Air Pollutant Emissions at 

Flowback/Produced Wastewater 

Treatment Plants 

Water Management for Shale Plays

Pittsburgh, PA

Dean Ritts, PE, CEM

Principal Engineer – USAC Water

WorleyParsons, Houston, Texas, USA
Why is this important?

 Wastewater Treatment Plants are considered sources of Air 

Pollutants, specifically hazardous air pollutants (HAPs) and 

volatile organic compounds (VOCs)

 Industrial Wastewater Treatment Plants are recognized by EPA 

as being sources of HAP and VOC emissions and refineries, 

chemical manufacturing facilities, pharmaceutical manufacturing 

facilities, etc.

 As such, there are many Air Regulations in effect, with more 

coming, that target Wastewater Treatment Plants, both industrial 

and non-industrial treatment plants
Key Acronyms

 CAA – Clean Air Act

 HAP – Hazardous Air Pollutant

 MACT – Maximum Achievable Control Technology

 NESHAP – National Emission Standard for Hazardous 

Air Pollutant

 NSPS – New Source Performance Standards

 VOC – Volatile Organic Compounds

 CWA – Clean Water Act

 NPDES – National Pollutant Discharge Elimination 

System
What Should You Know?

 Key Regulations in Effect

 MACT/NESHAP Standards

 NSPS

 State/Local Standards

 Air Permitting Issues – Do they impact you?

 Steps in Preparing an Emissions Estimate for 

your Wastewater Treatment Plant
POTW

MACT

The Wastewater Compliance Universe

MON

HON

BWON

(FF)

Pharma 

MACT

PAI

MACT

Rfy

MACT

YYY

CWA

CAA
Regulatory Drivers

 40 CFR Part 63 – hazardous air pollutants

 NSR for Upstream O&G – 40 CFR Part 60 OOOO

 Product Storage Tanks & Flowback Water Storage Tanks

 Water storage tanks and treatment units

 State Regulatory Agencies

 Local Regulatory Agencies
What Kinds of Emissions Might 

You Have?

 Air toxics (HAPS)

 Do you treat wastewater with solvents in it? Are they hazardous 

chemicals?

 Criteria pollutants - particulate matter, NOx, SOx, VOCs

 Do you have baghouses, boilers, emergency generators, flares, 

heaters, and mobile equipment?

 Greenhouse Gases – CO2 and Methane
Mechanisms to Create Air 

Pollutant Emissions

 Volatilization of organic compounds from treatment 

processes

 Volatilization of organic compounds from collection 

systems

 Amounts emitted depend on loading, turbulence, system 

design

 Combustion devices emit criteria pollutants and 

hazardous air pollutants

 Solids handling operations create dust (Particulate 

Matter)
Sources of Air Pollutants

 Equalization Tanks

 Headworks

 Grinders

 Flumes

 Wet wells

 Lift stations

 Collection systems

 Primary settling tanks

 Aeration basins

 Biofilters

 API separators

 Boilers

 Emergency generators

 Aerobic digesters

 Disinfection systems

 Nutrient addition (ammonia)

 Chlorine removal

 Steam strippers
Sources of Air Pollutant 

Emissions
Sources of Air Pollutant 

Emissions
Sources of Air Pollutant 

Emissions
Sources of Air Pollutant 

Emissions
How do you estimate emissions?

 Modeling: EPA Water9 version 2; TOXCHEM

 Spreadsheet calculations: RWET

 Stack testing

 Ambient Air Monitoring

 Isolation Flux Chambers

 Mass balance based on material usage

 Influent wastewater characteristics and flow monitoring

 Emissions Factors based on Production
Four Steps to Evaluate HAP/VOC 

Emissions from a Flowback and Produced 

Water Treatment Plant (WTP)

 Step 1 -Evaluate wastewater characteristics for 

compounds with high vapor pressures (typically organics 

and reduced sulfur compounds)

 Step 2 - Based on design flows and actual flows 

calculate emissions assuming all influent compounds 

volatilizes (within reason, e.g. benzene, toluene) - worst 

case scenario
Four Step Process (continued)

 Step 3 - Refine Estimate by Modeling - System design 

specifications and dimensions, collection system 

information

 Step 4 - Air Pollutant Emissions Sampling - equipment 

intensive, analytically challenging, sophisticated sample 

plan, trained sampling team
Importance of Accurate Estimates of Air Pollutant 

Emissions from Wastewater Treatment Plants

 Permit Compliance

 Regulatory Compliance

 Increasingly lower emissions limits for HAPS and VOCs 

are being established by regulatory agencies

 Increasing control requirements

 Greenhouse gas limits

 Risk assessments

 Facilitate better compliance strategy
Importance of Wastewater 

Characterization

 Use the most accurate data available for:

 Flow rate

 Chemical Characterization

 Wastewater variation (avg vs max)

 Well data for produced water and flowback water

 Water quality data for influent that will enter the IWTP

 DO NOT RELY ON A SINGLE SAMPLE SET!
Example – Benzene Emissions 

from a Treatment Tank

 Lb/day load = X ppm chemical x Q mgd x 8.34

 For Benzene example of 10 ppm in 100,000 gpd 

wastewater, the daily load is:

 8.34 lb/day = 10 x 0.1 mgd x 8.34

 IF 80% is shown to be emitted to the air then the daily benzene 

emission is 6.67 lb/day

 That is 0.27 lb/hr
Example – continued

 But what if the next day your wastewater sample has 

100 ppm benzene? Then the daily load is 83.4 lb/day 

and the emissions could be 66.7 pounds/day or 2.7 lb/hr
The Relevance Factor

 So 66.7 lbs per day? Big deal?

 Yes, it can be a REAL big deal when you consider that 

many air permits are written with maximum HOURLY 

limits (not daily maximum like in a NPDES permit)

 Some benzene permit limits are as low as 0.1 lbs/hr so 

66.7 lbs per day is well over a 0.1 lb/hr emission rate. 

You are out of compliance with your permit.

 Here is the limit from a Permit by Rule form for TCEQ –

0.17 lb/hr

PBR Emission Limits

Chemical PBR Claimed L, mg/m3 Emission Limit

(E = L/K), lb/hr

Emission Limit

tpy

Actual Emissions

lb/hr

Actual Emissions

tpy

benzene §106.262 3 0.17 0.73 0.01 0.04

BTEX (as benzene) §106.262 3 0.17 0.73 0.05 0.20

C6-C12 Hydrocarbons (as 

naphtha)

§106.261 350 6.0 5.0 0.06 0.28

C12-C28 Hydrocarbons (as 

kerosene)

§106.262 100 5.56 5.0 0.06 0.25
The Ramification Factor

 Now extrapolate 66.7 lb/day benzene emitted to 365 

days and the benzene emission rate is 12.2 tpy.

 This puts the treatment unit in the Major Source category 

and it will require a Title V operating permit

 This is a regulatory program you really want to avoid and 

you have been pulled in because one 100 ml sample 

came back with a high benzene concentration
KNOW Your Wastewater for Air 

Permitting Purposes

 Take the time to properly characterize your wastewater stream;

 Document your sources of information and keep it available since 

recordkeeping is a key part of air permit compliance;

 Make sure that critical wastewater characterization equipment such 

as water meters and wastewater flow meters are well maintained 

and operational; and,

 Remember that existing wastewater data can be used but it may 

have been collected for different reasons and probably will not give 

you all the information you need.
Understand the Differences Between the Requirements of 

an Air Permit and a NPDES or SIU Permit

 For a NPDES Permit Application you can often take one sample set.

 For air permitting data, if you rely on ONE sample to develop your 

permit limits then you may be far off of the mark and left with a 

permit limit you will never make.

 Be cognizant of the different data needs for each permit type (e.g., 

maximum daily average vs hourly maximum emission rate) and how 

you gather that data. 
Texas Air Emission Limits for O&G Facilities

TCEQ Air Quality Permit Regulations for Oil and Gas Operations

Permit-By-Rule Emission Limits:

< 250 tpy of carbon monoxide (CO) or nitrogen oxides (NOx);

< 25 tpy of VOCs, SO2 or PM10; 

< 25 tpy of total HAPS (such as BTEX) and 10 tpy of individual HAPS; and

< 25 tpy of any other air pollutant except CO2, water, nitrogen, methane 

(CH4), ethane (C2H6), hydrogen (H2) or oxygen (O2).

Reference: Title 30, Texas Administrative Code, Ch. 106, Subchapter A.

2

5

11-

OctImportance of Obtaining Reliable Air Pollutant 

Emissions Values Can Not Be Overstated

 Data is used by regulators to set air permit limits

 Data is used by regulators to push for air pollution 

controls on wastewater treatment systems (e.g., covers)

 Data is used by regulators to develop regional air 

pollution control strategies and emission limits
Conservative Assumptions in 

Estimating Procedure with Water9

 NO LOSS of HAP as it passes through 3-phase 

separator at CPF, the implication being the WTP sees 

full load of HAP instead of 30 to 50% of load.

 Double concentration of highest TAP concentration 

recorded

 Run all estimates with maximum flowrates
Case Study for Flowback and Produced Water Treatment 

System Planned for EagleFord: Model Inputs

Case Study Values have been altered from project values
Flowback and Produced Water 

Treatment System
Water9 v2 Model Drawing of Case 

Study WTP

PW Model
Water9 v2 Model Drawing of WTP

FB Model
Case Study: Results Summary

Scenario 1 - Design Case

Compound*

Design Case Produced 

Water

Design Case Flowback 

Water

WW System Emissions (Total)

ppm ppm

(lb/hr) (tons/year)

Benzene 70.00 15.00 5.8E-03 2.1E-02

Toluene 500.00 26.00 3.15E-02 1.38E-01

Ethyl benzene 40.00 10.00 8.5E-03 3.9E-02

Xylene 772.00 60.00 5.1E+00 1.9E+01

Total VOCs 4.65E+00 2.15E+01

Total HAPs 4.66E+00 2.15E+01

Total Emissions 4.66E+00 2.15E+01

* HAPs and VOCs

Scenario 1 illustrates the design case with high concentration levels and peak flow of 235 gpm.
Scenario 2 – Double Design 

Case

Scenario 2 - Doubled Design Case Concentrations

Compound*

Double Design Case 

Concentrations Produced 

Water

Double Design Case 

Flowback Water

WW System Emissions (Total)

ppm ppm (lb/hr) (tons/year)

Benzene 140.00 30.00 1.1E-02 4.7E-02

Toluene 1000.00 52.00 5.9E-02 2.6E-01

Ethyl benzene 80.00 20.00 1.5E-02 6.6E-02

Xylene 1600 120.00 1.1E+01 5.0E+01

Total VOCs 1.1E+01 5.0E+01

Total HAPs 1.1E+01 5.0E+01

Total Emissions 1.1E+01 5.0E+01

* HAPs and VOCs

Scenario 2 illustrates doubling the design case concentration levels with peak flow of 235 gpm.
Scenario 2 Without Well #1

Scenario 3 - Doubled Design Case Concentrations Without Outlier Well #1

Compound*

Doubled Design Case 

Concentrations 

Produced Water

Double Design Case 

Concentrations 

Flowback Water

WW System Emissions (Total)

ppm ppm

(lb/hr) (tons/year)

Benzene 60.80 30.00 5.7E-03 2.5E-02

Toluene 150.60 52.00 1.1E-02 4.9E-02

Ethyl benzene 20.10 20.10 5.8E-03 2.6E-02

Xylene 376.00 122.00 3.5E+00 1.6E+01

Total VOCs 3.5E+00 1.6E+01

Total HAPs 3.5E+00 1.6E+01

Total Emissions 3.5E+00 1.6E+01

* HAPs and VOCs

Scenario 3 illustrates doubling the design case concentration levels without outlier well with peak flow of 235 gpm.
Scenario 4 – Expected 

Operating Conditions

Scenario 4 - Expected Operating Conditions

Compound*

Average Case 

Produced Water

Average Case 

Flowback Water

WW System Emissions (Total)

ppm ppm (lb/hr) (tons/year)

Benzene 15.00 10.00 1.5E-04 6.0E-04

Toluene 50.00 14.00 1.8E-04 6.5E-04

Ethyl benzene 11.00 8.00 3.9E-05 1.5E-04

Xylene 85.00 15.20 4.6E-01 2.2E+00

Total VOCs 4.6E-01 2.2E+00

Total HAPs 4.6E-01 2.2E+00

Total Emissions 4.6E-01 2.2E+00

* HAPs and VOCs

Scenario 4 illustrates expected operating conditions with flowrate of 149 gpm.
Document your Results

 Water9 v2 Model Results are Provided in an Excel Sheet 

that has:

 Model Input

− Wastewater Concentration Data

− Wastewater Flowrate Data

− Assumptions

− Treatment System Data

− EPA Default Data

 Model Output for All units, All compounds

 Model Output summarized for each operating condition

 Annual Average Emissions (TPY)

 Maximum Hourly Emissions (lb/hr)
Case study: 2

nd Look –

additional light VOCs

Scenario 4 - Expected Operating Conditions

Compound*

Average Case Produced Water Average Case Flowback 

Water WW System Emissions (Total)

ppm ppm (lb/hr) (tons/year)

Benzene* 16.00 10.40 1.2E-04 5.4E-04

Toluene * 53.00 12.00 1.5E-04 6.7E-04

Ethyl benzene* 10.00 9.00 4.1E-05 1.8E-04

Xylene * 81.00 17.20 2.8E-01 1.2E+00

Butane 5.50 5.4E-08 2.4E-07

Heptane 0.81 8.8E-08 3.8E-07

Hexane * 2.15 2.3E-07 1.0E-06

Octane 0.29 4.3E-08 1.9E-07

Pentane 1.59 2.2E-06 9.4E-06

Propane 8.19 2.2E-08 9.4E-08

Decane (C10 

&C10+) 1.57 2.4E-06 1.0E-05

Isopentane 1.87 1.1E-07 4.6E-07

Isobutane 32.74 9.9E-07 4.4E-06

Total VOCs 2.9E-01 1.5E+00

Total HAPs 2.9E-01 1.5E+00

* HAPs and VOCs

VOC

Scenario 4 illustrates expected operating conditions with flowrate of 149 gpm.
Main Data Requirements

 Flowback/Produced Water Compounds – compounds, 

compounds characteristics, flow rates

 System Design and Layout

 Unit dimensions
Recap: Estimation Methods

 Modeling: EPA Water9 version 2; TOXCHEM

 Spreadsheet calculations: RWET

 Stack testing

 Ambient Air Monitoring

 Isolation Flux Chambers

 Mass balance based on material usage

 Influent wastewater characteristics and flow monitoring

 Emissions Factors based on Production
Recap: Know Your Data Quality

 Need correct data for system design

 Need correct data for wastewater characteristics

 Need correct data for compound characteristics

 Need to determine when/where it is acceptable to accept 

model default values
Identify Critical and Controlling 

Parameters

 High energy inputs to the system

 Compound properties

 Wind speed

 Specific treatment units

 Temperatures (wastewater and ambient)
Summary: UCOG IWTP Results

 VOC emissions range from < 1 tpy to >50 tpy

 Individual HAP emissions can range from <1 tpy to >10 

tpy

 BTEX are important as contributors to HAPS

 Light hydrocarbons are important contributors to VOC 

emissions

 Emissions depend on the treatment system design

 Emissions depend on the concentration of volatile 

compounds

 Emissions depend on flowrate of wastewater through 

system