (scfm). 

The nature of SVE and MPE off-gas depends on the nature and distribution of VOCs in the 

subsurface. Site attributes and physical properties of the chemical constituents in the off-gas directly 

influence the selection of the off-gas treatment technology and overall remediation strategy. Some 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 ES-2

important site attributes include the presence of nonaqueous-phase liquid (NAPL) sources of 

contamination and the capacity of the soil to release vapors (soil permeability). Physical properties of 

the chemical compounds that influence the treatment technology include molecular weight and Henry’s 

law constants. 

Treatment technologies for off-gas treatment are categorized into the following four groups in this 

report: 

1. Thermal – An oxidation process in which the temperature is increased to destroy vapor-phase 

contaminants; for this report, internal combustion engine (ICE) is included as a thermal 

technology 

2. Adsorption – A process separating contaminants using a medium or matrix 

3. Biological – Use of living organisms that consume or metabolize chemicals in the off-gas 

4. Emerging technologies – Including photocatalytic and non-thermal plasma treatment, which 

destroy contaminants using ultraviolet (UV) light and electrical energy, respectively 

This document presents the state of the practice for these technologies based on applicability, 

limitations, performance, engineering considerations, residuals management, cost and economics, and 

developmental status. This information is intended to assist project managers and engineers in 

evaluating and selecting appropriate off-gas treatment technologies for SVE systems. The information 

is also applicable to vapors generated from groundwater treatment systems. Project managers and 

engineers seeking guidance for design and operation of such systems should consult engineering 

manuals and other specific guidance documents referred to throughout this document. 

Thermal treatment technologies include thermal oxidation and ICEs. Thermal oxidation systems 

(including direct-flame, flameless, and catalytic oxidizers) can treat a broad range of SVE off-gas 

streams and are often chosen for their reliability in achieving high VOC destruction and removal 

efficiencies (DRE). These systems can be designed to oxidize 95 percent to more than 99 percent of 

the influent VOCs. Target contaminant groups in SVE off-gas suitable for thermal oxidation treatment 

include non-halogenated VOCs, semivolatile organic compounds (SVOC), and fuel hydrocarbons at a 

wide range of concentrations. Specific classes of compounds readily destroyed in thermal oxidizers 

include alcohols, aliphatics, aromatics, esters, and ketones. If halogenated compounds are present in 

the SVE off-gas (chlorinated VOCs such as trichloroethene [TCE]), acid gases may be generated and 

require further treatment. Limitations of thermal oxidation for treating SVE off-gas include its 

comparatively high capital expense and the potentially high cost of energy to heat the incoming SVE 

off-gas. ICEs can treat high VOC concentrations and achieve relatively high DREs. 

Adsorption systems are most effective (in terms of both cost and waste management) for 

remediation projects involving moderate flow rates and dilute contaminant concentrations (less than 

100 parts per million by volume [ppmv]). Well-designed adsorption systems may achieve 95 to 98 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 ES-3

percent DREs at input concentrations of 500 to 2,000 ppmv. At lower concentrations, removal is 

generally greater than 98 percent. The main limitation of this technology is the high operating cost 

associated with adsorbent replacement or regeneration when high influent concentrations are treated. 

Granular activated carbon (GAC) is the most common adsorbent used to treat SVE off-gas. Although 

GAC systems can be used to treat a wide range of VOCs, they are not effective for treating VOCs with 

high polarity (such as alcohols and organic acids) or high vapor pressures (highly volatile compounds 

such as vinyl chloride, methyl tert-butyl ether [MTBE], and methylene chloride). System flow rates can 

range from 100 to 60,000 scfm. The relatively low initial capital cost of carbon adsorption systems 

makes them particularly attractive for short-term SVE applications when dilute concentrations of VOCs 

are present. High humidity diminishes the adsorptive ability of GAC. Zeolites and polymers are two 

other types of adsorbents. Their adsorptive capacities are not as affected by high humidity levels, but 

their costs are significantly higher than GAC costs. Highly polar and volatile VOC degradation 

products, such as vinyl chloride, formaldehyde, sulfur compounds, and alcohols, are better adsorbed 

by hydrophilic zeolites than by GAC. Polymeric adsorption is applicable to a wide range of VOCs and 

chlorinated VOCs at a wide range of vapor flow rates. 

Biofiltration can be used to treat relatively dilute VOC concentrations (typically less than 1,500 total 

ppmv). If optimum conditions are maintained, a properly designed biofilter may achieve greater than 

90 percent and sometimes greater than 95 percent DREs. System flow rates typically range from 20 to 

500 scfm. Specific classes of compounds readily biodegradable by biofilters include mono-aromatic 

hydrocarbons, alcohols, aldehydes, and ketones. Biofiltration is most effective in treating vapor 

streams from SVE systems remediating leaking USTs at gas stations. At these sites, destruction of 

aliphatic petroleum hydrocarbons and aromatic compounds (such as benzene, toluene, ethylbenzene, 

and xylene [BTEX] compounds) is required. One limitation is that the technology is sensitive to 

variations in operating parameters, such as moisture content, temperature, pH, and nutrient levels. A 

limited number of biofiltration systems are currently being used for SVE applications. 

Emerging technologies for SVE off-gas treatment include non-thermal plasma, photolytic and 

photocatalytic, membrane separation, gas absorption, and vapor condensation technologies. These 

technologies have not been used widely for SVE off-gas treatment. Also, several of the technologies 

are mostly in the research and development stage. Non-thermal plasma treatment can achieve high 

DREs for a wide range of chemicals, including aromatic VOCs (such as BTEX) and chlorinated VOCs. 

This treatment can address a wide range of concentrations but only at lower flow rates than thermal 

and adsorption technologies. Photolytic and photocatalytic technologies are effective for treating a 

broad range of halogenated and non-halogenated VOCs, aromatic and aliphatic hydrocarbons, 

alcohols, ethers, ketones, and aldehydes. These technologies work best on concentrated VOC waste 

streams (1 to 3,000 ppmv) at low flow rates. Membrane separation is best suited for chlorinated and 

non-chlorinated VOC concentrations exceeding 1,000 ppmv (up to 10,000 ppmv). The capacity of 

current systems generally ranges from 1 to 100 scfm. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 ES-4

Thermal oxidation and carbon adsorption are the two most common technologies used for SVE off-gas 

treatment. These two technologies are robust, demonstrated, and mature vapor treatment methods 

that can address a wide variety of contaminants and concentrations. At present, the selection of offgas treatment technologies for SVE is based on cost and operational considerations that differentiate 

thermal oxidation and carbon adsorption systems. Although many factors affect the cost of off-gas 

treatment, the general rule for selecting thermal oxidation or carbon adsorption is that dilute off-gases 

are more cost-effectively treated by carbon adsorption. Thermal oxidation becomes more 

cost-effective for off-gases that contain greater concentrations of vapor contaminants. Some sites 

have both thermal oxidation and GAC systems. Thermal oxidation systems have been used to treat 

higher initial concentrations and are replaced by GAC systems once concentrations have decreased. 

The following table summarizes evaluation factors for selecting thermal oxidation and carbon 

adsorption technologies. 

Although thermal oxidation and carbon adsorption are currently the most common treatment 

technologies for SVE off-gas, some emerging technologies presented in this document have the 

potential to be cost-effective alternatives to thermal oxidation and carbon adsorption. In the future, 

cost-efficiency improvements or reports of new, positive experiences in applying these alternatives 

may result in their selection more frequently. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 ES-5

Evaluation Factors for Thermal Oxidation and Carbon Adsorption Technology Selection 

Factor Thermal Oxidation Carbon Adsorption 

Concentration More commonly used for higher 

contaminant concentrations (> 500 

ppmv); treatment costs per pound of 

contaminant decrease as VOC 

concentrations increase because less 

supplemental energy is required per 

pound removed 

More frequently used for dilute vapor 

concentrations (< 1,000 ppmv); 

treatment costs per pound of 

contaminant tend to stay same or 

increase as concentration of vapors 

increase because carbon replacement 

frequency increases 

O&M requirements Tends to require more labor and more 

skilled labor to operate because of 

safety considerations 

Tends to be simpler and less laborintensive to operate and maintain 

unless vapor-phase concentrations 

are high and “breakthrough” occurs 

frequently 

Safety More safeguards necessary if it is 

possible for off-gases to reach high 

concentrations (significant fractions of 

the lower explosive limits of the 

contaminants in the vapor); formation 

of dioxins and furans is possible if not 

properly operated 

Tends to be very safe under most 

conditions; however, high levels of 

ketones or similar compounds may 

pose a fire hazard 

Chlorinated vs. 

non-chlorinated VOCs 

Less commonly used for chlorinated 

VOCs because of formation of 

hydrochloric acid during vapor 

combustion, which requires special 

acid-resistant materials for piping and 

equipment after combustion chamber

Equally applicable to chlorinated and 

non-chlorinated VOCs; acid formation 

not typically an issue 

Variety of compounds 

that can be treated 

Except for acid formation during 

combustion of chlorinated VOCs, wide 

variety of compounds can be treated 

Not all compounds adsorb well to 

activated carbon (depends on sorptive 

capacity); some common compounds 

(such as vinyl chloride) not readily 

treated; therefore, each compound in 

off-gas must be considered 

Capital vs. O&M costs Equipment significantly more 

expensive to purchase than carbon 

units; however, at high VOC 

concentrations, O&M costs lower than 

carbon units 

Capital costs fairly low; O&M costs 

proportional to off-gas flow rates and 

vapor concentrations 

Notes: 

O&M = Operation and maintenance 

ppmv = Part per million by volume 

VOC = Volatile organic compound 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 1-1

1.0 INTRODUCTION 

This document discusses the state of the practice of off-gas treatment technologies for remediation 

systems currently being used to clean up hazardous waste sites, with a focus on soil vapor extraction 

(SVE) systems. The following sections discuss the purpose of this document, summarize the 

document structure, present sources of information, discuss types of environmental remediation 

systems, present a summary of the off-gas treatment technologies evaluated, and discuss the 

evaluation criteria. 

1.1 Purpose 

This document provides information on a wide variety of technologies available for the treatment of 

chemical contaminants in the off-gas emissions of site remediation systems currently used in the 

environmental remediation industry. The off-gas treatment technologies evaluated include common 

practices as well as innovative emerging alternatives to represent the state of the practice. The 

technologies are analyzed based on a number of criteria, such as applicability, limitations, 

performance, and engineering considerations (see Section 1.6). This information is intended to assist 

project managers and engineers in the evaluation and selection of appropriate off-gas treatment 

technologies for SVE systems. The information is also applicable to vapors generated from multiphase extraction (MPE) and groundwater treatment systems. It is recommended that project 

managers and engineers seeking guidance on the design and operation of such off-gas treatment 

systems consult engineering manuals or other specific guidance documents for specific technologies 

of interest. Reference materials on the design and use of SVE systems are listed below. 

• U.S. Army Corps of Engineers’ (USACE) “Soil Vapor Extraction and Bioventing Engineer 

Manual” - http://www.usace.army.mil/inet/information/usace-docs/eng-manuals (USACE 2002) 

• USACE’s “Multi-Phase Extraction Engineer Manual” - 

http://www.usace.army.mil/inet/information/usace-docs/eng-manuals (USACE 1999) 

• U.S. Environmental Protection Agency’s (U.S. EPA) “Presumptive Remedies: Site 

Characterization and Technology Selection for CERCLA Sites with Volatile Organic 

Compounds in Soils” - http://www.epa.gov/superfund/resources/presump/finalpdf/scts.pdf 

(U.S. EPA 1993b) 

• U.S. EPA’s “Air Pollution Control Cost Manual” - http://www.epa.gov/ttn/catc/dir1/c_allchs.pdf 

(U.S. EPA 2002) 

• U.S. EPA’s “Development of Recommendations and Methods to Support Assessment of Soil 

Venting Performance and Closure” - 

http://www.epa.gov/ada/download/reports/epa_600_r01_070.pdf (U.S. EPA 2001b) 
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1.2 Document Structure 

This document has eight sections, including this introduction (Section 1.0). This introduction presents 

the purpose of this document, the document structure, sources of information, types of environmental 

remediation systems, summary of off-gas treatment technologies evaluated, and evaluation criteria. 

Section 2.0 presents background information that provides a basis for applying technology screening 

to specific sites and treatment system applications. This section also includes a brief discussion on the 

history of SVE treatment and the estimated number of systems in operation. It discusses typical 

sources of contamination and chemicals treated by SVE and related environmental remediation 

systems, how these chemicals generally behave in the environment, and the site characteristics that 

make sites amenable to SVE and related treatments. Treatment system components, monitoring 

parameters, and their general operation are also discussed to provide a clear understanding of how 

off-gas treatment systems physically interact with remediation systems. Regulatory requirements 

applicable to SVE systems are also discussed. 

Sections 3.0 through 6.0 provide evaluations of the various off-gas treatment technologies based on 

the defined evaluation criteria (see Section 1.6). Each section pertains to a particular technology 

group. Specifically, Section 3.0 evaluates thermal treatment technologies (including direct-flame 

thermal oxidation, flameless thermal oxidation, catalytic oxidation, hybrid thermal/catalytic oxidation, 

and heat recovery) and internal combustion engines (ICE). Section 4.0 discusses adsorption 

technologies and three major sorbents: activated carbon, zeolites, and synthetic polymers. Section 5.0 

evaluates biofiltration. Section 6.0 focuses on emerging technologies, including non-thermal plasma, 

photolytic and photocatalytic processes, membrane separation, and other technologies. 

Section 7.0 summarizes the information provided in this document. References used to prepare this 

document and cited in text are listed in Section 8.0. 

1.3 Sources of Information 

The information presented in this document comes from a number of sources (see Section 8.0). In 

general, every attempt has been made to use peer-reviewed information and technical literature from 

authoritative sources, including technical papers, textbooks, research articles, technology reviews, 

U.S. EPA and other agency documents, and commercial database searches. In certain 

circumstances, Web sites were consulted for recent data, and where applicable, they are cited. For 

emerging and innovative technologies, vendor information was obtained from papers, Web sites, and 

personal communications. 
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1.4 Types of Environmental Remediation Systems 

SVE systems generate off-gases by removing soil vapors directly from the subsurface or by 

transferring the contaminants to a vapor phase from an adsorbed phase on soil particles or from 

dissolved-phase (aqueous) or free-product liquids. These soil vapors often contain volatile organic 

compounds (VOC) that cannot be discharged to the atmosphere without treatment. A similar 

environmental remediation system that also generates vapors that require treatment is MPE, 

sometimes referred to as dual-phase extraction (DPE). Other systems that can create a vapor-phase 

off-gas requiring treatment are air strippers from groundwater pump-and-treat processes, ex-situ soil 

pile treatments, and bioventing systems. There are some exceptions to the need for treating off-gas 

(such as when the site-specific nature of the subsurface or operational flow of a system yields 

concentrations of off-gas chemicals that do not result in health effects or nuisance odors). Even in 

these cases, however, local ordinances may preclude direct venting of off-gases to the atmosphere. 

Treatment of remediation system off-gases is important. In many instances, the VOCs in the off-gases 

are chemicals that are hazardous because of toxicity (acute or long-term carcinogenicity), ignitability, 

or other reasons. The goal of off-gas treatment is to improve the quality of off-gas for release to the 

atmosphere, with minimal impact to human health and the environment. 

1.5 Summary of Off-Gas Treatment Technologies Evaluated 

Currently available commercial technologies for off-gas treatment can be categorized into three main 

groups: thermal treatment, adsorption, and biofiltration. Thermal treatment is an oxidation process in 

which the temperature is increased to destroy contaminants in the vapor phase. Adsorption 

technologies involve the separation of contaminants by medium or matrix. Biofiltration treatment 

technologies take advantage of living organisms that consume or metabolize off-gas chemicals prior to 

off-gas discharge. Other emerging technologies include photolytic treatment and non-thermal plasma 

treatment, which destroy contaminants using ultraviolet (UV) light or electrical energy. Although these 

technologies are used for industrial applications, they are considered emerging because of limited SVE 

applications to date. 

1.5.1 Thermal Treatment 

Thermal treatment of contaminants is a frequently used technology combusting or oxidizing VOCs to 

byproducts such as carbon dioxide and water. Section 3.0 discusses this technology in detail. 

Specific applications include direct-flame thermal oxidation, flameless thermal oxidation, catalytic 

oxidation, hybrid thermal/catalytic oxidation, heat recovery and ICEs. Thermal (non-catalytic) 

processes typically operate at temperatures exceeding 1,400 °F, with a typical minimum residence 

time over 0.5 seconds. Regenerative and recuperative system designs can be used to minimize 

operating costs by recovering a portion of the energy generated in the combustion process to preheat 

influent gases. Catalytic oxidation, the most commonly used thermal treatment off-gas technology, 
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uses a catalyst and heat to oxidize contaminants in the vapor stream (U.S. EPA 2004). The low VOC 

concentrations typically seen in SVE applications often require additional fuel or heat input to sustain 

oxidation. Halogenated contaminants, however, also form halogen acids, which cannot be discharged 

directly into the atmosphere. The acids also tend to “poison” the catalyst. Traditionally, the gas stream 

must be passed through a scrubber to remove the acid. In addition, thermal treatment of chlorinated 

organic compounds has the potential to form various products of incomplete combustion (for example, 

carbon monoxide). 

1.5.2 Adsorption 

The simplest process for off-gas treatment discussed in this document is adsorption of contaminants. 

Section 4.0 discusses this technology in detail. Adsorption refers to the attraction and retention of 

organic contaminants to the surface of an adsorbent material. The key feature of the adsorbent 

material is that it is extremely porous and provides a large surface area for adsorption to take place. 

Although this technology concentrates contaminants and can be used to treat reasonably large 

volumes of off-gas, ultimately the adsorbent must either be disposed of as a hazardous waste or 

regenerated, thus releasing the contaminants for further treatment. Activated carbon is the most 

widely used adsorbent material. Other adsorbent materials include alumino-silicate crystal structures 

known as “zeolites” and synthetic polymers. Selection of an appropriate adsorbent material is primarily 

a function of the contaminant to be adsorbed, although the adsorption capacity of certain materials 

may be adversely affected by the relative humidity of the off-gas stream. 

1.5.3 Biofiltration 

Biofiltration processes are used to biologically degrade VOCs present in remediation off-gas streams. 

Section 5.0 discusses this technology in detail. Off-gas streams are passed through a bed or vessel of 

biologically active filter material. The organic VOCs in the vapor stream are adsorbed onto the filter 

media and subsequently degraded to inert compounds, usually carbon dioxide and water, by the 

microorganisms. Various biofiltration media (organic or synthetic) can be used for these systems. 

1.5.4 Emerging Technologies 

A number of new or emerging technologies are either not commercially available or do not have 

substantial performance information available for SVE applications. Nonetheless, depending on the 

site circumstances, these technologies may warrant consideration for an SVE practitioner. These 

technologies are briefly summarized below and discussed in detail in Section 6.0. 
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1.5.4.1 Non-Thermal Plasma 

Non-thermal plasma technologies are relatively new technologies and involve the destruction of vaporphase compounds by ionizing the compounds in the gas stream, thereby creating a plasma. This 

process typically occurs at temperature ranging from 90 to 250 °F (hence the term “non-thermal”). By 

adjusting conditions under which plasma formation occurs, the free radicals formed recombine into 

(generally) non-toxic simple compounds that can be released to the atmosphere. Specific plasma 

technologies include silent discharge plasma, gas-phase corona reactor, tunable hybrid plasma, and 

low-pressure surface wave plasma. The techniques differ principally in the (1) methods of applying 

electric fields to impart energy to the air stream and destroy the VOCs, and (2) amount of energy 

required. Each technique uses a power source and a flow-through reactor vessel. 

1.5.4.2 Photolytic and Photocatalytic Technologies 

Photolytic technologies use UV light to ionize the VOC-laden vapor stream. The UV light furnishes 

energy to excite and break the molecular bond of the VOCs creating free radicals. Photocatalytic 

technologies are similar to photolytic technologies; however, UV light is used in the presence of a 

catalyst (usually titanium dioxide [TiO2]). The reaction resulting from the contact between the UV light 

and the catalyst produces hydroxyl radicals that destroy the VOC. The use of the catalyst allows VOC 

destruction to occur at, or near, room temperature. 

1.5.4.3 Membrane Separation 

Membrane separation technologies remove VOC vapors from the air/vapor mixture by passing the 

waste stream through a membrane designed to be more permeable to organic molecules than to air. 

These membrane separation systems can be configured as single-stage systems (typically used to 

treat concentrated industrial process streams) or as two-stage systems (commonly used to treat more 

dilute streams generated from site remediation). 

1.5.4.4 Other Technologies 

Other emerging technologies discussed in this document include gas absorption and vapor 

condensation technologies, which are summarized below. 

Gas Absorption 

Gas absorption technologies involve processes in which contaminant vapors are dissolved into a liquid 

solution. This process is referred to as “scrubbing.” In some cases, the contaminant in the vapor 

stream chemically reacts with a component of the absorbent solution (for example, acid gases are 

absorbed into a caustic solution). The main difference between absorption and adsorption is that 
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absorption incorporates the contaminant into the absorbent medium, while adsorption takes place on 

the medium. 

Vapor Condensation 

During treatment of contaminated vapor streams, cooling or condensation of the vapor-phase 

contaminants can be used as a stand-alone control system or as an enhancement to other vapor 

control technologies. In a condensation system, the contaminant-laden vapor stream is cooled below 

the dew point of the contaminants. The vapor-phase contaminants are then condensed and can be 

collected for recycling or disposal in the liquid phase. 

1.6 Evaluation Criteria 

Each technology-specific section of this document provides a description of the technology and a 

generalized graphical representation of a typical system. Criteria are then presented to support an 

evaluation of the off-gas treatment technologies. 

Applicability: This criterion evaluates the types of contaminants treated by each technology and the 

system’s ability to effectively treat a variety of contaminant types as well as mixed waste streams within 

a wide range of contaminant concentrations and flow rates. This criterion also addresses the use of 

the technology for industrial applications and environmental remediation systems, as well as the use of 

a particular technology in a treatment train with other technologies. 

Limitations: This criterion addresses whether the technology is easily used with SVE or other 

remediation systems and regulatory awareness and acceptance of the technology. This criterion also 

evaluates technology reliability and operation and maintenance (O&M) requirements for the technology 

to achieve its design performance standard (that is, considerations such as ease of maintenance and 

downtime). This criterion also considers technical limitations such as moisture content, flow rate, and 

residence time. 

Performance: This criterion allows evaluation of technology performance, including proven destruction 

and removal efficiency (DRE) values for each technology based on available literature and vendor 

data. This criterion considers performance monitoring procedures and equipment, and the ability of 

each technology to handle mass loading. Issues related to start-up and maintenance of steady-state 

conditions are also discussed. 

Engineering Considerations: This criterion considers various required engineering design parameters 

and their impact on the optimization of a specific technology. Problems typical in the operation of a 

system and means to circumvent these problems are discussed under this criterion. 
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Residuals Management: This criterion evaluates whether the technology generates treatment 

residuals (including solid, liquid, and vapor waste streams) that require additional treatment or 

disposal. Residual management environmental concerns include the potential of a technology to 

generate products of incomplete combustion (PIC) and toxic breakdown products, including dioxins 

and furans. 

Cost and Economics: This criterion evaluates technology costs, relative capital investment costs, and 

O&M costs associated with each technology. When cost information is limited, factors that significantly 

affect the cost of each technology are considered, and case-specific examples are presented as 

benchmark applications representing particular conditions. 

Developmental Status: This criterion addresses the commercial availability of the technology. Also 

considered are the status of the technology (indicated by regulatory approval or conference 

proceedings), availability of equipment, ongoing research projects, and pilot-scale and full-scale 

systems in place. Vendors for the technologies are also presented under this criterion. 
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2.0 BACKGROUND 

Off-gas treatment systems are used to prevent or minimize the discharge of vapor-phase chemicals 

associated with SVE or other environmental remediation systems. Untreated vapor-phase chemicals 

may pose risks or be a nuisance to surrounding human or environmental receptors. To support 

evaluation of off-gas treatment technologies and provide a frame of reference for understanding the 

nature of the off-gases, this section discusses the state of the practice of off-gas treatment, the history 

of SVE, SVE and MPE treatment systems, sources of contaminants typically present in off-gases 

treated using SVE and associated off-gas characteristics, and regulatory requirements. 

2.1 State of Practice for Off-Gas Treatment 

This document provides information regarding off-gas treatment technologies ranging from 

conventional to innovative methods. The state of the practice is quite varied. For example, of the 170 

Superfund remedial action sites having SVE systems with reported information from 1982 to 2002, 24 

systems do not include off-gas treatment of discharges. When off-gas treatment is used, the primary 

treatment documented is adsorption with activated carbon. For Superfund sites with data available, 70 

percent of off-gas control systems for ongoing remediation projects use carbon adsorption. The 

second most popular method is thermal oxidation at 25 percent (U.S. EPA 2004). Alternatives to 

thermal oxidation and carbon adsorption are being developed; however, experience with these 

emerging technologies for off-gas treatment is currently limited within the Superfund program. 

2.2 History of Soil Vapor Extraction 

SVE is one of the most efficient and cost-effective methods of removing VOCs from contaminated soils 

(USACE 2002). In the United States, SVE is an accepted technology that has been used at landfill 

sites and at leaking underground storage tank (UST) sites since the 1970s. As early as 1972, Duane 

Knopik began using SVE to clean up gasoline that had leaked from a UST at his service station in 

Forest Lake, Minnesota. By 1982, Knopik had used his by-then patented system at approximately 100 

sites throughout the United States. Other early developers of SVE systems in the late 1970s and early 

1980s included Oil Recovery Systems, Inc.; Exxon Company USA; Shell Oil Company; Upjohn 

Company; and the American Petroleum Institute (Thornton and Wootan 1982). More recently, the 

practice of SVE has been applied to remediate non-petroleum related hazardous waste sites. In 1993, 

U.S. EPA designated SVE as a presumptive remedy for Superfund sites contaminated with VOCs 

(U.S. EPA 1993b). 

MPE is, in many ways, a very similar technology to SVE and has similar off-gas control requirements. 

MPE uses a combination of SVE and liquid (both water and nonaqueous-phase liquid [NAPL]) 

extraction at the same well. MPE was used as a remediation technology as early as the mid-1980s 

(Blake and Gates 1986). In 1997, U.S. EPA identified MPE as a presumptive remedy for VOCs in soil 
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and groundwater (U.S. EPA 1997a). MPE is often considered an enhanced application of SVE (U.S. 

EPA 1997c). 

The status of these technologies as presumptive remedies, their cost-effectiveness, and the ease with 

which they are employed have resulted in their frequent application at contaminated sites. As of 2004, 

approximately 222 Superfund sites had SVE and MPE identified as remedial technologies 

(approximately 15 percent of all sites) (U.S. EPA 2004). SVE is also widely used in Europe and is 

considered standard protocol in Germany (Hiller 1991). 

Many of the treatment technologies described in this document also have been applied to industrial 

processes that emit off-gases. Because of the pollution control needs of industries, commercialization 

of treatment technologies often begins with industrial uses and then becomes applied for site 

remediation. Some of the performance data from industrial systems are directly applicable to 

remediation systems. For these reasons, where applicable and necessary, information from industrial 

off-gas treatment systems is included in this document. 

2.3 Soil Vapor Extraction and Multi-Phase Extraction Treatment Systems 

The following sections discuss SVE and MPE treatment system components and operations. 

2.3.1 Treatment System Components 

An SVE system consists of one or more extraction wells screened in the unsaturated zone connected 

to a regenerative blower or vacuum pump to withdraw the soil vapor. Sometimes, SVE systems 

include air-injection or passive-pressure venting wells to allow inflow of air to the subsurface. A lowpermeability cap at the ground surface may also be incorporated into the design to prevent the flow of 

atmospheric air into the SVE wells. An air-water separator (moisture knock-out pot) is typically 

installed upstream of the blower or pump to remove excess liquid water in the soil vapor to avoid 

damage to equipment. SVE induces air flow in the unsaturated zone by creating a pressure gradient 

through the withdrawal of air from the wells. The unsaturated zone air flow results in desorption of 

contaminants from the surface of soil particles, volatilization of contaminants dissolved in groundwater, 

and evaporation of NAPLs. Figure 2-1 shows a typical SVE system. 
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Figure 2-1. Typical Soil Vapor Extraction System 

Source: U.S. EPA 1991c 

MPE applications use the pressure gradient in a more aggressive manner. MPE systems incorporate 

a modification of the conventional SVE technology. MPE systems simultaneously extract both 

groundwater and soil vapor. A vacuum is placed on the extraction well, which facilitates inflow of 

groundwater and NAPLs to the well. This technique may make extraction of liquids in low-permeability 

soils feasible when liquid flow rates would normally be very slow. MPE systems often employ high 

vacuum (greater than 12 inches of mercury) pumps. A suction pipe can be lowered into the extraction 

well to extract soil vapor and groundwater, or a submersible pump can be used to draw the water table 

down while the vacuum removes soil vapor. Figure 2-2 shows a typical MPE system. 
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Figure 2-2. Typical Multi-Phase Extraction System 

Source: U.S. EPA 1997a 

The final component in both SVE and MPE systems is generally the off-gas treatment system. Off-gas 

treatment is necessary for almost all systems. Although off-gas treatment does not directly affect the 

rate of site remediation, technical issues and costs related to the off-gas treatment process are often 

major factors when considering whether to apply SVE or MPE at a site. The off-gas treatment portion 

of the SVE system can be either a minor or a significant portion of the remediation capital costs; 

however, off-gas treatment is typically the most expensive portion of the system O&M costs. 

2.3.2 Treatment System Operations 

A typical SVE or MPE system can operate at a site for approximately six months to several years, 

depending on the site, the system, and the remediation goals. Sites with the most stringent soil 

cleanup criteria or with very large contaminant masses, however, may employ SVE systems for much 

longer. Off-gas streams generated from site remediation systems are generally low- to medium-flow 

streams containing dilute concentrations of VOCs. The vapor flow rates for most SVE systems 

generally range from 100 to 1,500 standard cubic feet per minute (scfm). In comparison, industrial 

vapor treatment applications, which can have many attributes similar to SVE off-gas treatment 

applications, usually treat much greater volumes of air, can have flow rates of 100,000 scfm or more, 

and address average concentrations of vapor contaminants much higher than those generated by 

remedial treatment processes (Rafson 1998). 
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Separation and recovery of chemicals present in the vapor streams of industrial processes may result 

in economic benefit to the manufacturing operation by recycling recovered solvents or other materials 

back into the manufacturing process. Contaminants recovered from site remediation activities rarely 

can be reused or recycled for any significant economic benefit. SVE off-gas streams typically contain 

a mixture of VOCs, do not have consistent influent VOC concentrations over time, and have high 

water-vapor content. Based on these factors, distinctly different strategies may be used to treat offgases from site remediation versus industrial process systems. 

2.4 Contaminant Sources and Off-Gas Characteristics 

Numerous potential sources of contamination of soil and groundwater may require treatment by SVE 

or MPE systems, including spills or leaks from USTs and aboveground storage tanks (AST). Other 

major sources of contaminants treatable by these systems include the following industrial operations: 

semiconductor, plastics, and adhesives manufacturing; metal fabrication shops; chemical blending 

processes; and dry cleaning. Private and military landfills created from uncontrolled waste disposal 

are also candidate sites for SVE treatment and methane collection through vapor extraction. 

In general, chemicals that can be treated by SVE and MPE systems include halogenated and nonhalogenated VOCs, some semivolatile organic compounds (SVOC), and certain other polar and nonpolar organic compounds. Examples of these compounds include common chemical mixtures such as 

gasoline, kerosene, jet fuels, and “lighter” fuel oils (such as No. 2 heating oil). Specific chemicals that 

are treated using SVE include benzene, toluene, ethylbenzene, and xylene (BTEX), major components 

of most fuels. Other VOCs include styrene, chlorobenzene, chloroform, carbon tetrachloride, 

tetrachloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE), vinyl chloride, trichloroethane 

(TCA), dichloroethane (DCA), ethylene dibromide, methylene chloride, acetone, methyl ethyl ketone 

(MEK), and methyl isobutyl ketone (MIBK). 

VOCs can be released into the subsurface environment dissolved in water if (for example) the release 

is from a wastewater stream. More typically, VOCs are released into the subsurface environment as 

NAPLs. As the NAPL travels through the soil, residual “droplets” of NAPL are left in the pores of soil 

and, if released in sufficient quantity, may form subsurface “pools.” The NAPL will adsorb to soil, 

dissolve into groundwater, or volatilize into soil gas, depending on the individual physical and chemical 

properties of the NAPL and its interaction with the subsurface environment. The extent to which the 

NAPL becomes distributed into these various subsurface environmental “compartments” governs the 

subsequent behavior of the contaminants and the nature of the remediation system for off-gas 

treatment. 

Organic liquids that are less dense than water are called light nonaqueous-phase liquids (LNAPL), and 

liquids more dense than water are called dense nonaqueous-phase liquids (DNAPL). This density 

determines how the NAPL is distributed in the subsurface environment as the NAPL achieves 

equilibrium after release. Most chlorinated VOCs (such as TCE) are DNAPLs, and most nonchlorinated VOCs (such as benzene and fuel oil) are LNAPLs. At significant quantities, DNAPLs tend 
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to sink through both unsaturated and saturated permeable soils until they reach the lowest point on the 

top of a confining layer. LNAPLs sink through unsaturated permeable soils and will float on the water 

table, migrating to the lowest water table elevation. Capillary forces can trap NAPLs in porous media 

above or below the water table as the groundwater elevation changes over time. 

The distribution of a contaminant after a release is also determined by complex interaction of the 

chemical properties of the contaminant (such as solubility and volatility), the properties of the soil (such 

as heterogeneity and permeability), and the quantity and duration of the release. Both chlorinated and 

non-chlorinated VOCs have relatively high vapor pressures, which is the characteristic that governs the 

extent to which they can be removed from soil as vapor. The vapor-phase concentrations of VOCs 

extracted and the change in these concentrations over time varies significantly between sites, 

depending on initial subsurface distribution and many chemical and soil properties. Despite a wide 

variety of patterns of contaminant distribution, SVE and MPE systems commonly remove VOCs as 

vapors that typically require additional treatment. 

The nature of the off-gas streams from remediation systems is governed by distribution of the VOCs in 

the subsurface. Some important site characteristics that significantly affect the off-gas streams and 

subsequent selection of off-gas treatment technologies are listed below. These attributes may also 

influence the overall remediation strategy for a site. 

• Sites with a relatively small VOC mass in the vadose zone tend to have lower off-gas VOC 

concentrations that decline rapidly. 

• Sites with substantial quantities of residual NAPL in unsaturated soil will tend to have high 

VOC off-gas concentrations for sustained periods of time. The concentration will only 

significantly decline once the NAPL is removed. Substantial quantities of NAPL are more 

common at petroleum release sites with LNAPL than at sites with chlorinated VOCs (for 

example, DNAPL) contamination. 

• Off-gas concentrations at sites where contamination is present in high-permeability, coarsegrained soils (such as sand and gravels) will tend to decrease more rapidly than at sites with 

less permeable soils because VOCs are more readily removed from permeable soils; 

therefore, in high-permeability soil, the nature of the off-gas will tend to change more rapidly. 

• The off-gas composition at a site where the release was a mixture (such as jet fuel) will change 

over time as the more volatile compounds are more quickly “stripped” from the subsurface; 

therefore, compounds that are the basis of selection of an off-gas treatment technology may 

change during the progress of the remediation. 

• MPE is often applied to sites that contain accessible pools of NAPL, either floating LNAPL or 

DNAPL trapped above a confining layer (either above or below the water table). The high 

vacuums applied to achieve MPE generally enhance the volatilization of VOCs entering the 

extraction wells as liquid; therefore, off-gas streams from MPE systems tend to have high VOC 

concentrations that remain high until the NAPL has been remediated. 
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Physical and chemical properties of the chemical constituents in the SVE off-gas also influence the 

selection of treatment technologies. Several general characteristics of SVE off-gas are summarized 

below. Specific off-gas properties relevant to each technology are discussed in Sections 3.0 through 

6.0. 

• The relative humidity of off-gas is typically very close to 100 percent. Some technologies (such 

as carbon adsorption) are less effective with high relative humidity, but others (such as 

biofiltration) are effective at high relative humidity. 

• Henry's law determines the extent of volatilization of a contaminant dissolved in water. The 

higher the Henry's law constant for a given compound, the higher its concentration will be in 

the extraction air stream. 

• Vapor pressure governs the tendency of a solid or liquid to evaporate. Vapor pressure is the 

force per unit area exerted by the vapor of the chemical in equilibrium with its solid or liquid 

form. Vapor pressure is applicable when NAPL is present and increases with increasing 

temperature. 

• Boiling point indicates the temperature at which a compound's vapor pressure equals the 

vapor pressure of the atmosphere, which at sea level is approximately 760 millimeters of 

mercury (mm Hg). Inducing a vacuum in soil causes the pressure in the air-filled soil pores to 

decrease, leading in turn to a lowering of the boiling point and an increase in volatilization of 

the contaminant (USACE 2002). 

• The molecular weight of compounds impacts adsorption ability. Adsorbents have different 

capacities for adsorbing different contaminants. Generally, high molecular weight 

compounds will adsorb better than low molecular weight compounds. 

• Compounds exhibit different lower explosive limit (LEL) concentrations in air. Compounds with 

low LEL concentrations will require more dilution air to prevent explosive SVE air streams. 

2.5 Regulatory Requirements 

The requirements for application of SVE, MPE, and other systems for site remediation as well as the 

requirements for control of vapor-phase discharges from these systems are based on applicable 

regulations promulgated by federal, state, and local regulatory agencies. These regulations generally 

fall into the following three categories of requirements: 

1. Control of off-gases to a best available control technology (BACT) standard (usually 95 percent 

contaminant destruction standard) 

2. Exemption for treatment of SVE off-gases due to acceptably small contaminant emission rates 

(compared to industrial process emission rates) 
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3. Need for treatment of off-gases based on site-specific considerations (risks), including the type 

and concentration of contaminant(s) being treated and the distance to the nearest potentially 

impacted receptor 

At the federal level, the 1990 amendments to the Clean Air Act (CAA) introduced new requirements for 

the control of hazardous air pollutant (HAP) emissions. These amendments identify 189 chemicals as 

HAPs as well as several sources of HAP emissions. All sources of HAPs within each category must 

be treated to a maximum achievable control technology (MACT) standard defined for that specific 

category. Many HAPs are likely to be extracted by SVE, MPE, and groundwater pump-and-treat 

systems that use air strippers. In addition, site remediation has been identified as a source category 

for HAP emissions. However, air emissions from site remediation regulated by rules established under 

federal corrective authorities are exempt from the site remediation National Emission Standard for 

Hazardous Air Pollutant (NESHAP). Cleanups conducted under State or voluntary cleanup programs 

are not exempted, and therefore, may be subject to the site remediation NESHAP. 

The emission rates observed at SVE and MPE remediation sites are generally much lower than the 

rates defined under the MACT standards. The CAA stipulates that MACT standards apply when the 

off-gas is emitted from a "major" source, one that has the potential to emit 10 tons per year or more of 

a listed pollutant or 25 tons per year or more of a combination of pollutants. Although many of the 

compounds extracted in a typical SVE remediation system may be HAPs, emission rates are typically 

less than the 10- or 25-ton-per-year thresholds. For example, in order to meet the major source permit 

10-ton-per-year threshold, a system treating 900 scfm of vapor would require a HAP concentration of 

at least 1,000 parts per million (ppm) as methane after any control system. 

U.S. EPA, a state, or local agency may implement the air toxics program for control of HAPs. The 

CAA allows U.S. EPA to delegate authority to a state or local agency to implement an air toxics 

program in lieu of the federal program. Many states have specified emission control equipment and 

specific emission limits for sources emitting VOCs and other HAPs. State requirements are frequently 

more stringent than federal requirements, and many states have regulations and standards not 

addressed under federal regulations. 

Some state agencies allow SVE-type systems to operate without off-gas treatment if the annual 

discharge of individual compounds or total mass is below a set default value or if the system can be 

shown to pose no significant risk to human health or the environment. Thus, regulations applicable to 

SVE off-gas treatment can be site-specific. For example, the Massachusetts Contingency Plan (MCP) 

provides graphs that indicate when off-gas treatment is, and is not, required for five specific groups of 

chemicals (such as gasoline). The criteria used for this distinction are chemical type, mass emission 

rate, and distance to the nearest receptor; however, in all cases, the MCP requires that SVE systems 

include an off-gas treatment component for the first 1,500 hours of operation to “guard against 

transient discharge anomalies” that can occur during initial SVE system operation. This provision 

reflects the observed phenomenon that SVE systems typically extract high concentrations of VOCs in 

the early stages of operation, followed by a steep decline in VOC removal rates (Massachusetts 

General Laws 1994). 
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3.0 THERMAL TREATMENT TECHNOLOGIES 

Thermal treatment technologies destroy hydrocarbons in a vapor stream at elevated temperatures 

(exceeding 600 °F) by combusting or oxidizing the hydrocarbons to carbon dioxide and water. The 

general chemical reaction for hydrocarbon oxidation is as follows: 

 HC + O2 + heat Æ CO2 + H2O + heat (1) 

where 

HC = Hydrocarbon 

O2 = Atmospheric oxygen 

CO2 = Carbon dioxide 

H2O = Water 

Heat is required to achieve the necessary temperature for this reaction to occur; the required 

temperature is a function of several factors, including the presence of a catalyst. The heat shown on 

the right side of the equation represents the heat released by the reaction. Hydrocarbons in off-gas 

from SVE and other environmental remediation systems are typically in the form of VOCs as described 

in Section 2.0. 

Thermal oxidation systems and ICEs are the two different types of thermal VOC treatment 

technologies discussed in this section. Additional information about these technologies is presented in 

the “Soil Vapor Extraction and Bioventing Engineer Manual” (USACE 2002) and “Air Pollution Control 

Cost Manual” (U.S. EPA 2002). 

3.1 Thermal Oxidation 

Thermal oxidation systems are commonly used to control VOC vapors generated from soil and 

groundwater remediation systems as well as to treat a wide variety of vapor streams from chemical 

processing, painting, printing, and pharmaceutical industrial sources. As of 1997, approximately 6,000 

thermal oxidation systems were in operation worldwide and approximately 20 percent of these systems 

were used for remediation (ENSR 2000a and b). A survey of Superfund sites from 1982 through 2002 

shows that thermal treatment (primarily catalytic oxidation) was the selected technology for SVE offgas treatment for 25 percent of the 170 sites at which the type of off-gas treatment was reported (U.S. 

EPA 2004). The Federal Remediation Technologies Roundtable (FRTR), a collaborative group of 

government agencies involved in hazardous waste site cleanup, estimates that over 20 firms currently 

manufacture catalytic oxidation systems specifically for remedial activities (FRTR 2004). 
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3.1.1 Technology Description 

A thermal oxidation unit typically consists of a fan or blower to move VOC-laden air; a filter-mixer to 

mix the VOC-laden air; a fan to supply combustion air; a combustion unit (reaction chamber) consisting 

of a refractory-lined chamber and one or more burners; heat recovery equipment (heat exchanger); 

and a stack for atmospheric discharge of the treated exhaust (Naval Facilities Engineering Service 

Center 2004). In some cases, post-oxidation treatment units are included. Figure 3-1 shows a 

generalized flow diagram for a typical SVE off-gas thermal oxidation system. 

Figure 3-1. Generalized Flow Diagram of Typical Thermal Oxidation System 

Source: Modified from U.S. EPA 2002 

Processes in dashed boxes are not applicable to all oxidation systems. 

Three general types of thermal oxidation systems are available for controlling VOC emissions1: 

• Direct-flame thermal oxidizers (DFTO) 

• Flameless thermal oxidizers (FTO) 

• Catalytic oxidizers (Cat-Ox) 

Key variables within each type of thermal oxidation system include the following: 

• Extent and type of heat recovery system 

• Post-oxidation treatment requirements (such as scrubbing) 

• Metallurgy requirements for equipment 

• Utility energy cost and source (heat from electricity, propane, natural gas, or fuel oil) 

 

1

A fourth thermal treatment technology that is not strictly a thermal oxidation process is the ICE, which is discussed in 

Section 3.2. 
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Each type of system operates somewhat differently, but all types destroy contaminants by raising the 

temperature of the gas stream to a sufficient level to promote oxidation (or combustion) of 

contaminants to carbon dioxide and water as shown in Equation (1). The VOCs in the off-gas act as a 

fuel source for the oxidation reaction. If the VOC concentrations are relatively low, there may be 

insufficient “fuel” to promote oxidation, and auxillary fuel must be added (such as natural gas or “fuel 

oil”). Sometimes the VOC concentrations are too high for the oxidation reaction to proceed safely, and 

the off-gas must be diluted with ambient air. 

High VOC concentrations are typically treated using non-catalytic thermal oxidation systems, and lower 

VOC concentrations are often treated using Cat-Ox systems. DFTO and FTO non-catalytic thermal 

oxidizers typically operate at 1,200 to 2,000 °F (Alley 1998), and Cat-Ox systems typically operate at 

500 to 900 °F. If the off-gas contains sufficient concentrations of VOCs to sustain the high operating 

temperatures, a catalyst is not needed. Where the off-gas VOC concentrations are relatively low, a 

catalyst will lower the amount of auxiliary fuel that must be supplied to sustain the oxidation reaction, 

and operational expenses will be reduced compared to a non-catalytic system. 

In a complete oxidation reaction, VOCs are oxidized to carbon dioxide and water. If halogenated 

compounds are present in the SVE off-gas (such as a chlorinated VOC like TCE), the resulting 

combustion products can include acid gases (such as hydrochloric acid). The acid gases require 

further treatment with an acid scrubber (for example, caustic solution to neutralize the acid) prior to 

discharge. The efficiency of a thermal oxidation system is a function of “the three Ts”: temperature, 

time (residence time in the combustion zone), and turbulence (mixing of the VOCs and oxygen) as 

discussed in Section 3.1.4. 

The following sections discuss DFTOs, FTOs, Cat-Ox, hybrid thermal/Cat-Ox systems, and recovery of 

heat generated during thermal oxidation. 

3.1.1.1 Direct-Flame Thermal Oxidizers 

The simplest type of thermal oxidizer is a DFTO system without heat recovery (no heat exchanger 

following the combustion chamber). These systems are sometimes called “direct-fired thermal 

oxidizers” or “afterburners” and do not use a catalyst. In a DFTO system, contaminated vapor from 

SVE system off-gases enters a reaction chamber, where an open flame burns the vapors and 

additional fuel. VOCs are oxidized as they are exposed to the necessary oxidation temperatures. 

3.1.1.2 Flameless Thermal Oxidizers 

Combustion in FTO systems occurs within a chemically inert, porous ceramic bed heated to oxidation 

temperatures. In FTO systems, the mixing zone is where the fuel is pre-mixed with off-gas at the inlet 

of the reactor before it passes through a pre-heated ceramic matrix, which heats the organic vapors 

(Thermatrix 2005). Once the vapors reach oxidation temperature, they auto-ignite in the system’s 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

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reaction zone. The energy released by combustion heats the ceramic matrix of the mixing zone. If the 

waste stream has sufficient energy content, then the system is self-sustaining after it reaches its 

operating temperature. One vendor claims that VOC vapor streams with as little as 10 British thermal 

units per cubic foot (BTU/cubic ft) can sustain the temperatures necessary for oxidation (Thermatrix 

2005). Like DFTO systems, this approach does not use a catalyst to aid combustion. Figure 3-2 

presents a schematic diagram of an FTO system. 

Figure 3-2. Flameless Thermal Oxidizer System 

Source: Thermatrix 2005 

System operates with straight-through flow and no heat recovery 
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3.1.1.3 Catalytic Oxidizers 

A Cat-Ox system uses a catalyst and heat to oxidize SVE off-gas contaminants. The catalyst enables 

the oxidation reaction to occur at much lower temperatures than required by non-catalytic thermal 

oxidation systems. Most Cat-Ox systems operate at 500 to 900 °F depending on the compound being 

oxidized and the catalyst. The addition of a catalyst accelerates the rate of oxidation by adsorbing 

oxygen and the contaminant on the catalyst surface, where they react to form carbon dioxide and 

water. As with non-catalytic thermal oxidation systems, when chlorinated VOCs are present in the 

contaminant off-gas stream, acid gas is produced. 

The most common catalysts for VOC combustion are the noble metals platinum, palladium, and 

rubidium. The metal is deposited on an aluminum oxide-coated ceramic or stainless steel substrate to 

produce the monolith (or honeycomb) form of the catalyst. Other types of catalyst are base-metal 

catalysts, such as chromium- or manganese-oxides. Base-metal catalysts are usually supplied in 

pelletized form (Alley 1998). Metal oxide catalysts are typically used to treat vapor streams containing 

chlorinated compounds (U.S. EPA 1991a). 

In a Cat-Ox system, the contaminated SVE off-gas stream is preheated prior to its interaction with the 

catalyst. The off-gas stream then enters the catalytic chamber and is evenly distributed over the 

catalytic beds. An exothermic combustion reaction takes place and the treated vapor (combustion 

products and byproducts) is then discharged. 

The primary advantage of Cat-Ox systems compared to other thermal oxidation systems is the lower 

energy requirement resulting from lower oxidizing temperatures. Consequently, operating costs are 

lower. Also, less costly metallurgy can be used in Cat-Ox systems than in non-catalytic oxidizers 

because of the lower operating temperatures. In addition, the catalyst increases the rate of reaction, 

which lowers the residence time of the contaminants in the reaction chamber and allows use of a 

smaller reaction chamber than with non-catalytic systems to process the same flow rate. 

3.1.1.4 Hybrid Thermal/Catalytic Oxidizer Systems 

Because off-gas from SVE systems fluctuates in concentration over time, hybrid thermal/Cat-Ox 

systems (also called hybrid systems) are designed to allow the SVE system to be operated at a higher 

temperature without a catalyst (thermal mode) in the initial stages of an SVE project when the 

extracted VOC contaminant concentrations are usually at their highest. When the VOC concentrations 

in the SVE off-gas decrease, the hybrid system converts to operate as a Cat-Ox system. Many 

vendors offer hybrid oxidizer systems. Figure 3-3 shows a typical hybrid system. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

March 2006 3-6

Figure 3-3. Hybrid Thermal/Catalytic Oxidizer System 

Source: Anguil 2004 

3.1.1.5 Heat Recovery 

Thermal and Cat-Ox systems can be designed to reuse heat generated during combustion. Heat 

recovery methods use either recuperative or regenerative heat exchangers to reuse heat energy, 

reduce the residence time of the waste stream in the system, and reduce fuel costs. Heat recovery 

can be applied to both catalytic and non-catalytic oxidation systems. 

Recuperative Thermal Oxidizers

Figure 3-4 shows a recuperative thermal oxidizer. In these systems, combustion takes place, and then 

the hot, treated vapor from the reaction or catalytic chamber is directed to a heat exchanger shell. 

Incoming SVE off-gas flows through the tubes of the shell-and-tube heat exchanger, and heat is 

exchanged from the hot, treated vapor stream to the cool, incoming untreated vapor stream. The 

streams do not mix. Because the influent stream is preheated in the heat exchanger, less fuel is 

needed to bring the contaminated vapor up to oxidation temperature. Recuperative systems can 

recover up to 70 percent of the heat of the reaction chamber effluent (U.S. EPA 2002); therefore, they 

require substantially less energy than non-recuperative systems. When chlorinated solvents are 

extracted, the design of recuperative heat exchangers generally limits flue gas cooling to avoid 

condensing acid gases onto heat exchanger surfaces and minimize corrosion. 
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Figure 3-4. Recuperative Thermal Oxidizer 

Source: Rafson 1998 

Cool off-gas enters the system and flows directly through the tubes of the heat exchanger for heating before entering the reaction 

chamber. The flame in the reaction chamber oxidizes the contaminants and generates a hot vapor stream that flows through the shell 

of the heat exchanger to be cooled before discharge. The heat recovered in the heat exchanger is used to heat the incoming off-gas. 

Regenerative Thermal Oxidizer

Regenerative thermal oxidizers (RTO) use more complex and more efficient heat exchange systems 

than recuperative systems to heat incoming SVE off-gas and promote the oxidation reaction. These 

units consist of a series of beds of heat-resistant media (such as ceramic) that absorb energy from the 

hot combustion product outlet gas. That energy is then used to preheat the incoming SVE off-gas. 

Figure 3-5 shows a typical regenerative thermal oxidizer system. The inlet gas first passes through a 

hot bed of ceramic transfer blocks that heats the stream to its ignition temperature (and cools the 

bed). If the desired temperature is not attained, a small amount of auxiliary fuel is added in the 

combustion chamber. The hot gases then react, releasing energy in the combustion chamber and 

while passing through another bed of ceramic transfer blocks, thereby heating the second bed to the 

combustion chamber outlet temperature. The process flows are then switched: now the inlet stream 

is fed to the hot bed. This cyclic process affords very high energy recovery (up to 95 percent) and 

may occur several times each hour (U.S. EPA 2002). It should be noted that complex controls and 

large, high-temperature valves add to the capital cost of these types of systems. However, the lower 

expense for energy can off-set these higher capital costs in some cases. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

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Figure 3-5. Regenerative Thermal Oxidizer 

Source: Rafson 1998 

Off-gas first passes through a hot bed of ceramic transfer blocks that heats the off-gas to its ignition temperature (and cools the 

bed). The hot gases then react and release energy while passing through another bed of ceramic transfer blocks, which is heated 

by the reaction. The process flows are then switched, and the incoming off-gas is fed to the hot bed. 

3.1.2 Applicability 

Thermal oxidizers are often chosen for their reliability in achieving high VOC DREs. Thermal 

oxidizers are capable of treating waste streams containing a wide range of VOC concentrations if 

sufficient oxygen is present. Generally, thermal oxidizers easily reach DREs higher than 99 percent 

except for RTOs, which achieve somewhat lower DREs. Thermal oxidation is a dependable 

treatment method when strict release standards must be achieved. In addition, thermal treatment 

can produce a buoyant plume that disperses well in the atmosphere. 

Target contaminant groups for SVE off-gas thermal oxidation include non-halogenated VOCs, 

SVOCs, and fuel hydrocarbons (FRTR 2004). Specific classes of compounds readily destroyed in 

thermal oxidizers include alcohols, aliphatics, aromatics, esters, and ketones. Halogenated VOCs 

are also readily destroyed; however, post-treatment is often required (typically a caustic scrubbing 

system) to capture acid gases formed by the oxidation of halogenated compounds. The type of 

thermal oxidation system most appropriate for a specific site depends on the type(s) and 

concentration(s) of contaminant(s); off-gas extraction flow rate; required DRE; and the British 

thermal unit (BTU) value of the contaminant streams to be treated. 
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The influent concentration to a thermal oxidizer system is often expressed as the waste stream’s BTU 

content or a percentage of the treated contaminant stream’s LEL. The LEL is defined as the minimum 

concentration of a chemical vapor in atmospheric air (for example, 21 percent oxygen at 20 °C) that is 

sufficient to support combustion (“burning”). For safety reasons, the influent concentrations to an 

oxidizer are generally limited to a fraction of the LEL. Concentrations below the LEL can be obtained 

by diluting the SVE off-gas with ambient air using an LEL detector to regulate the amount of ambient 

air added. For SVE systems, influent concentrations are typically limited to 10 to 25 percent of the LEL 

to ensure safe operation. In industrial settings where the influent concentrations are more predictable, 

thermal oxidation systems are often safely operated at higher fractions of the LEL, sometimes higher 

than 50 percent of the LEL. It is important to recognize that different gases have different LELs. Table 

3-1 lists LELs (presented as percent-of-gas-in-air) for an assortment of hydrocarbons commonly 

extracted by SVE systems. 

Table 3-1. Lower Explosive Limits for Selected Hydrocarbons 

Hydrocarbon LEL in Air (%)a Hydrocarbon LEL in Air (%)a

Methane 5.0 Gasoline 1.4 

Ethane 3.0 Acetone 2.5 

Propane 2.1 Methanol 6.0 

n-Butane 1.6 Toluene 1.1 

n-Pentane 1.5 m- and p-Xylenes 1.1 

n-Hexane 1.1 1,4-Dioxane 2.0 

Benzene 1.2 Methyl ethyl ketone 1.4 

Source: NIOSH 2005, Canadian Association of Petroleum Producers 1999 

Notes: 

LEL = Lower explosive limit 

ppmv = Part per million by volume 

a One percent (%) in air is equivalent to 10,000 ppmv. For example, the LEL for methane can also be expressed as 50,000 

ppmv. 

As described above, the thermal oxidation system selection process for a remediation site is affected 

by a number of important factors. Some generalizations can be made for when each type of system is 

most appropriate; however, almost all SVE off-gases can be treated using the different thermal 

oxidation systems discussed. In general, DFTO systems are best suited for low-flow, high-BTU, and 

high-VOC concentration SVE off-gases (such as vapors extracted from a recent gas station release). 

In contrast, FTOs are most often used for high-flow, low-BTU, and low-VOC concentration off-gas 

streams (such as industrial sites with historic solvent contamination). Cat-Ox systems are used for 

both low- and high-flow rates, with relatively low-BTU and low-VOC concentration streams. These 

generalizations are somewhat different for remediation systems (in which flow rates range from 100 to 

1,500 scfm) than for industrial systems, which typically have higher flow rates of up to 100,000 scfm. 

The higher flow-rate industrial off-gas streams are generally treated using non-catalytic oxidation 

systems. 
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The likelihood that influent VOC concentrations to the thermal treatment system will decrease over 

time, thereby affecting both cost to operate and achievable DREs, is an important consideration for 

SVE off-gas application selection. This consideration must be accounted for in the engineering and 

economic analysis to select both thermal treatment as well as the specific type of thermal treatment 

system. For this reason, some practitioners choose hybrid thermal treatment systems that can be 

readily converted from thermal to Cat-Ox mode. 

Also, because thermal oxidizers treat relatively small off-gas flow rates generated by most SVE 

systems and are in use for a limited period of time (usually less than 5 years), many SVE practitioners 

manage the challenge of decreasing concentrations by employing a mobile system. For example, a 

portable, skid-mounted RTO system can be used during initial treatment when concentrations are 

highest. Then, depending on the expected rate of decrease in VOC concentrations over time, the 

regenerative system can be removed and replaced by a portable Cat-Ox or other treatment system. 

Another option is to rent a thermal treatment unit to treat higher concentrations and then switch to a 

more permanent treatment system (such as carbon adsorption) once concentrations have decreased. 

3.1.3 Limitations 

Thermal oxidation is a mature technology that has been successfully implemented at many sites. It is 

a robust and well understood technology and thus readily implemented at SVE sites. Nevertheless, 

there are some technical and logistical limitations to using thermal oxidation. A significant limitation of 

thermal oxidation for treating SVE off-gas is the comparatively high capital expense as well as the 

often high cost for energy to heat the incoming SVE off-gas. For example, compared to the use of 

activated carbon for treating off-gas, the cost of auxiliary fuel (utility costs) can often exceed the cost of 

carbon replacement, making thermal oxidation cost-prohibitive. 

There are also environmental regulatory concerns about using thermal oxidizers. Although thermal 

oxidizers are capable of treating waste streams containing virtually any VOC concentrations, 

incomplete combustion will occur if sufficient oxygen is not present or if the “three Ts” are not 

adequately addressed (temperature, time, and turbulence; see Section 3.1.4). Other potential 

limitations include community acceptance and ability to obtain the necessary permits for operation. 

The oxidation process can result in treated vapor streams that contain undesirable byproducts. 

Generally, there are two classes of oxidation byproducts: products of complete combustion (PCC) and 

PICs. The PCCs are carbon dioxide and water. These compounds are innocuous and can be 

discharged directly to the atmosphere. Other compounds generated can include nitrogen oxides (NOx) 

and/or sulfur oxides (SOx) (if sulfur compounds such as hydrogen sulfide are extracted from the 

subsurface); however, these compounds are not typically formed during SVE off-gas treatment using 

thermal oxidizers. At sites where chlorinated or fluorinated hydrocarbons are present in the off-gas, 

hydrochloric or hydrofluoric acid gases may be produced from combustion. The acid gas produced 

can generally be treated using a mild caustic scrubbing solution following oxidation, although metal 

compatibility must be considered during equipment selection. For example, the presence of these 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

March 2006 3-11

compounds requires specialty steel, which greatly increases the capital cost for the system and makes 

heat recovery less cost-effective or even impractical. Hastelloy® is one example of a common upgrade 

material used when acid gases are produced. 

PICs are undesirable and include untreated VOCs, carbon monoxide, and toxic compounds, such as 

polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF), collectively 

referred to as PCDD/Fs (see Section 3.1.6 for additional information). The presence of significant 

concentrations of PICs often results from thermal oxidation systems that are not operated properly. 

Proper operation of the thermal oxidizer or additional vapor treatment should assist in minimizing the 

production of PICs. 

Cat-Ox systems require more maintenance than non-catalytic thermal systems. An operating problem 

associated with Cat-Ox systems is their vulnerability to chemicals and/or particulate matter that 

“poison” the surface of the catalyst. Poisons mask active sites or change the selectivity of the catalyst 

for particular reactions or reaction types. Poisons can also deactivate the catalyst by physically 

preventing interaction of VOCs and the catalyst. Compounds poisonous to catalysts include halogens; 

metals; and silicon-, phosphorus-, or sulfur-containing compounds. Catalysts can also be deactivated 

if the vapor stream becomes too hot, either chemically or physically altering the catalyst. Deactivated 

catalysts must be regenerated or disposed and replaced. 

Certain aliphatic compounds, such as propane, ethane, and methane, combust at higher temperatures 

(more than 900 °F) than typical for Cat-Ox systems (Rafson 1998). To heat the waste stream to 

sufficient temperatures to destroy these compounds would risk thermally deactivating the catalyst. In 

addition, energy costs to heat the waste stream to such temperatures could render the technology 

uneconomical. Thorough characterization of the vapor stream is required to ensure that the catalyst is 

compatible with the contaminants. 

Finally, thermal oxidizers are generally more complex to start up and operate than carbon treatment 

systems and require more maintenance. More operator training is typically required than for 

adsorption systems to address safety considerations, management of auxiliary fuel, and other system 

issues. When an SVE system is first turned on, the extracted vapor concentrations often exceed 10 to 

25 percent of the LEL for the SVE off-gas mixture, particularly at fuel release sites. Consequently, a 

trained operator is necessary to ensure that sufficient clean ambient air (often termed “dilution air”) is 

added to the extracted vapor stream to maintain the oxidizer influent vapor concentrations at safe 

levels. As the extracted vapor concentrations decrease, less dilution air is necessary, until eventually 

none is needed. 

3.1.4 Performance 

Thermal oxidation systems are robust and can treat a broad range of SVE off-gas streams. The 

systems can be designed to oxidize 95 percent to greater than 99 percent of the influent VOCs, 
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independent of the mass of contaminants entering the oxidizer (sometimes referred to as the “mass 

loading”). 

RTOs have somewhat lower efficiencies because of the alternating flow between beds; however, a 

survey of four systems showed DREs ranging from 98 to 99.8 percent for low-concentration inlet 

streams of less than 100 parts per million by volume (ppmv) (U.S. EPA 1995b). According to 

Heumann (1997), catalytic systems are designed to achieve DREs from 90 to 98 percent. U.S. EPA 

(1995b) reported DREs ranging from 72 to 99.6 percent for several sites where Cat-Ox was being 

used. Lower DREs were generally seen at sites where higher concentrations of chlorinated 

hydrocarbons were present. 

The performance of a thermal oxidation system is largely defined by the DRE, and DRE depends 

primarily on three essential factors (the “three Ts”): 

1. Temperature - achieving and maintaining the required oxidation temperature 

2. Time - allowing sufficient residence time for the reaction to occur 

3. Turbulence - ensuring sufficient reactant mixing 

To improve performance and increase the DRE, it is necessary to increase the temperature of the 

reaction, increase the residence time, or increase both. Increasing the residence time generally 

requires reducing flow (extraction) rates, which would in turn increase the duration of remediation. 

Thus, to avoid reducing extraction rates, it is generally necessary to raise the system temperature to 

achieve higher DREs. Similarly, to address fluctuations in concentrations and composition of off-gas 

streams, the oxidation temperature is the easiest operating parameter to adjust to achieve the desired 

DRE. The operating temperature of the unit can be adjusted by changing the amount of heat added to 

the system. For DFTO and FTO systems, the flow of auxiliary fuel to the combustion chamber or 

reaction zone can be adjusted. For Cat-Ox systems, the amount of time the waste stream is 

preheated prior to entering the catalyst bed can be adjusted. 

Table 3-2 lists the theoretical non-catalytic oxidation temperatures for select VOCs required for 99 

percent DRE for residence times of 0.5, 1, and 2 seconds. The chlorinated compounds in this table 

require significantly higher temperatures for oxidation. It is important to note that the oxidation 

temperatures listed are idealized values. Because most waste streams contain mixtures of VOCs, the 

actual oxidation temperatures required will vary and usually can only be determined after system startup. Field test data show that non-catalytic thermal oxidizers should generally be run at a temperature 

of approximately 1,600 °F and with a residence time of at least 0.75 second to ensure a 98 percent 

DRE for non-halogenated VOCs (U.S. EPA 1995b). Temperatures of near 2,000 °F are required for 

waste streams containing chlorinated VOCs. 
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Table 3-2. Required Oxidation Temperatures to Achieve 99% Destruction and Removal 

Efficiencies for Select Compounds 

Theoretical Combustion Temperature (°F) 

Compound 

Residence Time of 

0.5 Second 

Residence Time of 

1.0 Second 

Residence Time of 

2.0 Seconds 

Acetic anhydride 805 771 738 

Acetonitrile 1,997 1,887 1,786 

Acrylonitrile 2,056 1,909 1,778 

Aniline 1,440 1,402 1,366 

Azomethane 892 866 841 

Benzene 1,622 1,538 1,461 

Butene 1,708 1,653 1,600 

Carbon tetrachloride 1,987 1,822 1,679 

Chloroform 1,262 1,217 1,174 

Dichlorobenzene 1,668 1,583 1,504 

Dichloromethane 1,602 1,553 1,506 

Ethane 1,994 1,816 1,662 

Ethyl chlorocarbonate 1,028 972 920 

Ethyl nitrile 692 666 641 

Ethyl peroxide 463 442 423 

Ethylene dibutyrate 985 938 894 

Ethylidene dichloride 1,341 1,293 1,246 

Hexachlorobenzene 1,802 1,711 1,626 

Hexachlorobutane 1,553 1,502 1,453 

Hexachloroethane 1,438 1,347 1,265 

Methane 1,822 1,742 1,667 

Methyl iodide 1,034 966 959 

Monochlorobenzene 2,028 1,838 1,675 

Nitrobenzene 1,355 1,316 1,279 

Paracetaldehyde 796 770 744 

Pentachlorobiphenyl 1,404 1,367 1,331 

Propylene oxide 1,312 1,272 1,233 

Pyridine 2,041 1,856 1,697 

Tetrachlorobezene 1,895 1,761 1,642 

Tetrachloroethene 2,062 1,922 1,798 

Toluene 1,379 1,334 1,292 

Trichlobenzene 1,653 1,567 1,487 

Trichloroethane 1,336 1,262 1,194 

Vinyl chloride 1,415 1,373 1,334 

Source: Alley 1998 
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3.1.5 Engineering Considerations 

Engineering considerations include design issues and operating issues as discussed below. 

3.1.5.1 Design Issues 

The proper size for a thermal oxidation system is governed by several factors. The required 

performance of the system is largely defined by the DREs required. The DREs achieved depend 

primarily on the “three Ts” (temperature, time, and turbulence). Temperature can be adjusted in the 

field, but the size of the combustion chamber impacts residence time and is designed by the 

manufacturer. Thermal treatment systems should be designed so that the required combination of 

temperature, residence time, and turbulence can be achieved under all expected flow and 

concentration conditions; however, if insufficient time or attention is devoted to adequate system 

design, then thermal oxidizer systems can fail and, in some cases, equipment can catch fire or melt. 

Most thermal oxidizers used for industrial processes have a life expectancy of approximately 20 years 

(ENSR 2000b). Because SVE remediation systems typically operate for 5 years or less, owners and 

operators of thermal oxidation systems will often move the systems after completing treatment at each 

site. The most common heat recovery technique for SVE off-gas treatment is to use hot exhaust gas 

from the combustion chamber to preheat the cooler incoming vapor stream; however, thermal 

oxidizers for SVE systems can be integrated into operating facilities that may have other desirable 

uses for the recovered heat (such as to heat boilers). Heat recovery systems can range from simple 

shell-and-tube heat exchangers to complex systems of ceramic beds with sophisticated controls and 

valves. In general, the greater the degree of heat recovery, the greater the capital cost and the more 

complex the system. The cost of heat recovery can be a significant portion of the overall installed cost 

of the thermal treatment system. The benefits of heat recovery (lower energy costs and ancillary use 

of recovered heat) must be weighed against the capital cost for the heat recovery system. 

For most SVE applications, when the system is not integrated into an operating facility (such as a 

chemical plant, oil refinery, or manufacturing facility), there are no practical applications for facility 

reuse of recovered heat. In these cases, the capital cost of heat recovery must be weighed against the 

cost savings in decreased energy required to heat SVE off-gases in the thermal treatment system. 

The PCCs for VOCs are carbon dioxide and water. When chlorinated or fluorinated hydrocarbons are 

present in the waste stream, hydrochloric or hydrofluoric acid gas may be produced by combustion. 

The acid gas produced can be treated using a mild caustic scrubbing solution before the treated vapor 

stream is discharged to the atmosphere; however, the presence of these compounds requires 

specialty steel for construction, which greatly increases capital costs. This requirement for special 

materials resulting from the need to treat acid gases also makes heat recovery much less costeffective or sometimes even impractical when such contaminants are present. 
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3.1.5.2 Operating Issues 

Thermal oxidation systems are successful at treating most SVE off-gases if the proper temperature, 

residence time, and turbulence are maintained. Optimization of these parameters is usually not 

difficult, and thermal oxidation systems often require only minimal maintenance after start-up; however, 

if thermal oxidation systems are not properly operated, they can form PICs, including PCCD/Fs (see 

Section 3.1.6). 

Cat-Ox systems are also able to maintain high DREs, but these systems typically require more 

maintenance and are more sensitive to changing VOC concentrations and process conditions than 

non-catalytic thermal oxidation systems. In addition, the potential for catalyst deactivation is a major 

concern. Recent generations of catalysts have much longer lives and greater poison resistance than 

their predecessors, and have greater capabilities, including the destruction of chlorinated organic 

materials (ICAC 2005). A catalyst bed should last for approximately 5 years of continuous operation 

(Rafson 1998). Catalyst life may exceed 5 years with proper washing and regeneration techniques. 

Proper mixing of the waste stream with oxygen (turbulence) is a critical factor in achieving and 

maintaining thermal oxidation DREs. The theoretical oxygen requirement for complete oxidation of a 

given compound can be determined stochiometrically from Equation (1) in Section 3.0. If the SVE offgas stream does not contain sufficient oxygen and additional oxygen is required for complete 

oxidation, then ambient air can be mixed with the SVE off-gas before the gas enters the combustion 

chamber. In the early stages of operation of an SVE system, free product and VOC-saturated soils 

can be present. During this period, the oxygen content of the extracted vapors is often relatively low 

and the VOC concentrations are relatively high. Under these conditions, addition of ambient air will 

improve the DREs of the thermal oxidizer (both catalytic and non-catalytic). Further, the maximum 

desired influent vapor concentration is typically 10 to 25 percent of the LEL (see Section 3.1.2). For 

both of these reasons, substantial dilution air is often required in the initial stages of system operation. 

3.1.6 Residuals Management 

Residuals associated with thermal oxidation systems include PCCs and PICs. Their management is 

discussed below. Cat-Ox systems have an additional residual management concern, which includes 

the disposal of spent catalysts. 

3.1.6.1 Products of Complete Combustion 

The high operating temperatures of thermal oxidizers can cause the formation of undesirable 

byproducts that require additional treatment. High temperatures are conducive to the production of the 

NOx gases, nitric oxide and nitrogen dioxide. Nitric oxide forms at temperatures above 1,300 °F when 

molecular nitrogen (from the air) and oxygen react as follows: 
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 N2 + O2 ↔ 2NO (2) 

Nitric oxide and oxygen can react at cooler temperatures, generally below 1,300°F, to form nitrogen 

dioxide as follows: 

 NO + 1⁄2 O2 ↔ NO2 (3) 

NOx gases initiate the formation of photochemical smog, and at high atmospheric levels trigger 

respiratory-related health problems. NOx gases are not commonly produced from SVE applications; 

however, significant NOx can be formed from VOCs that contain nitrogen. Because of the relatively 

low volume of SVE off-gases treated by thermal oxidation, the contribution of NOx to atmospheric 

levels is relatively small compared to other combustion sources. SOx could similarly be formed during 

SVE off-gas treatment if sufficient sulfur-containing compounds are present and sufficiently high 

temperatures; however, suitable conditions for SOx formation are extremely rare or non-existent during 

SVE off-gas treatment. 

3.1.6.2 Products of Incomplete Combustion 

The most common PIC generated by thermal oxidation is carbon monoxide (CO). PCDD/Fs may also 

be formed as PICs, and the concern over their potential formation has become a limitation for using 

thermal oxidizers at some sites. To minimize the formation of PICs, most commercial thermal 

oxidation systems are designed to have sufficient fuel/air mixing processes and operate at high 

temperatures (U.S. EPA 1997b). 

U.S. EPA (2003) describes three primary scenarios for the formation of PCDD/Fs during combustion 

processes: 

1. Incomplete destruction of PCDD/Fs present in the source materials during combustion 

2. Formation of PCDD/Fs during combustion from precursor compounds, which can be 

chlorinated aromatic (“ringed”) hydrocarbons that may be structurally similar to PCDD/Fs 

3. Formation of PCDD/Fs from simple organic compounds in the ductwork downstream of the 

combustion chamber as the treated gas stream is cooled (referred to as “de novo synthesis”) 

Dioxin formation is catalyzed by the following additional conditions (Alley 1998; U.S. EPA 1996): 

• Temperatures between 480 and 840 °F (they showed the highest concentrations of PCDD/F); 

• The presence of particulate matter in the waste gas that contains a metal as an element oxide 

or other compound; or 

• A transition metal catalyst (such as copper, iron, zinc, nickel, manganese, chromium, titanium, 

or vanadium) in the gas phase. 
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PCDD/F formation has long been a concern in municipal refuse incineration, where parameters that 

promote their formation are often ideal. Incinerators, including industrial, municipal, and stationary 

Resource Conservation and Recovery Act (RCRA) hazardous waste incinerators, may process 

materials containing PCDD/Fs. By improving combustion efficiencies that reduce PIC formation, these 

incinerators have been able to reduce emissions to acceptable levels (Alley 1998). 

Incomplete combustion is generally due to inadequate air (oxygen), mixing, and/or temperature 

conditions to support the complete destruction of PCDD/Fs during combustion. Processing of 

chlorinated or fluorinated compounds at these incinerators can form PCDD/F precursors with 

temperatures at about 930 o

F. This temperature range can occur for fairly long periods in refuse 

incinerator heat recovery systems, where high-pressure steam generation is common. In addition, 

gases within these incinerators typically have significant time after combustion in the so-called de novo

temperature range (between 480 to 840 o

F). This temperature range is typical in refuse incinerator 

semi-dry scrubbing units. Refuse incinerators also have the necessary particulate matter for de novo

synthesis. These incinerators produce a substantial amount of particulate matter containing very high 

concentrations of heavy metals. The combustion flue gases can contact these particulates and 

catalyze PCDD/F formation reactions. Additional information about PCDD/F formation is presented in 

U.S. EPA’s “The Inventory of Sources and Environmental Releases of Dioxin-Like Compounds in the 

United States: The Year 2000 Update” (U.S. EPA 2005b). 

Although there are well-documented reports of PCDD/F emissions from refuse incinerators as well as 

a variety of chemical and manufacturing industry sources, data for SVE off-gas thermal oxidizers are 

limited (U.S. EPA 2003). The limited data available are summarized below for potential PCDD/F 

formation as a result of thermal oxidation of SVE off-gases for the three primary scenarios listed 

previously. 

Scenario 1 - Because SVE systems are not used typically to treat sites where PCDD/F contamination 

is present, this scenario is not probable. Limited measurements of PCDD/F emissions from thermal 

oxidizers used to treat SVE off-gases indicate that when aromatic compounds are present in the SVE 

off-gases (either extracted from the ground or from ambient dilution air), they are destroyed within the 

oxidizer (Hart 2004). 

Scenario 2 - This scenario is not likely at most SVE sites because the most common compounds 

considered precursors of PCDD/Fs are chlorobenzenes, chlorophenols, and polychlorinated biphenyls 

(PCB), which are not frequently present at sites remediated using SVE (U.S. EPA 2003). Of these 

compounds, only the lower molecular weight compounds, including mono-chlorobenzene, phenol, and 

possibly dichlorobenzene, are sufficiently volatile to be extracted by an SVE system. In addition, 

thermal oxidation for SVE systems operating at 930 °F lasts a very short period when flue gas 

quenching is applied and a relatively short period when heat recovery systems are used. 
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Scenario 3 - Chlorinated VOCs commonly observed at SVE remediation sites are chlorinated 

aliphatics, such as chloroethanes and chloroethenes. These compounds are not typically identified as 

precursors for PCDD/F formation, but they may be candidates for source material for de novo

synthesis. This scenario is the most likely mechanism for PCDD/F formation for SVE systems using 

thermal treatment units (Hart 2004). However, design temperatures and residence times can be 

increased to limit PIC formation when chlorinated compounds are combusted. Also, wet scrubbing 

systems are generally used when chlorinated VOCs are combusted. These systems operate below 

200 °F, rapidly cooling combustion gases below the de novo synthesis temperature range and 

minimizing dioxin formation potential. 

3.1.7 Cost and Economics 

Thermal treatment is generally the most cost-effective remedial technology for SVE off-gas treatment 

when high VOC concentrations are involved, and this technology offers the advantage of permanent, 

efficient contaminant destruction within a relatively short time-frame. The costs to design, build, install, 

and operate thermal treatment systems for SVE off-gases are driven by several factors, including the 

expected waste stream flow rate, waste stream composition and changes in composition over time 

(which impacts metallurgy and post-combustion treatment requirements), the degree of heat recovery, 

and the required DRE. 

It is difficult to compare costs for different thermal treatment systems because of the large cost impact 

of key site-specific variables (such as degree of heat exchange, type and cost of energy available, and 

concentrations and types of contaminants treated). Standard cost curves depicting cost versus 

capacity are generally not available for these reasons, and costs are quoted on a site-specific, caseby-case basis; however, some cost guidelines based on case studies are provided for both capital and 

operating costs. 

3.1.7.1 Capital Cost 

Capital costs include the design, procurement, construction, and installation costs for the thermal 

treatment system. Key variables that influence capital costs include the following: 

• SVE off-gas flow rate and composition 

• Presence or absence of a catalyst 

• Degree of heat recovery desired 

• Requirements for management of residuals (post-thermal treatment requirements) 

• Expected life (and reuse) of the equipment 
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A key management tool for capital cost control is flexibility – the portability of the system for use at 

other SVE off-gas treatment sites (or for other remediation applications, such as treatment of vapors by 

an air stripper) will allow some capital costs to be shared by various users. 

The FRTR estimates the following capital equipment costs for a range of thermal oxidation unit sizes. 

Equipment costs range from $20,000 for a 100-scfm unit, $100,000 for a 500-scfm unit, and $200,000 

for a 2,000-scfm unit (FRTR 2004). Actual capital costs can vary substantially from these estimates, 

depending on the variables described above. The case studies summarized below provide some 

insight into the capital costs for specific site conditions. 

Case Study 1: An electric-powered Cat-Ox system was used for five years to treat off-gas from an 

SVE system in Massachusetts. The main contaminant of concern was toluene. Electricity rather than 

natural gas was selected for supplemental heat energy because of availability issues and ease of use. 

The Cat-Ox system had a capacity 300 scfm, a process catalytic temperature of 630 °F and a 65 

percent effective air-to-air heat exchanger. The cost to purchase the oxidizer was $42,000, and the 

installation cost was $6,000 (ENSR 2004c). 

Case Study 2: A natural gas-fueled thermal oxidizer (non-catalytic) was used for one year for an SVE 

system at an active retail petroleum station in California. The main contaminant in the SVE off-gas 

was gasoline. The specification for the oxidizer included a capacity rating of 300 scfm, a combustion 

chamber process temperature of 1,460 °F and no heat recovery (no heat exchanger). The cost to 

purchase the oxidizer was $32,000, and the installation cost was $8,000. The salvage value of the 

oxidizer after 1 year was about half its original purchase price (ENSR 2004c). 

Case Study 3: A natural gas-fueled Cat-Ox system was installed to treat PCE vapors from an SVE 

system. The design flow rate was 500 scfm. A polyvinyl chloride scrubber was included to treat acid 

gases from the oxidizer, and there was no heat recovery system. The purchase price was 

approximately $250,000, including the scrubber. The DRE for the system was increased from 91 

percent at the start to 99 percent over the first 7 months by adding additional ceramic heat recovery 

elements, effectively increasing the reaction temperature without increasing utility fuel consumption 

(ENSR 2004c). 

3.1.7.2 Operating Cost 

Operating cost is the cost to implement the system once it is installed. Key variables that influence 

operating cost include the type, price, and amount of energy required (most common are electric, 

propane, and natural gas); the extent of safety systems incorporated into the thermal treatment unit 

(which can, for example, influence the amount of energy required if off-gas stream dilution is required 

for safety reasons); the level of O&M required; and, for Cat-Ox systems, the cost of maintaining, 

disposing of, and replacing catalyst. It is important to consider operating costs during SVE off-gas 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

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treatment analysis and selection because these are often the predominant costs over the lifetime of the 

SVE application. 

Typically, the annual energy cost for a Cat-Ox system operating at 100 to 200 scfm ranges from $20 to 

$40 per scfm for natural gas- or propane-fired systems and from $50 to $100 per scfm for electrically 

heated systems. Natural gas costs to run a 100-scfm oxidizer will generally range from $2,000 to 

$4,000 per year, and the energy costs for the same system using electricity for supplemental heat 

would cost $5,000 to $10,000 per year. For gasoline and petroleum compounds, higher contaminant 

concentrations would result in lower fuel use and therefore lower annual operating costs (ENSR 

2004c). 

Energy costs can also be expressed on a daily basis. Typical daily energy costs for a Cat-Ox system 

alone operating at 100 to 200 scfm may range from $8 to $15 per day (for natural gas or propane-fired 

systems) and $20 to $40 per day (for electrically heated systems). A 100-scfm oxidizer operating for 

250 days per year will therefore generally entail $2,000 to $5,000 of annual energy costs, depending 

on whether natural gas or electricity is used for supplemental fuel (ENSR 2004c). In addition, energy 

costs will fluctuate with market conditions. 

The case studies summarized below provide some insight into the operating costs for site-specific site 

conditions. 

For Case Study 1 discussed in Section 3.1.7.1 above, the typical off-gas flow rate was 250 scfm, with a 

maximum contaminant rate of 22 pounds per day (260 ppmv toluene). The remediation system ran 

approximately 70 percent of any year because of seasonal high water tables and winter weather. 

Electric utility rates were 6.5 cents per kilowatt-hour (not including monthly service charges). A total 

mass of 10,000 pounds of contaminant was destroyed over the life of the remediation project. The 

daily electrical cost for running the oxidizer was between $35 per day for near-zero concentrations in 

the off-gas to $25 per day for the highest concentrations of contaminants in the off-gas. The total 

electricity cost for the oxidizer over 5 years was $42,000 (ENSR 2004c). 

For Case Study 2 discussed in Section 3.1.7.1 above, the typical extracted flow from the subsurface 

was 230 scfm, with a maximum contaminant extraction rate of 130 pounds per day (1,600 ppmv) for 

several weeks. One year later, the concentrations fell below 10 pounds per day, at which time the 

oxidizer was replaced with an activated carbon system. During the year of operation, the oxidizer 

destroyed 16,000 pounds of gasoline vapor. The fuel cost for the oxidizer over the year was $48,000, 

or an average cost of approximately $130 per day. If the oxidizer had used electricity for heating the 

off-gas, the average daily cost would have been $350 per day (at an electricity cost of 6.5 cents per 

kilowatt-hour) (ENSR 2004c). 

For Case Study 3 discussed in Section 3.1.7.1 above, natural gas consumption was approximately 

$190 per day ($69,000 per year) at an equivalent fuel cost of 6.5 cents per kilowatt-hour (ENSR 

2004c). 
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3.1.8 Developmental Status 

Thermal oxidation is a well established, effective, and frequently cost-competitive treatment technology 

for SVE off-gases. Research and development for thermal oxidation are primarily focused on 

technology refinements. 

There are many vendors of thermal oxidation systems, although not all vendors supply systems for use 

in SVE off-gas treatment. An example of a Web site that contains information on treatment technology 

vendors is http://www.cluin.org/vendor. 

Additional information and case studies are also available on the FRTR Web site at the following 

address, http://www.frtr.gov/matrix2/section3/sec3_int.html. 

Often the vendors differ in the ancillary equipment offered in a package (for example, heat exchangers, 

concentrators, post-oxidation treatment systems, and control systems). For SVE applications, 

portability is a key factor, and packages on skids and trailers are readily available. 

3.2 Internal Combustion Engines 

The ICE is a thermal treatment technology used to treat SVE off-gases extracted from petroleumcontaminated sites. ICEs are most effective in controlling emissions when high VOC concentrations 

are present in the SVE off-gas. These systems are primarily used for tank degassing operations in the 

initial stage of SVE operation and at gas station sites with substantial amounts of “floating” or residual 

NAPLs present in unsaturated soil. 

The use of an ICE to treat contaminated off-gases can provide site-specific advantages. The 

contaminated off-gas stream is mixed in the carburetor of the engine with air and, if necessary, 

additional fuel, which is then combusted normally in the engine. The advantages of using an ICE for 

destruction of VOCs in contaminated off-gas streams include simplicity of operation, production of 

useful mechanical energy from the engine that can be used to extract the vapors from the wells (no 

additional blower or pump needed), relatively high DREs, and the ability to operate at very high influent 

conditions throughout the explosive range of the contaminants. Numerous standard fuels, including 

natural gas and propane, can provide the auxiliary fuel used in the engine. This technology is normally 

not used to treat chlorinated VOC compounds unless they are co-mingled with petroleum VOCs. The 

commercial vendor Remediation Services, International (RSI, formerly VR Systems Inc.), estimates 

that 200 to 300 ICE systems are currently being used to treat SVE off-gases (ENSR 2004a). A survey 

of Superfund sites from 1982 through 2002 shows that thermal treatment using ICE technology was 

selected for SVE off-gas treatment in only 1 percent of more than 170 sites listed with off-gas 

treatment (U.S. EPA 2004). 
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3.2.1 Technology Description 

Currently, RSI is the only commercial vendor of ICE remediation equipment. This company 

manufactures three system sizes to handle SVE off-gas flow rates from 65 to 500 cubic feet per minute 

(cfm) and to produce vacuums of up to 18 inches of mercury. Figure 3-6 shows a picture of a 

representative ICE system. 

The systems manufactured by RSI are Ford® gasoline-powered engines. Hydrocarbon vapors 

extracted from the subsurface by SVE are combusted as fuel to run the ICE. The ICE provides a 

method for treating SVE off-gases and also can generate the vacuum required for vapor extraction 

from the subsurface; however, the achievable vacuum may not be sufficient for all applications. The 

exhaust gases from the ICE can be passed through a standard automobile catalytic converter for 

complete oxidation or activated carbon before discharge to the atmosphere. Depending on the 

concentration and BTU value of the influent off-gas stream, supplemental fuel such as propane or 

natural gas may be used to maintain smooth operation of the engine because hydrocarbon 

concentrations may fluctuate during SVE system operation (RSI 2005). 

Figure 3-6. Internal Combustion Engine Remediation System 

Source: RSI 2005 

3.2.2 Applicability 

The main factors in determining the applicability of ICE for a particular site are the contaminant’s 

chemical characteristics, the anticipated influent off-gas concentration, and the permeability of the soil 

being treated. ICE systems are used only to treat non-chlorinated VOCs and are becoming more 

common at MPE sites where free-product recovery is being conducted. ICE is most effective at 
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treating high concentrations of VOCs in off-gases (typically 3,000 ppmv or greater). Off-gas with low 

contaminant concentrations (less than 1,000 ppmv) may not be suited for ICE technology. 

The combustion of halogenated VOCs produces corrosive gases that can destroy engine components 

and the catalytic converter. Most ICE systems can handle influent vapor concentrations well above the 

LEL (for example, about 1 percent for gasoline in air) and even approaching the upper explosive limit 

(UEL) (for example, about 7 or 8 percent for gasoline in air). If the sites soils have very low 

permeability (an intrinsic permeability less than 10-10 square centimeters), then the 18 inches of 

mercury vacuum provided by the ICE intake may not be sufficient for SVE operation; however, the ICE 

can generate power for a separate, high-vacuum SVE blower to treat lower permeability soils and still 

provide treatment of the extracted vapors (RSI 2005). ICE units are self-contained, requiring no 

outside power source. This feature is particularly useful in areas where electric power is not readily or 

economically available. 

3.2.3 Limitations 

As previously discussed, ICEs are not capable of treating halogenated VOCs. Other limitations are 

listed below (AFCEE 1998; Archabal and Downey 1994). 

• Relative humidity should be less than 95 percent. High humidity vapors may condense 

following a decrease in temperature or an increase in pressure. Condensation can reduce the 

efficiency of off-gas treatment and degrade system components. An air-water separator can 

be added to minimize condensation. 

• ICEs often have limited warranties. A demonstration project by the Air Force Center for 

Environmental Excellence (AFCEE) found that an ICE used for SVE off-gas treatment can last 

15,000 hours with factory-recommended maintenance (equivalent to 1.7 years of continuous 

operation) (AFCEE 1998). ICEs can be rebuilt as necessary. 

• ICE systems can generate nuisance noise conditions if operated near residential areas. (The 

nuisance can be mitigated with noise suppression insulation and privacy fences.) 

• ICE units are complex systems that require skilled technicians to maintain, and as ICE units 

age, more intensive maintenance is required. 

3.2.4 Performance 

ICEs can effectively treat relatively high concentrations of non-chlorinated VOCs, achieving DREs 

generally between 90 and 98 percent (USACE 2002); however, they have been reported to achieve 

greater than 99 percent DRE for petroleum hydrocarbon vapors given proper air-to-fuel ratios 

(Archabal and Downey 1994). If the air-to-fuel ratio is not correct (for example, if the oxygen or 

hydrocarbon content of the SVE off-gas changes substantially), the DRE drops accordingly. ICE 

systems are most cost-effective when treating vapor streams with high VOC concentrations. Influent 
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concentrations of 30,000 to 40,000 ppmv are usually sufficient to maintain smooth system operation 

without requiring supplemental fuel.

Since 1993, ICE technology has been tested at more than 25 U.S. Air Force Base sites. Each of these 

systems was supplied by RSI. At Patrick Air Force Base (AFB) in Florida, an ICE was used to treat 

gasoline vapors and achieved greater than 99 percent DRE for BTEX compounds and over 96 percent 

DRE for total volatile hydrocarbons (AFCEE 1998; Archabal and Downey 1994). The ICEs operated at 

three AFB sites in Arizona and one AFB site in Washington, DC, achieved DREs of over 99 percent for 

vapor streams containing both gasoline and JP-4 constituents (Archabal and others 1997a and b). 

3.2.5 Engineering Considerations 

RSI’s ICE systems are designed as fully automated systems using data loggers to monitor and control 

system performance. The ICE operating parameters that are monitored regularly by the data logger 

include oil pressure and temperature, coolant temperature, exhaust temperature, percent oxygen in the 

exhaust, extraction flow rate, applied vacuum, air-to-fuel ratio, supplemental fuel consumption, and 

engine operating time. Adjustments of the air-to-fuel ratio and supplemental fuel input rate can be 

made automatically by the on-board computer to maintain acceptable DREs (Archabal and Downey 

1994). 

ICEs require routine maintenance based on hours of engine operation (similar to automobile engines). 

Long-term ICE maintenance includes battery and catalytic converter replacement every 5,000 to 

10,000 hours of operation (Archabal and others 1997a and b; Parker 1992). Also, complex 

maintenance may be required to maintain the proper air-to-fuel ratio, and to troubleshoot the on-board 

computer or data logger when excessive heat, dust, or engine vibrations are present. 

RSI offers several enhancements to its ICE units, including a generator module that can produce up to 

25 kilowatts of power per engine, a load module to provide backpressure and increase VOC DRE, and 

integrated extraction/treatment systems such as vacuum stripping and MPE systems. 

3.2.6 Residuals Management 

With the exception of lubricant oils and coolants (which are recyclable), no major ancillary waste 

streams requiring further treatment or disposal are associated with the ICE technology. As with 

thermal oxidizers, most of the contaminants present in the influent off-gas and the supplemental fuel 

are combusted in the ICE. Any remaining hydrocarbons in the ICE exhaust are oxidized using a 

standard catalytic converter. 

Combustion byproducts such as CO, NOx, and SOx can be generated in an ICE as in a thermal 

oxidizer; however, these compounds are not typically a problem for SVE off-gas treatment using ICE. 

If an ICE is operating at 99 percent DRE, then the production of byproducts may not be an issue; 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

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however, because ICEs require monitoring and maintenance to remain properly “tuned” (as with a car 

engine), the formation of these byproducts may increase between maintenance periods. 

3.2.7 Cost and Economics 

The capital cost of an ICE is most dependent on the flow volume it is designed to handle. The overall 

cost to treat hydrocarbon vapors depends on factors such as the cost and amount of auxiliary fuel 

required. 

ICE appears to be a cost-effective off-gas treatment technology under suitable site conditions. In 

general, the ICE technology becomes more cost-effective with higher influent VOC concentrations. 

The AFCEE (1998) report suggests that the cost for SVE off-gas treatment for a 100-scfm extraction 

rate and 1,000 ppmv of average total vapor hydrocarbon content is about the same as the cost for 

thermal oxidation, about 20 percent more than for a Cat-Ox system, and about 40 percent of the cost 

of treatment using granular activated carbon (GAC). At a 5,000 ppmv average total hydrocarbon 

content, costs for ICE and thermal or Cat-Ox systems for the 100-scfm rate are about the same and 

about 10 percent of the cost of GAC treatment (depending on the cost of fuel for the different thermal 

treatment systems). 

3.2.8 Developmental Status 

ICE systems are commercially available for treating non-chlorinated site remediation off-gases. Based 

on available data, ICE appears to be a cost-effective technology. This cost-effectiveness is likely due 

to the fact that the system provides a method for treating remediation off-gases and also generates the 

vacuum required for SVE system operation. 
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4.0 ADSORPTION TECHNOLOGIES 

Adsorption is a traditional technological approach for treating off-gases containing VOCs, both in 

industrial processes and soil remediation applications. Of SVE projects at Superfund sites that had 

data available about off-gas treatment, 70 percent used adsorption (U.S. EPA 2004). The adsorbent 

most often used to treat SVE off-gas is activated carbon. Other adsorbents include alumino-silicate 

“zeolites” and synthetic polymers. All three classes of materials treat the contaminated waste stream 

by capturing and removing the VOCs through physical adsorption. 

In adsorption, the pollutant is collected on the surface (primarily the internal surface) of a granule, 

bead, or crystal of sorbent medium. The adsorbed compound is held physically and somewhat 

loosely, and can be released (desorbed) relatively easily by either heat or vacuum processes. 

Each material has a different adsorption capacity referred to as the “adsorption isotherm.” This 

capacity is measured in pounds of pollutant adsorbed per pound of adsorbent at a given temperature. 

This isotherm is a function of the contaminant concentration (or partial pressure) in the vapor, the 

temperature, the total ambient pressure, and the adsorptive areas of the medium that the VOCs can 

reach. Because carbon, zeolites, and polymers each have different pore sizes and surface areas vary, 

the adsorption isotherm is different for each material and each type of pollutant. These factors dictate 

the amount of contaminant that each sorbent can adsorb. Selection of an appropriate adsorbent 

material is primarily a function of the contaminant to be adsorbed, but the adsorption capacity of certain 

sorbents may be reduced by the relative humidity of the gas stream. 

Carbon was the first material observed to have a large surface area and the property of decreasing a 

chemical’s ability to escape the closer it approaches the carbon’s internal surface area. Zeolite and 

polymer adsorbents have more recently been found to also have large internal surface areas with this 

property. Each class of adsorbent material has advantages for particular applications. Activated 

carbon has a long history of usage and hundreds of case studies, while other sorbents have been 

applied to SVE treatment system off-gases only more recently. Additional information about 

adsorption technologies can be found in U.S. EPA’s “Technical Bulletin: Choosing an Adsorption 

System for VOC: Carbon, Zeolite, or Polymers” (U.S. EPA 1999) and USACE’s “Engineering and 

Design – Adsorption Design Guide” (USACE 2001). 

4.1 Activated Carbon Adsorption 

Activated carbon is an excellent adsorption medium because of its large specific surface area and 

micropores (smaller than the smallest pores manufactured in polymer adsorbents) of 2 to 500 

angstroms (Å) (1 angstrom = 1 x 10-10 meter). Activated carbon typically has surface areas ranging 

from 800 to 1,400 square meters per gram (Alley 1998). In vapor-phase activated carbon adsorption, 

contaminants are removed from a vapor stream by physical adsorption onto the surface of “activated” 

carbon pellets, beads, granules, or powder. Because the granular form of activated carbon is 
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generally used as the adsorbent, the systems are referred to as GAC systems. Before use, the carbon 

is activated by high-temperature steam pyrolysis in an oxygen-limited environment of coal, wood, bark, 

coconut husks, and other materials to remove all volatile material as a gas or vapor and to leave only 

the carbon. Various starting materials produce GAC with slightly different properties to address 

specific applications. For example, coconut-based carbons generally perform better at higher humidity 

than coal-based carbons. Following activation, carbon may be partially oxidized to enlarge its pores 

prior to use. 

4.1.1 Technology Description 

The treatment process of carbon adsorption is relatively simple. The off-gas from an SVE system (or 

the vapor emissions of an industrial process) is blown or sucked by blowers and vacuum pumps 

through the activated carbon. The vapor-phase contaminants flow through a packed bed or vessel 

containing activated carbon and are adsorbed onto the surface of the carbon until the concentration of 

VOCs in the effluent from the carbon bed exceeds acceptable levels. Most adsorption systems consist 

of one or more vessels connected in series or in parallel. These vessels can be cartridges, canisters, 

drums, tanks, or bins. Typical equipment sizes include 55-gallon drums containing 150 pounds of 

carbon; 8-foot-diameter containers with approximately 4,000 pounds of carbon; and 12-foot-diameter 

containers or larger containing more than 10,000 pounds of carbon. Activated carbon typically sorbs 

10 to 20 percent of its weight; however, when relative humidity is above 50 percent, this capacity is 

reduced by sorption of water. 

The adsorbent system employs fixed, moving, or fluidized beds. In fixed-bed systems, the adsorbent 

is contained within a square or cylindrical chamber, and the contaminated vapor is directed vertically 

downward or horizontally through the chamber. Fixed-bed adsorption systems are the most common 

for SVE off-gas treatment. In moving-bed systems, the adsorbent is contained between two coaxial 

rotating cylinders, and the vapor flows between the two cylinders. As the cylinders rotate, part of the 

adsorbent is regenerated, while the rest continues to remove contaminants from the vapor stream. In 

fluidized-bed systems, the contaminated vapor flows upward through the adsorbent vessel. As the 

adsorbent becomes saturated, it slowly migrates downward in the vessel to a surge bin, where it is 

passed to a regeneration chamber and finally to the top of the adsorbent chamber for reuse. Figure 

4-1 shows a typical regenerable system. 

Activated carbon may be regenerated once it has reached its adsorption capacity. Some types of 

carbon systems are non-regenerable (for example, carbon canisters), and others are regenerable (for 

example, fixed beds). Once saturated with sorbed compounds, the carbon in regenerable systems is 

treated to remove the adsorbed chemicals and allow the carbon to be used again. Regeneration is 

performed by changing the conditions in the bed to desorb the contaminants from the carbon. This 

process is achieved by increasing the temperature using hot air or steam, decreasing the partial 

pressure, or introducing a stronger adsorbed material to displace the VOCs (Rafson 1998). 
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Regenerable carbon can be regenerated in place (generally by steam or hot air regeneration) or at an 

off-site regeneration facility, or it can be disposed when spent. In on-site regenerable systems, one or 

more carbon vessels are typically kept in operation while others are regenerated. Systems using 

steam for on-site regeneration typically include a boiler, a feed water supply and treatment system, a 

condenser, a separator, storage for the recovered contaminants (either a tank or drums), and a 

source of drying air, such as process gas exiting an on-line adsorber or compressor. Systems using 

hot gas for on-site regeneration include gas storage (either in cylinders or tanks) or on-site gas 

generators, heaters for the gas, condensers, and contaminant storage equipment. A separator is 

not usually required for gas systems because the condensate is a single organic phase, but a fan or 

pump for the cooling fluid may be needed. Regeneration may result in some contaminants 

remaining adsorbed and unaltered within the carbon. The adsorption capacity of the carbon will 

likely be reduced by these residual contaminants. The number of times carbon can be regenerated 

is based on these residual contaminants, and when the adsorption capacity becomes too low, the 

carbon requires replacement (USACE 2001). 

Figure 4-1. Typical Regenerable Activated Carbon System 

Source: FRTR 2004 
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4.1.2 Applicability 

GAC systems are robust in that they are not sensitive to rapid changes in extracted vapor 

concentration. Another attribute of GAC systems is that they can be used to treat a wide range of 

VOCs, although some highly polar and/or volatile VOCs (such as vinyl chloride) and VOCs having 

smaller molecules (such as methanol and formaldehyde) do not adsorb well. Thus, GAC systems can 

be used to capture and control most VOC and chlorinated VOC emissions from both site soil 

remediation and industrial processes. GAC can also be impregnated to remove additional 

contaminants, such as hydrogen sulfide, mercury, and ammonia (Calgon 2005; General Carbon 

Corporation 2005). GAC systems are more efficient in removing non-polar organics than either zeolite 

or synthetic polymer systems. 

For high flow-rate industrial applications, GAC systems may be used with other technologies, such as 

condensation or thermal treatment. In some industrial applications, GAC systems act as a 

concentrator to make condensation or thermal treatment more cost-effective. The most common use 

of activated carbon in conjunction with another technology is as a "polishing" process. That is, GAC is 

often used after less effective off-gas treatment technologies such as biofiltration or condensation to 

achieve VOC discharge limits. 

The relatively low initial capital cost of carbon adsorption systems makes them particularly attractive for 

short-term SVE off-gas treatment applications where dilute concentrations of VOCs are present; 

however, carbon usage increases as the concentration of the vapor to be treated or flow rate of the offgas increases (increasing O&M costs). 

A carbon bed will adsorb more VOCs at high vapor concentration than at low concentration; however, 

sustained high vapor concentration will result in faster carbon “breakthrough” (resulting in 

unacceptable VOC concentrations in the effluent). Adsorption systems are most effective (in terms of 

both cost and waste management) in remediation projects involving dilute contaminant concentrations 

(less than 100 ppmv) and moderate flow rates. These relatively low concentrations may be difficult or 

uneconomical to meet using another technology. Adsorption is also common when expected VOC 

emissions are in the range of 500 to 5,000 ppmv initially but are not expected to remain high for long 

periods (Govind and others 1994). For higher concentrations or extended periods of high 

concentrations, thermal treatment (see Section 3.0), membrane separators (see Section 6.3), and 

condensers (see Section 6.4.2) may be more economically feasible. 

4.1.3 Limitations 

In general, GAC is a very robust and cost-effective vapor treatment technology; therefore, it is routinely 

used for vapor treatment applications, particularly SVE off-gas treatment. There are, however, a 

variety of limitations to the use of GAC for SVE off-gas treatment that are important considerations. 
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The main limitation of this technology is the high operating costs associated with adsorbent 

replacement or regeneration when high influent concentrations are present. The use of more 

capital-intensive, on-site regenerative systems can help minimize these costs. Carbon should be 

replaced every 6 months to 5 years, depending on the frequency of regeneration and temperatures at 

which the system operates (U.S. EPA 1999). 

Carbon is neither fully hydrophobic nor hydrophilic and has an affinity for both polar and non-polar 

molecules. Because of its affinity for polar molecules, water is easily adsorbed by activated carbon. 

This factor causes high-humidity off-gas streams to diminish carbon’s adsorptive ability because the 

carbon will preferentially adsorb polar water molecules and a layer of water will form on the surface of 

the carbon. This water film hinders the natural attraction forces of the VOCs to carbon. When the 

contaminated vapor has a relative humidity of greater than 50 percent, zeolite or synthetic polymers 

may be considered as more appropriate adsorbents because they have less affinity for water than 

activated carbon. However, these materials may not be as cost-effective as adding dehumidification 

equipment. 

Temperatures above 100 °F in gas streams also can significantly reduce carbon’s adsorption capacity. 

Activated carbon generally is not used with thermal treatment technologies because of the inherent 

high efficiencies of thermal units and their associated high-temperature effluent. 

Carbon usage increases as the concentration of contaminants in the vapor increases. The associated 

cost of carbon disposal or regeneration also increases as a function of the influent vapor concentration; 

therefore, other vapor treatment alternatives may be more cost-competitive at the higher vapor 

concentrations typically encountered during the initial phases of SVE system operation. Adsorption 

systems can be designed to handle high flow rates and high vapor concentrations, but the costs 

associated with frequent carbon replacement or regeneration may make the use of other off-gas 

treatment technologies (such as thermal oxidation) more attractive. 

Activated carbon is not effective for VOCs with high polarity, such as alcohols and organic acids, or 

high vapor-pressure (highly volatile) compounds, such as vinyl chloride, methyl tert-butyl ether 

(MTBE), or methylene chloride. 

When treating VOCs that are monomers for plastics (such as styrene), the system design needs to 

consider the possibility of polymerization reactions on the GAC bed. This situation may result in 

bridging and clumps in the bed. Polymerization reactions generally require heat input; therefore, this 

factor is of particular concern with on-site regenerable GAC systems. 

Another concern is that certain chemicals, once adsorbed, can cause carbon bed fires. Most 

adsorption of VOCs by activated carbon is exothermic (energy-releasing). Because the heat of 

adsorption is especially high with ketones (such as MEK and MIBK), aldehydes, and similar organic 

compounds (Naujokas 1985), the heat released during sorption can cause carbon to auto-ignite and 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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produce a bed fire. Therefore, special fire protection measures may be needed when treating these 

compounds (USACE 2001). 

Effluent streams can be treated until the concentrations (in gas or the adsorption bed) reach 25 

percent of the VOC’s LEL. At these concentrations, fire safety issues arise and the off-gas must be 

diluted with ambient (clean) air. For most VOCs, 25 percent of the LEL would be in the range of 2,500 

to 10,000 ppmv (see Table 3-1). 

4.1.4 Performance 

Well-designed adsorber systems should achieve 95 to 98 percent DREs at input concentrations of 500 

to 2,000 ppmv (U.S. EPA 1999). At lower concentrations, DREs are generally greater than 98 percent. 

These levels of removal will be achieved until the adsorption capacity of the sorbent bed is reached. 

The carbon adsorption capacity is defined as the mass of contaminant that can be adsorbed onto a 

given mass of carbon until breakthrough occurs. 

There is no theoretical method that consistently and accurately predicts the performance of adsorption 

systems (Rafson 1998). Carbon adsorption is based on the principle of equilibrium partitioning from 

the vapor phase to the surface of the carbon. The carbon adsorption capacity is strongly influenced by 

the contaminant concentration in the process stream and the temperature at which the adsorption is 

taking place. In general, the higher the concentration of contaminant in the vapor stream, the higher 

the contaminant adsorption capacity of the carbon. Conversely, the higher the temperature, the lower 

the adsorption capacity. 

Most carbon manufacturers have empirical adsorption isotherm data (adsorption capacity as a function 

of concentration at a constant temperature) used to predict when the adsorption capacity of a particular 

adsorbent will be reached for specific contaminants at varying influent concentrations. 

As discussed above, the performance of a GAC system in particular can be greatly influenced by the 

relative humidity of the vapor stream (FRTR 2004). Thus, a moist, hot regenerated carbon bed will not 

remove VOCs as effectively as cool dry carbon. Steam regenerable carbon systems typically include a 

drying cycle using clean ambient air for regeneration. Moisture in the GAC bed can also promote 

biological growth on the carbon, which can reduce the surface area in the bed available for sorption 

and provide resistance to air flow through the bed. High particulate loading from influent vapor can 

also reduce flow through the bed. 

4.1.5 Engineering Considerations 

As described in Section 4.1.4, the time until carbon breakthrough occurs may be short for several 

reasons, and frequent carbon replacement or regeneration may be required (for example, when high 

influent vapor concentrations, poorly sorptive compounds [such as MTBE], or very high humidity vapor 
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are present). Two or more GAC beds are typically aligned in series to prevent contaminant emissions 

from exceeding local regulatory requirements if breakthrough in the primary treatment vessel occurs. 

Most fixed-bed adsorption systems consist of one or more adsorbent bed vessels connected in series 

or in parallel. Series arrangements permit more contact time for the contaminant and the adsorbent 

and allow breakthrough monitoring of the effluent of the primary vessel without risking contaminant 

emissions to the atmosphere. Parallel arrangements accommodate higher flow rates and increase the 

overall adsorption capacity of the system. For multi-vessel systems, flexible piping is generally used to 

allow easy interchange between parallel or series configurations. 

Adsorption isotherm data are available from manufacturers for many compounds, and these should be 

consulted during system design. For special circumstances, batch isotherm tests can be used to 

determine system size and carbon usage requirements. 

Linear bed velocities for carbon adsorption typically range from 8 to 100 feet per minute (fpm), 

although depending on the system, velocities as high as 200 fpm have been achieved. Residence 

times are typically a few seconds, but they can be up to hundreds of minutes (U.S. EPA 1991b). 

High temperature and relative humidity in the process stream can reduce the adsorption capacity of 

the system; however, short-term fluctuations in temperature and moisture will not result in significant 

system upsets. Moisture separators are typically used to remove entrained water droplets prior to 

carbon treatment. Although higher temperatures can also reduce adsorption capacity, this effect is 

less severe; therefore, designs sometimes incorporate a small reheater to reduce incoming humidity. 

More efficient humidity control can be accomplished by cooling prior to the moisture separator followed 

by reheating to a lower temperature. For SVE applications, particulate concentrations are typically not 

of concern, so particle filtration “upstream” of the GAC units is not common. In addition, bed plugging 

or masking from particulate matter or biological growth can also diminish the adsorption capacity by 

reducing the carbon surface area available for adsorption (Rafson 1998). 

A fire hazard can also exist during the virgin operating cycle because there is little moisture present on 

the carbon to act as a heat sink and because more contaminants are likely to be adsorbed during this 

cycle than any other cycle. This situation occurs during the initial operation of an SVE system when 

VOC concentrations are high. Because the sorption reaction is exothermic (as described in Section 

4.1.3), the adsorption of relatively large amounts of VOCs on dry virgin carbon can release enough 

energy to raise the temperature of the vapor from 100 to over 250 °F, which can melt plastic piping at 

the outlet of the adsorber. This heat can also cause an explosion or fire hazard, especially if the 

vapors contain flammable or combustible compounds (Rafson 1998). This situation is especially an 

issue when ketones, aldehydes, and similar organic compounds are treated. Most carbon bed fires 

occur when units stay stagnant for prolonged periods of time. To minimize the potential for these fires, 

one or more of the following can be conducted: (1) lowering the temperature by convectional and 

evaporational cooling, (2) maintaining flow at more than 2 fpm, and (3) removing oxygen from the 

system using nitrogen or water (Naujokas 1985). 
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Carbon beds requiring regeneration need to be regenerated in cycles of adsorption and desorption 

until they achieve a stable amount of adsorption capacity and desorption. Because there is equilibrium 

between the captured and escaping molecules, carbon (and some polymers) continue to recapture 

molecules in micropores during regeneration; therefore, after regeneration, carbon is capable of 

adsorbing only 50 percent of the amount adsorbed by the virgin material due to these residual 

contaminants (U.S. EPA 1999). The number of times carbon can be regenerated is based on these 

residual contaminants, and when the adsorption capacity becomes too low, the carbon will need to 

be replaced. This issue must be considered when sizing an adsorbent bed (USACE 2001). 

The traditional carbon adsorber regeneration system uses steam to raise the temperature of the 

adsorbed VOCs and boil them from the carbon. This process is known as “thermal swing 

regeneration” because the temperature usually swings during the process from ambient temperatures 

to 250 to 350 °F. After desorption, cooling air blown into the carbon bed in place of the steam helps to 

dry the bed and remove any excess water that may inhibit future adsorption. If air flow and 

concentration vary, a sensor indicates when regeneration has been accomplished. Thermal 

regeneration has also been performed using other methods such as microwaves and heated nitrogen. 

Purifics ES Inc. also has a patent pending on a technology called Phase Extraction Technology, which 

uses heat to regenerate activated carbon filter beds to treat VOCs and SVOCs (Purifics 2006). 

To enhance solvent recovery from carbon when chemical reuse may be considered and to minimize 

contamination of the solvent by steam, vacuum regeneration may be used. This process uses a 

vacuum pump to lower the pressure below the vapor pressure of the VOCs adsorbed, which causes 

the VOCs to boil off without the use of heat. Sometimes, a purge gas is used to enhance the flow of 

the VOCs. The VOCs can then be separated from the vacuum flow by condensation, compression, or 

membrane separation. If the purity of the separated VOCs is too low to make separation practical, 

they can be destroyed by incineration. 

4.1.6 Residuals Management 

The carbon from SVE off-gas adsorption treatment systems is most often taken off site for 

regeneration or disposal. Less commonly, adsorbents can be regenerated on site. During 

regeneration, the contaminants are thermally desorbed and must be further treated using some other 

technology (usually incineration). Spent carbon designated for disposal may need to be managed as a 

hazardous waste (FRTR 2004). 

4.1.7 Cost and Economics 

The capital cost of a carbon adsorption system is almost directly proportional to the off-gas flow rate 

and concentration. The total cost of an activated carbon system is generally driven by the amount of 

carbon used (an O&M cost), which is a function of the amount of time the SVE system is operated. 

The amount of carbon needed also impacts the capital costs in terms of blower pressure drop, bed 
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size, and, for on-site regenerable systems, boiler and condenser size. Based on a 20,000-cfm fixed 

bed system, Rafson (1998) indicates that the purchase price of an activated carbon adsorption system 

ranges from $20 to $25 per cfm. 

Portable canister systems are typically used for SVE off-gas treatment applications and generally have 

lower purchase costs than permanent, large-scale industrial systems. A typical 180-pound canister 

costs approximately $600 for the vessel, carbon, and connections, without taxes, freight, or installation. 

Table 4-1 lists costs for larger sorption systems (RTN 1999a). These costs include initial filling with 

carbon. The cost range is based on whether virgin or regenerated carbon is initially provided and is 

consistent with cost estimates provided by other carbon vendors. 

Table 4-1. Estimated Capital Cost Range for Vapor-Phase Activated Carbon Treatment Units 

(Including Carbon) 

Canister Size Diameter 

(feet) 

Capability 

(cfm) Capital Cost Range 

1,000-pound unit 8 6,000 $6,000 - $7,500 

2,000-pound unit 12 8,000 $7,000 - $8,500 

Source: RTN 1999a 

Calgon reports typical installation costs of $3,200 and $4,600 for the 1,000- and 2,000-pound units, 

respectively. Activated carbon replacement cost ranges from $0.85 to $2.00 per pound, depending on 

mesh size, activity preparation, and volume, with a median price of $1.50 per pound. Reactivated 

carbon replacement costs per pound would be approximately 25 percent less. Annual maintenance 

costs range from 3 to 10 percent of the installed capital costs (RTN 1999a). 

4.1.8 Developmental Status 

Activated carbon adsorption is a mature, demonstrated, established technology and is frequently part 

of remedial designs. This readily available technology is offered by many vendors. 

4.2 Zeolite Adsorption 

In a zeolite adsorption treatment system, hydrophobic zeolite packing is the adsorption medium 

instead of activated carbon. The process components of a zeolite adsorption system are similar to 

those of a GAC adsorption system in that contaminants are captured and removed from a vapor 

stream through physical adsorption. 

4.2.1 Technology Description 

Zeolites act like a reverse filter to capture small molecules while letting larger molecules pass through. 

They are sometimes referred to as “molecular sieves” because of their crystalline structures with 
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uniform and regularly spaced pores (U.S. EPA 1998). The pore sizes of zeolites can range from 

approximately 8 Å (Munters Zeol) to 13 Å (Alley 1998). Zeolite crystals also have specific surface 

areas of approximately 1,200 square meters per gram, which is comparable to the surface area of 

activated carbon (Alley 1998). Zeolites are also capable of selective ion exchange. 

Natural zeolites are hydrophilic, anhydrous, aluminosilicate minerals found in volcanic rock and ancient 

seabed sedimentary formations. Approximately 40 natural zeolites have been identified (U.S. EPA 

1998). Synthetic zeolites may be either hydrophilic or hydrophobic and are manufactured to have 

specific properties. For example, synthetic zeolites can be made hydrophobic to provide them with an 

affinity for non-polar compounds such as many VOCs or can be chemically-enhanced to target specific 

contaminants. Zeolites (both natural and synthetic) are packaged in a variety of container sizes for 

vapor stream treatment of SVE off-gases, or they can be placed into containers at the point of use. 

Once the sorbent bed is saturated, the zeolite material can be regenerated and reused. However, the 

desorbed contaminants must subsequently be disposed of or treated by some other process. 

Zeolites may be synthesized to have specific properties. For example, zeolites can be made 

hydrophobic to provide them with an affinity for non-polar compounds such as many VOCs or can be 

chemically-enhanced to target specific contaminants. Zeolites (both natural and synthetic) are 

packaged in a variety of container sizes for vapor stream treatment of SVE off-gases, or they can be 

placed into containers at the point of use. Once the sorbent bed is saturated, the zeolite material can 

be regenerated and reused. However, the desorbed contaminants must subsequently be disposed of 

or treated by some other process. 

Because of zeolite’s ability to adsorb at high humidity, its resistance to burning, its higher DRE for 

VOCs at lower concentrations, and its more complete regeneration, zeolites could be more 

advantageous in certain system applications than activated carbon. Figure 4-2 shows a typical zeolite 

adsorption system. The system shown includes a zeolite rotary concentrator, two heat exchangers, 

and an optional catalyst. 

4.2.2 Applicability 

Zeolite adsorption appears to have limited use in soil and groundwater remediation applications, 

although it is widely applied in air pollution control technologies for industrial applications. Currently, 

there are no reported applications of zeolite adsorption to treat SVE off-gases, according to the 

references used for this report. However, Munters Corporation has a system operating at a U.S. 

government-owned facility to treat chlorinated vapors generated from a groundwater remediation 

project (ENSR 2003b). 
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Figure 4-2. Zeolite Adsorption System 

Source: Munters Corporation 2005 

Zeolite systems appear to be primarily used in industrial applications as concentrator systems in 

conjunction with thermal oxidation (U.S. EPA 1995). Inorganic crystalline zeolite is the only 

hydrophobic adsorbent not damaged by temperatures of up to 1,800 °

F (Munters Corporation 2005). 

Zeolites can be used to treat vapor streams containing NOx emissions, most chlorinated VOCs, and 

non-chlorinated VOCs. According to U.S. EPA (1995), Munters Corporation’s hydrophobic zeolites 

can also be used to effectively treat high boiling-point solvents. Highly polar and volatile VOC 

degradation products such as vinyl chloride, formaldehyde, sulfur compounds, and alcohols are better 

adsorbed by hydrophilic zeolites than by activated carbon (U.S. EPA 1995). Hydrophilic zeolites 

impregnated with potassium permanganate are also effective at removing polar substances such as 

sulfur compounds, alcohols, vinyl chloride, and formaldehyde. 

Current zeolite systems generally treat high-flow and very-low concentration vapor streams less 

commonly encountered for SVE off-gas treatment applications. This may explain their limited use for 

SVE off-gas treatment to date. The influent concentration range for 32 Munters Corporation systems 

reported in U.S. EPA (1995) ranged from 20 to 150 ppmv for industrial influent vapor streams 

containing petroleum compounds, paint solvents, and plastic fumes. Some of the zeolite adsorption 

systems identified in U.S. EPA (1995) were stand-alone, fixed-bed systems with flow rates ranging 

from 3,000 to 30,000 cfm. 
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An advantage of hydrophobic zeolite adsorption over carbon adsorption systems is that it can be 

applied to humid vapor streams without adversely affecting the working capacity of the adsorption bed. 

The zeolite’s pore size is determined by its strictly regular crystalline structure, whereas carbon has a 

broad range of pore sizes. Carbon tends to adsorb large highly volatile organic compounds such as 

naphthalene, but zeolite does not adsorb large molecules. Highly volatile contaminants will therefore 

gradually block smaller carbon pores, preventing further adsorption and decreasing carbon’s 

adsorptive capacity. Highly volatile organic compounds are also difficult to desorb. In contrast, the 

narrow pore size of zeolite does not allow organic molecules to enter the zeolite structure (Munters 

Corporation 2005). 

Zeolite has a greater sorption capacity than carbon because of its more consistent pore size. This 

uniformity can be particularly important in VOC abatement applications that require a high percentage 

removal rate from low-concentration inlet streams, such as those commonly associated with SVE 

applications, at relatively low capital and operating costs (Munters Corporation 2005). 

4.2.3 Limitations 

Typically, zeolites can adsorb molecules up to approximately 8 Å in diameter such as formaldehyde, 

methane, acetone, MEK, vinyl chloride, phenol, and styrene; however, certain contaminants tend to 

polymerize on this adsorbent, complicating the removal and ultimate destruction of the contaminant. 

Styrene, for example, polymerizes to polystyrene, which has a high boiling point and large molecular 

weight, making it relatively impossible to desorb without very high temperatures. The use of high 

temperatures to desorb contaminants requires fuel and leads to higher operating costs for zeolite 

systems used to treat these types of chemicals. 

Zeolite has a non-linear adsorption isotherm relative to vapor pressures for the molecules for which it 

has an affinity. This non-linearity makes either carbon or polymers the better adsorbent when the 

vapor pressure (or concentration) is higher. 

Zeolites are generally not useful in treating vapor streams with a wide assortment of contaminants 

because not all contaminants may be removed from the stream. Carbon or synthetic polymer 

adsorbents are better suited to treat multi-contaminant waste streams because these adsorbents have 

a wider range of pore sizes. In addition, zeolites allow larger molecules to pass through and the 

potential exists for large-sized molecules not retained in the zeolite to be discharged to the 

atmosphere. 

4.2.4 Performance 

The performance of zeolite adsorption systems is a function of the chemical and physical properties of 

both the adsorbent and influent. The ability of the zeolite to adsorb certain compounds depends on its 

adsorption area and whether the pore size of the sieve is large enough to allow the targeted 
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compounds to pass through. Molecules much larger than 8 Å in diameter are not adsorbed effectively. 

Typical DREs for zeolite adsorption systems range from 95 to 98 percent at inlet concentrations 

ranging from 500 to 2,000 ppmv in air. Zeolite adsorption has been shown to reduce VOC inlet 

concentrations of 400 to 2,000 ppmv to less than 50 ppmv. In some cases, VOC concentrations have 

been reduced to 20 ppmv (U.S. EPA 1999). 

Compared to carbon adsorption, zeolite adsorbers have even higher adsorption capacity at influent 

concentrations less than 100 ppmv (U.S. EPA 1999). At higher inlet concentrations, the relative 

adsorption capacity of activated carbon becomes greater compared to zeolites. The adsorption 

capacity of zeolites is not adversely affected until the relative humidity of the influent streams reaches 

approximately 90 percent (U.S. EPA 1995). Figures 4-3 and 4-4 compare the relative adsorption 

capacities of activated carbon and Munters Zeol (a proprietary product) in terms of inlet concentration 

and relative humidity, respectively. 

The effectiveness of zeolite adsorbent is monitored similarly to GAC’s effectiveness by comparing 

influent and effluent concentrations. As with carbon adsorption, there are limitations to total mass 

loading and as the adsorptive capacity is exhausted, breakthrough occurs, requiring bed change-out or 

regeneration. Zeolite adsorbents have no particular or unique start-up or steady-state use issues, and 

the material is reliable if properly applied. 

Figure 4-3. Comparison of Inlet Concentration Effects on Adsorption Capacity of Adsorbents 

Source: U.S. EPA 1995b 

Note: 

ppm = Part per million 
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Figure 4-4. Comparison of Humidity Effects on Adsorption Capacity of Adsorbents 

Source: U.S. EPA 1995b 

4.2.5 Engineering Considerations 

Regeneration of a zeolite consists of heating or evacuating the zeolite to desorb the hydrating water or 

other material from the adsorption area, similar to carbon regeneration. As with GAC systems, the 

predicted time to breakthrough of a zeolite adsorbent can be determined using empirical adsorption 

isotherm equilibrium data, expected or actual system flow rates, and influent concentrations. In 

choosing zeolites, the effective pore size (sometimes referred to as “window” size) and the effective 

molecular diameter for each VOC to be removed must be known. It is necessary to accurately match 

the pore (or window) size with the VOC molecular diameter to ensure that the VOC will be trapped 

within the bed rather than escape because the pores are too small. 

The adsorption capacity or adsorption isotherm (pounds of VOCs adsorbed per pound of adsorbent) is 

a function of concentration, temperature, area, and pressure. This capacity must be evaluated based 

on the VOCs, flow rates, and expected treatment duration. The critical temperature and pressure (for 

polymerization and oxidation) for catalyzing reactions must be known for each VOC and each zeolite to 

design the system properly and avoid such reactions. The effective bed life or “working capacity” for a 

zeolite adsorption system is substantially longer than for activated carbon. Zeolites have a more 

uniform pore size than carbon or polymers and therefore are able to adsorb about 90 percent of the 

amount of contaminant mass during subsequent regenerations as the virgin material. In fact, zeolite 

replacement after regeneration occurs very rarely. Zeolite beds can withstand high desorption 
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temperatures of up to 1,000 °F, which allows for better VOC desorption during regeneration (U.S. EPA 

1999). For safety reasons, the VOC level after the vapor stream has been concentrated by zeolite 

treatment should remain below one-fourth of the LEL for that mixture of compounds. 

Compounds not easily adsorbed by activated carbon or zeolite may be target contaminants for 

chemically-enhanced zeolite treatment. For example, zeolite impregnated with potassium 

permanganate specifically targets acid gases and low molecular-weight aliphatics such as 

formaldehyde, acetone, and vinyl chloride. The zeolite acts as a carrier vehicle for the potassium 

permanganate and provides surface area for the oxidizing reactions to take place between the 

contaminants and the potassium permanganate. Zeolite is chosen as the substrate over other 

materials such as sand or activated alumina because of its porous structure, superior crush strength, 

and lower cost. Six percent of the weight of the final impregnated zeolite material is potassium 

permanganate. The ultimate density of the impregnated material is 60 pounds per cubic foot, which is 

approximately twice the density of activated carbon. The higher density of the impregnated zeolite is a 

factor in the material’s longevity because it is less apt to break apart or turn to dust than activated 

carbon or activated alumina substrates. One manufacturer’s field studies show that 70 pounds of 

impregnated zeolite will remove 1 pound of vinyl chloride from a vapor stream (Hydrosil International 

2005). 

Potassium permanganate-impregnated zeolite is applied most often for the removal of vinyl chloride 

from vapor streams. When hydrated, potassium permanganate forms the products potassium 

hydroxide, manganese tetraoxide, and manganese dioxide. Manganese tetraoxide reacts with vinyl 

chloride to form potassium chloride and carbon dioxide. The carbon dioxide is released while the 

potassium chloride resides in the zeolite. For greatest efficiency, vinyl chloride removal systems may 

have two or three adsorbent beds placed in series. The first is an activated carbon bed that removes 

most non-polar VOCs from the vapor stream but leaves vinyl chloride unaffected. The vapor stream 

passes next through a bed of impregnated zeolite, where the vinyl chloride is removed by reactions 

with the potassium permanganate. A third bed of activated carbon captures any breakthrough from 

the previous two beds. 

4.2.6 Residuals Management 

Similar to carbon adsorption, zeolite technology requires sequential replacement or regeneration to 

remove and destroy VOCs sorbed onto the zeolite bed. This requirement generates residuals that 

may be classified as hazardous waste requiring further treatment or destruction. 

Regeneration can be achieved by either vacuum adsorption or temperature swing adsorption. In 

vacuum adsorption, a vacuum pump lowers the pressure of the adsorbent to below the vapor pressure 

of the contaminants. Consequently, the contaminants boil off the adsorbent without an increase in 

temperature. In temperature swing adsorption, the adsorption bed is subjected to steam or a different 

source of heat, allowing the contaminants to boil off. During regeneration, the temperature swings 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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from ambient temperatures to 250 to 350 °F (U.S. EPA 1999). The concentrated air stream is often 

fed into an oxidizer where the contaminants are destroyed. 

4.2.7 Cost and Economics 

The cost of manufactured hydrophobic zeolites ranges from $6.50 to $40.00 per pound. Generally, a 

minimum of 1 ton of zeolite is needed for treatment of VOCs, and this amount can handle up to 8,000 

cfm (U.S. EPA 1998). Similar to activated carbon systems, zeolite system costs depend greatly on 

factors such as vapor flow rate and VOC concentrations. As examples, a unit sized for an influent 

concentration of 110 ppmv costs $1,000 to $1,500 per ton of VOCs removed. This cost increases to 

approximately $3,250 per ton of VOCs removed for a stream with an initial concentration of only 20 

ppmv (Munters Corporation 2005). By comparison, activated carbon costs range from $1.40 to $2 per 

pound ($2,800 to $4,000 per ton) (U.S. EPA 1998). 

A zeolite rotary concentrator system sized for a 970-scfm vapor flow rate with a less than 110-ppmv 

VOC influent concentration costs approximately $67,000. Including a Cat-Ox that would then thermally 

treat the desorbed VOCs would increase the cost to approximately $160,000. A fuel savings of 80 

percent can be achieved by a zeolite rotary concentrator/oxidizer system compared to a conventional 

catalytic oxidation system without the zeolite rotary concentrator (Munters Corporation 2005). 

Zeolite systems regenerated using pressure swing adsorption at room temperatures do not require a 

source of heat during regeneration and therefore would allow some cost savings over activated carbon 

systems. As described in Section 4.2.3, certain compounds tend to polymerize on the zeolites, 

requiring added costs for desorption. 

4.2.8 Developmental Status 

Zeolites are naturally occurring and manufactured materials used extensively in several commercial 

applications. Zeolites are used fairly extensively for industrial air pollution control; therefore, the 

technology is relatively well developed. As described in Section 4.2.2, however, there are few, if any, 

commercial applications of zeolite adsorption for SVE off-gas treatment. 

Because zeolites have had limited use for treating SVE off-gases, there is limited regulatory 

awareness of this adsorbent. Zeolites challenge activated carbon as the preferred adsorbent in certain 

fixed-bed adsorbers. Zeolite has reportedly been used instead of carbon in systems in Europe (U.S. 

EPA 1998). 

Vendors of this technology are listed below. This list is not meant to be comprehensive and was 

developed from mention in literature and from Web sites providing descriptions of technologies 

applicable to SVE off-gas treatment. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use.
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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• Munters Corporation; Amesbury, MA; www.munters.com

• Hydrosil International, Ltd.; Elgin, IL; www.hydrosilintl.com

• PQ Corporation; Berwyn, PA; www.pqcorp.com

• Grace Davison; Columbia, MD; www.gracedavison.com

• Dedert Corporation; Olympia Fields, IL; www.dedert.com

• Dürr Industries; Plymouth, MI; www.durr.com/en/

• Zeolyst International; Valley Forge, PA; www.zeolyst.com

4.3 Synthetic Polymer Adsorption 

Polymer adsorption is similar to carbon and zeolite adsorption in that contaminants are captured and 

removed from a vapor stream through physical adsorption. The process components are similar to 

those of a GAC or zeolite adsorption system. In general, polymeric adsorbents (such as plastics, 

polyesters, polyethers, or rubbers) are not considered highly selective of the VOCs adsorbed. They 

are more expensive than carbon but do not need replacement as frequently. Relative to carbon, they 

are less sensitive to humidity, less subject to fire, and less subject to loss of integral structure. The 

polymers can be regenerated as the other adsorbents can, and synthetic polymers have been 

observed to desorb more quickly than carbon. 

4.3.1 Technology Description 

Polymer adsorption uses synthetic polymer adsorbent packing instead of activated carbon or zeolites. 

Polymeric adsorbents have manufactured pores, and pore sizes range from macro-porous through 

molecular sizes. The smallest pore size is still larger than the micropores of activated carbon (U.S. 

EPA 1999). Polymer adsorbents are used as granules or beads and are usually plastic. 

The use of polymer adsorbents for SVE off-gas treatment is not particularly well developed compared 

to the use of activated carbon, but polymer adsorbents are more commonly used than zeolites. Dow 

Chemical Company developed a polymer adsorbent (DOWEX OPTIPORETM) in 1997 designed for 

adsorption of chlorinated VOCs from SVE and groundwater off-gases generated from site remediation 

(RTN 1999a). Thermatrix Inc. (Thermatrix; formerly PURUS, Inc.) is currently marketing the PADRE®

system for SVE off-gas treatment in the United States. In general, the system resembles other 

activated carbon and zeolite filter bed systems but uses a hydrophobic polymer adsorption medium 

made by Dow Chemical Company. A regeneration loop feed from the adsorbent bed allows air flow 

through a chiller and condenser, where contaminant chemicals are separated and drain to a waste 

storage tank. Figure 4-5 shows a typical polymer adsorption system. 
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Figure 4-5. Polymer Adsorption System 

Source: AFCEE 1995 

4.3.2 Applicability 

Because polymeric adsorption is non-selective in nature, this technology is applicable to a wide range 

of VOCs and chlorinated VOCs and has been shown to be effective in various applications, including 

SVE off-gas treatment. Thermatrix reports that the PADRE® system treated off-gases from an air 

stripper containing PCE, TCE, TCA, and DCE at concentrations ranging from 1.8 to 9 ppmv (RTN 

1999a). Other chemicals successfully treated using this polymeric adsorption technology include 

freons, toluene, xylenes, aldehydes, ketones, and alcohols. 

Polymer adsorption systems can be applied to a wide range of vapor flow rates. One manufacturer 

reports that its systems have been used to treat vapor flow rates ranging from 100 to 10,000 scfm and 

can accommodate mass loadings of up to 30 pounds of VOCs per hour (RTN 1999a). 

Polymer adsorbents are much less sensitive to humidity compared to activated carbon, so they can be 

applied to off-gas streams with humidity greater than 90 percent without the loss of adsorption 

efficiency. Polymers are usually hydrophobic and do not adsorb water readily; however, they can be 

specifically made with other affinities. For example, amphophilic block co-polymers can have both 

hydrophilic and hydrophobic affinities (U.S. EPA 1999). Polymers, like carbon, have a linear 

adsorption isotherm relative to vapor pressure (concentration) of VOCs; therefore, they are similar in 

usefulness when concentrations of influent vapors are higher. 
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The polymer bed capacity is somewhat lower than for zeolites but somewhat higher than for carbon. 

Desorption time is less than for carbon or zeolite systems, and desorption requires lower temperatures. 

Polymer adsorbents are also less prone to fires than carbon and are more structurally stable. 

4.3.3 Limitations 

Use of polymer adsorbents is not as effective for low VOC concentrations as carbon and zeolites since 

polymer sorbents have linear adsorption isotherms relative to influent VOC concentrations. The cost of 

polymer adsorbents is in the range of zeolites. For example, DOWEX OPTIPORETM is $14 per pound. 

Also, technical experience and results for polymer adsorbents in SVE off-gas applications are limited. 

One manufacturer’s system is not appropriate for air streams containing vinyl chloride because it is 

difficult to maintain this compound in a condensed form during the system’s regeneration cycle (RTN 

1999a). 

4.3.4 Performance 

Polymer adsorption systems appear to be capable of achieving DREs greater than 95 percent. The 

PADRE®

 process is an ex situ off-gas treatment technology marketed by Thermatrix. As of 1995, 30 

field installations of this system existed, and the system is still commercially available today. This 

system includes a regenerative component and involves one on-line treatment bed for influent air, and 

another bed undergoes a desorption cycle. A controller switches the beds between adsorption and 

desorption cycles. The desorption cycle uses a combination of heat, pressure, and nitrogen purge 

gas. Contaminants are removed, condensed, and transferred as a liquid to a storage tank for 

reclamation and disposal. The PADRE® system has been demonstrated to achieve 95 to more than 

99 percent removal of several chlorinated VOCs, including TCE, PCE, DCE, and TCA. Outlet 

concentrations were all reported as not detected (RTN 1999a). 

A field demonstration of the PADRE®

 system was conducted in 1994 at Vandenburg AFB in California 

to treat hydrocarbon vapors. Maximum soil gas concentrations at this site were 54,000 ppmv for 

hydrocarbons and 400 ppmv for benzene. Treatment flow rates ranged from 20 to 49 scfm during the 

110-day demonstration. Average removal rates for the PADRE® system were greater than 98 percent 

for total hydrocarbons and greater than 99 percent for benzene. The cost of the demonstration was 

$23 per kilogram of hydrocarbon removed (AFCEE 1995). 

General Electric Company evaluated more than 100 adsorbents compared to activated carbon. 

Commercial polyether/polyester block co-polymers and rubber were shown to have a high capacity for 

adsorbing chlorinated VOCs in saturated vapors, but their capacity at low concentrations was orders of 

magnitude below that of the other types of sorbents. This characteristic would severely limit the value 

of synthetic polymers in environmental remediation applications. One polymeric sorbent, DOWEX 

OPTIPORETM, was observed to be an excellent alternative to activated carbon. In general, it desorbed 

faster and at lower temperatures than carbon and had at least as great or larger adsorption capacity in 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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either high or low humidity (U.S. EPA 1999); however, the maximum temperature for desorption must 

exceed the boiling point for the VOC and not exceed the polymer’s melting point. 

Polymer materials can be regenerated using a combination of relatively low-temperature heat 

(compared to GAC), pressure, and a nitrogen purge gas. Polymers vary from 50 to 90 percent in their 

ability to adsorb contaminants after regeneration compared to the virgin material; carbon only has a 50 

percent adsorption capacity after regeneration. Adsorbent beds used in the PADRE® system have 

been recycled on a test basis more than 2,000 times, with no measurable loss of adsorption capacity. 

Similar to zeolite adsorbents, polymeric adsorbents rarely need replacement (only slightly more 

frequently than zeolite) because they are less sensitive to humidity and less subject to fire, crumbling, 

or powdering (U.S. EPA 1999). 

4.3.5 Engineering Considerations 

The use of polymer adsorption systems is similar to that of carbon and zeolite adsorption systems. 

Polymer adsorption systems can be designed as stand-alone units or combined in series and used as 

add-on polishing treatment for less effective technologies. The effect of regeneration (as discussed in 

Section 4.1.5) should be considered when the size of the synthetic polymer adsorption bed is 

designed. This effect will reduce the working capacity and can therefore require up to twice as much 

adsorbent in the bed. The capacity of the bed is based on the chemicals in the contaminant stream, 

the concentrations of the chemicals, the air flow rate, the working capacity, and the regeneration 

frequency. Because polymer regeneration is not needed as often as carbon regeneration, system 

designers should perform a cost-benefit analysis to determine if off-site remote regeneration or on-site 

regeneration is the most desirable. 

4.3.6 Residuals Management 

A residuals management concern for polymeric adsorption is that, like other adsorption technologies, 

VOCs are captured and removed from air only. Follow-up treatment of the desorbed contaminants 

may be required. The use of low temperatures during regeneration allows for more efficient 

reclamation of solvents and other recyclable materials. 

4.3.7 Cost and Economics 

Limited cost information is available for polymer adsorbents. Information provided by a U.S. EPA 

technical bulletin comparing carbon, zeolites, and polymer adsorbents indicates that the cost of 

synthetic polymers is as much as 20 times that of activated carbon (U.S. EPA 1999). DOWEX 

OPTIPORETM is $14 per pound (RTN 1999a). The replacement frequency of polymer sorbents is far 

less than for the other adsorbents. 
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Factors that significantly impact unit prices include initial contaminant concentration, volume of gas 

stream to be treated, and target cleanup levels. For example, polymeric adsorption is generally not 

cost-effective for streams with VOCs concentrations of less than 100 ppmv. 

4.3.8 Developmental Status 

Polymeric adsorption systems are a readily available technology for a variety of industrial applications 

but do not appear to be used very frequently for SVE off-gas treatment applications. Consequently, 

there is not much experience in the use of these sorbents for remediation applications. As of 

September 1994, there were 23 PADRE® systems installed across the country (AFCEE 1995). 

Polymeric adsorption systems likely have not been used as often for SVE off-gas treatment because of 

the higher costs of the sorbent material and the substantial historical use of carbon for SVE off-gas 

treatment. 

In 1995, a site demonstration was performed at McClellan AFB in California. The 2-month study was 

conducted using an elastomeric polymer filter medium to treat VOCs and petroleum hydrocarbons in 

the vapor phase from a SVE system in operation at the site. The filter medium was a blend of crosslinked polymer and activated carbon (PetroLOKTM PL22 by Advanced Water Systems). The site 

contained soils impacted with TCE, cis-DCE, benzene, and vinyl chloride. One-hundred-pound 

canisters placed between an air-water separator and a blower were tested and compared. The 

canisters included the polymer/carbon blend and the polymer alone operating at flow rates of 11 to 35 

cfm and inlet VOC concentrations of 350 to 740 ppmv. The polymer/carbon blend medium had a DRE 

of 89 percent during the test but a maximum adsorptive capacity of only 15 percent. When the inlet 

concentration was diluted 50 percent, the DRE increased to 99 percent. The polymer medium alone 

was not observed to remove any VOCs from the off-gas stream. The overall performance of the 

polymer/carbon media was about the same as the GAC used for the existing SVE system. 

Under the U.S. EPA’s “Waste Reduction Evaluation at Federal Sites,” the PADRE® system was 

successfully demonstrated at the Tinker AFB for the capture and recovery of MEK used in paint 

stripping (RTN 1999a). 

Vendors of this technology are listed below. This list is not meant to be comprehensive and was 

developed from available literature and from Web sites providing descriptions of technologies 

applicable to SVE off-gas treatment. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use. 

• Thermatrix (formerly Purus Inc.); Blue Bell, PA; www.thermatrix.com

• Advanced Water Systems; Woodinville, WA; www.advanced-water.com

• DOW Chemical Company; Midland, MI; www.dow.com
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5.0 BIOFILTRATION TECHNOLOGIES 

Biofiltration processes biologically degrade VOCs present in remediation off-gas streams using 

microorganisms located on a fixed media. Throughout the 1960s, 1970s, and 1980s, biofiltration has 

been used increasingly throughout Europe (particularly in Germany and the Netherlands) and Japan 

as an accepted technology for controlling VOCs and odors. The technology has treated various VOCs 

and odor-producing compounds (such as hydrogen sulfide, mercaptans, and air toxics) from a variety 

of sources, including wastewater treatment plants and chemical and food processing facilities. By the 

1990s, as many as 500 biofilters were in operation throughout Germany and the Netherlands (Leson 

and Winer 1991). 

Use of biofiltration as an air pollution control technology began in the United States during the 1990s. 

The technology has been used to treat odors and HAPs from several sources, including composting 

facilities, biological wastewater treatment plants, petroleum refineries, commercial bakeries, and 

chemical manufacturing facilities. In the past 5 to 10 years, a number of different industries in the 

United States have accepted biofiltration as an alternative to thermal and sorptive VOC control 

technologies. Biofiltration can be a low-cost technology for treating off-gases generated from SVE 

systems when BTEX components are the contaminants of concern, and DREs are generally greater 

than 90 percent. Biofiltration offers the advantage of destroying the contaminant compared to carbon 

adsorption, which merely transfers the contaminant to an adsorptive medium requiring further 

treatment or disposal. However, according to several principal companies installing biofiltration units, 

fewer than 20 full-scale biofiltration systems are currently treating off-gas vapors from soil and 

groundwater remediation applications. In addition, many projects conducive to SVE and biofiltration 

are also candidates for bioventing, unless air injection is infeasible for practical or safety reasons. 

5.1 Technology Description 

Biofiltration occurs within a bed or vessel of biologically active filter material called the “treatment cell.” 

This filter material acts as a support matrix for the location and attachment of microorganisms that 

eventually form an aqueous biofilm. The microorganisms can include bacteria, heterotrophs, 

oligotrophs, and fungi, and can be naturally occurring or added to the filter. The biofilm resides in the 

moisture (water) layer, which coats the filter. Sometimes the filter also provides nutrients for the 

microorganisms. As the contaminated vapor stream passes through the treatment cell, the filter does 

not trap contaminants as in adsorption but rather retains them so that the organic contaminants diffuse 

through the biofilm formed around the solid filter material. The microorganisms within the biofilm obtain 

primary energy and carbon by oxidizing (consuming) the organic contaminants. The resultant end 

products are usually carbon dioxide and water, plus mineral salts if the constituents contain molecules 

such as sulfur, nitrogen, and chloride. The microorganisms regenerate themselves and ultimately die 

and are recycled. 
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A typical biofiltration system (both for SVE off-gas and industrial vapor stream treatment) involves the 

general configuration and process components summarized below (Swanson and Loehr 1997). 

Particulate Removal: Pre-treatment of the waste stream for particulate removal is performed to protect 

downstream system components from particle build-up or clogging. Pre-treatment can be 

accomplished with simple screen filters, more complex Venturi scrubbers, or electrostatic precipitators. 

Flow Equalization: Depending on the design of the biofiltration system, the performance of the system 

and microorganisms are often slow to respond to transient spikes in influent VOC concentrations. The 

system can also be upset by such “shocks;” therefore, a flow equalization vessel (sometimes 

containing GAC) leading to some type of diffuser may be used to dampen spikes and better distribute 

peak VOC loadings on the biofilter unit. Uniform distribution of the influent stream is required to ensure 

that the vapor contact time with the biofilm is adequate for consistent contaminant DREs. 

Humidification Regulation: Humidity is the single most important parameter affecting biofilter 

performance. The influent vapor stream’s relative humidity should be as close to 99 percent as 

possible. Moisture content is typically maintained by humidifying the influent vapor stream before it 

enters the filter bed using equipment such as quench ducts, atomizing nozzles, and packed towers. 

This technique prevents moisture and chemicals targeted for treatment from being stripped from the 

filter material as the vapor stream passes through it. The moisture is also necessary to allow the 

microorganisms (and thus biofilm) to develop. Many systems use an overhead spraying mechanism to 

add moisture and nutrients to the filter material. 

Temperature Regulation: Temperature regulation is also important for both the microorganisms and 

efficient adsorption of the contaminants in the biofilter. For example, mesophilic bacteria that operate 

in biofilters have an optimal temperature range of 50 to 105 °F. Metabolic rates of bacteria can double 

with each 10 °F increase in temperature. Temperature and humidity of the influent vapor stream are 

somewhat interdependent. The influent vapor stream may need to be heated or cooled to the optimal 

temperature for microbial activity prior to being passed through the biofilter. Most SVE off-gas 

treatment systems are exposed to ambient conditions and often require heat input unless insulated 

adequately. Heating can be accomplished by injecting steam or by direct heating of the air stream 

using natural gas or electricity. Cooling can be accomplished with evaporative or forced cooling (heat 

exchanger). Humidification and temperature regulation are generally performed during the same step, 

although heating of the gas stream may sometimes precede humidification. 

Residence Time in Filter Bedding: A typical parameter frequently used to describe biofilters is empty 

bed contact time (EBCT). EBCT represents a standard measure of gas residence time and is used to 

compare the design of different biofilters and the effects of different mass loadings within the same 

biofilter. EBCT is the bulk volume of the filter material divided by the volumetric flow through the 

system; therefore, the inlet gas flow rate (blower design) may affect the mass transfer driving force 

from vapor to biofilm and require adjustments in residence time. The average gas residence time is a 

function of the porosity and moisture content of the filter bedding material. Together, these parameters 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 5-3

comprise the effective porosity of the media. Choice of biofilter media can affect pressure drop and 

compaction rates, which can vary substantially for different media and impact residence time. Actual 

residence time will vary depending on how evenly the air flows through the biofilter. An EBCT of 1 to 2 

minutes is generally considered the upper limit for cost-effective treatment using biofiltration (Leson 

and Smith 1997). Lesser residence times may not result in sufficient treatment, and longer times could 

make the biofilter too big to be cost-effective. 

Measurement and Control: Depending on the location and sensitivity of the biofiltration system, 

operation and performance measurement and programmable logic controller equipment may be 

necessary. Information can be directly observed at the system or by remote monitoring and controlled 

using system collection and data acquisition designs. Typical measurement parameters include pH, 

temperature, head loss, moisture, air flow rate, influent and effluent concentrations, biomass, and 

DREs. 

Biofiltration reactor units can vary greatly in size and shape depending on the contaminants to be 

removed from the waste stream and the required DRE. Most biofiltration systems used to treat lowflow remediation off-gas streams are fully enclosed vessels or columns. Column systems may range 

in size from a few inches to a few feet in diameter and from approximately 3 to 10 feet in height. Many 

small systems employ 55-gallon drums packed with filter media. One or more biofiltration vessels can 

be stacked or aligned in series to improve performance. Other types of biofiltration systems are opentop bed systems. Typical SVE system flow rates range from 100 to 1,500 scfm. Large systems are 

usually rectangular and resemble light industrial/commercial buildings. These systems are mainly 

used to treat industrial process emissions with high flow rates. One of the largest industrial biofiltration 

systems in the world is located in Germany and has 85,000 cubic feet of filter material. In the 

Netherlands, there are biofiltration systems with flow rates of up to 240,000 cfm. Figure 5-1 shows a 

schematic diagram of a typical biofiltration process. 

Figure 5-1. Typical Biofiltration System 

Source: U.S. EPA 1995b 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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5.2 Applicability 

Biofiltration has been used successfully in commercial applications in the United States and Europe 

but does not have a long history or a large number of case studies for use with SVE systems. 

Commercial applications include petrochemical, furniture manufacturing, various food industry, and 

printing operations. Vendor information indicates that biofilters can also be used to control chemical 

and odor emissions from a variety of manufacturing operations, including petroleum refining, chemical 

processing, wood and paper processing, wastewater treatment, and paint spraying (RTN 1999a). 

Specific classes of compounds that are readily biodegradable in biofilters include mono-aromatic 

hydrocarbons, alcohols, aldehydes, and ketones. Biofilters have also been used with mixtures of 

VOCs including compounds such as hydrogen sulfide that create problematic byproducts when 

oxidized, such as sulfuric acids that kill the microbes. In such cases, two-stage reactors can be 

constructed. The first stage can use (1) non-compacting inert media that are acid-resistant to treat the 

low-pH hydrogen sulfide or (2) calcium carbonate minerals to neutralize the pH. The second stage 

then treats the other VOCs at a more neutral pH. Depending on the application, biofilters can also be 

designed to accept continuous low flows of aqueous waste streams. These types of biofilters are also 

known as “biotrickling filters.” Biotrickling filters can operate as up-flow reactors with open tops and as 

enclosed, insulated reactors with engineered media. 

In the last few years, approximately 100 smaller systems (less than 500 cfm) have been used to treat 

landfill gas collected using SVE as well as gasoline vapors from SVE systems at many U.S. service 

stations (Skladany and others 1995; Bohn Biofilter Corporation 2005). Biofiltration is most effective in 

treating vapor streams from SVE systems remediating leaking USTs at gas stations. These systems 

require the destruction of aliphatic petroleum hydrocarbons as well as aromatic compounds such as 

BTEX compounds. Bacteria degrade simple short-chained compounds first, then successively larger 

compounds (such as C-8 aliphatics), then aromatics. Hydrocarbons up to the heavier constituents in 

JP-4 and JP-5 jet fuels (which are likely to be removed by a typical SVE system) are the simplest 

compounds to treat using biofiltration. Higher molecular weight compounds require longer EBCT for 

complete oxidation, making removal of these compounds by biofiltration somewhat less economical. 

Biofilters may be used to treat relatively dilute VOC concentrations of typically less than 1,500 total 

ppmv, although concentrations as high as 5,000 ppmv can be treated. Treatment efficiency depends 

on the mix of the compounds and the duration of the mass load. If concentrations vary widely in the 

influent over time, the bacteria may not adapt to the variety of compounds or high concentrations. 

Higher concentrations may become toxic to microorganisms and inhibit biodegradation of the waste 

stream (Leson and Winer 1991; RTN 1999b, c, and h). 

Biofilter systems have been designed and installed to treat vapor flow rates ranging from less than 100 

to approximately 240,000 cfm (RTN 1999a). For most SVE applications, system flow rates typically 

range from 100 to 1,500 scfm. In general, higher vapor or mass flow rates require larger biofilter 

designs to maintain the required DRE and necessary EBCT. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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SVE off-gas treatment systems are typically designed with emphasis on high initial expected off-gas 

contaminant concentrations and an assumed short duration of SVE system operation. Based on these 

criteria, biofiltration would likely be limited for most SVE off-gas applications; however, off-gas 

concentrations generally quickly decline to substantially less than initial levels. In addition, SVE 

remediation systems are often operated for longer than initial design expectations. When these factors 

are taken into account, biofiltration may be applicable to a greater number of sites. 

Biofiltration systems can be designed to operate with other off-gas treatment technologies as well. The 

most common companion technology used with biofiltration is activated carbon to "polish" the biofilter 

effluent. In this way, temporary system upsets are readily managed. 

5.3 Limitations 

A significant limitation to biofiltration is its sensitivity to variations in operating parameters such as 

moisture content, temperature, pH, and nutrient levels. This sensitivity is likely because the system 

incorporates living microorganisms. Constant monitoring and maintenance of these operating 

parameters is necessary to sustain the microorganisms and achieve the required contaminant removal 

rates. System performance can also be upset by a build-up of biomass within the treatment cell; 

therefore, frequent backwashing of the system is required. 

Most biofiltration systems in operation are used for odor reduction for wastewater treatment facilities 

and food processing operations or as an air pollution control technology for VOC emissions from 

industrial operations. Therefore, another limitation is the lack of data associated with the limited 

number of systems used specifically for SVE applications. 

Most biofilter systems experience an initial period of low or no contaminant removal while the 

microorganisms acclimate to the contaminants and multiply to sufficient quantities. This initial 

acclimation period can be as long as 30 days; therefore, biofilters are not reliable for maximum 

contaminant DREs after installation. Interruptions in system operation will also result in brief periods of 

reduced DREs immediately after system restart. For these reasons, biofilters may not be appropriate 

when intermittent or cycled operation of an SVE system is planned (such as during the later stages of 

site remediation). 

As discussed in Section 5.2, biofiltration is limited to relatively low concentrations of total VOCs (0 to 

5,000 ppmv, with an optimum concentration of no more than 1,500 ppmv). In addition, depending on 

the mass loading and design of the treatment system, a relatively stable variety of influent constituents 

that are degradable is also required. Excessive influent concentrations could be managed by diluting 

the vapor stream with ambient air to avoid this limitation. This dilution may cause the volumetric flow 

rates to increase and the size of the unit to provide the necessary EBCT may no longer be 

economically feasible or practical for the location. 
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The type of contaminants targeted for treatment is also very important. For example, a 50,000-cfm 

vapor stream with an ethylbenzene concentration of only 50 ppm could be treated using biofiltration, 

but the residence time would be about 4 minutes and therefore would not be feasible given economic 

limitations. In contrast, a system with the same flow rate and 50-ppm concentration of MEK would only 

require a residence time of 25 seconds, making biofiltration a very feasible treatment option. 

Another limitation is that DREs can drop as the concentration of the vapor stream drops. Depending 

on the VOCs involved, the mass transfer limitation, and not the biodegradation rate, would control the 

biofilter at low concentrations. For volatile but only slightly soluble VOCs, therefore, diffusion into the 

biofilter may decrease, causing a decrease in DRE. 

Based on the DREs that are attainable for a given biofilter design and influent vapor stream, this 

technology may not be effective in achieving strict regulatory emission standards that exceed 99.8 

percent, for example. This technology can be successful at attaining performance-based reductions 

such as those of some agencies for limiting the total mass discharged to a certain number of pounds of 

VOCs per year. 

Chlorinated VOCs such as PCE and TCE are not effectively treated by biofiltration. These VOCs are 

not highly water-soluble or easy to biodegrade. Highly aerobic conditions achieve good removal of 

light to moderately halogenated organics. Heavily halogenated species appear to require anaerobic or 

co-metabolic (added methane, propane, or aromatics) conditions for at least the first stages of 

decomposition. The limited data regarding the use of biofiltration to treat chlorinated VOCs indicates 

that biofiltration is not yet applicable to these compounds on a commercial basis. Likewise, the current 

generation of biofilters does not address other complex compounds on a commercial scale. One 

exception is MTBE, which studies have shown is destroyed at a high efficiency rate by biofilters (Fortin 

and Deshusses 1999). 

5.4 Performance 

Consistent DREs of greater than 90 percent have been achieved for many common air pollutants in 

industrial off-gas treatment systems. Information regarding performance of biofilters for SVE off-gas 

treatment is only now becoming available as their use increases. To monitor performance, various 

methods are used, including interval and continuous monitoring of influent, effluent, or both. U.S. EPA 

Methods 25 and 25A are often used to monitor total VOCs. Methane is poorly filtered out of most air 

streams and can yield false-positive results because methane is not usually of regulatory concern. In 

addition, some analytical detectors have a low response to certain chemicals, and high relative 

humidity can affect instrument readings. Gas chromatography/mass spectroscopy testing can yield 

better confirmation of influent and effluent concentrations. 

Mass loading is an important parameter that directly relates to the performance of a biofilter. Mass 

loading is the amount of contaminant that can be applied per volume of filter material within a given 

time period. It is typically expressed in units of grams per cubic meters per hour (g/m3

/hr) and ranges 
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from approximately 10 to 150 g/m3/hour. SVE biofiltration systems, which often treat slightly less 

biodegradable hydrocarbons than alcohols and ketones, usually achieve the lower end of this range. 

The mass-loading rate of a particular system is dictated by the required DRE (Swanson and Loehr 

1997). 

The maximum elimination capacity (MEC) is a measure of a biofilter’s capacity to remove 

contaminants at a given mass-loading rate and is used to define the effectiveness of biofiltration 

systems. MECs vary for different chemicals, filter materials, and operating conditions, and are ideally 

equal to or slightly greater than the mass loading rate. Mass loading rates that exceed the MECs may 

result in system clogging and/or toxic conditions that inhibit biodegradation (RTN 1999g). In addition, 

regardless of the DREs of a particular biofilter, the filter will have a finite mass removal limit before it 

requires recharging and re-start-up. This limit depends on compaction, longevity of filter media, and 

limitations on maximum biological growth. 

At optimum conditions, biofiltration has been demonstrated to effectively treat VOC waste streams; 

however, biofilters are relatively sensitive to deviations in system inputs. Fluctuations in contaminant 

concentrations, air stream moisture content, and flow rates will all substantially affect biofilter 

performance. For example, a laboratory study of BTEX removal using biofiltration reported greater 

than 95 percent removal for all BTEX compounds at an EBCT as low as 1 minute and a mass loading 

of 4.2 kilograms of chemical oxygen demand per cubic meter per day (Sorial and others 1997). The 

overall DRE dropped to 88 percent when the mass loading was increased to 6.2 kilograms of chemical 

oxygen demand per cubic meter per day, and the EBCT decreased to 40 seconds. Another study 

reported DREs of greater than 95 percent for the following compounds when the influent 

concentrations of these constituents were below 300 ppmv: styrene, methanol, ethanol, formaldehyde, 

MEK, ethyl acetate, ammonia esters, ethers, and phenol (RTN 1999h). 

Provided that optimum conditions are maintained, a properly designed biofilter should achieve greater 

than 90 and perhaps more than 95 percent DRE. Vendors report better than 99 percent removal of 

hydrogen sulfide, 95 to 99 percent removal of odors (ENSR 2004b), and greater than 90 percent 

removal of VOCs (RTN 1999c). The high DREs occur, however, only after steady-state operating 

conditions have been achieved (Heumann 1998). 

DREs for biofilters are very much related to the biofilter matrix used. Certain materials provide more 

optimum porosity, a wider microorganism population, more adsorption capacity, or better pH buffering. 

In bench-scale testing, a number of organic biofilter media were evaluated for the treatment of 

hydrocarbon fuel vapors from both jet fuel and diesel fuel (Hodge and others 1991). Activated carbon 

was a better filter medium than soil, diatomaceous earth, or a combination of carbon and 

diatomaceous earth. Jet fuel vapors were removed more rapidly than diesel fuel vapors, and the 

concentrations declined linearly, signifying zero-order kinetics with respect to the substrate 

concentration. Chang and Devinny (1996) document a bench-scale study in which GAC, bark 

compost, and yard compost were evaluated for biofiltration effectiveness on JP-4 jet fuel vapors. GAC 
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had a good efficiency and tolerated load shocks well, whereas yard compost had slow acclimation, 

good shock recovery, excellent DRE (more than 99 percent) but significant compaction. 

DREs also vary depending on the type of contaminants treated. Three published studies indicate that 

biofilters can be expected to have DREs of 90 percent or greater for BTEX compounds in off-gas from 

SVE systems but less for total VOCs and total petroleum hydrocarbon (TPH) removal (Leson and 

Smith 1997; Wright and others 1997; Swanson and Loehr 1997). The reduction in DRE depends on 

the site-specific mass transfer from the gas phase to the filter media. Leson and Smith (1997) present 

results from a field pilot study funded by the Petroleum Environmental Research Forum, a consortium 

of several major oil companies that confirmed aromatic removal greater than 90 percent at residence 

times of less than 1 minute. The removal of total VOCs, however, only exceeded 70 percent and 

required substantially longer residence times (Skladany and others 1995). 

The DREs for biofilters used for eight gasoline service station SVE systems in Arizona averaged 90 

percent for TPH, 95 percent for BTEX, and 80 percent for aliphatic petroleum VOCs (Bohn Biofilter 

Corporation 2003). Biofiltration research for MTBE, another compound often present in SVE off-gas 

from petroleum remediation, has begun to show promise. Fortin and Deshusses (1999) report greater 

than 97 percent removal of MTBE when the system was allowed an acclimation period of 6 months. 

From August 1995 to April 1996, a full-scale biofilter was contracted by Black and Veatch to treat jet 

fuel from an enhanced SVE system at March AFB, Site 18 (Chang and Devinny 1996; TRG 2004). 

Bench-scale modeling tests showed compost media to be effective. The full-scale biofilter (18 cubic 

yards) was operated at 100 to 180 cfm continuously 24 hours per day, 7 days per week. Influent 

concentrations ranged from 128 to 1,400 ppmv, and the biofilter had an acclimation period of 2 to 3 

weeks. The DRE of the full-scale biofilter was greater than 90 percent; however, the project was not 

run long enough to determine the longevity of the filter media. Key performance factors included 

consistent airflow, proper insulation of the biofilter, maintenance of adequate moisture conditions, and 

prevention of short-circuiting of the influent air. 

Many chlorinated VOCs are not effectively treated by conventional biofilters because of the function of 

the microbial degradation mechanisms required (anaerobic or co-metabolic). Chlorinated VOCs can 

degrade the biofiltration performance for non-chlorinated VOCs (Heumann 1998). Biofiltration of 

chlorinated VOCs is a field of ongoing research. Aerobic biodegradation of the most prevalent 

chlorinated VOCs (such as TCE) extracted by SVE systems occurs co-metabolically. Cox and others 

(1998) indicate that under appropriate reduction-oxidation and substrate conditions, the DRE for TCE 

could exceed 95 percent for a co-metabolic biofilter intermittently fed toluene as a co-metabolite; 

however, data from several studies showed relatively poor DREs for these types of systems (the 

maximum DRE reported was 74 percent) (Lewis and McPherson 1996; Devinny and Hodge 1995; 

Tonga and Magar 1997). 
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5.5 Engineering Considerations 

Moisture content, temperature, pH, nutrient levels, contaminant concentrations, air flow, and influent 

constituents all must be continuously monitored and maintained at optimum levels for proper biofilter 

performance. Deviations from optimum ranges can result in system upset. Because biofiltration 

systems are sensitive to relatively minor fluctuations of these parameters and because many 

parameters are inter-related, significant engineering effort is required to specify and design a proper 

biofilter for a site-specific application. 

Three important engineering considerations that must be known or at least estimated prior to use of a 

biofilter for SVE off-gas treatment are (1) the chemical constituents in the vapor, (2) the concentrations 

of the system influent constituents, and (3) the allowable effluent concentrations required to meet 

regulatory standards. Pretreatment of the inlet stream can be performed using GAC to prevent 

unacceptable discharges during the initial acclimation period of the biofilter and to control influent 

concentration spikes. However, use of GAC with a biofilter could increase both the capital and 

operating costs. 

If particulates are present in the SVE off-gas, they may clog either a GAC pre-filter or the biofilter. A 

simple bag or screen mesh filter can be used to filter particulates, which are typically measured using 

U.S. EPA Method PM-10. More complex systems for particulate removal of industrial off-gases include 

Venturi scrubbers or electrostatic precipitators. Additionally, water-soluble condensed chemical 

constituents may accumulate in pre-filters, requiring monitoring and management. For off-gases with 

particulates that are both filterable and condensable, monitoring can be performed using U.S. EPA 

Method 5 for the front-half of the filter and U.S. EPA Method 202 for back-half condensables. Filters 

used in certain applications may also become clogged from slime growth of microorganisms caused by 

high humidity. 

Moisture content is probably the most critical parameter associated with biofilter performance. Biofilter 

DRE is directly related to the health of the biodegrading microbial population, which is related to the 

filter media moisture content. Insufficient moisture can inhibit diffusion of the contaminants through the 

biofilm. If conditions are extremely dry, channelization of the media and excessive fungal growth can 

occur. For these reasons, the influent air should remain moist at all times and be brought to saturation 

humidity to prevent wide fluctuations in moisture content. Relative humidity also changes with the 

ambient conditions; therefore, these parameters should be measured and logged. Depending on the 

level of humidification necessary, atomizing nozzles or packed towers can be used. If the media dries 

out, spray irrigation can be used. However, humidifiers can also cause slime growth to develop on the 

packed beds of the biofilter. This growth can be removed by cleaning, washing, and re-loading the 

bedding material. However, the problem often recurs. Biocides typically used in cooling towers are 

not recommended as carryover to the biofilter media because they can impair the microorganisms. 

Conversely, too much moisture can also adversely affect system performance. Elevated moisture 

levels reduce the interfacial area between the biomass and the vapor stream, which reduces the ability 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 5-10

of oxygen and contaminants to diffuse through the biofilm. Elevated moisture content can also reduce 

the effective porosity of the media and impede gas flow so that anaerobic zones develop. Decreased 

porosity can result in increased back-pressure and increased overall operating cost of the system. 

Biofilter moisture targets are within the range of 40 to 60 percent relative humidity (Swanson and Loehr 

1997; Leson and Winer 1991). De-misters can help prevent excess water carryover into the filter 

media, often by decreasing the off-gas velocity in ducts or pipes carrying the off-gas to the biofilter 

(PPC Biofilter Inc. 2004). Maintaining uniform moisture in the media is often more important than 

trying to keep an optimal target moisture level; however, this situation depends on the biofilter media. 

Moisture content also plays a role in modifying the influent gas temperature. Most gas streams do not 

automatically fit operational requirements and must be conditioned before entering the biofilter. The 

temperature of the system should be between 77 and 95 °F for optimal microbial biodegradation 

(Swanson and Loehr 1997). Based on the minimum air temperatures necessary for optimum biofilter 

operation and the effect moisture has on temperature when it is added to maintain humidity, a 

minimum wet-bulb temperature of 70 °F should be specified. 

Wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the 

thermometer bulb wrapped in muslin that is kept wet. The evaporation of water from the thermometer 

has a cooling effect, so the temperature indicated by the wet-bulb thermometer is less than the 

temperature indicated by a dry-bulb (normal unmodified) thermometer. The rate of evaporation from 

the wet-bulb thermometer depends on the humidity of the air; evaporation is slower when the air is 

already full of water vapor. For this reason, the difference in the temperatures indicated by the two 

thermometers gives a measure of atmospheric humidity. An increase of the wet-bulb temperature of 

1 °F can typically increase DRE by 1 percent. When air is cold and dry, wet-bulb thermometer 

temperature can be increased by injecting steam into the gas. Within limits, the higher the operating 

temperature, the more active the bacteria and the shorter the residence time necessary, thereby 

requiring less biofilter mass. Metabolic rates of bacteria can double with about every 10 °F increase in 

temperature. Although increasing temperature will enhance the physical properties of the biofilm 

(solubility, molecular diffusivity, and bacterial metabolism), it will adversely affect the physical 

properties of the contaminants in the gas state (vapor pressure and Henry’s law constant). 

Temperature adjustments are usually performed during or prior to the humidification step. If the 

temperature is greater than 105 °F, the air stream requires cooling such as with a heat exchanger or 

packed cooling tower. If the temperature is less than 50 °F, the influent requires heating such as with 

steam or a heater. The influent stream temperature and the exothermic biological reactions that occur 

within the filter affect the system temperature. Elevated temperatures can result in evaporation of 

moisture from the filter bed, thereby reducing system DRE. Any heat generated during biological 

reaction can be recovered and used to heat the system influent stream; however, because most SVE 

off-gas treatment systems are exposed to ambient conditions, they more often require heat input 

unless they are well insulated. 
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The filter materials used in the design of a biofiltration system should have the following properties 

(Swanson and Loehr 1997): 

• High moisture and nutrient retention properties to sustain microbial growth 

• A large specific surface area to allow sufficient contact between the vapor-phase compounds 

and the biologically active material present in the biofilm 

• Adequate and relatively uniform porosity to promote even distribution of air flow through the 

filter materials and to minimize pressure drop through the system 

• Ability to maintain a relatively constant and neutral pH 

• A low bulk density to minimize the potential for compaction of the material during use 

Two types of media are typically used within the filter bed: (1) organic biofilters consisting of soil and 

compost, leaves, wood, bark, peat, or other organic materials are one common form, and (2) synthetic 

filter media consists of inert agents such as perlite, vermiculite, polystyrene spheres, or GAC 

(Swanson and Loehr 1997). 

The specific advantages and disadvantages summarized below are associated with each type of filter 

material. 

• Organic filter materials have an inherent nutrient supply, so the addition of nutrients is often not 

necessary. The nutrients within the reactor will be exhausted eventually, and then 

supplemental nutrients are necessary or the filter bed must be replaced. Organic filter 

materials are better at retaining moisture than synthetic filter materials. 

• Organic filter materials typically have a wide range of indigenous microorganisms; therefore, 

shorter acclimation periods are required. 

• Organic filters generally eliminate the need to dispose of spent filter material as a regulated 

solid waste; however, this advantage depends on site-specific situations. Used compost can 

generally be returned to the earth without environmental impact (Leson and Winer 1991). 

• Organic filter materials are subject to compaction (“aging”), which reduces the porosity of the 

material (Sorial and others 1997; Swanson and Loehr 1997); therefore, organic filter materials 

need to be replaced more often than synthetic materials. 

• Synthetic filter materials are more durable and may have higher surface areas for contact 

between the biomass and the vapor-phase contaminants. These materials typically require the 

addition of nutrients with the humidifying water spray. 

Some vendors have used soil as a biofilter media. However, soil has a large pressure drop and is not 

economically feasible on large gas streams (10,000 to 100,000 cfm) because the fan blowers required 

must be very large resulting in significant power consumption. Organic media are relatively dense and 
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March 2006 5-12

therefore require a larger footprint for a given system. It is possible to produce biofilters that are 

stackable in series up to 15 to 20 feet tall in order to reduce the footprint of the system. However, 

because of density and potential compaction of the biofilter media, consistent air velocities become an 

issue. For example, stacking compost or peat biofilters is not practical because air velocities greater 

than 10 fpm usually cannot be sustained. Also, wood-based media experience more fungal growth, 

which can increase the pressure drop over time. To overcome these problems, a portion of inert 

media can be added to create an engineered media, which would decrease compaction and the 

resulting pressure drop and allow use of a smaller fan with a smaller electrical cost. Engineered media 

allow velocities of greater than 20 fpm but are often hydrophobic and do not have uniform moisture 

holding capacity. Media with a very large void fraction may attain velocities of 40 fpm but experience a 

net loss of mass transfer of vapor-phase constituents from the gas phase to the biofilm. 

The microorganisms used for a biofilter can come from a number of sources. Often, they are present 

in the compost media itself and are used as the inoculum because of their diverse existing population. 

In other cases, microorganisms are derived from sludges or acclimated cultures derived from the 

contaminated soil or groundwater. Some acclimation period is always necessary, whether for the 

organisms to become stable in a consortium of other organisms or for them to multiply and seed the 

entire biofilter with enough biomass to biodegrade the influent effectively. In very few cases, seeding 

of unique strains of organisms into the biofilter may be required, such as when degradation of 

recalcitrant compounds is necessary. Reseeding may be necessary if toxic shock loads or other 

hostile environments (such as low pH) affect the microorganisms. 

As described above, biological activity is directly related to temperature. DREs of greater than 98 

percent at a temperature of 95 oF have been observed to drop to 70 percent as the temperature fell 

below 65 °F. This factor is an important design consideration because ambient air to decrease 

contaminant concentrations may also decrease the temperature of the off-gas. The supply of nutrients 

to a growing and metabolizing population of microorganisms is important in sustaining biological 

activity within the filter material. Nutrient addition is a well-known process, and specific guidelines on 

the required level of nutrients to support microbial health are available in numerous texts on the 

subject. Because the biofilter is continually used, the nutrients available to the microorganisms may no 

longer be sufficient for the biomass or may require supplementation. When filter media or biomass 

become exhausted, the biofilter is often replaced or recharged and restarted. 

Generally, optimum microbial degradation occurs at pH readings of 7 to 8. Biodegradation of VOCs 

can produce acidic intermediate byproducts that can reduce the pH of the system and upset system 

performance. To overcome the potential for pH reduction within the biofilter, buffering materials (such 

as lime, marl, and crushed oyster shells) are often added to the filter media. Buffering solution can be 

added along with moisture and nutrient additions by overhead spraying mechanisms. System 

performance can also be affected by build-up of biomass within the reactor vessel (such as slime 

growth or fungal mats), which reduces the filter bed surface area available for biodegradation (Alonso 

and others 1998) and also increases the pressure drop over the filter bed. 
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5.6 Residuals Management 

Depending on the type of waste being treated, degradation of organic contaminants can result in the 

generation of acidic intermediates or byproducts. Acidic intermediates occur when the mass loading 

(VOC concentration) exceeds the MEC. In such cases, the contaminant is only partially degraded and 

acidic intermediates accumulate. Devinny and Hodge (1995) identify the formation of acetaldehyde 

and acetic acid when laboratory-scale biofilters were overloaded with ethanol. When the system 

operated under well-balanced influent concentrations, the acetaldehyde and acetic acid were rapidly 

degraded and acidic intermediate products were avoided. 

Acidic end-products may also result from the biodegradation of some pollutants, including sulfur or 

nitrogen-containing compounds and chlorinated hydrocarbons (Leson and Winer 1991). The products 

that can form include sulfuric acid, nitric acid, and hydrochloric acid (Swanson and Loehr 1997). If this 

build-up becomes excessive and the biofilter becomes inactive, the entire filter media may require 

disposal. This situation is however, an extreme scenario. 

As discussed in Section 5.5, use of compost biofilters eliminates the need to dispose of spent filter 

material such as activated carbon as a solid or hazardous waste. If an activated carbon pre-filter is 

used, it must be handled as either a hazardous or special waste, depending on the chemical 

concentrations it has adsorbed. Dust and particulate filters can usually be managed as general trash. 

After the filter media are spent, if no contaminants of concern adsorbed to the media, the media can be 

disposed as universal waste or recycled. For example, compost media can be reused as regular 

compost at nurseries or as landfill cover material. 

Biofiltration systems may also require periodic backwashing to remove excess biomass or slime. 

Alonso and others (1998) found that build-up of excess biomass in a laboratory-scale biofilter led to 

significant loss of system DRE resulting from channeling or short-circuiting. The handling of biomass 

wastes depends on the waste composition and local ordinances, but after dewatering, these wastes 

may be managed as either a hazardous, special, or municipal waste. 

If any chemicals condense out of the influent gas, they will generate a waste stream that may require 

additional management. This situation is not common but occurs when the influent gas stream is 

much warmer than the ambient temperature of the biofilter. During cooling (either naturally or forced), 

heavier molecular weight compounds can condense in ducts and piping leading into the biofilter. In 

addition, condensates can form on pre-filters that are not heated. Typically, these condensates are in 

liquid form and can also develop during humidification. System operators must manage and properly 

dispose of condensate and other liquid residuals based on their chemical composition and local 

regulatory requirements. Depending on the flow volume and composition, discharge to a publicly 

operated treatment works may be an option. 
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5.7 Cost and Economics 

Biofiltration is often more economical than activated carbon or thermal treatment when airflow rates are 

high and concentrations are less than 3,000 ppm (or more frequently, below 1,000 ppm). Start-up 

costs are generally comparable to conventional technologies such as carbon adsorption and thermal 

treatment, but O&M costs are substantially lower. The lower operating costs for biofilter systems are 

the main advantage of these systems. 

Capital costs depend on the size of the reactor, which depends on the flow rate, chemical composition, 

and chemical concentrations. Operating costs largely correspond to electricity and natural gas 

consumption, water usage, and disposal of the media. These factors in turn are also related to the 

size of the unit. In general, most operating costs come from operating humidifier pumps and blower 

fans. 

Cost data for biofilter treatment of SVE off-gas are limited because not many projects use biofilters as 

part of a SVE system. FRTR (2004) reports a unit cost for biofiltration of $0.60 to $1.50 per 100,000 

cubic feet of off-gas treated. This cost combines both capital and operational costs into the unit cost 

and appears to be for a large system and independent of the concentrations in the air stream. FRTR 

(2004) also reports a unit cost (capital and operation costs included) of $2.27 to $4.55 per pound of 

contaminant treated. However, these cost ranges are not necessarily applicable to SVE systems, and 

project-specific details are unknown, such as filter media, capital or operational cost breakdown, 

contaminants, or concentrations. 

The following examples of biofiltration projects are provided to further illustrate the range of costs 

relative to key factors in biofiltration treatment. 

Cost estimates were developed for off-gas treatment at an industrial printing press operation with a 

flow rate of 6,000 cfm and eight separate VOCs in the influent at concentrations of 0.1 pound per hour 

(lb/hr) (methanol) to 19.2 lb/hr (propanol). Biofiltration and regenerative Cat-Ox systems were 

compared, and biofiltration was observed to be the most economical alternative for treating the off-gas 

stream. The total cost for the biofiltration system over 5 years was $413,500, which included $342,000 

for capital costs; $4,300 for operating costs; and $50,000 for media replacement over 5 years. The 

total cost for the regenerative Cat-Ox system over 5 years was $628,200, which included $350,000 for 

capital costs; $43,400 for operating costs; and $61,200 for catalyst replacement over 5 years. The low 

operating cost of biofiltration reduced the 5-year costs significantly (PPC Biofilter Inc. 2004). 

Another biofiltration project included eight separate biofilter systems installed at former and operating 

service stations in Arizona to address SVE off-gas associated with leaking USTs. Each system had a 

capacity of approximately 50 cfm drawing vapors from one to three wells using compost as the biofilter 

media. BTEX DRE was 95 percent, TPH DRE was 90 percent, and aliphatic DRE was 80 percent. 

The duration of these projects ranged from 12 to 36 months, and closure at the sites was completed. 
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Capital costs for biofiltration at each site were $8,000, with an operating cost of approximately $200 per 

month (Bohn Biofilter Corporation 2003). 

5.8 Developmental Status 

Biofiltration is a readily available technology offered by several vendors. Off-the-shelf units are 

available for smaller flow rates (up to 5,000 cfm); larger flow rates require custom-designed steel or 

synthetic tanks, masonry structures, or in-ground beds. Currently, commercial biofilters are only 

applicable for SVE systems that treat non-chlorinated VOCs. Several companies and universities are 

currently researching the use of biofilters to treat chlorinated VOCs and MTBE. 

Because many factors require evaluation to engineer a biofilter for a particular application, a pilot test 

of a smaller unit (approximately 250 cfm) for an acclimation period of 2 weeks and an operation period 

of 60 days is often warranted. Based on the results of such a test, applicability of a biofilter to a sitespecific air stream can be determined, operating parameters can be modified, and the full-scale 

biofilter system can be designed. Typical costs for pilot-plant operation range from $15,000 to 

$25,000, and several vendors have packaged plants for rent to accomplish the pilot testing. 

Vendors of this technology are listed below. This list is not meant to be comprehensive and was 

developed from mention in literature and from Web sites providing descriptions of technologies 

applicable to SVE off-gas treatment. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use. 

• Air & Water Solutions, Inc.; Nutley, NJ; www.cleanairplant.com

• Biocube, Inc. (formerly EG&G Biofiltration); Victor, NY; www.biocube.com

• Bio Reaction Industries, LLC; Tualatin, OR; www.bioreaction.com

• Bohn Biofilter Corporation; Tucson, AZ; www.bohnbiofilter.com

• Envirogen/Shaw; Lawrenceville, NJ; www.shawgrp.com

• Matrix Environmental Technologies Inc.; Orchard Park, NY; www.matrixbiotech.com

• PPC Air Pollution Control; Longview, TX; www.ppcbio.com

• The Reynolds Group; Tustin, CA; www.reynolds-group.com
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6.0 EMERGING TECHNOLOGIES 

A number of emerging technologies that have been applied and developed show promise for treating 

SVE off-gases. They include non-thermal plasma, photolytic and photocatalytic technologies, 

membrane separation, absorption, and vapor condensation technologies. Information on the use of 

these emerging technologies for SVE off-gas treatment is limited compared to the more 

commercialized processes described in Sections 3.0 through 5.0; therefore, the format of this section 

is modified to discuss only the technology description and applicability; developmental status; 

performance; and engineering considerations, limitations, and costs. 

Because these technologies are more in the research and testing stage for SVE applications, the 

information presented herein may change more rapidly than for more commercialized technologies; 

therefore, if any of these emerging technologies are of particular interest, it may be desirable to 

perform additional literature investigation to explore new relevant information on the performance, 

limitations, and state of development for these technologies. 

6.1 Non-Thermal Plasma Technologies 

Plasma is a gaseous state where molecules or atoms are broken apart to form ions, radicals, and free 

electrons. In general, non-thermal plasma techniques do not use direct heat or flame but instead use 

electric fields or energetic beams to destroy vapor-phase contaminants by breaking the molecules into 

radicals and highly energetic electrons. Non-thermal plasmas operate at 85 to 250 °F but can produce 

the same radicals and other reactive species normally associated with high-temperature reactions. 

High-temperature plasma techniques may operate at 3,600 to 18,000 °F. High-temperature plasmas 

are more commonly used in industrial applications, allow either liquid or gaseous feed streams, and 

completely destroy any complex molecules. 

Under suitable operating conditions in non-thermal plasma treatment, the free radicals formed during 

the process recombine into non-toxic, simple compounds that can be released to the atmosphere. 

Non-thermal plasma treatment can achieve high DREs for a wide range of chemicals in off-gases 

generated from SVE systems. 

6.1.1 Technology Description and Applicability 

Variations of non-thermal plasma technology include “silent discharge plasma,” “tunable hybrid 

plasma,” “electron beam,” “low-pressure surface wave plasma,” and “gas-phase corona.” Each 

technique uses an electric power source to generate the electric field or energetic beam and a flowthrough reactor vessel where the SVE off-gas is treated. These technology variants differ principally in 

(1) the temperature at which the contaminated vapor is treated and (2) the type and magnitude of the 

energy applied to ionize the gas. 
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Silent discharge plasma technology operates at ambient temperatures and uses a 15- to 18-kilovolt 

(kV) alternating electric field. 

Tunable hybrid plasma technology uses a 150- to 175-kV electron beam to create the plasma in the 

gas stream. Varying the electron energy and density accommodates varying contaminant levels and 

types. This technique requires a virtually dry gas stream. 

In electron beam technology, electrons are generated in a vacuum tube and the resultant beam is 

used to bombard off-gas vapors flowing through a reaction chamber to ionize the gas stream into 

plasma. Water vapor in the gas can interfere with the effectiveness of the technique but is sometimes 

useful in creating additional hydroxyl ions. 

The low-pressure surface wave plasma reactor requires the contaminated gas stream to be below 

ambient pressure. It uses a high-frequency radio source to ionize gas and create the plasma. 

The gas-phase corona reactor is similar to the silent discharge plasma reactor but includes a bed of 

dielectric beads to provide more discharge paths and to catalytically assist decomposition. The system 

operates at ambient conditions and requires a 25- to 30-kV alternating electric field. 

The principle components of the non-thermal plasma system are the reactor vessel and, if chlorinated 

VOCs are treated, an acid scrubber. The reactor vessel is either a glass or stainless-steel vessel, 

usually cylindrical and scaled to handle anticipated off-gas flow rates. The reactor vessel (sometimes 

called the “reactor cell”) is connected to the power supply and control system for corona production. 

As contaminants pass through this chamber, they are converted to radicals. The free radicals 

recombine further downstream and either emerge from the stack of the reactor directly into the 

atmosphere (as in the case of non-halogenated VOCs) or continue through an acid scrubber that 

removes halogen acids (formed when chlorinated VOCs are treated). Multiple reactor cells can be 

arranged in series or in parallel either to increase the DREs for the contaminants, handle higher 

volumes of gas, or both. Co-located with the reactor cell array are the high-voltage power supply and 

necessary electrical control systems. All of the equipment can be mounted in a trailer to make it 

transportable. 

Most often, the results of non-thermal plasma treatment are benign compounds such as carbon 

dioxide, water vapor, or low concentrations of NOx. When acids are generated (typically when 

chlorinated VOCs are present in the influent), the effluent must pass through a scrubber before being 

released to the atmosphere. The scrubber usually contains a mild caustic solution for acid removal but 

can contain any solution components necessary to remove hazardous contaminants. 

The off-gas influent to the reactor is typically filtered and dehumidified. Filtering prevents the reactor 

from clogging, the electrodes from being coated and damaged, and the air pump (vacuum blower) 

from being damaged. Dehumidification minimizes corrosion and protects the electric field from 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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disruption. An air cooler (heat exchanger) may also be added to the gas stream influent treatment 

train prior to the reaction vessels if a hot inlet gas is treated or may be required to cool the actual 

equipment for certain forms of non-thermal plasma treatment, although this situation would not typically 

be a factor for SVE systems. 

Of all these forms of non-thermal plasma technology, the two most tested in site remediation 

applications are those using an alternating electric field: the silent discharge plasma and the gas-phase 

corona reactor. These technologies are further discussed below. 

6.1.1.1 Silent Discharge Plasma 

In silent discharge plasma treatment, the contaminated off-gas stream is routed from the SVE blower 

to a Pyrex® reaction chamber. The chamber is sandwiched between two electrodes attached to a 

high-voltage power supply. When a high-voltage alternating current (AC) electric field is applied 

across the electrodes, microbursts of plasma form in the chamber. These plasma bursts create free 

radicals of the contaminants in the gas stream. The free radicals recombine to form harmless organic 

molecules, mostly carbon dioxide and water vapor. Chlorinated VOCs in the gas stream will also 

produce halogen acids that must later be removed by an acid scrubber. The plasma chambers are 

small because of the small free path length of the plasma, but the chambers can be replicated 

inexpensively and placed in parallel to handle large volumes of gas. 

6.1.1.2 Gas-Phase Corona Reactor 

In a gas-phase corona reactor, contaminated influent gas is forced through a reaction chamber, where 

it reacts with plasma containing high-energy electrons. The reaction of the organic compounds in the 

vapor stream and the electrons typically produces carbon dioxide and water, and in the case of 

halogenated hydrocarbons, halogen acids. 

Two types of gas-phase corona reactors are typically used for off-gas treatment: a packed bed corona 

reactor or a pulsed corona reactor. The packed bed corona reactor contains a bed of dielectric 

packing material (see Figure 6-1). The packing material may be as simple as glass beads or may 

consist of more exotic materials such as Perovskite (BaTiO3). These more exotic materials possess 

catalytic properties and increase the effectiveness of this technology but also increase costs. Wiremesh electrodes are located on each end of the packing bed and are attached to a high-energy AC 

power source. As the gas flows through the packing material, a high-voltage, low-current AC is applied 

to the electrodes. The electrons generated from the resulting discharges of the bead “capacitors” 

create radicals out of the contaminants in the gas stream. According to Current Environmental 

Solutions (CES), the packing material performs the following three critical functions (ENSR 2004d): 

1. Amplifies the electric fields between the beads by refracting the electric currents as much as 

10 to 250 times more than would otherwise be possible 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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2. Acts as a catalyst for chemical reactions with contaminants in the vapor stream 

3. Provides adsorption sites for contaminants, which effectively increases the residence time of 

the contaminant in the reaction vessel 

Figure 6-1 shows a simplified schematic diagram of packed bed corona reactor treatment. 

Figure 6-1. Packed Bed Corona Reactor 

Source: CES 2005 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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Pulsed corona reactors consist of a single wire centered between two steel electrodes within an 

unfilled tube. The wire is connected to a high-voltage spark gap power supply. As the contaminated 

vapor stream flows through the reactor vessel, nanosecond-length corona pulses are used to ionize 

the organic vapors. 

Non-thermal plasma technology can effectively treat a variety of compounds, including aromatic VOCs 

(such as BTEX) and chlorinated VOCs (such as chlorinated ethenes and ethanes, and freons). CES 

reports that vapor stream concentrations ranging from 10 to 10,000 ppmv have been successfully 

treated with generally greater than 99 percent DREs (ENSR 2004d). Because efficiency does not 

depend on high temperatures, other benefits include rapid start-up and low operating costs, with much 

lower requirements for supplemental fuel or energy during treatment than required for thermal 

treatment alternatives. Korzekwa and others (1998) report DREs somewhat lower than 99 percent for 

pulsed corona reactors and determined that the amount of energy necessary for destruction is affected 

by the relative humidity of the gas stream (that is, higher humidity causes lower DREs). More 

significantly, their study shows that the amount of energy required to destroy VOCs is highly 

compound-specific. For example, freon requires more than an order of magnitude higher energy input 

than toluene under similar treatment conditions. 

6.1.2 Developmental Status 

Non-thermal plasma technology has the potential to compete with traditional off-gas treatment 

technologies because of its potential to operate for long periods without requiring much O&M and its 

potential to be scaled up to handle large contaminant loads (Chapman and others 1997). Although the 

technology has been the process of choice for the production of ozone in industrial settings for several 

years, it needs further development, refining, and marketing for use in SVE off-gas treatment 

applications. 

The silent discharge plasma technology was originally developed by the U.S. Department of Energy’s 

(DOE) Los Alamos National Laboratory (LANL) in the early 1990s. The technology was field tested on 

an SVE system at McClellan AFB from October 1995 to January 1996. In the test, cells were 

constructed of Pyrex® and sandwiched between electrodes. The Pyrex® cells provided the dielectric 

barrier. Forty such cells were connected in series to form the primary treatment chain during this field 

test of the silent discharge plasma equipment. Results indicated greater than 95 percent DREs for 

several chlorinated hydrocarbons. The process has not subsequently been successfully 

commercialized (ENSR 2004d). 

The gas-phase corona technique has been used as an industrial pollution control technology for years 

to control NOx, SOx, and mercury compounds (among other pollutants) and is sometimes used in 

conjunction with electrostatic precipitators and electron beams. Performance information available for 

gas-phase corona technology, although limited, indicates that the technology is capable of achieving 

DREs of more than 99 percent for chlorinated VOCs with proper gas residence time and voltage within 

the reactor vessel. 
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In 1992, Battelle Pacific Northwest Laboratories (Battelle) completed bench-scale testing of the gasphase corona technology. This testing demonstrated the ability of the corona reactor to destroy up to 

1,500 ppmv TCE with DREs of greater than 99 percent. Battelle reported that DREs were found to be 

a function of residence time, concentration, and applied voltage (RTN 1999d). Later studies by Nunez 

and others (1993) revealed that compound-specific DREs could be correlated to the ionization 

potential (IP) of that compound. The lower the IP of a particular compound, the higher the DRE at 

constant operating conditions. 

Battelle also conducted pilot-scale field testing of gas-phase corona technology for site remediation at 

the DOE Savannah River site in Aiken, South Carolina (see Section 6.1.3 below). This technology is 

being marketed for full-scale site remediation applications by CES in partnership with Battelle. In 1998, 

gas-phase corona technology was applied at bench scale for the treatment of SVE off-gases at the 

Silresim Superfund Site in Lowell, Massachusetts. The technology proved successful in destroying 

chlorinated and fluorinated contaminants but was not tested during subsequent treatment studies at 

the site. A pilot system is now developed and marketed by CES for treating automotive and diesel 

exhausts. CES has indicated that a full-scale demonstration of this technology will continue to be 

marketed, but to date, such a demonstration has not been performed (ENSR 2003a). 

The other non-thermal plasma technologies (tunable hybrid plasma, electron beam, and low-pressure 

surface wave plasma) are not commercialized and are still in the research and development stages. 

6.1.3 Performance 

In 1993, Battelle continued bench-scale research of gas-phase corona technology and performed a 

pilot-scale test at the DOE Savannah River site in Aiken, South Carolina. The test was performed on 

TCE and PCE vapors from an SVE system at concentrations of 130 and 720 ppmv, respectively. The 

reported results show that DREs for TCE and PCE could be raised to greater than 99.9 percent for gas 

residence times of 1.8 and 7.7 seconds, respectively. DREs increased as the residence time 

increased (RTN 1999d). 

Battelle has also tested this technology for a variety of other compounds. Table 6-1 summarizes 

typical DREs for the other compounds tested by Battelle. As the table shows, the technology was 

able to achieve 99 percent DREs or greater for several contaminants and greater than 95 percent

DREs for all but one of the contaminants tested. 
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Table 6-1. Gas-Phase Corona Plasma Reactor Technology Results 

Compound Destruction and Removal Efficiency (%) 

Overall 97 

Acetone 97.9 

Benzene 97.85a

Carbon tetrachloride >99a

1,1-Dichloroethane >96.6 

1,2 Dichlorobenzene >97.0 

1,1-Dichloroethene >97.4 

cis-1,2-Dichloroethene >95.3 

Ethylbenzene >96.3 

GD (Nerve Agent) >99.8a

Methane >97a

Methylene chloride 95a

Naphthalene >99.9a

Phosgene 99.84a

Tetrachlorothene 99.5 - 99.9a

1,1,1-Trichloroethane 95.4 

1,2,4-Trimethylbenzene >98.2 

Trichlorotrifluoroethane 67a

Semivolatile organic compounds >99.9 

Trichloroethene 99.6 - 99.9a

Total nonmethane organic compounds 97.0 

Xylenes 99.4 

Others (hydrogen cyanide, etc.) >99a

Note: 

a Reported result obtained using a packed bed corona reactor 

Source: RTN 1999d 

The McClellan AFB study notes that average DREs as high as 97 percent were demonstrated with 

dehumidified and hydrogen-enriched influent in the silent discharge reactor. DREs dropped to less 

than 90 percent in the presence of significant humidity in the gas stream; therefore, the gas stream 

should be dehumidified for optimal operation (Chapman and others 1997). An electron beam benchscale study at McClellan AFB indicated DREs of 93 to 99 percent for a mixed VOC and chlorinated 

VOC waste stream from an SVE off-gas (Chapman and others 1997). Non-thermal plasma technology 

has proven effective in treating a wide range of contaminants, including those shown in Table 6-1, 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-8

which are typical of SVE off-gases as well as for treating chlorofluorocarbons, SOx, NOx, and PCB 

surrogates. 

At the current stage of development, the gas-phase corona technology is limited to relatively low flow 

rates (less than 20 cfm) for both industrial and remediation applications of single-unit systems. 

Systems can be designed with several corona reactors in parallel to increase treatment capacity. At 

the DOE Savannah River site, 21 parallel reactors achieved a maximum treatment rate of 105 scfm. 

Battelle indicates that plans have been developed for a commercial system capable of treating 250 cfm 

using six reactors (ENSR 2003a). 

6.1.4 Engineering Considerations, Limitations, and Cost 

As of the mid-1990s, none of the systems had been demonstrated at the scale necessary for practical 

remediation systems. The chief deficiency was that the systems only operated at about 10 scfm. 

Although this flow rate is low, certain SVE remediation systems operate in this range. Placing reactors 

in parallel can increase flow capacity. 

A general disadvantage of the systems is that they may generate products that require further 

treatment. Depending on plasma temperature and input energy, free radicals formed within the 

plasma can sometimes recombine into compounds more toxic than their precursors. This situation 

results in the need for continuous monitoring of the exhaust products and, in many cases, the need for 

off-gas scrubbers. For specific applications, however, tailoring system conditions can result in very 

high DREs with a low probability that further off-gas treatment will be required (except for halogenated 

contaminants that result in acid formation). Contaminant destruction is linked to the treatment rate. 

For greater DREs, lower treatment rates (flow rates) may be required (RTN 1999d). Because the 

technology is not fully developed, not all potential problems associated with full-scale operation are 

well understood, especially with regard to full control of the process and the impact of corrosiveness. 

To date, no significant environmental concerns are associated with this technology. Production of NOx 

and ozone is low to moderate. At higher temperatures (and power consumption), ozone production 

ceases but NOx and nitric acid production increases (RTN 1999a). No evidence has been found of the 

production of dioxin or furans with this treatment technology. The potential for production of toxic or 

carcinogenic compounds in the effluent stream depends on the influent compounds and therefore 

possible free radical combinations that can occur. The use of free radical scavengers such as 

hydrogen and water vapor in the influent minimizes the formation of such toxics. This technology’s low 

operating temperatures generally preclude the formation of toxics. 

As with most off-gas treatment technologies, the system produces halogen acids from halogenated 

contaminants and requires an acid scrubber when these contaminants are present. Depending on the 

other contaminants collected in the scrubber solution, the subsequent liquid waste stream may require 

testing and treatment prior to disposal. Dehumidification (air-water separator) condensate, if any, 

should be managed separately. 
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For gas-phase corona technology, power consumption is expected to be the primary operating cost, 

with consumption rates of 12 to 15 kilowatts at 25 to 30 kV. As with Cat-Ox, a significant capital cost 

could be for acid-resistant catalyst beads in the reactor. Although not necessary to the operation of the 

process, catalyst beads make the process more efficient and allow larger mass flow rates (higher 

concentrations, lower residence times, or both), thereby allowing smaller reaction vessel size. The unit 

cost of gas-phase corona technology is also very dependent on the contaminant mass loading. 

Cummings and Booth (1997) used data obtained from field testing of the corona technology and 

additional data provided by Battelle to estimate the unit cost (price per pound) of off-gas treatment by 

gas-phase corona technology over a range of influent concentrations and flow rates. The calculated 

estimates ranged from approximately $1 per pound at a 500-cfm flow rate and 1,000-ppmv influent 

vapor concentration to $34 per pound for a 100-cfm flow rate and 50-ppmv influent vapor concentration 

(the influent vapor stream was assumed to contain a ratio of 70 percent PCE to 30 percent TCE). 

These estimates are supported by the data presented by Chapman and others (1997) from field testing 

of the silent discharge plasma technology (which is almost identical to the gas-phase corona 

technology) at McClellan AFB in 1995 and 1996. At a relatively low contaminant mass loading rate of 

0.05 kilogram per hour, the operating costs were estimated to be about four times as great as for a 

Cat-Ox because of energy requirements. Capital costs were estimated to be double those of a Cat-Ox 

with the same capacity. Energy costs are a primary challenge for this non-thermal plasma technology. 

Costs are contaminant-specific; the lower the contaminant bond strength (IP) and gas stream loading 

rate, the lower the cost (RTN 1999a). Conversely, cost information provided by CES indicates that at a 

relatively high contaminant mass loading rate of approximately 6 kilograms per hour of benzene 

(equivalent to SVE treatment of a 2,000-ppm influent stream at a 500-cfm flow rate), the cost of nonthermal plasma treatment could be approximately one-fourth that of Cat-Ox treatment (ENSR 2003a). 

Taking into account all capital and O&M costs, FRTR (2004) recently estimated that gas-phase corona 

technology costs $10 per pound of contaminant. It is clear from the wide variations in estimates that 

additional data are necessary to develop a better understanding of costs relative to performance and 

associated limitations. 

Developers of this technology are listed below. This list is not meant to be comprehensive and was 

developed from mention in literature and from Web sites providing descriptions of technologies 

applicable to SVE off-gas treatment. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use. 

• Current Environmental Solutions; Kennewick, WA; gas-phase corona; www.cesiweb.com

• ENV America; Irvine, CA; silent discharge plasma; www.envamerica.com

• Los Alamos National Laboratory; Dr. Louis A. Rosocha; gas-phase corona; e-mail address: 

rosocha@lanl.gov
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

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• Thermo Electron, Inc.; Waltham, MA; tunable hybrid plasma; www.thermo.com

• First Point Scientific, Inc.; Agoura Hills, CA; electron beam and corona discharge; 

www.firstpsi.com

6.2 Photolytic and Photocatalytic Technologies 

Light energy in the UV wavelength spectrum can be used to oxidize or otherwise break down vaporphase organic contaminants in air streams. Two related processes, photolytic destruction and 

photocatalytic destruction, use this phenomenon to treat off-gases generated from SVE systems. 

These two processes have also been proposed to improve general indoor air quality by removing 

VOCs and tobacco smoke from the air as well as to mitigate NOx in industrial flue gas streams (U.S. 

EPA 2000a, b, c, and d). Although these proposals are encouraging, these technologies have not 

been fully developed or widely used for SVE off-gas treatment applications. 

6.2.1 Technology Description and Applicability 

Both photolytic and photocatalytic destruction technologies use an ionization source of UV light or 

near-UV light (light having a wavelength of 150 to 350 nanometers [nm]) to ionize components of a 

vapor stream (such as oxygen and VOCs), causing the formation of reactive free radicals. Other 

wavelengths could provide more energetic photons for higher-bonding energy compounds, but for the 

purpose of destroying VOCs and chlorinated VOCs, the wavelength ranges of UV light and near-UV 

light are sufficient. 

In photolytic destruction systems, radicals are formed in reactors made of quartz. These highly 

energetic free radicals then recombine to form mostly harmless products such as water vapor and 

carbon dioxide. In addition to the UV energy source and the quartz reactor, photolytic destruction can 

also require follow-on treatment processes to manage other products that are formed. For example, 

oxygen radicals can combine with nitrogen (from air) to form NOx. Also, as with other technologies, 

destruction of chlorinated VOCs produces halogenated acids (such as hydrochloric acid). The less 

desirable reaction byproducts may be released directly to ambient air at small concentrations or be 

removed by equipment such as acid scrubbers. 

Photocatalytic processes similarly use UV light as an ionization source to create free radicals but also 

use a catalyst to promote reactions. In this process, radicals are formed either in the gas stream 

(when the contaminant molecule absorbs the UV light energy directly) or when the influent vapor 

stream contacts the catalyst (usually TiO2). The catalyst is activated by the UV light energy. The 

radicals then recombine to form water vapor, carbon dioxide, and, if chlorinated VOCs are treated, 

halogenated acids. 

The process components of UV oxidation systems vary depending on the technology developer and 

whether photolytic or photocatalytic processes are used. One photolytic destruction system developed 
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by Process Technologies, Inc. (PTI), feeds off-gas from an SVE system to a sorption chamber 

containing a fluidized bed of adsorbent material. In the sorption chamber, the contaminants are 

prepared for photolysis by concentrating the vapor stream. The adsorbent material is continuously 

moved to a separate chamber where the VOCs are desorbed from the adsorbent by steam stripping. 

Ultimately, the adsorbent must be replaced. The VOC-laden air stream resulting from the steam 

stripping is sent to a reaction chamber with a special cement liner for irradiation with UV light in the 

150- to 254-nm wavelength range. The UV light generates free radicals and destroys the VOCs, 

converting them to carbon dioxide and water. 

A representative implementation of a photocatalytic system, developed by Matrix Photocatalytic, Inc. 

(no longer in business), feeds off-gas from an SVE system directly into a reaction chamber. The offgas contacts a TiO2-coated fiberglass open-cell mesh in the reaction chamber and becomes irradiated 

with UV light, which causes free radicals to form. Generally, the mesh is wrapped around the UV lamp 

and the air stream flows between the UV lamp and the catalyst mesh. Other photocatalytic systems 

use reaction vessels containing patented catalyst beads spread on trays irradiated by a UV lamp (RTN 

1999a). In addition, a photocatalytic system, developed by Purifics ES Inc., uses the TiO2 catalyst in a 

slurry form (Purifics 2006). 

Photolytic and photocatalytic technologies effectively treat a broad range of halogenated and nonhalogenated VOCs, including PCE, TCE, and vinyl chloride, which are common SVE off-gas 

components. Other VOCs that can be destroyed by these processes include aromatic and aliphatic 

hydrocarbons, alcohols, ethers, ketones, and aldehydes. Photolytic technology works best on 

concentrated VOC waste streams (1 to 3,000 ppmv) at low flow rates; therefore, this technology is 

applicable for some SVE off-gas treatment scenarios. 

6.2.2 Developmental Status 

Research and development on photolytic and photocatalytic technologies have advanced to the point 

that the technologies are used for industrial air treatment; however, no operating SVE systems appear 

to be using UV technologies for off-gas treatment. However, photolytic and photocatalytic technologies 

have been tested on several SVE off-gas streams and have performed well. Examples of successful 

demonstrations of photolytic and photocatalytic technologies are described below. Also, photocatalytic 

technologies are widely used for treating water in industrial and remediation applications. 

6.2.2.1 Photolytic Treatment 

The PTI photolytic destruction technology was tested as part of the U.S. Navy Environmental 

Leadership Program at Naval Air Station (NAS) North Island’s Site 9 SVE system. The system was 

installed to treat a 250-cfm slip stream from the operating SVE system already installed on site. 

Compounds treated in the system included halogenated and non-halogenated VOCs, such as 

1,2-DCE, TCE, PCE, toluene, and octane. During the 4-month test, the system successfully treated 

VOCs in the SVE off-gas from an average concentration of 192 ppmv to below the maximum allowable 
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emissions of 25 ppmv. The average total DRE for VOCs was 95 percent. The U.S. Navy estimated 

the cost of scaling up the system to 3,000 scfm to treat all SVE off-gas from NAS North Island’s Site 9 

would be $3.77 per pound of VOCs treated (U.S. Navy 2002). PTI’s system had similar positive results 

in the mid-1990s at other test sites, including McClellan AFB in California and Hill AFB in Utah; 

however, PTI is no longer in business, and its technology is not currently commercially marketed. 

In October 1991, Thermatrix began a field test of its photolytic oxidation technology at the Lawrence 

Livermore National Laboratory Superfund site in California under the Superfund Innovative Technology 

Evaluation (SITE) Demonstration Program. The photolytic process treated flows of up to 500 cfm and 

used a xenon pulsed-plasma flash lamp to emit UV light at high densities. Initial TCE concentrations 

were approximately 250 ppmv and were reduced by more than 99 percent under all conditions except 

when the lowest flash frequencies and fewest lamps were used (U.S. EPA 1993a). 

6.2.2.2 Photocatalytic Treatment 

The AIR-I process developed by KSE, Inc. (KSE) is an adsorption integrated reaction in which a 

contaminated air stream flows into a photocatalytic reactor where VOCs are trapped on the surface of 

a proprietary catalytic adsorbent. This process was successfully tested at the laboratory scale on 

chlorinated VOCs at inlet concentrations of 1 to 3,000 ppmv. The process was then demonstrated as 

part of a groundwater remediation demonstration project at Dover AFB in Delaware, where effluent air 

from a groundwater stripper was treated. Results indicated over 99 percent removal of DCA from air 

containing 1 ppmv and saturated with water vapor. 

The photocatalytic destruction process has been tested for SVE off-gas treatment during remediation 

demonstrations. KSE’s AIR-II process was accepted into the SITE Emerging Technology Program in 

1995. This process was initially evaluated at full-scale operation for treating SVE off-gas at Loring 

AFB. The DRE for PCE exceeded 99.8 percent (U.S. EPA 2005a). A study at the former National 

Environmental Test facility at McClellan AFB was conducted from October 1995 to January 1996. This 

study demonstrated that KSE’s AIR-II process had VOC DREs as high as 98 percent. 

Recently, KSE conducted research under the National Institute of Environmental Health and Sciences, 

Superfund Basic Research Program to evaluate TCE destruction and byproducts of their AIR process. 

More than 40 different catalysts containing copper, manganese, cerium, cobalt, tungsten, TiO2, tin 

oxide, and noble metals such as platinum were included in the study. The research brief for this study 

(number 131) can be found at http://www-apps.niehs.nih.gov/sbrp/researchbriefs. 

Another demonstration project was conducted at the Stamina Mills Superfund site in North Smithfield, 

Rhode Island, from August to October 1999. A 700-scfm photocatalytic oxidizer developed by KSE 

and manufactured by Trojan Technologies, Inc. (Trojan), treated SVE off-gases containing primarily 

TCE at influent concentrations of 10 to 78 ppmv. DREs exceeded 99.6 percent for TCE, even at a 

maximum flow rate of 0.72 lb/hr. Unexpectedly, however, chloroform was emitted as a byproduct of 
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the reaction. KSE subsequently reported that chloroform formation can be reduced through the use of 

different photocatalysts or reactor configurations (U.S. EPA 2001a). 

The photocatalytic technology has been commercialized by a number of companies, including KSE 

through its licensee, Trojan; Purifics ES Inc.; and Zentox Corporation. Trojan reported fielding four 

systems for industrial applications and providing quotes on dozens of others; however, Trojan is not 

currently constructing or selling systems for SVE off-gas treatment (U.S. EPA 2001a). Purifics ES Inc. 

reports a number of photocatalytic deployments; however, almost all of these applications are for 

treating ground water or process water (Purifics 2006). 

More novel ideas being evaluated for photocatalytic systems include coating glass fibers with TiO2 and 

filling the reaction vessel with UV-lit fibers. Another approach being researched is a solar-powered 

version in which UV light from the sun activates the TiO2. 

6.2.3 Performance 

Table 6-2 summarizes data from KSE for its photocatalytic technology and patented catalyst. This 

mixture of chemicals may not accurately reflect a typical mixture or many of the individual compounds 

often present in SVE off-gases. 

Table 6-2. Representative Destruction and Removal Efficiencies for Photocatalytic Treatment 

of Volatile Organic Compound-Contaminated Air 

Compound Destruction and Removal Efficiency (%)

Average total volatile organic compounds 95 - >99 

Dichloroethene >99 

1,2- Dichloroethane >99 

Formaldehyde 100 

Acetaldehyde 99 

Trichloroethene 84 - >99 

1,1,1- Trichloroethane >99 

Toluene 96 

Vinyl chloride >99 

Hexane 99 

Carbon monoxide 94 

Ammonia 100 

Source: RTN 1999e 
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During the first McClellan AFB project, PTI’s photolytic system achieved SVE off-gas VOC DREs as 

high as 98 percent under high-concentration, low flow-rate conditions. The technology was shown to 

be effective in treating a wide range of contaminants. Table 6-3 summarizes the VOCs treated for this 

demonstration and DREs for each contaminant. 

Table 6-3. Destruction and Removal Efficiencies for Photolytic Treatment of Soil Vapor 

Extraction Contaminants from McClellan Air Force Base 

Compound Destruction and Removal Efficiency (%) 

Average total volatile organic compounds 98 

1,1- Dichloroethane >81.7 

1,1-Dichloroethene >85.7 

cis-1,2-Dichloroethene >83.4 

Ethylbenzene >87.7 

Methylene chloride >67.4 

Toluene >98.6 

Trichloroethene >99.5 

1,1,1-Trichloroethane 84.5 

Xylenes >97.7 

1,2,4-Trimethylbenzene >95.3 

Methylisobutylketone >95.7 

Acetone >99.9 

Semivolatile organic compounds 97.9 

Source: Chapman and others 1997 

Photolytic and TiO2-based photocatalytic oxidation does not effectively destroy long-chain (greater 

than six carbon atoms) alkanes. When TiO2 catalysts are used, the alkanes partially oxidize and then 

foul the catalysts. This situation may not apply to the patented Trojan/KSE catalysts, which are not 

primarily composed of TiO2. The technologies have not been extensively tested on substituted 

aromatics and seem to have lower DREs for BTEX-like compounds. To treat compounds with higher 

energies of dissociation (such as TCA and methylene chloride), the energy of the UV light photons 

must be increased (Chapman and others 1997). At influent temperatures above about 150 °F, the 

efficiency of the UV lamps drops. Tables 6-2 and 6-3 indicate how DREs for different compounds vary 

for a particular application and off-gas stream. For example, the McClellan AFB photolytic DRE for 

TCE was greater than 99 percent, but the DREs for methylene chloride and TCA were less than 90 

percent, which may be insufficient at some sites to meet performance standards. 
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Overall, photolytic and photocatalytic technologies have low environmental impact (minimal formation 

of waste byproducts), which makes them an attractive SVE vapor treatment method. In early tests, 

however, a tendency was noted for the process to produce low concentrations of phosgene as the 

residence time of chlorinated contaminant vapor in the reactor increased, although dangerous levels 

were never reached (CDC 2005). 

Over time, these technologies have become relatively simple to implement and generally do not 

produce waste products that require further treatment. Hydrochloric acid formed from the treatment of 

chlorinated VOCs may require removal by an acid scrubber. Alternatively, hydrochloric acid may be 

treated by absorption and neutralization in cement liners used in the reactors. The cement liner would 

ultimately become saturated with neutralized halogens and require replacement. Waste from the liner 

can be disposed of as solid waste. 

The effluent from the oxidation/destruction chamber is generally benign enough for direct release to 

the atmosphere; however, the potential generation of other waste products (such as chloroform) may 

represent a disadvantage with this technology. If an adsorbent is used as a fluidized bed in the 

process, it can be regenerated and ultimately disposed of as solid waste. 

6.2.4 Engineering Considerations, Limitations, and Cost 

Photolytic and photocatalytic systems require relatively little maintenance or replacement parts. The 

systems are designed for unattended operation, and any wastes produced may be readily managed. 

Catalysts typically last 2 or 3 years, and UV lamps, which are widely available and inexpensive, require 

replacement approximately annually. 

There are engineering concerns associated with photolytic and photocatalytic technologies. In 

photocatalytic systems, the UV lamps must illuminate and activate the catalyst, which must be thin 

enough for the UV light to penetrate it. Large influent mass loadings therefore require large catalytic 

surface areas for photocatalytic oxidation to occur. In addition, the influent must be filtered to prevent 

the degradation of the catalyst by particulates entrained in the vapor stream. The internal equipment 

of photolytic and photocatalytic systems is sometimes difficult to reach, which could prove troublesome 

if maintenance or periodic replenishment is required. In larger systems that process larger mass loads 

of contaminants, reaction vessels may require cooling systems because catalytic reactions generate 

heat. Cooling water may be considered a waste stream but could be recirculated. 

Although the basic photo-energetic concept of these technologies is relatively straightforward, the 

related processes used by some systems can add complexity. For example, systems that temporarily 

fix contaminants onto adsorbents require intricate air-flow configurations to properly fluidize the bed. 

These same types of systems, which then use steam to regenerate the adsorbents, must be 

engineered so that the steam does not affect photolysis. Further, energy required to generate the 

steam adds cost. Depending on the system design and generation of halogen acids, periodic 

replacement and disposal of liners and adsorbents may be necessary. Finally, more energy and 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-16

additional equipment may be needed for UV light production and supplemental water cooling to 

remove heat (if larger vessels or higher mass loadings are used). 

The photolytic destruction process is not cost-effective compared to activated carbon treatment for offgases with contaminant inlet concentrations of less than 100 ppm. Energy costs to operate the UV 

lamps are the principal economic driver. The technology appears to be relatively independent of 

contaminant loading except for technologies that require replacement of reactor liners. Operating 

costs for photolytic systems are about two-and-a-half times as great as for an acid-resistant Cat-Ox 

system, because of energy costs. Capital costs for photolytic systems are estimated to be twice those 

of Cat-Ox systems of the same capacity (Chapman and others 1997). 

Compared to thermal Cat-Ox systems, photocatalytic technology has higher capital costs but lower 

O&M costs according to a former vendor of this technology (RTN 1999a). Operating energy costs may 

be sufficiently lower so that the higher capital cost is recouped over the life of the unit. For one 

photocatalytic technology project, $1.5 million was reportedly saved over the life of the project 

compared to the use of GAC (Brunet and others 1999). However, because few examples of this 

technology have been used for SVE off-gas treatment, the life of such a unit and its components 

cannot be reliably stated. Maintenance costs are expected to be small and are mostly associated with 

yearly replacement of UV lamps and biennial replacement of catalyst. 

Developers of this technology are listed below. This list is not meant to be comprehensive and was 

developed from mention in literature and from Web sites providing descriptions of technologies 

applicable to SVE off-gas treatment. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use.

• KSE Inc.; Amherst, MA; photocatalytic; www.kse-online.com

• Purifics ES Inc.; London, Ontario, Canada; photocatalytic; www.purifics.com

• Trojan Technologies Inc.; London, Ontario, Canada; photocatalytic; www.trojanuv.com

• Zentox Corporation; Newport News, VA; photocatalytic; www.zentox.com

6.3 Membrane Separation Technologies

Membrane separation technologies have limited use for treating SVE off-gases from site remediation 

applications. Membrane separation techniques are used extensively in industrial applications to 

remove VOCs from process air emissions. One major use of this technology is to separate carbon 

dioxide from natural gas. The membrane separation process combines compression and 

condensation with membrane separation, and results in significantly more solvent recovery than 

compression and condensation alone. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-17

6.3.1 Technology Description and Applicability 

Membrane separation technology involves separation of VOCs from the contaminated vapor stream by 

passing the stream through a membrane designed to be up to 100 times more permeable to organic 

molecules than to air. Membrane separation systems are typically spiral-wound modules ranging from 

4 to 8 inches in diameter and 3 feet in length. As the feed gas enters the module, a partial vacuum on 

the other side of the membrane sheet draws the organic vapors through the spiral membrane to a 

central permeate collection pipe. The purified air stream excluded by the membrane exits the 

separator. This stream, sometimes called the “residue stream,” may require additional treatment by 

activated carbon prior to discharge to the atmosphere (MTR 2004). 

Membrane separation systems can be configured either as single-stage systems, which are typically 

used to treat concentrated vapor streams, or as two-stage systems, which are used to treat more dilute 

vapor streams. In the single-stage system, the contaminated vapor stream is initially routed through a 

condenser, where a portion of the organic vapor is cooled and condensed. The liquid condensate is 

recovered for recycling or reuse. The uncondensed air-vapor mixture is then routed to a membrane to 

separate the VOCs from the air stream. Two effluent streams are generated from the membrane: a 

purified stream containing low levels of VOC vapor that may be discharged to the atmosphere or 

further treated by activated carbon, and a concentrated vapor stream that is recirculated back to the 

condenser. It is uncommon for SVE off-gas streams to be concentrated enough for treatment by a 

single-stage system; however, SVE off-gas for systems treating substantial amounts of NAPL and 

significant VOC concentrations may be appropriate for such treatment. 

Two-stage systems are used when the vapor concentration in the off-gas stream is too dilute for an 

effective initial condensation step prior to separation. As the air-vapor mixture passes through the first 

of two membranes, most of the VOCs are separated from the clean air, which is discharged to the 

atmosphere. The VOC-containing permeate is then compressed and condensed, typically producing 

water and minor amounts of liquid VOCs. The bleed stream from the condenser passes through a 

second membrane that again produces a clean air discharge and a highly concentrated VOC 

permeate that is compressed and condensed. The liquid condensate from this second stage may be 

suitable for recycling, and the bleed stream from the second condenser is recirculated through the first 

membrane. In general, two-stage systems are too costly to be practical for most SVE applications. 

Figure 6-2 shows a typical two-stage membrane separation system schematic diagram. 

According to Membrane Technology and Research Inc. (MTR), its VaporSep® technology can treat a 

variety of VOCs, including chlorinated and non-chlorinated VOCs typically associated with SVE offgases. The single-stage membrane separation process was initially developed to remove and recover 

gasoline hydrocarbon vapors from fuel storage tanks at retail service stations. Other commercial 

applications involve recovery of vinyl chloride monomer from industrial processes. In these situations, 

the VaporSep® system can recover up to 100 lb/hr of the monomer. The technology can also be used 

to treat a wide range of vapor concentrations, which may make it applicable to both start-up and longterm SVE off-gas treatment applications. Compounds that can be recovered include common SVE off-
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-18

gas constituents such as benzene, carbon tetrachloride, chlorofluorocarbons, methanol, methylene 

chloride, PCE, TCA, TCE, toluene, and vinyl chloride. 

Figure 6-2. Typical Two-Stage Membrane Separation System 

Source: DOE 2001 

According to MTR, the VaporSep® system works best at influent concentrations of 100 to 10,000 

ppmv. Single-stage systems are best suited for concentrations exceeding 1,000 ppmv. Two-stage 

systems can be used to treat influent vapor concentrations less than 1,000 ppmv. Membrane 

separation systems have been designed for industrial applications to treat vapor flow rates of up to 

3,500 scfm and have been tested at influent concentrations of up to 100,000 ppmv; therefore, the 

technology should be adaptable for larger SVE systems, but treatment from typical low flow-rate SVE 

systems (less than 200 cfm) may not be cost-effective. 

6.3.2 Developmental Status 

A two-stage VaporSep® system was tested in a pilot-scale application as part of the U.S. EPA SITE 

Demonstration Program in 1994 (U.S. EPA 1994). The technology was also field-demonstrated on a 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-19

SVE system removing carbon tetrachloride and chloroform from subsurface soils at the DOE Hanford 

Reservation site in Richland, Washington. The influent concentrations were 1,000 ppmv, and the 

observed DRE was 95 percent (FRTR 2004). This technology was also tested for a SVE system at 

McClellan AFB in California. The demonstration at this site was conducted from March 1999 through 

May 2000, and initial chlorinated VOC concentrations ranged from 23 to 101 ppm (DOE 2001). 

Commercial installations of VaporSep® systems have been exclusively for industrial applications. 

6.3.3 Performance 

This technology has limited use in treating off-gases from site remediation applications, because it 

becomes more cost-effective when solvent recovery is desirable. During the U.S. EPA SITE study, the 

average DREs for perchloroethylene, hexane, and iso-octane were approximately 90 percent at 

influent concentrations ranging from 43 to 890 ppmv (U.S. EPA 1994). Depending on performance 

standards, these reductions may not yield acceptable concentration levels. The concentrations of 

organic compounds in the purified air stream are generally low enough for discharge to the 

atmosphere but can be further treated by other methods, if necessary. During the DOE demonstration, 

DREs of more than 95 percent were achieved by a system treating a stream containing 200 to 1,000 

ppmv of carbon tetrachloride (FRTR 2004). The demonstration at McClellan AFB reduced chlorinated 

VOC concentrations to less than 10 ppm before performance was impacted due to the presence of 

carbon dioxide, which was not accounted for in the design, and mechanical issues (the presence of oil 

and water in the system) (DOE 2001). 

6.3.4 Engineering Considerations, Limitations, and Costs 

As discussed in Section 6.3.1, single-stage systems are best suited for influent concentrations 

exceeding 1,000 ppmv (up to 10,000 ppmv). The capacity of current systems ranges from 1 to 100 

scfm, although MTR has constructed a unit capable of processing up to 140 scfm of feed air. 

Two-stage systems can be used to treat influent vapor concentrations less than 100 ppmv; however, at 

these levels, carbon adsorption may be more cost-effective because low-volume, low-concentration 

conditions do not yield significant solvent recoveries. 

The capital cost of the two-stage system tested at the DOE Hanford Reservation site capable of 

handling 7,000 scfm was approximately $2.5 million. This system treated SVE concentrations of 200 

to 1,000 ppmv of carbon tetrachloride and chloroform. More than 95 percent of the carbon 

tetrachloride was recovered as liquid, and the remaining VOCs (2 to 25 ppmv) were treated using 

activated carbon prior to discharge. More generalized capital costs are expected to range from $400 

to $1,000 per scfm of system capacity, with the cost per unit volume generally decreasing as treatment 

capacity increases (MTR 2004). 

O&M costs for the DOE Hanford Reservation site system were reportedly $6,000 per year for actual 

membrane maintenance and for assumed replacement every 3 years. At the test site, overall costs 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-20

were estimated to be $2,000 to $5,000 per pound of VOCs recovered, although the complete life cycle 

of the technology for this application has not been evaluated (FRTR 2004). Operation costs are 

generally quoted as $0.50 to $1.00 per 1,000 scfm treated. Costs associated with the demonstration 

at McClellan AFB (with a design flow rate of 100 scfm) were scaled up for a 500-scfm membrane 

separation system. For the larger system, estimated capital costs were $660,000 and O&M costs 

were $675,000 per year assuming membrane replacement every 3 years (DOE 2001). 

The only known developer of this technology is listed below. This list is not meant to be 

comprehensive and was developed from mention in literature and from Web sites providing 

descriptions of technologies applicable to SVE off-gas treatment. Mention of trade names or 

commercial products does not constitute endorsement or recommendation for use.

• Membrane Technology and Research, Inc.; Menlo Park, CA; VaporSep® Membrane; 

www.mtrinc.com

6.4 Other Technologies 

Gas absorption and vapor condensation technologies, with further development, are two other 

technologies that may be applicable under certain circumstances for treating SVE off-gas. These 

technologies are used in industrial applications and in some cases for regenerating activated carbon or 

other adsorbents. In other cases, they may be combined with other technologies already presented in 

this document. Because gas absorption and vapor condensation technologies are not frequently used, 

detailed information with respect to the evaluation criteria discussed in Section 1.6 was not available. 

This section presents a description of each technology and, when available, a brief discussion of 

technology performance during field applications. 

6.4.1 Gas Absorption Technologies 

In gas absorption processes, contaminant vapors are dissolved into a liquid solution. This process is 

referred to as “scrubbing” and is widely used in both industrial and site remediation applications as an 

ancillary treatment process to eliminate acid gases formed during thermal treatment; however, the 

technology is rarely used as the primary treatment method for site remediation off-gases. 

Absorption and adsorption are related physical-chemical processes. The fundamental difference 

between absorption and adsorption is that absorption involves the dissolving and/or mixing of the 

contaminant into the absorbent (usually liquid) medium, and adsorption involves the attraction and 

adhering of vapor or liquid to the surface of an adsorbent medium such as activated carbon. During 

the gas absorption process, contaminated vapor flows through a column in a counter-current direction 

to that of a low-volatility scrubbing solution (solvent). The scrubbing solution is usually distributed 

through the column as a spray or mist to maximize contact between the vapor and liquid phases. The 

concentration gradient between the gas and liquid phases and vapor-liquid equilibrium processes drive 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-21

dissolution of the gas-phase contaminants into the liquid solution; therefore, for this approach to work, 

it is necessary to select a suitable solvent for the chemical(s) targeted for recovery or removal from the 

air stream. 

In some cases, the contaminants in the vapor stream chemically react with a component of the 

absorbent solution to form chemically inert compounds (for example, acid gases absorbed into a 

caustic solution). In other cases, the contaminant simply dissolves into the absorbent solution, which 

requires further treatment or disposal. Gas absorption system configurations can include packedtower, sieve plate/tray-tower, Venturi, and fluidized bed configurations. Some solvents used include 

water, mineral oils, and non-volatile petroleum oils. The most typical solvent used in absorption 

applications is water, which works well for polar compounds but not for most compounds targeted in 

SVE remedies. Specialized, low-volatility, non-polar organic solvents can also be used, and these 

types of solvents are most applicable to SVE off-gas treatment applications. 

In 1989, the ChemtactTM absorption process marketed by Quad Environmental Technologies 

Corporation was used as part of a demonstration project. In this technology, a proprietary chemical 

absorbent solution treats the contaminated vapor stream in a once-through (non-regenerable) 

absorption process. The liquid absorbent is dispersed through the reaction chamber as finely 

atomized droplets (less than 10 microns in diameter). In the 1989 demonstration, mobile ChemtactTM

systems capable of treating 800 to 2,500 cfm treated a vapor stream containing unspecified 

hydrocarbons plus phenol and formaldehyde. The process was reported to have achieved 85 to 100 

percent DREs for the hydrocarbons and 94 percent DREs for both phenol and formaldehyde. During 

other projects, the DRE for benzene was effectively 100 percent; however, the toluene DRE was only 

50 to 93 percent. The ChemtactTM technology may no longer be commercially available. Attempts to 

contact the vendor have been unsuccessful (RTN 1999i). 

In the early 1990s, the University of Connecticut developed a process called “Contaminant Absorption 

and Recovery.” This process involves absorption of vapor-phase contaminants using a non-volatile 

liquid solvent (typically corn oil). The liquid solvent then passes through a second column, where the 

organic contaminants are stripped from the solvent at elevated temperatures and reduced pressure. 

This second process produces a low-flow vapor stream that can be condensed, and the liquid 

contaminants are recovered for recycling or disposal. The solvent solution can then be recirculated 

through the process (RTN 1999j). Water entrained in the soil vapor decreases the efficiency of this 

technology; therefore, SVE systems would need to include an efficient air-water separator to avoid this 

problem. 

Most research on this technology has been at the laboratory scale; however, a diffused bubblecontactor and a packed column contactor were each investigated for pilot-scale off-gas treatment at a 

sparge-and-vent remediation site. Off-gas VOCs from the system were successfully treated using both 

contactors. The TCE DRE was greater than 95 percent for both configurations. A patent was issued 

to the University of Connecticut for this technology in 1993, and a single field-scale application was 

conducted at that time; however, this technology was not subsequently commercialized (RTN 1999j). 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-22

One developer of this technology is listed below. This list is not meant to be comprehensive and was 

developed from mention in literature and from Web sites providing descriptions of technologies 

applicable to SVE off-gas treatment. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use.

• Environmental Research Institute, University of Connecticut; Storrs, CT 

In addition, gas absorption systems are commonly available from many vendors that serve 

manufacturing installations and the chemical processing industry. 

6.4.2 Vapor Condensation Technologies 

When treating contaminated vapor streams, the vapors can be cooled to condense the contaminants 

using a stand-alone control system or as an enhancement to other off-gas treatment technologies. 

Removing water vapor from the off-gas (as condensate) enhances the effectiveness of subsequent offgas treatment methods. Stand-alone condensation systems are typically used when the air stream 

contains concentrations of contaminants greater than 5,000 ppm (Govind and others 1994) or when it 

is economically desirable to recover the organic contaminant contained in the vapor stream for reuse 

or recycling (similar to vapor recovery systems for industrial processes). This process is generally not 

applied to SVE systems because the recovered products are generally not economically viable; 

however, if concentrations during the initial stages of an SVE system are high or the contaminated 

stream is not a mixture but consists of a single chemical, this process could be applicable. Particular 

applications that use condensation for SVE treatment generally use heat (such as steam or electrical 

heat) to enhance the effectiveness of SVE. The off-gases extracted by such systems are generally hot 

and have high vapor concentrations. 

Other configurations of vapor condensation include adsorbing or otherwise concentrating compounds 

from low-concentration vapors using another technology and then performing condensation for 

recovery for disposal or recycling as a feedstock into an ongoing industrial process (such as at an oil 

refinery). Recovered chemicals can also be used as a supplemental fuel source for off-gas treatment 

such as with ICE (see Section 3.2). 

In a condensation system, the contaminant-laden vapor stream is cooled below the dew point of the 

contaminants (assuming that the VOCs have sufficiently high dew points to allow condensation). 

Depending on the composition and concentration of contaminants, the temperature required for 

condensation may be extremely low. The vapor-phase contaminants, once condensed, can be 

collected for recycling or disposal. Methods used to cool the vapor stream may include the use of 

liquid nitrogen, mechanical chilling, or a combination of the two. The primary advantage of using a 

vapor condensation system for SVE off-gas treatment is that such a system is quite safe, even when 

applied to off-gases whose concentrations approach or exceed the LELs of the constituents. In 

contrast, oxidation systems must have significant protection and safeguards to ensure safe operation 

at LEL-range concentrations. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-23

Pure-phase product recovery is possible using vapor condensation for SVE off-gas treatment; 

however, materials that are readily condensable do not usually volatilize well at typical soil 

temperatures. This technology is better suited to applications where heating is used to increase the 

hydrocarbon removal rate from the subsurface. 

Although vapor condensation technologies are not commonly used for SVE systems, demonstrations 

of this application have been performed. The BRAYSORB system marketed by NUCON International, 

Inc. (NUCON), was demonstrated at the DOE Savannah River site in South Carolina in 1993 and at a 

Chevron facility in California in 1994. In the BRAYSORB process, the low-concentration vapor stream 

is adsorbed onto activated carbon to concentrate the vapor stream prior to condensation. Two carbon 

units are provided in series. Initially, only the first carbon canister is used until the concentration in the 

effluent exceeds a predetermined level. When saturation of the first carbon unit is achieved, the unit is 

taken off line for regeneration and the second carbon unit is used. This technology is efficient only for 

compounds that are reasonably well adsorbed on activated carbon and subsequently can be stripped 

at temperatures lower than 350 °F. The typical flow rate for this system is 8,000 to 10,000 scfm. 

During regeneration, hot nitrogen is passed through the spent carbon unit and solvents are desorbed 

from the carbon. The nitrogen and solvent vapors are compressed and cooled using a regenerative 

heat exchanger. Following the heat exchanger is a separator, where greater than 90 percent of the 

solvents in the vapor stream are condensed. Additional solvent is condensed by passing it through the 

compressor. After regeneration, the carbon bed is cooled and made ready for adsorption (RTN 1999f). 

NUCON has developed a mobile carbon regeneration system to perform the necessary regenerations. 

At the DOE Savannah River site, vapor condensation used for the recovery of SVE off-gas from 

treatment of soils impacted by machine shop wastes was performed at a flow rate of 250 scfm using a 

mobile unit. TCE and PCE concentrations in the influent were 2,000 and 5,000 ppm, respectively. 

Treatment using vapor condensation resulted in effluent TCE and PCE concentrations of 20 and 50 

ppm, respectively, or DREs of 99 percent (although the actual solvent recovery efficiency was not 

reported). Another application occurred at a Chevron site (a gasoline service station with an existing 

SVE system). The off-gas was treated at 500 scfm with an influent concentration of total BTEX of 900 

ppm. The resultant effluent had a BTEX concentration of 20 ppm, which correlates to a DRE of 98 

percent, during an 18-month period (RTN 1999f). 

This technology is considered to have low capital costs, because the absorber vessels can be 

constructed of less expensive materials. No corrosion results from condensing steam and soluble 

acids. This factor also reduces operating costs. Unlike conventional steam regeneration of activated 

carbon, no contaminated water stream is generated by this technology. Through the use of heat 

interchangers, this process results in reductions of up to 75 percent in energy requirements compared 

to conventional systems (NUCON 2005). Total costs for the DOE Savannah River site system (10 

wells) were approximately $25,000 per well for operating costs and $125,000 per well for capital costs. 

Each regeneration performed at the Chevron site cost $1,800, and the total cost for the unit at the 

Chevron site was $32,400 (RTN 1999f). 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 6-24

The only known developer of this technology is listed below. This list is not meant to be 

comprehensive and was developed from mention in literature and from Web sites providing 

descriptions of technologies applicable to SVE off-gas treatment. Mention of trade names or 

commercial products does not constitute endorsement or recommendation for use.

• NUCON International Inc.; Columbus, OH; BRAYSORB process; www.nucon-int.com

In addition, vapor condensing equipment is commonly available from many vendors that serve 

manufacturing installations and the chemical processing industry. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 7-1

7.0 SUMMARY 

A variety of technologies are capable of treating off-gas from SVE remediation systems. By far, the 

most common types of off-gas treatment systems use activated carbon adsorption and thermal 

oxidation (primarily catalytic oxidation). Other commercially available but less frequently used 

technologies include biofiltration and ICEs. Emerging technologies have been explored for 

remediating SVE off-gas in recent years, although none have matured to become significant 

commercially available options. 

This document presents available information for various technologies based on applicability, 

limitations, performance, engineering considerations, residuals management, cost and economics, and 

developmental status. This information is intended to present the state of the practice of off-gas 

treatment technologies for SVE systems. The information is also applicable to vapors generated from 

MPE and groundwater treatment systems. The document is intended to assist remediation project 

managers and engineers in evaluating and selecting appropriate off-gas treatment technologies for 

these systems. Project managers and engineers seeking guidance for the design and operation of 

such systems should consult engineering manuals or other specific guidance documents for specific 

technologies of interest (see Section 1.1 for examples). 

Selection of the most appropriate off-gas treatment alternative is a function of the following factors: 

• Types of contaminants present in the extracted vapor stream 

• Vapor concentration(s) 

• System flow rates 

• Anticipated duration of system operation 

• Local air emission and treatment requirements 

• Estimates of how the off-gas concentration will change during remediation 

Table 7-1 provides general concentration ranges of SVE off-gas applicable for commercially available 

technologies. As the table shows, more than one technology may be applicable for treating a specific 

off-gas. For example, SVE system off-gas concentrations of 500 ppmv of petroleum hydrocarbons 

(VOCs) could be treated by carbon adsorption, thermal oxidation, or biofiltration. Other less readily 

available technologies discussed in this report might also be applicable. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 7-2

Table 7-1. Generalized Volatile Organic Compound Influent Concentration Ranges for 

Commercially Available Technologies 

Technology Influent Volatile Organic Compound 

Concentration (ppmv) 

Carbon Adsorption <5,000 

Thermal Oxidation 100 – 4,000 

Biofiltration <1,000 

Notes: 

ppmv = Part per million by volume 

Source: USACE 2002 

In most instances, remediation practitioners evaluate and select thermal oxidation or carbon adsorption 

at sites with SVE systems. These two technologies are both robust, demonstrated, mature off-gas 

treatment methods that can treat a wide variety of contaminants. Compared to thermal oxidation or 

carbon adsorption, the other technologies presented in this document are generally cost prohibitive or 

are not sufficiently demonstrated for SVE off-gas treatment except under very specific circumstances. 

Currently, selection of off-gas treatment technology is based on cost and operational considerations 

that differentiate thermal oxidation and carbon adsorption systems. Although many factors affect the 

cost of an off-gas treatment system, the general rule for selecting between thermal oxidation and 

carbon adsorption is that more dilute off-gases are more cost-effectively treated by carbon adsorption. 

Thermal oxidation becomes more cost-effective when off-gases contain higher concentrations of vapor 

contaminants. Table 7-2 summarizes evaluation factors for selecting thermal oxidation and carbon 

adsorption technologies. 

Although thermal oxidation and carbon adsorption are currently the most common treatment 

technologies for SVE off-gas, some emerging technologies presented in this document have the 

potential to be cost-effective alternatives to thermal oxidation and carbon adsorption. In the future, 

cost-efficiency improvements or reports of new, positive experiences in applying these alternatives 

may result in their selection more frequently. With development, some of the emerging technologies, 

such as non-thermal plasma and photolytic technologies, have the potential to be cost-effective 

alternatives to thermal oxidation and carbon adsorption for SVE off-gas treatment. 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice

March 2006 7-3

Table 7-2. Evaluation Factors for Thermal Oxidation and Carbon Adsorption Technology 

Selection 

Factor Thermal Oxidation Carbon Adsorption 

Concentration More commonly used for higher 

contaminant concentrations (> 500 

ppmv); treatment costs per pound of 

contaminant decrease as VOC 

concentrations increase because less 

supplemental energy is required per 

pound removed 

More frequently used for dilute vapor 

concentrations (< 1,000 ppmv); 

treatment costs per pound of 

contaminant tend to stay same or 

increase as concentration of vapors 

increase because carbon replacement 

frequency increases 

O&M 

requirements 

Tends to require more labor and more 

skilled labor to operate because of safety 

considerations 

Tends to be simpler and less laborintensive to operate and maintain unless 

vapor-phase concentrations are high 

and “breakthrough” occurs frequently 

Safety More safeguards necessary if it is 

possible for off-gases to reach high 

concentrations (significant fractions of 

the lower explosive limits of the 

contaminants in the vapor); formation of 

dioxins and furans is possible if not 

properly operated 

Tends to be very safe under most 

conditions; however, high levels of 

ketones or similar compounds may pose 

a fire hazard 

Chlorinated vs. 

non-chlorinated 

VOCs 

Less commonly used for chlorinated 

VOCs because of formation of 

hydrochloric acid during vapor 

combustion, which requires special acidresistant materials for piping and 

equipment after combustion chamber

Equally applicable to chlorinated and 

non-chlorinated VOCs; acid formation 

not typically an issue 

Variety of 

compounds that 

can be treated 

Except for acid formation during 

combustion of chlorinated VOCs, wide 

variety of compounds can be treated 

Not all compounds adsorb well to 

activated carbon (depends on sorptive 

capacity); some common compounds 

(such as vinyl chloride) not readily 

treated; therefore, each compound in offgas must be considered 

Capital vs. O&M 

costs 

Equipment significantly more expensive 

to purchase than carbon units; however, 

at high VOC concentrations, O&M costs 

lower than carbon units 

Capital costs fairly low; O&M costs 

proportional to off-gas flow rates and 

vapor concentrations 

Notes: 

O&M = Operation and maintenance 

ppmv = Part per million by volume 

VOC = Volatile organic compound 
Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice 

8-1 March 2006 

8.0 REFERENCES 

Agency for Toxic Substances and Disease Registry (ATSDR). 2001. “Health Consultation: Site 

Summary Outlining Public Health Issues from February 2001 Site Visit, Edwards Air Force Base, 

Los Angeles, Kern and San Bernardino Counties, California.” May. 

Air Force Center for Environmental Excellence (AFCEE). 1995. “A Performance and Cost Evaluation 

for Purus PADRE® Regenerative Resin System for the Treatment of Hydrocarbon Vapors from 

Fuel-Contaminated Soils.” April. 

AFCEE. 1998. “Final Comprehensive Technical Report for the Evaluation of Soil Vapor Extraction 

and Treatment using Internal Combustion Engine Technology.” Technology Transfer Division. 

July. 

Alley, E. Roberts, and Associates, Inc. (Alley). 1998. “Chapter 22, Incineration of Gaseous 

Emissions.” Air Quality Control Handbook. Pages 22.1 through 22.26. McGraw Hill. 

Alonso, C., and others. 1998. “Dynamic Mathematical Model for the Biodegradation of VOCs in a 

Biofilter: Biomass Accumulation Study.” Environmental Science and Technology, Volume 32, 

Number 20. Pages 3118 through 3123. 

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EPA-542-R-05-028

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