Materials Characterization Paper 

In Support of the 

Final Rulemaking: 

Identification of Nonhazardous Secondary Materials That Are Solid Waste 

Wastewater Treatment Sludge 

February 3, 2011 

==================================================================== 

1. Definition of Wastewater Treatment Sludge 

The suspended and dissolved solids generated in the wastewater treatment process are called 

sludges or sewage sludge. Sludge may also be defined as any solid, semisolid or liquid material 

generated from a wastewater treatment process. This sludge is also commonly known as 

biosolids. Wastewater treatment operations require careful management of sludge, not only after 

removal from the treatment process, but also during the treatment process: sludge is a critical 

biologically active mix of water, organic matter (derived from human wastes, food wastes, etc.), 

inorganic solids (including trace elements), dead and alive micro-organisms (including 

pathogens), and trace contaminants (e.g., chemicals). Some sludge is routinely recycled within 

the treatment facility process to optimize operations. However, as sludge builds up, batches of it 

are regularly removed from effluent treatment operations. This “raw” sludge is typically two to 

three percent solids and 97 to 98 percent water and must be further treated to be utilized in a 

beneficial manner. Most sludge comes from primary settling tanks (“clarifiers”) and/or 

secondary settling tanks and, in most cases, is a slightly thick, gray-bottom liquid. In this 

analysis, wastewater treatment sludge includes materials generated after both primary and 

secondary treatment stages. 

Sludges generated from the pulp and paper industry are discussed in the Materials 

Characterization Paper entitled, “Forest-Derived Biomass and Pulp and Paper Residues.” 

2. Annual Quantities of Wastewater Treatment Sludge Generated and Used: 

(1) Sectors that generate Wastewater Treatment Sludge: 

The NAICS code for sewage treatment facilities is 221320. 

(2) Quantities and Prices of Wastewater Treatment Sludge Generated: 

• Data sources for the 2002-2003 period differ in their assessment of the generation 

of wastewater treatment sludge in the U.S. Based on the range of values reported 

by these sources, an estimated 5 to 7 million tons were generated annually in the 

U.S. during this period. (City of Toronto 2003; WEF 2002; NBP 2005, p. 19-1). 

• Nationwide data on the market value of wastewater treatment sludge are not 

readily available. On a local scale, a 1996 article by BioCycle magazine

estimated that biosolids produced by the Hampton Roads Sanitation District in 

Virginia was worth approximately $20 per dry ton of biosolids (or $95 per acre 

for land application) (NBP 2005). More recent estimates of the economic value 

of biosolids were not readily available. 
Wastewater Treatment Sludge 

2

 

(3) Trends in Generation of Wastewater Treatment Sludge: 

• The readily available data do not show a clear trend in the generation of 

wastewater treatment sludge. 

3. Uses of Wastewater Treatment Sludge 

(1) Fuel uses: 

• Wastewater treatment sludge may be used as a fuel in cement kilns (NAICS 

327310 cement industry). The available data do not indicate how widespread this 

practice is. 

• Various sources indicate that between 15 and 22 percent of wastewater treatment 

sludge is incinerated (WEF 2002; City of Toronto 2003; UNEP). Approximately 

150 municipalities in the U.S. - representing approximately 250 installations - 

engage in this practice (City of Toronto 2003). However, when wastewater 

treatment sludge is burned in dedicated wastewater treatment sludge incinerators, 

it is most likely not used as a substitute for conventional fossil fuels that 

incinerators may burn to maintain efficient combustion.1

 

• There are two fundamental options for recovery of energy from dried biosolids: 

1) utilize an existing offsite “host” process in which the biosolids replace a 

portion of the prime fuel; and 2) construct a dedicated plant onsite to produce 

energy for either onsite use or export (ENERGOs, 2007). 

• Atlanta-based EnerTech Environmental recently developed the “Slurry-CarbTM” 

process that converts biosolids into a renewable fuel called E-Fuel. The E-Fuel 

product derived from the Slurry-CarbTM process is suitable for gasification, cofiring, use in cement kilns or utilization in industrial and utility boilers (PWM 2007; 

EnerTech 2008). The cost of the Slurry-Carb process is approximately $72 per 

ton (PWM 2007). The first Slurry-Carb facility is scheduled to open in the Los 

Angeles area in 2008 and will convert 675 wet tons of sludge per day into 

approximately 145 tons per day of E-fuel, which will be sold to a local cement 

kiln as a substitute for coal (PWM 2007). 

• Several companies have begun to develop gasification technologies that treat 

biosolids, converting them into energy and inert ash (Patten, 2010). 

• Incineration of wastewater treatment sludge is regulated under 40 CFR 503 

Subpart E. These regulations cover pollutant limits; general management 

practices; and monitoring, reporting, and recordkeeping requirements (EPA 

2004). 

(2) Non-Fuel Uses of Wastewater Treatment Sludge

• According to EPA data summarized by the Water Environment Federation, more 

than 60 percent of wastewater treatment sludge produced in the United States is 

 1 We note, however, that a public comment submitted to EPA in response to the Advanced Notice of 

Proposed Rulemaking (ANPRM) for the Identification of Non-hazardous Materials That Are Solid Waste indicated 

that a sanitary district in California would require additional natural gas if it were unable to incinerate waste water 

treatment sludge for energy recovery (Craig 2009). 
Wastewater Treatment Sludge 

3

beneficially used on land following treatment, frequently by alkaline addition 

(WEF 2002). Similarly, another data source indicates that 63 percent of biosolids 

in the U.S. were land applied in 1998, with this figure expected to reach 66 

percent in 2005 and 70 percent in 2010 (Oleszkiewicz and Mavinic 2002). 

• Nearly 42 percent of all U.S. wastewater treatment solids are land applied as 

fertilizer (Clackamas County 2006). Based on the figures presented in the 

previous bullet, this represents approximately two-thirds of the total percentage 

land applied. 

• Data published by the United Nations Environment Program (UNEP) suggest that 

approximately 36 percent of wastewater treatment sludge generated in the mid1990s was beneficially used for agricultural applications (land applied) and 10 

percent was managed through other methods (UNEP). Examples of land 

application uses include: 

o Agricultural cropland application, through liquid injection, or surface 

spreading followed by incorporation into the soil; 

o Commercial sale as a fertilizer or soil conditioning material, particularly for 

horticultural and landscaping applications; 

o Rangeland and pasture application to improve available grazing; 

o Remediation of contaminated areas such as mine sites; 

o Soil amendment and recovery of marginal land; and 

o Land application in reforested areas (UNEP). 

• A potential future use of wastewater treatment sludge is as an anaerobic agent to 

optimize the natural degradation of waste in municipal solid waste landfills. (NBP 

2005, p 19-1). 

• Land application of wastewater treatment sludge is regulated under 40 CFR 503. 

These regulations cover pollutant limits; general management practices; and 

monitoring, reporting, and recordkeeping requirements (EPA 2004). The 

pollutant limits are listed below in Exhibit 4. 

(3) Quantities of Wastewater Treatment Sludge Landfilled 

• Based on EPA estimates from the 2000-2002 period, approximately 14 to 17 

percent of wastewater treatment sludge is landfilled, and, of this, approximately 3 

percent is used as daily or final cover in landfills (EPA as cited in WEF 2002 and 

NBP 2005, p. 19-1). 

• Data published by UNEP suggest that 38 percent of wastewater treatment sludge 

was disposed in landfills in the mid-1990s (Chang, Pang and Asano 1996). 

(4) Quantities Wastewater Treatment Sludge Stockpiled/Stored

• The available information sources do not suggest that wastewater treatment 

sludge is stockpiled or stored. 
Wastewater Treatment Sludge 

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Exhibit 1: Overview of Generation and Use - Wastewater Treatment Sludge 

Commodity 

Annual 

Quantity 

Generated 

Beneficial Use

Annual 

Quantity 

Landfilled 

Annual 

Quantity 

Incinerated 

Annual 

Quantity in 

Other Uses 

(landfill 

cover) 

Fuel 

(cement 

kiln)

Land Applied 

--------------------------------- Short Tons --------------------------------

Biosolids 

(Wastewater 

Treatment Sludge) 

5 to 7 

million dry 

tons 

Quantity 

data 

unavailable 

60%+ 14 to 17% 15 to 22% 

Quantity 

data 

unavailable 

Sources:

 City of Toronto 2003, NBP 2005, Oleszkiewicz and Mavinic 2002, UNEP, and WEF 2002 

4. Management and Combustion processes 

(1) Types of Units

• Fluidized Bed Incineration (FBI) 

• Multiple Hearth Furnace (MHF) 

• Cement Kilns 

(2) Sourcing information

• Municipal and food wastewater treatment plants. 

(3) Processing Information: 

• Most often, wastewater treatment sludge is treated in either an aerobic or 

anaerobic digester to stabilize the material and reduce pathogen concentrations 

(disease-causing organisms). A variety of other treatment options may be used to 

ensure that sludge meets federal and state requirements for beneficial use. Upon 

satisfying these requirements, the sludge is considered a “biosolid” (NEBRA 

2005). 

• The Slurry-CarbTM technology converts biosolids to fuel using the following 

processes: 

o Biosolids are subjected to pressure and heat; 

o Upon reaching the desired reaction temperature, the biosolids break down into 

carbon and light gases; 

o The result is a slurry with molecules that are much smaller than the original 

biosolids and very high in energy; 

o The slurry is dewatered to 50 percent total solids through centrifugation rather 

than evaporation, allowing it to dry using approximately two-thirds less 

energy than conventional drying methods; 

o The end product, a renewable fuel called E-Fuel, is an alternative to fossil 

fuels (EnerTech 2008). 

• Energy recovery technologies can be classified into sludge-to-biogas processes, 

sludge-to-syngas processes, sludge-to-oil processes, and sludge-to-liquid 

processes. The technologies available for resource recovery include those to 

recover phosphorus, building materials, nitrogen, volatile acids, etc (WERF, 

2008a). 
Wastewater Treatment Sludge 

5

(4) Changes in Technology to improve use as a fuel: 

• Information is not readily available on specific technological developments to 

improve the use of wastewater treatment sludge in combustion applications. The 

Lehigh Cement Company, however, began performing tests in 2004 on the use of 

dried sewage pellets from two Baltimore treatment plants to replace some of the 

coal burned by Lehigh’s kiln near Union Bridge. Lehigh’s parent corporation, 

Heidelberger Zement, and other cement makers have been using sludge pellets for 

about 20 years in Europe. Approximately 55 dry tons of pellets are produced 

each day at the Back River and the Patapsco wastewater treatment plants in 

Baltimore (Hand-Smith 2007, p5). 

(5) State status of Combustion as beneficial use: 

• At this stage, we have not identified any comprehensive information on states 

approving use of wastewater treatment sludge as fuel, but we have not performed 

an exhaustive investigation of state activities and regulations. In comments 

submitted in response to the Proposed Rule, the Virginia Department of 

Environmental Quality indicated that it supports the recovery of energy from 

sewage sludge (Steers, 2010). 

5. Wastewater Treatment Sludge Composition and Impacts 

(1) Composition of Commodity

• The Slurry-CarbTM process recently developed by EnerTech Environmental, 

produces a final product called “E-Fuel” from biosolids that can be sold as a 

renewable fuel with approximately 7,000 BTU per pound (EnerTech 2008).

• According to the Water Environment Foundation, the energy value of biosolids is 

approximately 5,500 kcal/kg (10,000 Btu per pound) of dry volatile solids or 

2,500 to 3,000 kcal/kg (4,500 to 5,500 Btu per pound) of total dry matter (WEF 

2000). Exhibit 2 presents separate Btu values for different types of wastewater 

treatment sludge. 

Exhibit 2: Btu Value of Different Types of Wastewater Treatment Sludge 

Wastewater Treatment Sludge 

Material 

Heating Value 

(Btu per pound of dry solids) 

Fine Screenings 9,000 

Grit 4,000 

Grease and Scum 16,700 

Dewatered Raw Biosolids 10,300 

Chemical Precipitated Biosolids 7,500 

Dewatered Digested Biosolids 5,300 

Source:

NBP 2005, p 15-10 

 
Wastewater Treatment Sludge 

6

• Exhibit 3 compares the results of several surveys of the chemical composition of 

biosolids, as well as constituent data for sewage sludge from select facilities 

across the U.S. The 40-City Study, which was conducted in 1979 and 1980, 

examined the solids generated at 40 publicly-owned wastewater treatment 

facilities, while the National Sewage Sludge Survey conducted in 1989 

summarizes the results of testing conducted at over 200 wastewater treatment 

facilities (NBP 2005, pp. 3-1 to 3-2). While dated, the results of the two surveys 

indicate that, in almost every case, the observed concentrations in the National 

Sewage Sludge Survey are less than those in the 40-city study, suggesting that 

concentrations fell during the 1980s (NBP 2005, p.3-2). Copper and mercury 

concentrations, however, appear to have increased during this period. The 

increase in copper concentrations most likely reflects increased use of copper 

plumbing in new homes. There is no clear explanation for the increase in the 

average mercury concentration (NBP 2005, p.3-2). Many concentration levels 

fell further by 2006, according to the Targeted National Sewage Sludge Survey 

cited by the National Association of Clean Water Agencies (Kirk 2010). 
Wa

7

Exhibit 3: Composition of Municipal Wastewater Treatment Sludges 

Element 

 Wastewater Treatment Sludge (All Measurements in mg/kg Dry Weight) 

40-City Study1

(1980) 

National 

Sewage Sludge 

Study1 (1989) 

Targeted 

National 

Sewage Sludge 

Survey2

(2006 – 2007) 

WWTS: 

Central 

Contra Costa 

Sanitation 

District3

(2010) 

Cedar Rapids Water Pollution 

Control Facilities: 

b

 

Belt Filter 

Cake4 (2009, 

2010) 

Centrifuge 

Filter Cake4

(2009 - 2010) 

Arsenic 9.9 6.7 6.9 <0.88 10.2 10.35 Cadmium 69 6.9 2.6 <1.03 1.71 2.13 Chromium 429 119 80 13.31 25.6 17.525 Copper 602 741 Not Available Not Available 150 224.5 Lead 369 134.4 76 <9.95 19.7 22.175 Mercury 2.8 5.2 1.2 0.85 0.15 0.2125 Molybdenum 17.7 9.2 Not Available Not Available 6.2 12.475 Nickel 135.1 42.7 48 <10.8 18.3 11.7 Selenium 7.3 5.2 7 2.75 10.2 13.195 Zinc 1,594 1,202 Not Available Not Available 785 996 Sources:1: NBP 2005, pp. 3-1 to 3-2. 2: Kirk 2010, p. 14. 3: Swanson 2010, p. 3. 4: Hershner 2010, p. 3. 5: Brady 2010, p. 3.
Wastewater Treatment Sludge 

8

• In accordance with Part 503, all biosolids applied to the land must be within the limits 

listed in Exhibit 4. 

Exhibit 4: Pollutant Limits for Biosolids Applied to the Land 

Pollutant 

Ceiling Concentration Limits for All Biosolids Applied to Land 

(milligrams per kilogram)a 

Arsenic 75 

Cadmium 85 

Chromium 3000 

Copper 4300 

Lead 840 

Mercury 57 

Molybdenumb 75 

Nickel 420 

Selenium 100 

Zinc 7500 

Applies to: All biosolids that are land applied 

From Part 503 Table 1, Section 503.13 

Source:

EPA 2004

Notes:

a

Dry-weight basis 

b

As a result of the February 25, 1994, Amendment to the rule, the limits for molybdenum were 

deleted from the Part 503 rule pending EPA reconsideration. 

(2) Impact Information

• Cost Impacts: The net cost impacts associated with the beneficial use of 

wastewater treatment sludge as a fuel in cement kilns depends on the value of the 

fuel that this material replaces and the cost of processing the material prior to its 

use as a fuel. Information on these processing costs is not readily available, but 

we estimate that the fuel savings associated with using wastewater treatment 

sludge as a substitute for conventional fuels would be $2.23 per MMBtu, 

assuming that the wastewater treatment sludge would replace coal (EIA 2007, 

Table ES1).2,3

 2

 To express this value as dollars per MMBtu, we assumed 22,473,000 Btu per short ton of coal (EIA 2005, 

Table A5). 

3 In a comment submitted to EPA following the release of the ANPRM, a sanitary district in California 

indicated that the cost of incinerating its waste treatment sludge is, on an annual basis, $4.3 million less than the cost 

of landfilling this material. Additionally, incineration of this wastewater treatment sludge reportedly saves the 

district $320,000 per year in energy costs, assuming the district would use natural gas in place of the sludge (Craig 

2009).
Wastewater Treatment Sludge 

9

• Emissions Impacts of Using Wastewater Treatment Sludge as a Fuel in Cement 

Kilns: As indicated above, an unknown quantity of wastewater treatment sludge 

may be beneficially used as a fuel in cement kilns. Information is not available on 

the emissions associated with this activity, but we were able to identify 

information on the emissions associated with the incineration of wastewater 

treatment sludge, as summarized in Exhibit 5. The exhibit also summarizes the 

combustion-related emissions for coal, distillate oil, residual oil, and natural gas. 

• Upstream Emissions Impacts: Use of wastewater treatment sludge as a 

replacement for traditional primary fuels in cement kilns eliminates the 

environmental impacts associated with extraction and processing of the traditional 

fuels. Exhibit 5 lists the quantities of cradle-to-gate (i.e., combustion plus 

upstream) emissions for coal, the fuel typically used by cement kilns, and other 

fossil fuels based on typical processes in the United States in the late 1990s. 
Wa

10

Exhibit 5: Comparative Impacts of Sludge Incineration versus Alternative Primary Fuels 

Pollutant 

Sludge 

Incineration Coal Distillate Fuel Oil Residual Fuel Oil 

Combustion 

(Incineration) Combustion 

Combustion 

plus 

Upstream Combustion 

Combustion 

plus 

Upstream Combustion 

Combustion 

plus 

Upstream C

------------------------------------------------------- lb./MMBtu ---------------------------------

Criteria Pollutants PM2.5 0.025 - - - - - - PM10 0.043 0.054 0.054 0.011 0.011 0.093 0.093 PM, unspecified 0.010 - 0.246 - 0.012 - 0.012 NOx 0.201 0.482 0.504 0.173 0.234 0.367 0.428 VOCs 0.008 0.006 0.014 0.001 0.363 0.002 0.367 SOx 0.28 1.446 1.469 0.209 0.394 1.593 1.781 CO 0.8 0.068 0.085 0.036 0.082 0.033 0.079 Pb 3.1x10-3 8.93x10-6 9.19x10-6 4.60x10-6 5.61x10-6 5.80x10-5 5.90x10-5 Hg - 2.05x10-6 2.14x10-6 1.58x10-6 1.77x10-6 8.67x10-6 8.85x10-6 Greenhouse Gases CO2 - 210.7 214.7 161.2 180.1 173.3 192.5 CH4 3.023x10-4 0.007 0.458 3.74x10-4 0.03 0.007 0.036 N2O - 0.003 0.003 - 2.01x10-5 - 2.07x10-5 MTCO2E/MMBtu 0.096 0.102 0.073 0.082 0.079 0.088 Source:

Franklin Associates 1998. Notes:

“-” signifies data not available; may equal zero. The sludge incineration numbers are based on a combination of averages, including the average controlled emissions rates from 12 sou

incineration technologies (multiple hearth, fluidized bed, and electric). Additionally, a midpoint value of 10,350 BTUs/lb heating valu

treatment sludge was used (heating values ranged from 4,000 btu/lb to 16,700 btu/lb). 

The emission information presented in this table is derived from Life Cycle Inventory (LCI) data, as compiled by Franklin Associates. quantifies resource inputs, energy requirements, and releases to the air, water, and land for each step in the manufacture of a product or extraction of the raw materials to ultimate disposal. The LCI can be used to identify those system components or life cycle steps that arto environmental burdens such as energy use, solid waste, and atmospheric and waterborne emissions. Uncertainty in an LCI is due to 

input uncertainties and data variability. 

There are several life cycle inventory databases available in the U.S. and Europe. For this paper, we applied the most readily available 

most consistent with the materials and uses examined. These LCI data rely on system boundaries as defined by Franklin Associates, as 

documentation for this database, available at: http://www.pre.nl/download/manuals/DatabaseManualFranklinUS98.pdf. 
Wastewater Treatment Sludge 

11

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