Waste 8-1

8. Waste 

Waste management and treatment activities are sources of greenhouse gas emissions (see Figure 8-1). Landfills 

accounted for approximately 17.5 percent of total U.S. anthropogenic methane (CH4) emissions in 2011, the third

largest contribution of any CH4 source in the United States. Additionally, wastewater treatment and composting of 

organic waste accounted for approximately 2.8 percent and less than 1 percent of U.S. CH4 emissions, respectively. 

Nitrous oxide (N2O) emissions from the discharge of wastewater treatment effluents into aquatic environments were 

estimated, as were N2O emissions from the treatment process itself. N2O emissions from composting were also 

estimated. Together, these waste activities account for less than 2 percent of total U.S. N2O emissions. Nitrogen 

oxides (NOx), carbon monoxide (CO), and non-CH4 volatile organic compounds (NMVOCs) are emitted by waste 

activities, and are addressed separately at the end of this chapter. A summary of greenhouse gas emissions from the 

Waste chapter is presented in Table 8-1 and Table 8-2.

Figure 8-1: 2011 Waste Chapter Greenhouse Gas Sources 

Box 8-1: Methodological Approach for Estimating and Reporting U.S. Emissions and Sinks

In following the UNFCCC requirement under Article 4.1 to develop and submit national greenhouse gas emission 

inventories, the emissions and sinks presented in this report and this chapter, are organized by source and sink 

categories and calculated using internationally-accepted methods provided by the Intergovernmental Panel on 
8-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

Climate Change (IPCC).246 Additionally, the calculated emissions and sinks in a given year for the United States

are presented in a common manner in line with the UNFCCC reporting guidelines for the reporting of inventories 

under this international agreement.247 The use of consistent methods to calculate emissions and sinks by all nations 

providing their inventories to the UNFCCC ensures that these reports are comparable. In this regard, U.S. emissions 

and sinks reported in this inventory report are comparable to emissions and sinks reported by other countries. 

Emissions and sinks provided in this inventory do not preclude alternative examinations,248 but rather this inventory 

presents emissions and sinks in a common format consistent with how countries are to report inventories under the 

UNFCCC. The report itself, and this chapter, follows this standardized format, and provides an explanation of the 

IPCC methods used to calculate emissions and sinks, and the manner in which those calculations are conducted.

Overall, in 2011, waste activities generated emissions of 127.7 Tg CO2 Eq., or just under 2 percent of total U.S. 

greenhouse gas emissions.

Table 8-1: Emissions from Waste (Tg CO2 Eq.)

Gas/Source 1990 2005 2007 2008 2009 2010 2011

CH4 164.0 130.5 129.8 131.9 131.4 124.7 120.8

Landfills 147.8 112.5 111.6 113.6 113.3 106.8 103.0

Wastewater Treatment 15.9 16.5 16.6 16.6 16.5 16.4 16.2

Composting 0.3 1.6 1.7 1.7 1.6 1.5 1.5

N2O 3.8 6.4 6.7 6.8 6.7 6.8 6.9

Domestic Wastewater 

Treatment 3.5 4.7 4.8 4.9 5.0 5.1 5.2

Composting 0.4 1.7 1.8 1.9 1.8 1.7 1.7

Total 167.8 136.9 136.5 138.6 138.1 131.4 127.7

Note: Totals may not sum due to independent rounding.

Table 8-2: Emissions from Waste (Gg)

Gas/Source 1990 2005 2007 2008 2009 2010 2011

CH4 7,810 6,217 6,183 6,280 6,258 5,936 5,751

Landfills 7,037 5,357 5,314 5,409 5,397 5,083 4,907

Wastewater Treatment 758 785 791 791 786 779 770

Composting 15 75 79 80 75 73 74

N2O 12 21 21 22 22 22 22

Domestic Wastewater 

Treatment 11 15 16 16 16 16 17

Composting 1 6 6 6 6 5 6

Note: Totals may not sum due to independent rounding.

Carbon dioxide, CH4, and N2O emissions from the incineration of waste are accounted for in the Energy sector 

rather than in the Waste sector because almost all incineration of municipal solid waste (MSW) in the United States 

occurs at waste-to-energy facilities where useful energy is recovered. Similarly, the Energy sector also includes an 

estimate of emissions from burning waste tires and hazardous industrial waste, because virtually all of the 

combustion occurs in industrial and utility boilers that recover energy. The incineration of waste in the United States 

 

246 See http://www.ipcc-nggip.iges.or.jp/public/index.html.

247 See http://unfccc.int/national_reports/annex_i_ghg_inventories/national_inventories_submissions/items/5270.php.

248 For example, see http://www.epa.gov/aboutepa/oswer.html.
Waste 8-3

in 2011 resulted in 12.4 Tg CO2 Eq. emissions, nearly half of which is attributable to the combustion of plastics. For

more details on emissions from the incineration of waste, see Section 3.3.

It is additionally noted that in this chapter methodological guidance was taken from the 2006 IPCC Guidelines for 

National Greenhouse Gas Inventories. This latest guidance from the IPCC best represents the understanding of 

emissions profiles from activities in the waste sector. The use of the most recently published calculation 

methodologies by the IPCC, as contained in the 2006 IPCC Guidelines for waste source categories is fully in line 

with the IPCC good practice guidance for methodological choice to improve rigor and accuracy. In addition, the 

improvements in using the latest methodological guidance from the IPCC has been recognized by the UNFCCC’s 

Subsidiary Body for Scientific and Technological Advice in the conclusions of its 30th Session249, Numerous U.S. 

inventory experts were involved in the development of the 2006 IPCC Guidelines, and their expertise has provided 

this latest guidance from the IPCC with the most appropriate calculation methods that are then used in this chapter.

Box 8-2: Waste Data from the Greenhouse Gas Reporting Program

On October 30, 2009, the U.S. EPA published a rule for the mandatory reporting of greenhouse gases from large 

GHG emissions sources in the United States. Implementation of 40 CFR Part 98 is referred to as EPA’s

Greenhouse Gas Reporting Program (GHGRP). 40 CFR part 98 applies to direct greenhouse gas emitters, fossil 

fuel suppliers, industrial gas suppliers, and facilities that inject CO2 underground for sequestration or other 

reasons and requires reporting by 41 industrial categories. Reporting is at the facility level, except for certain 

suppliers of fossil fuels and industrial greenhouse gases. In general, the threshold for reporting is 25,000 metric 

tons or more of CO2 Eq. per year. 

EPA’s GHGRP dataset and the data presented in this inventory report are complementary and, as indicated in the 

respective planned improvements sections for source categories in this chapter, EPA is analyzing how to use 

facility-level GHGRP data to improve the national estimates presented in this inventory. Most methodologies 

used in EPA’s GHGRP are consistent with IPCC, though for EPA’s GHGRP, facilities collect detailed 

information specific to their operations according to detailed measurement standards. This may differ with the 

more aggregated data collected for the inventory to estimate total, national U.S. emissions. It should be noted that 

the definitions for source categories in the GHGRP may differ from those used in this inventory in meeting the 

UNFCCC reporting guidelines. In line with the UNFCCC reporting guidelines, the inventory report is a 

comprehensive accounting of all emissions from source categories identified in the IPCC guidelines. Further 

information on the reporting categorizations in EPA’s GHGRP and specific data caveats associated with 

monitoring methods in EPA’s GHGRP has been provided on the EPA’s GHGRP website.250

EPA presents the data collected by EPA’s GHGRP through a data publication tool251 that allows data to be 

viewed in several formats including maps, tables, charts and graphs for individual facilities or groups of facilities. 

 

249 These Subsidiary Body for Scientific and Technological Advice (SBSTA) conclusions state, “The SBSTA acknowledged 

that the 2006 IPCC Guidelines contain the most recent scientific methodologies available to estimate emissions by sources and 

removals by sinks of greenhouse gases (GHGs) not controlled by the Montreal Protocol, and recognized that Parties have gained

experience with the 2006 IPCC Guidelines. The SBSTA also acknowledged that the information contained in the 2006 IPCC 

Guidelines enables Parties to further improve the quality of their GHG inventories.” See 

<http://unfccc.int/resource/docs/2009/sbsta/eng/03.pdf>

250 See 

<http://www.ccdsupport.com/confluence/display/ghgp/Detailed+Description+of+Data+for+Certain+Sources+and+Processes>.

251 See <http://ghgdata.epa.gov>.
8-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

8.1 Landfills (IPCC Source Category 6A1)

In the United States, solid waste is managed by landfilling, recovery through recycling or composting, and 

combustion through waste-to-energy facilities. Disposing of solid waste in modern, managed landfills is the most 

commonly used waste management technique in the United States. More information on how solid waste data are 

collected and managed in the United States is provided in Box 8-3 and Box 8-4. The municipal solid waste (MSW) 

and industrial waste landfills referred to in this section are all modern landfills that must comply with a variety of 

regulations as discussed in Box 8-5. Disposing of waste in illegal dumping sites is not considered to have occurred 

in years later than 1980 and these sites are not considered to contribute to net emissions in this section for the 

inventory time frame of 1990 to 2011. MSW landfills, or sanitary landfills, are sites where MSW is managed to 

prevent or minimize health, safety, and environmental impacts. Waste is deposited in different cells and covered 

daily with soil; many have environmental monitoring systems to track performance, collect leachate, and collect 

landfill gas. Industrial waste landfills are constructed in a similar way as MSW landfills, but accept waste produced 

by industrial activity, such as factories, mills, and mines.

After being placed in a landfill, organic waste (such as paper, food scraps, and yard trimmings) is initially 

decomposed by aerobic bacteria. After the oxygen has been depleted, the remaining waste is available for 

consumption by anaerobic bacteria, which break down organic matter into substances such as cellulose, amino acids, 

and sugars. These substances are further broken down through fermentation into gases and short-chain organic 

compounds that form the substrates for the growth of methanogenic bacteria. These methane- (CH4) producing 

anaerobic bacteria convert the fermentation products into stabilized organic materials and biogas consisting of 

approximately 50 percent biogenic carbon dioxide (CO2) and 50 percent CH4, by volume. Landfill biogas also 

contains trace amounts of non-methane organic compounds (NMOC) and volatile organic compounds (VOC) that 

either result from decomposition by-products or volatilization of biodegradable wastes (EPA 2008). 

Methane and CO2 are the primary constituents of landfill gas generation and emissions. However, the 2006 

Intergovernmental Panel on Climate Change (IPCC) Guidelines set an international convention to not report 

biogenic CO2 released due to landfill decomposition in the Waste sector (IPCC 2006). Carbon dioxide emissions 

are estimated and reported for under the Land Use/Land Use Change and Forestry (LULUCF) sector (see Box 8-6). 

Additionally, emissions of NMOC and VOC are not estimated because they are considered to be emitted in trace 

amounts. Nitrous oxide (N2O) emissions from the disposal and application of sewage sludge on landfills are also not 

explicitly modeled as part of greenhouse gas emissions from landfills. N2O emissions from sewage sludge applied 

to landfills as a daily cover or for disposal are expected to be relatively small because the microbial environment in 

an anaerobic landfill is not very conducive to the nitrification and denitrification processes that result in N2O 

emissions. Furthermore, the 2006 IPCC Guidelines (IPCC 2006) did not include a methodology for estimating N2O 

emissions from solid waste disposal sites “because they are not significant.” Therefore, only CH4 generation and 

emissions are estimated for landfills under the Waste sector. 

Methane generation and emissions from landfills are a function of several factors, including: (1) the total amount of 

waste-in-place, which is the total waste landfilled annually over the operational lifetime of a landfill; (2) the 

characteristics of the landfill receiving waste (e.g., composition of waste-in-place, size, climate, cover material); (3) 

the amount of CH4 that is recovered and either flared or used for energy purposes; and (4) the amount of CH4

oxidized as the landfill gas passes through the cover material into the atmosphere. Each landfill has unique 

characteristics, but all managed landfills practice similar operating practices, including the application of a daily and 

intermediate cover material over the waste being disposed of in the landfill to prevent odor and reduce risks to 

public health. Based on recent literature, the specific type of cover material used can affect the rate of oxidation of 

landfill gas (RTI 2011). The most commonly used cover materials are soil, clay, and sand. Some states also permit 

the use of green waste, tarps, waste derived materials, sewage sludge or biosolids, and contaminated soil as a daily 

cover. Methane production typically begins one or two years after waste is disposed of in a landfill and will continue 

for 10 to 60 years or longer as the degradable waste decomposes over time. 

In 2011, landfill CH4 emissions were approximately 103.0 Tg CO2 Eq. (4,907 Gg of CH4), representing the third 

largest source of CH4 emissions in the United States, behind natural gas systems and enteric fermentation. 

Emissions from MSW landfills, which received about 69 percent of the total solid waste generated in the United 

States, accounted for about 94 percent of total landfill emissions, while industrial landfills accounted for the 

remainder. Approximately 1,900 to 2,000 operational MSW landfills exist in the United States, with the largest 
Waste 8-5

landfills receiving most of the waste and generating the majority of the CH4 emitted (EPA 2010; BioCycle 2010; 

WBJ 2010). Conversely, there are approximately 3,200 MSW landfills in the United States that have been closed 

since 1980 (for which a closure data is known, WBJ 2010). While the number of active MSW landfills has 

decreased significantly over the past 20 years, from approximately 6,326 in 1990 to approximately 2,000 in 2010, 

the average landfill size has increased (EPA 2010; BioCycle 2010; WBJ 2010). The exact number of active and 

closed dedicated industrial waste landfills is not known at this time, but the Waste Business Journal total of landfills 

that accept industrial and construction and demolition debris for 2010 is 1,305. 

The estimated annual quantity of waste placed in MSW landfills increased 26 percent from about 205 Tg in 1990 to 

258 Tg in 2011 (see Annex 3.13). Net CH4 emissions have fluctuated from year to year, but a slowly decreasing 

trend has been observed over the past decade despite increased waste disposal amounts. For example, from 1990 to 

2011, net CH4 emissions from landfills decreased by approximately 30 percent (see Table 8-3 and Table 8-4). This 

decreasing trend can be attributed to a 21 percent reduction in the amount of decomposable materials (i.e., paper and 

paperboard, food scraps, and yard trimmings) discarded in MSW landfills over the time series (EPA 2010) and an 

increase in the amount of landfill gas collected and combusted (i.e., used for energy or flared), resulting in lower net 

CH4 emissions from MSW landfills.252 For instance, in 1990, approximately 954 Gg of CH4 were recovered and 

combusted from landfills, while in 2011, approximately 8,177 Gg of CH4 were combusted, representing an average 

annual increase in the quantity of CH4 recovered and combusted from 1990 to 2011 of 11 percent (see Annex 3.13). 

In 2011, an estimated 71 new landfill gas-to-energy (LFGTE) projects and 29 new flares began operation (EPA 

2012). While the amount of landfill gas collected and combusted continues to increase every year, the rate of 

increase in collection and combustion no longer exceeds the rate of additional CH4 generation from the amount of 

organic MSW landfilled as the U.S. population grows. 

The total amount of MSW generated is expected to increase as the U.S. population continues to grow. The 

percentage of waste landfilled, however, may decline due to increased recycling and composting practices. 

Additionally, the quantity of recovered CH4 that is either flared or used for energy purposes is expected to 

continually increase as a result of 1996 federal regulations that require large MSW landfills to collect and combust 

landfill gas (see 40 CFR Part 60, Subpart Cc 2005 and 40 CFR Part 60, Subpart WWW 2005), as well as voluntary 

programs that encourage CH4 recovery and use such as EPA’s Landfill Methane Outreach Program (LMOP), and 

federal and state incentives that promote renewable energy (e.g., tax credits, low interest loans, and Renewable 

Portfolio Standards).

 

252 Due to a lack of data specific to industrial waste landfills, landfill gas recovery is only estimated for MSW landfills. 
8-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

Table 8-3: CH4 Emissions from Landfills (Tg CO2 Eq.)

Activity 1990 2005 2007 2008 2009 2010 2011

MSW Landfills 172.6 241.2 254.2 259.2 262.9 266.6 270.2

Industrial Landfills 11.6 15.4 15.5 15.7 15.8 15.9 16.0

Recovered

 Gas-to-Energy (13.3) (55.9) (62.6) (67.2) (74.2) (82.5) (88.0)

 Flared (6.7) (75.7) (83.2) (81.5) (78.6) (81.4) (83.7)

 Oxidizeda(16.4) (12.5) (12.4) (12.6) (12.6) (11.9) (11.4)

Total 147.8 112.5 111.6 113.6 113.3 106.8 103.0

Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.

a

Includes oxidation at both municipal and industrial landfills. Oxidation at MSW landfills is accounted for after 

CH4 recovery. 

Table 8-4: CH4 Emissions from Landfills (Gg)

Activity 1990 2005 2007 2008 2009 2010 2011

MSW Landfills 8,219 11,486 12,106 12,343 12,519 12,694 12,868

Industrial Landfills 554 732 740 746 752 758 761

Recovered

 Gas-to-Energy (634) (2,660) (2,980) (3,198) (3,532) (3,927) (4,190)

 Flared (321) (3,606) (3,961) (3,880) (3,743) (3,876) (3,986)

 Oxidizeda(782) (595) (590) (601) (600) (565) (545)

Total 7,037 5,357 5,314 5,409 5,397 5,083 4,907

Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.

a

Includes CH4 oxidation at municipal and industrial landfills. Oxidation at MSW landfills is accounted for after CH4

recovery.

Methodology 

CH4 emissions from landfills were estimated as the CH4 produced from MSW landfills, plus the CH4 produced by 

industrial waste landfills, minus the CH4 recovered and combusted from MSW landfills, minus the CH4 oxidized 

before being released into the atmosphere:

CH4,Solid Waste = [CH4,MSW + CH4,Ind − R] − Ox

where,

CH4,Solid Waste = CH4 emissions from solid waste

CH4,MSW = CH4 generation from MSW landfills,

CH4,Ind = CH4 generation from industrial landfills, 

R = CH4 recovered and combusted (only for MSW landfills), and

Ox = CH4 oxidized from MSW and industrial waste landfills before release to the atmosphere.

The methodology for estimating CH4 emissions from landfills is based on the first order decay model described by 

the IPCC (IPCC 2006). Methane generation is based on nationwide waste disposal data; it is not landfill-specific. 

The amount of CH4 recovered, however, is landfill-specific, but only for MSW landfills due to a lack of data 

specific to industrial waste landfills. Values for the CH4 generation potential (L0) and decay rate constant (k) used in 

the first order decay model were obtained from an analysis of CH4 recovery rates for a database of 52 landfills and 

from published studies of other landfills (RTI 2004; EPA 1998; SWANA 1998; Peer, Thorneloe, and Epperson 

1993). The decay rate constant was found to increase with average annual rainfall; consequently, values of k were 

developed for 3 ranges of rainfall, or climate types (wet, arid, and temperate). The annual quantity of waste placed 

in landfills was apportioned to the 3 ranges of rainfall based on the percent of the U.S. population in each of the 3 

ranges. Historical census data were used to account for the shift in population to more arid areas over time. An 

overview of the data sources and methodology used to calculate CH4 generation and recovery is provided below, 

while a more detailed description of the methodology used to estimate CH4 emissions from landfills can be found in 

Annex 3.13.
Waste 8-7

National MSW landfill waste generation and disposal data are obtained from the BioCycle State of Garbage surveys, 

published approximately every two years. The State of Garbage (SOG) survey is the only continually updated 

nationwide survey of waste disposed in landfills in the United States. The SOG surveys use the principles of mass 

balance where all MSW generated is equal to the amount of MSW landfilled, combusted in waste-to-energy plants, 

composted, and/or recycled (BioCycle 2010). This approach assumes that all waste management methods are 

tracked and reported to state agencies. Survey respondents are asked to provide a breakdown of MSW generated 

and managed by landfilling, recycling, composting, and combustion (in waste-to-energy facilities) in actual 

tonnages. The survey reported data are adjusted to exclude non-MSW materials (e.g., industrial and agricultural 

wastes, construction and demolition debris, automobile scrap, and sludge from wastewater treatment plants) that 

may be included in survey responses. All state disposal data are adjusted for import/export; imported waste is 

included in a particular state and exported waste is not. Where no waste generation data are provided by a state in 

the SOG survey, the amount generated is estimated using the average nationwide waste per capita rate multiplied by 

that particular state’s population. 

National landfill waste generation data for 1989 through 2008 were obtained from the SOG survey for every two 

years (BioCycle 2006, 2008, and 2010). National landfill waste generation data for the years in-between the 

BioCycle State of Garbage surveys (e.g., 2001, 2003, 2005, 2007, 2009, 2010, and 2011) were extrapolated based on 

BioCycle data and the U.S. Census population The most recent SOG survey was published in 2010 for the 2008 

year. Waste generation data will be updated as new reports are published. Because the SOG survey does not 

account for waste generated in U.S. territories, waste generation for the territories was estimated using population 

data obtained from the U.S. Census Bureau (2009, 2012) and national per capita solid waste generation from the 

survey (2010). 

Estimates of the quantity of waste landfilled from 1989 to the current inventory year are determined by applying a 

waste disposal factor to the total amount of waste generated (i.e., the SOG data). A waste disposal factor is 

determined for each year an SOG survey is published and equals the ratio of the total amount of waste landfilled to 

the total amount of waste generated. The waste disposal factor is interpolated for the years in-between the BioCycle

surveys, as is done for the amount of waste generated for a given survey year. 

Estimates of the annual quantity of waste landfilled for 1960 through 1988 were obtained from EPA’s 

Anthropogenic Methane Emissions in the United States, Estimates for 1990: Report to Congress (EPA 1993) and an 

extensive landfill survey by the EPA’s Office of Solid Waste in 1986 (EPA 1988). Although waste placed in 

landfills in the 1940s and 1950s contributes very little to current CH4 generation, estimates for those years were 

included in the first order decay model for completeness in accounting for CH4 generation rates and are based on the 

population in those years and the per capita rate for land disposal for the 1960s. For calculations in this inventory, 

wastes landfilled prior to 1980 were broken into two groups: wastes disposed in landfills (Methane Conversion 

Factor, MCF, of 1) and those disposed in dumps (MCF of 0.6). All calculations after 1980 assume waste is disposed 

in managed, modern landfills. Please see Annex 3.13 for more details. 

Methane recovery is currently only accounted for at MSW landfills since no comprehensive data regarding gas 

collection systems have been published for industrial waste landfills. The estimated landfill gas recovered per year at 

MSW landfills was based on a combination of three databases: the flare vendor database (contains updated sales 

data collected from vendors of flaring equipment), a database of landfill gas-to-energy (LFGTE) projects compiled 

by LMOP (EPA 2012), and a database developed by the Energy Information Administration (EIA) for the voluntary 

reporting of greenhouse gases (EIA 2007). Based on the information provided by the EIA and flare vendor 

databases, the CH4 combusted by flares in operation from 1990 to the current inventory year was estimated. 

Information provided by the EIA and LMOP databases were used to estimate CH4 combusted in LFGTE projects 

over the time series. The three databases were carefully compared to identify landfills that were in two or all three 

of the databases to avoid double or triple counting CH4 reductions. 

The flare vendor database estimates CH4 combusted by flares using the midpoint of a flare’s reported capacity while 

the EIA database uses landfill-specific measured gas flow. As the EIA database only includes data through 2006; 

2007 to 2011 recovery for projects included in the EIA database were assumed to be the same as in 2006. This 

quantity likely underestimates flaring because these databases do not have information on all flares in operation. 

The EIA database is no longer being updated and it is expected that data obtained from the EPA’s Greenhouse Gas 

Reporting Program (GHGRP) will serve as a supplemental data source for facility-reported recovery data. 

Additionally, the EIA and LMOP databases provided data on landfill gas flow and energy generation for landfills 

with LFGTE projects. If a landfill in the EIA database was also in the LMOP and/or the flare vendor database, the 
8-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

emissions avoided were based on the EIA data because landfill owners or operators reported the amount recovered 

based on measurements of gas flow and concentration, and the reporting accounted for changes over time. If both 

flare data and LMOP recovery data were available for any of the remaining landfills (i.e., not in the EIA database), 

then the emissions recovery was based on the LMOP data, which provides reported landfill-specific data on gas flow 

for direct use projects and project capacity (i.e., megawatts) for electricity projects. The flare data, on the other 

hand, only provide a range of landfill gas flow for a given flare size. Given that each LFGTE project is likely to also 

have a flare, double counting reductions from flares and LFGTE projects in the LMOP database was avoided by 

subtracting emission reductions associated with LFGTE projects for which a flare had not been identified from the 

emission reductions associated with flares (referred to as the flare correction factor). A further explanation of the 

methodology used to estimate the landfill gas recovered can be found in Annex 3.13.

A destruction efficiency of 99 percent was applied to CH4 recovered to estimate CH4 emissions avoided due to the 

combusting of CH4 in destruction devices, i.e., flares. The destruction efficiency value was selected based on the 

range of efficiencies (86 to 99 percent) recommended for flares in EPA’s AP-42 Compilation of Air Pollutant 

Emission Factors, Chapter 2.4 (EPA 2008), efficiencies used to establish new source performance standards (NSPS) 

for landfills, and in recommendations for shutdown flares used in LMOP.

Emissions from industrial waste landfills were estimated from industrial production data (ERG 2012), waste 

disposal factors, and the first order decay model. As over 99 percent of the organic waste placed in industrial waste 

landfills originated from the food processing (meat, vegetables, fruits) and pulp and paper industries, estimates of 

industrial landfill emissions focused on these two sectors (EPA 1993). There are currently no data sources that track 

and report the amount and type of waste disposed of in industrial waste landfills in the United States. Therefore, the 

amount of waste landfilled is assumed to be a fraction of production that is held constant over the time series as 

explained in Annex 3.13. The composition of waste disposed of in industrial waste landfills is expected to be more 

consistent in terms of composition and quantity than that disposed of in MSW landfills. 

The amount of CH4 oxidized by the landfill cover at both municipal and industrial waste landfills was assumed to be 

ten percent of the CH4 generated that is not recovered (IPCC 2006, Mancinelli and McKay 1985, Czepiel et al. 

1996). To calculate net CH4 emissions, both CH4 recovered and CH4 oxidized were subtracted from CH4 generated 

at municipal and industrial waste landfills. 

Uncertainty and Time-Series Consistency

Several types of uncertainty are associated with the estimates of CH4 emissions from MSW and industrial waste 

landfills. The primary uncertainty concerns the characterization of landfills. Information is not available on two 

fundamental factors affecting CH4 production: the amount and composition of waste placed in every MSW and 

industrial waste landfill for each year of its operation. The SOG survey is the only nationwide data source that 

compiles the amount of MSW disposed at the state-level. The surveys do not include information on waste 

composition and there are no comprehensive data sets that compile quantities of waste disposed or waste 

composition by landfill. Some MSW landfills have conducted detailed waste composition studies, but landfills in 

the United States are not required to perform these types of studies. The approach used here assumes that the CH4

generation potential and the rate of decay that produces CH4, as determined from several studies of CH4 recovery at 

MSW landfills, are representative of conditions at U.S. landfills. When this top-down approach is applied at the 

nationwide level, the uncertainties are assumed to be less than when applying this approach to individual landfills 

and then aggregating the results to the national level. In other words, this approach may over- and under-estimate 

CH4 generation at some landfills if used at the facility-level, but the end result is expected to balance out because it 

is being applied nationwide. There is also a high degree of uncertainty and variability associated with the first order 

decay model, particularly when a homogeneous waste composition and hypothetical decomposition rates are applied 

to heterogeneous landfills (IPCC 2006). 

Additionally, there is a lack of landfill-specific information regarding the number and type of industrial waste 

landfills in the United States. The approach used here assumes that the majority (99 percent) of industrial waste 

disposed of in industrial waste landfills consists of waste from the pulp and paper and food and beverage industries. 

However, because waste generation and disposal data are not available in an existing data source for all U.S. 

industrial waste landfills, we apply a straight disposal factor over the entire time series to the amount of waste 

generated to determine the amounts disposed. 
Waste 8-9

Aside from the uncertainty in estimating CH4 generation potential, uncertainty exists in the estimates of the landfill 

gas oxidized. A constant oxidation factor of 10 percent as recommended by the Intergovernmental Panel on Climate 

Change (IPCC) for managed landfills is used for both MSW and industrial waste landfills regardless of climate, the 

type of cover material, and/or presence of a gas collection system. The number of field studies measuring the rate of 

oxidation has increased substantially since the IPCC 2006 Guidelines were published and, as discussed in the 

Potential Improvements section, efforts are being made to review the literature and revise this value based on recent, 

peer-reviewed studies. 

Another significant source of uncertainty lies with the estimates of CH4 that are recovered by flaring and gas-toenergy projects at MSW landfills. Three separate databases containing recovery information are used to determine 

the total amount of CH4 recovered and there are uncertainties associated with each. The LMOP database and the 

flare vendor databases are updated annually, while the EIA database has not been updated since 2005 and will 

essentially be replaced by the GHGRP data for a portion of landfills (i.e., those meeting the GHGRP thresholds). To 

avoid double counting and to use the most relevant estimate of CH4 recovery for a given landfill, a hierarchical 

approach is used among the three databases. The EIA data are given precedence because CH4 recovery was directly 

reported by landfills, the LMOP data are given second priority because CH4 recovery is estimated from facilityreported LFGTE system characteristics, and the flare data are given third priority because this database contains 

minimal information about the flare and no site-specific operating characteristics (Bronstein et al., 2012). The IPCC 

default value of 10 percent for uncertainty in recovery estimates was used in the uncertainty analysis when metering 

of landfill gas was in place (for about 64 percent of the CH4 estimated to be recovered). This 10 percent uncertainty 

factor applies to 2 of the 3 databases (EIA and LMOP). For flaring without metered recovery data (approximately 34 

percent of the CH4 estimated to be recovered), a much higher uncertainty of approximately 50 percent was used 

(e.g., when recovery was estimated as 50 percent of the flare’s design capacity). The compounding uncertainties 

associated with the 3 databases leads to the large upper and lower bounds for MSW landfills presented in Table 8-5. 

The results of the IPCC Good Practice Guidance Tier 2 quantitative uncertainty analysis are summarized in Table 

8-5. In 2011, landfill CH4 emissions were estimated to be between 47.0 and 150.2 Tg CO2 Eq., which indicates a 

range of 54 percent below to 46 percent above the 2011 emission estimate of 103.0 Tg CO2 Eq.

Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills (Tg CO2

Eq. and Percent)

Source Gas

2011 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

(Tg CO2 Eq.) (Tg CO2 Eq.) (%)

Lower 

Bound

Upper 

Bound

Lower 

Bound

Upper 

Bound

Landfills CH4 103.0 47.0 150.2 -54% +46%

 MSW CH4 88.7 33.5 136.0 -62% +53%

 Industrial CH4 14.4 10.5 17.4 -27% +21%

a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.

QA/QC and Verification

A QA/QC analysis was performed for data gathering and input, documentation, and calculation. QA/QC checks are 

not performed on the published data used to populate the inventory data set, including the SOG survey data and the 

published LMOP database. A primary focus of the QA/QC checks was to ensure that CH4 recovery estimates were 

not double-counted and that all LFGTE projects and flares were included in the respective project databases. Both 

manual and electronic checks were made to ensure that emission avoidance from each landfill was calculated in only 

one of the three databases. The primary calculation spreadsheet is tailored from the IPCC waste model and has been 

verified previously using the original, peer-reviewed IPCC waste model. All model input values were verified by 

secondary QA/QC review.

Recalculations Discussion

When conducted, methodological recalculations are applied to the entire time-series to ensure time-series 

consistency from 1990 through the current inventory year. No methodological changes were made for this 
8-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

inventory, but the national landfill waste generation data for 2007, 2008, 2009, and 2010 were recalculated for states 

that did not report an amount of waste generated in the SOG 2010 survey. This recalculation was warranted after 

reviewing the waste generation and disposal trends over the time series, particularly for years after 2004 where a 

noticeable decrease in the amount of waste generated was calculated. For states that did not report an amount of 

waste generated in the 2010 survey (BioCycle 2010), the recalculations used the most recent SOG waste per capita 

data in the 2010 survey and state-specific generation rates from the previous SOG survey (BioCycle 2008). These 

recalculations resulted in a slight increase in the waste generated for 2007 through 2010. 

Planned Improvements

Improvements to the inventory being examined include incorporating data from the EPA’s GHGRP and recent peerreviewed literature, modifying the default oxidation factor applied to MSW and industrial waste landfills, and either 

modifying the bulk waste degradable organic carbon (DOC) value or estimating emissions using a waste-specific 

approach in the first order decay model. 

Beginning in 2011, all MSW landfills that accepted waste on or after January 1, 1980 and generate CH4 in amounts 

equivalent to 25,000 metric tons or more of carbon dioxide equivalent (CO2 Eq.) were required to calculate and 

report their greenhouse gas emissions to EPA through its GHGRP. The MSW landfill source category of the 

GHGRP consists of the landfill, landfill gas collection systems, and landfill gas destruction devices, including flares. 

Potential improvements to the inventory methodology may be made using the GHGRP data, specifically for inputs 

to the first order decay equation. The approach used by the inventory to estimate CH4 generation assumes a bulk 

waste-specific DOC value that may not accurately capture the changing waste composition over the time series (e.g., 

the reduction of organics entering the landfill environment due to increased composting, see Box 8-4). Using data 

obtained from the GHGRP and any publicly available landfill-specific waste characterization studies in the United 

States, the methodology may be modified to incorporate a waste composition approach or revisions may be made to 

the bulk waste DOC value currently used. Additionally, GHGRP data could be analyzed and a weighted average for 

the methane correction factor (MCF), fraction of CH4 (F) in the landfill gas, the destruction efficiency of flares, and 

the decay rate constant (k) could replace the values currently used in the inventory. 

The most significant contribution of the GHGRP data to the inventory is expected to be the amount of recovered 

landfill gas and other information related to the gas collection system (Bronstein et al., 2012). Information for 

landfills with gas collection systems reporting under the GHGRP will be incorporated into the inventory data set and 

the measured CH4 recovery data will be used for the reporting landfills in lieu of the EIA, LMOP, and flare vendor 

data. The GHGRP data undergo an extensive series of verification steps, are more reliable and accurate than the 

data currently used, and will reduce uncertainties surrounding CH4 recovery when applied to the landfills in the 

inventory data set (Bronstein et al., 2012).

In addition to MSW landfills, industrial waste landfills at facilities generating CH4 in amounts equivalent to 25,000 

metric tons or more of CO2 Eq. were required to report their GHG emissions beginning in September 2012 through 

EPA’s GHGRP. Similar data for industrial waste landfills as is required for the MSW landfills will be reported. Any 

additions or improvements to the inventory using reported GHGRP data will be made for the industrial waste 

landfill portion of the inventory. One possible improvement is the addition of industrial sectors other than pulp and

paper, and food and beverage (e.g., metal foundries, petroleum refineries, and chemical manufacturing facilities). 

Of particular interest in the GHGRP data set for industrial waste landfills will be the presence of gas collection 

systems since recovery is not currently associated with industrial waste landfills in the inventory methodology. It is 

unlikely that data reported through the GHGRP for industrial waste landfills will yield improved estimates for k and 

Lo for the industrial sectors. However, EPA is considering an update to the Lo and k values for the pulp and paper

sector and are currently gathering feedback from stakeholders. 

The addition of this higher tier data will improve the emission calculations to provide a more accurate representation 

of greenhouse gas emissions from MSW and industrial waste landfills, but potential improvements to the inventory 

will not occur until after the deferral of GHGRP equation inputs expires in March 2013 for both MSW and industrial 

waste landfills, or as early as the 1990 to 2013 inventory report. Facility-level reporting data from the GHGRP are 

not available for all inventory years as reported in this inventory; therefore, particular attention will be made to 

ensure time series consistency while incorporating data from EPA’s GHGRP that would be useful to improve the 
Waste 8-11

emissions estimates for MSW landfills. In implementing improvements and integration of data from the GHGRP, 

the latest guidance from the IPCC on the use of facility-level data in national inventories will be relied upon.253

As a first step toward revising the oxidation factor used in the inventory, a literature review was conducted in 2011 

(RTI 2011). A standard CH4 oxidation factor of 10 percent has been used for both industrial and MSW landfills for 

all inventory reports and is currently recommended as the default for well-managed landfills in the latest IPCC 

guidelines (2006). Recent comments on the inventory methodology indicated that a default oxidation factor of 10 

percent may be less than oxidation rates achieved at well-managed landfills with gas collection and control. The 

impact of different landfill cover types on the rate of oxidation warrants further investigation as well. 

Currently, one oxidation factor (10 percent) is applied to the total amount of waste generated nationwide. Changing 

the oxidation factor and calculating the amount of CH4 oxidized from landfills with gas collection and control 

requires the estimation of waste disposed of in these types of landfills. The inventory methodology uses waste 

generation data from the SOG surveys, which report the total amount of waste generated and disposed nationwide 

by state. In 2010, the State of Garbage survey requested data on the presence of landfill gas collection systems for 

the first time. Twenty-eight states reported that 260 out of 1,414 (18 percent) operational landfills recovered landfill 

gas (BioCycle 2010). However, the survey did not include closed landfills with gas collection and control systems. 

In the future, the amount of states collecting and reporting this information is expected to increase. The EPA’s 

GHGRP data set for MSW landfills could be used to fill in the gaps related to the amount of waste disposed in 

landfills with gas collection systems. Although the EPA’s GHGRP does not capture every landfill in the United 

States, larger landfills are expected to meet the reporting thresholds and will be reporting waste disposal information 

by year beginning in March 2013. After incorporating the EPA’s GHGRP data, it may be possible to calculate the 

amount of waste disposed of at landfills with and without gas collection systems in the United States, which will 

allow the inventory waste model to apply different oxidation factors depending on the presence of a gas collection 

system. 

While research findings indicate some evidence that landfills with gas collection and control achieve a 20 percent or 

higher oxidation rate, there is not sufficient certainty to adopt a higher oxidation rate at this time. It is expected that 

with increased reporting by states in the State of Garbage survey, as well as the data collected through EPA’s 

GHGRP, the oxidation rate for at least a subset of landfills may be increased in a future inventory. A continued 

effort will be made to review peer-reviewed field studies that focus on oxidation specifically to determine how 

oxidation is affected by the presence of a gas collection system and landfill cover type and whether increasing the 

oxidation factor is warranted for all or only a portion of landfills (e.g., open versus closed, or only those with gas 

collection systems).

Box 8-3: Nationwide Municipal Solid Waste Data Sources

Municipal solid waste generated in the United States can be managed through landfilling, recycling, composting, 

and combustion with energy recovery. There are two main sources for nationwide solid waste management data in 

the United States, 

 The BioCycle and Earth Engineering Center of Columbia University’s State of Garbage (SOG) in America 

surveys and 

 The EPA’s Municipal Solid Waste in The United States: Facts and Figures reports. 

The SOG surveys collect state-reported data on the amount of waste generated and the waste managed via different 

management options: landfilling, recycling, composting, and combustion. The survey asks for actual tonnages 

instead of percentages in each waste category (e.g., residential, commercial, industrial, construction and demolition, 

organics, tires) for each waste management option. If such a breakdown is not available, the survey asks for total 

tons landfilled. The data are adjusted for imports and exports so that the principles of mass balance are adhered to, 

whereby the amount of waste managed does not exceed the amount of waste generated. The SOG reports present 

survey data aggregated to the state level. 

 

253 See: http://www.ipcc-nggip.iges.or.jp/meeting/pdfiles/1008_Model_and_Facility_Level_Data_Report.pdf
8-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

The EPA Facts and Figures reports use a materials flow methodology, which relies heavily on a mass balance 

approach. Data are gathered from industry associations, key businesses, similar industry sources, and government 

agencies (e.g., the Department of Commerce and the U.S. Census Bureau) and are used to estimate tons of materials 

and products generated, recycled, or discarded nationwide. The amount of MSW generated is estimated by 

adjusting the imports and exports of produced materials. MSW that is not recycled, composted, or combusted is 

assumed to be landfilled. The data presented in the report are nationwide totals. 

The State of Garbage surveys are the preferred data source for estimating waste generation and disposal amounts in 

the inventory because they are considered a more objective, numbers-based analysis of solid waste management in 

the United States. However, the EPA Facts and Figures reports are useful when investigating waste management 

trends at the nationwide level and for typical waste composition data, which the State of Garbage surveys do not ask 

for. 

In this inventory, emissions from solid waste management are presented separately by waste management option, 

except for recycling of waste materials. Emissions from recycling are attributed to the stationary combustion of 

fossil fuels, and are presented in the stationary combustion chapter in the Energy sector, although the emissions 

estimates are not called out separately. Emissions from solid waste disposal in landfills and the composting of solid 

waste materials are presented in the Landfills and Composting chapters in the Waste sector of this report. In the 

United States, almost all incineration of MSW occurs at waste-to-energy facilities or industrial facilities where 

useful energy is recovered, and thus emissions from waste incineration are accounted for in the Incineration chapter 

of the Energy sector of this report. 

Box 8-4: Overview of the Waste Sector

As shown in Figure 8-2 and Figure 8-3, landfilling of MSW is currently and has been the most common waste 

management practice. A large portion of materials in the waste stream are recovered for recycling and composting, 

which is becoming an increasingly prevalent trend throughout the country. Materials that are composted would have 

normally been disposed of in a landfill. 

Figure 8-2: Management of Municipal Solid Waste in the United States, 2010 (BioCycle 

2010)
Waste 8-13

Figure 8-3: MSW Management Trends from 1990 to 2010 (EPA 2011)

Table 8-6 presents a typical composition of waste disposed of at a typical MSW landfill in the United States over 

time. It is important to note that the actual composition of waste entering each landfill will vary from that presented 

in Table 8-6. Understanding how the waste composition changes over time, specifically for the degradable waste 

types, is important for estimating greenhouse gas emissions. For certain degradable waste types (i.e., paper and 

paperboard), the amounts discarded have decreased over time due to an increase in recovery (see Table 8-6 and 

Figure 8-4). Landfill ban legislation affecting yard trimmings resulted in an increase of composting from 1990 to 

2008. Table 8-6 and Figure 8-4 do not reflect the impact of backyard composting on yard trimming generation and 

recovery estimates. The recovery of food trimmings has been consistently low. Increased recovery of degradable 

materials reduces the CH4 generation potential and CH4 emissions from landfills. 
8-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

Table 8-6: Materials Discarded in the Municipal Waste Stream by Waste Type, percent (EPA 

2011) 

Waste Type 1990 2005 2007 2008 2009 2010

Paper and Paperboard 24.5% 24.5% 21.7% 19.7% 14.8% 15.3%

Glass 5.7% 5.7% 5.5% 5.3% 5.0% 4.8%

Metals 7.7% 7.7% 7.9% 8.0% 8.0% 8.3%

Plastics 15.7% 15.7% 16.4% 16.0% 15.8% 16.3%

Rubber and Leather 3.5% 3.5% 3.6% 3.7% 3.7% 3.8%

Textiles 5.5% 5.5% 5.9% 6.2% 6.3% 6.4%

Wood 7.4% 7.4% 7.5% 7.6% 7.7% 7.8%

Othera 1.8% 1.8% 1.9% 1.9% 1.9% 1.9%

Food Scrapsb 17.9% 17.9% 18.2% 18.6% 19.1% 19.3%

Yard Trimmingsc 7.0% 7.0% 6.7% 6.6% 7.6% 8.1%

Miscellaneous 

Inorganic Wastes 2.1% 2.1% 2.1% 2.2% 2.2% 2.2%

a Includes electrolytes in batteries and fluff pulp, feces, and urine in disposable diapers. Details may 

not add to totals due to rounding. Source: EPA 2011.

b Data for food scraps were estimated using sampling studies in various parts of the country in 

combination with demographic data on population, grocery store sales, restaurant sales, number of 

employees, and number of prisoners, students, and patients in institutions. Source: EPA 2010.

c Data for yard trimmings were estimated using sampling studies, population data, and published 

sources documenting legislation affecting yard trimmings disposal in landfills. Source: EPA 2010.

Figure 8-4: Percent of Recovered Degradable Materials from 1990 to 2010, percent (EPA 2011)

Box 8-5: Description of a Modern, Managed Landfill

Modern, managed landfills are well-engineered facilities that are located, designed, operated, and monitored to 

ensure compliance with federal, state, and tribal regulations. Municipal solid waste (MSW) landfills must be 

designed to protect the environment from contaminants which may be present in the solid waste stream. 

Requirements for affected MSW landfills may include:

 Siting requirements to protect sensitive areas (e.g., airports, floodplains, wetlands, fault areas, seismic 

impact zones, and unstable areas)

 Design requirements for new landfills to ensure that Maximum Contaminant Levels (MCLs) will not be 

exceeded in the uppermost aquifer (e.g., composite liners and leachate collection systems) 
Waste 8-15

 Leachate collection and removal systems

 Operating practices (e.g., daily and intermediate cover, receipt of regulated hazardous wastes, use of 

landfill cover material, access options to prevent illegal dumping, use of a collection system to prevent 

stormwater run-on/run-off, record-keeping)

 Air monitoring requirements (explosive gases)

 Groundwater monitoring requirements

 Closure and post-closure care requirements (e.g., final cover construction), and

 Corrective action provisions.

Specific federal regulations that affected MSW landfills must comply with include the 40 CFR Part 258 (Subtitle D 

of RCRA), or equivalent state regulations and the New Source Performance Standards (NSPS) 40 CFR Part 60 

Subpart WW. Additionally, state and tribal requirements may exist. For more information regarding federal MSW 

landfill regulations, see http://www.epa.gov/osw/nonhaz/municipal/landfill/msw_regs.htm. 

Box 8-6: Biogenic Wastes in Landfills

Regarding the depositing of wastes of biogenic origin in landfills (i.e., all degradable waste), empirical evidence 

shows that some of these wastes degrade very slowly in landfills, and the C they contain is effectively sequestered in 

landfills over a period of time (Barlaz 1998, 2006). Estimates of C removals from landfilling of forest products, 

yard trimmings, and food scraps are further described in the Land Use, Land-Use Change, and Forestry chapter, 

based on methods presented in IPCC (2003) and IPCC (2006). 

8.2 Wastewater Treatment (IPCC Source 

Category 6B)

Wastewater treatment processes can produce anthropogenic CH4 and N2O emissions. Wastewater from domestic254

and industrial sources is treated to remove soluble organic matter, suspended solids, pathogenic organisms, and 

chemical contaminants. Treatment may either occur on site, most commonly through septic systems or package 

plants, or off site at centralized treatment systems. Centralized wastewater treatment systems may include a variety 

of processes, ranging from lagooning to advanced tertiary treatment technology for removing nutrients. In the

United States, approximately 20 percent of domestic wastewater is treated in septic systems or other on-site systems, 

while the rest is collected and treated centrally (U.S. Census Bureau 2011). 

Soluble organic matter is generally removed using biological processes in which microorganisms consume the 

organic matter for maintenance and growth. The resulting biomass (sludge) is removed from the effluent prior to 

discharge to the receiving stream. Microorganisms can biodegrade soluble organic material in wastewater under 

aerobic or anaerobic conditions, where the latter condition produces CH4. During collection and treatment, 

wastewater may be accidentally or deliberately managed under anaerobic conditions. In addition, the sludge may be 

further biodegraded under aerobic or anaerobic conditions. The generation of N2O may also result from the 

treatment of domestic wastewater during both nitrification and denitrification of the N present, usually in the form of 

urea, ammonia, and proteins. These compounds are converted to nitrate (NO3) through the aerobic process of 

nitrification. Denitrification occurs under anoxic conditions (without free oxygen), and involves the biological 

conversion of nitrate into dinitrogen gas (N2). N2O can be an intermediate product of both processes, but has 

typically been associated with denitrification. Recent research suggests that higher emissions of N2O may in fact 

originate from nitrification (Ahn et al. 2010). 

 

254 Throughout the inventory, emissions from domestic wastewater also include any commercial and industrial wastewater collected and cotreated with domestic wastewater.
8-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

The principal factor in determining the CH4 generation potential of wastewater is the amount of degradable organic 

material in the wastewater. Common parameters used to measure the organic component of the wastewater are the 

Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). Under the same conditions, 

wastewater with higher COD (or BOD) concentrations will generally yield more CH4 than wastewater with lower 

COD (or BOD) concentrations. BOD represents the amount of oxygen that would be required to completely 

consume the organic matter contained in the wastewater through aerobic decomposition processes, while COD 

measures the total material available for chemical oxidation (both biodegradable and non-biodegradable). Because 

BOD is an aerobic parameter, it is preferable to use COD to estimate CH4 production. The principal factor in 

determining the N2O generation potential of wastewater is the amount of N in the wastewater. The variability of N 

in the influent to the treatment system, as well as the operating conditions of the treatment system itself, also impact 

the N2O generation potential.

In 2011, CH4 emissions from domestic wastewater treatment were 0.36 Tg CO2 Eq. (360 Gg). Emissions remained 

fairly steady from 1990 through 1997, but have decreased since that time due to decreasing percentages of 

wastewater being treated in anaerobic systems, including reduced use of on-site septic systems and central anaerobic 

treatment systems. In 2011, CH4 emissions from industrial wastewater treatment were estimated to be 8.6 Tg CO2

Eq. (409 Gg). Industrial emission sources have increased across the time series through 1999 and then fluctuated up 

and down with production changes associated with the treatment of wastewater from the pulp and paper 

manufacturing, meat and poultry processing, fruit and vegetable processing, starch-based ethanol production, and 

petroleum refining industries. Table 8-7 and Table 8-8 provide CH4 and N2O emission estimates from domestic and 

industrial wastewater treatment. 

With respect to N2O, the United States identifies two distinct sources for N2O emissions from domestic wastewater: 

emissions from centralized wastewater treatment processes, and emissions from effluent from centralized treatment 

systems that has been discharged into aquatic environments. The 2011 emissions of N2O from centralized 

wastewater treatment processes and from effluent were estimated to be 0.3 Tg CO2 Eq. (1 Gg) and 4.9 Tg CO2 Eq. 

(15.7 Gg), respectively. Total N2O emissions from domestic wastewater were estimated to be 5.2 Tg CO2 Eq. (16.7 

Gg). N2O emissions from wastewater treatment processes gradually increased across the time series as a result of 

increasing U.S. population and protein consumption. 

Table 8-7: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Tg 

CO2 Eq.)

Activity 1990 2005 2007 2008 2009 2010 2011

CH4 15.9 16.5 16.6 16.6 16.5 16.4 16.2

Domestic 8.8 8.3 8.1 8.0 8.0 7.8 7.6

Industrial* 7.1 8.2 8.5 8.6 8.5 8.6 8.6

N2O 3.5 4.7 4.8 4.9 5.0 5.1 5.2

Domestic 3.5 4.7 4.8 4.9 5.0 5.1 5.2

Total 19.4 21.2 21.4 21.5 21.5 21.5 21.4

* Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, 

fruit and vegetable processing, starch-based ethanol production, and petroleum refining 

industries.

Note: Totals may not sum due to independent rounding.

Table 8-8: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Gg)

Activity 1990 2005 2007 2008 2009 2010 2011

CH4 758 785 791 791 786 779 770

Domestic 421 396 385 383 380 370 360

Industrial* 338 389 405 409 406 409 409

N2O 11 15 16 16 16 16 17

Domestic 11 15 16 16 16 16 17

* Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, fruit 

and vegetable processing, starch-based ethanol production, and petroleum refining industries.
Waste 8-17

Note: Totals may not sum due to independent rounding.

Methodology

Domestic Wastewater CH4 Emission Estimates

Domestic wastewater CH4 emissions originate from both septic systems and from centralized treatment systems, 

such as publicly owned treatment works (POTWs). Within these centralized systems, CH4 emissions can arise from 

aerobic systems that are not well managed or that are designed to have periods of anaerobic activity (e.g., 

constructed wetlands), anaerobic systems (anaerobic lagoons and facultative lagoons), and from anaerobic digesters 

when the captured biogas is not completely combusted. CH4 emissions from septic systems were estimated by 

multiplying the United States population by the percent of wastewater treated in septic systems (20 percent), an

emission factor (10.7 g CH4/capita/day) and converting that to Gg/year. Methane emissions from POTWs were 

estimated by multiplying the total BOD5 produced in the United States by the percent of wastewater treated centrally 

(80 percent), the relative percentage of wastewater treated by aerobic and anaerobic systems, the relative percentage 

of wastewater facilities with primary treatment, the percentage of BOD5 treated after primary treatment (67.5 

percent), the maximum CH4-producing capacity of domestic wastewater (0.6), and the relative MCFs for aerobic 

(zero or 0.3) and anaerobic (0.8) systems with all aerobic systems assumed to be well-managed. Methane emissions 

from anaerobic digesters were estimated by multiplying the amount of biogas generated by wastewater sludge 

treated in anaerobic digesters by the proportion of CH4 in digester biogas (0.65), the density of CH4 (662 g CH4/m3

CH4) , and the destruction efficiency associated with burning the biogas in an energy/thermal device (0.99). The 

methodological equations are: 

Emissions from Septic Systems = A

= USPOP × (% onsite) × (EFSEPTIC) × 1/10^9 × Days

Emissions from Centrally Treated Aerobic Systems = B

= [(% collected) × (total BOD5 produced) × (% aerobic) × (% aerobic w/out primary) + (% collected) × (total BOD5

produced) × (% aerobic) × (% aerobic w/primary) × (1-% BOD removed in prim. treat.)] × (% operations not well 

managed) × (Bo) × (MCF-aerobic_not_well_man) × 1/10^6

Emissions from Centrally Treated Anaerobic Systems = C

= [(% collected) × (total BOD5 produced) × (% anaerobic) × (% anaerobic w/out primary) + (% collected) × (total 

BOD5 produced) × (% anaerobic) × (% anaerobic w/primary) × (1-%BOD removed in prim. treat.)] × (Bo) × (MCFanaerobic) × 1/10^6

Emissions from Anaerobic Digesters = D

= [(POTW_flow_AD) × (digester gas)/ (per capita flow)] × conversion to m3 × (FRAC_CH4) × (365.25) × (density 

of CH4) × (1-DE) × 1/10^9

Total CH4 Emissions (Gg) = A + B + C + D

where,

USPOP = U.S. population

% onsite = Flow to septic systems / total flow

% collected = Flow to POTWs / total flow

% aerobic = Flow to aerobic systems / total flow to POTWs

% anaerobic = Flow to anaerobic systems / total flow to POTWs

% aerobic w/out primary = Percent of aerobic systems that do not employ primary treatment

% aerobic w/primary = Percent of aerobic systems that employ primary treatment

% BOD removed in prim. treat. = 32.5%

% operations not well managed = Percent of aerobic systems that are not well managed and in which 

some anaerobic degradation occurs

% anaerobic w/out primary = Percent of anaerobic systems that do not employ primary treatment

% anaerobic w/primary = Percent of anaerobic systems that employ primary treatment
8-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

EFSEPTIC = Methane emission factor (10.7 g CH4/capita/day) – septic systems

Days = days per year (365.25)

Total BOD5 produced = kg BOD/capita/day × U.S. population × 365.25 days/yr

Bo = Maximum CH4-producing capacity for domestic wastewater (0.60 kg 

CH4/kg BOD)

1/10^6 = Conversion factor, kg to Gg

MCF-aerobic_not_well_man. = CH4 correction factor for aerobic systems that are not well managed 

(0.3) 

MCF-anaerobic = CH4 correction factor for anaerobic systems (0.8)

DE = CH4 destruction efficiency from flaring or burning in engine (0.99 for 

enclosed flares)

POTW_flow_AD = Wastewater influent flow to POTWs that have anaerobic digesters (gal)

digester gas = Cubic feet of digester gas produced per person per day (1.0 

ft3/person/day) (Metcalf and Eddy 2003)

per capita flow = Wastewater flow to POTW per person per day (100 gal/person/day)

conversion to m3 = Conversion factor, ft3to m3(0.0283)

FRAC_CH4 = Proportion CH4 in biogas (0.65)

density of CH4 = 662 (g CH4/m3 CH4)

1/10^9 = Conversion factor, g to Gg

U.S. population data were taken from the U.S. Census Bureau International Database (U.S. Census 2012) and 

include the populations of the United States, American Samoa, Guam, Northern Mariana Islands, Puerto Rico, and 

the Virgin Islands. Table 8-9 presents U.S. population and total BOD5 produced for 1990 through 2011, while Table

8-10 presents domestic wastewater CH4 emissions for both septic and centralized systems in 2011. The proportions 

of domestic wastewater treated onsite versus at centralized treatment plants were based on data from the 1989, 1991, 

1993, 1995, 1997, 1999, 2001, 2003, 2005, 2007, 2009, 2011 American Housing Surveys conducted by the U.S. 

Census Bureau (U.S. Census 2011), with data for intervening years obtained by linear interpolation. The percent of 

wastewater flow to aerobic and anaerobic systems, the percent of aerobic and anaerobic systems that do and do not 

employ primary treatment, and the wastewater flow to POTWs that have anaerobic digesters were obtained from the 

1992, 1996, 2000, and 2004 Clean Watershed Needs Survey (EPA 1992, 1996, 2000, and 2004). Data for 

intervening years were obtained by linear interpolation and the years 2004 through 2011 were forecasted from the 

rest of the time series. The BOD5 production rate (0.09 kg/capita/day) and the percent BOD5 removed by primary 

treatment for domestic wastewater were obtained from Metcalf and Eddy (2003). The CH4 emission factor (0.6 kg 

CH4/kg BOD5) and the MCF used for centralized treatment systems were taken from IPCC (2006), while the CH4

emission factor (10.7 g CH4/capita/day) used for septic systems were taken from Leverenz et al. (2010). The CH4

destruction efficiency for methane recovered from sludge digestion operations, 99 percent, was selected based on the 

range of efficiencies (98 to 100 percent) recommended for flares in AP-42 Compilation of Air Pollutant Emission 

Factors, Chapter 2.4 (EPA 1998), efficiencies used to establish new source performance standards (NSPS) for 

landfills, and in recommendations for closed flares used by the Landfill Methane Outreach Program (LMOP). The 

cubic feet of digester gas produced per person per day (1.0 ft3/person/day) and the proportion of CH4 in biogas 

(0.65) come from Metcalf and Eddy (2003). The wastewater flow to a POTW (100 gal/person/day) was taken from 

the Great Lakes-Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers, 

"Recommended Standards for Wastewater Facilities (Ten-State Standards)” (2004).

Table 8-9: U.S. Population (Millions) and Domestic Wastewater BOD5 Produced (Gg)

Year Population BOD5

1990 253 8,333

2005 300 9,853

2007 305 10,039

2008 308 10,132

2009 311 10,220

2010 314 10,303

2011 316 10,377
Waste 8-19

Source: U.S. Census Bureau (2012); 

Metcalf & Eddy 2003.

Table 8-10: Domestic Wastewater CH4 Emissions from Septic and Centralized Systems 

(2011) 

CH4 emissions (Tg CO2 Eq.) % of Domestic Wastewater CH4

Septic Systems 5.0 66.4%

Centralized Systems 2.5 33.6%

Total 7.6 100%

Note: Totals may not sum due to independent rounding.

Industrial Wastewater CH4 Emission Estimates

Methane emission estimates from industrial wastewater were developed according to the methodology described in 

IPCC (2006). Industry categories that are likely to produce significant CH4 emissions from wastewater treatment 

were identified. High volumes of wastewater generated and a high organic wastewater load were the main criteria. 

The top five industries that meet these criteria are pulp and paper manufacturing; meat and poultry processing; 

vegetables, fruits, and juices processing; starch-based ethanol production; and petroleum refining. Wastewater 

treatment emissions for these sectors for 2011 are displayed in Table 8-11 below. Table 8-12 contains production 

data for these industries.

Table 8-11: Industrial Wastewater CH4 Emissions by Sector (2011) 

CH4 emissions (Tg CO2 Eq.) % of Industrial Wastewater CH4

Pulp & Paper 4.1 48%

Meat & Poultry 3.7 43%

Petroleum Refineries 0.6 7%

Fruit & Vegetables 0.1 1%

Ethanol Refineries 0.1 1%

Total 8.6 100%

Note: Totals may not sum due to independent rounding.

Table 8-12: U.S. Pulp and Paper, Meat, Poultry, Vegetables, Fruits and Juices, Ethanol, and 

Petroleum Refining Production (Tg)

Year

Pulp and 

Papera

Meat

(Live Weight 

Killed)

Poultry

(Live Weight 

Killed)

Vegetables, 

Fruits and 

Juices Ethanol

Petroleum 

Refining

1990 128.9 27.3 14.6 38.7 2.7 702.4

2005 131.4 31.4 25.1 42.9 11.7 818.6

2007 135.9 33.4 26.0 44.7 19.4 827.6

2008 134.5 34.4 26.6 45.1 26.9 836.8

2009 137.0 33.8 25.2 46.5 31.7 822.4

2010 137.0 33.7 25.9 43.2 39.5 848.6

2011 137.0 33.8 26.2 42.9 41.5 858.8

aPulp and paper production is the sum of woodpulp production plus paper and paperboard production.

Methane emissions from these categories were estimated by multiplying the annual product output by the average 

outflow, the organics loading (in COD) in the outflow, the percentage of organic loading assumed to degrade 

anaerobically, and the maximum CH4 producing potential of industrial wastewater (Bo). Ratios of BOD:COD in 
8-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

various industrial wastewaters were obtained from EPA (1997a) and used to estimate COD loadings. The Bo value 

used for all industries is the IPCC default value of 0.25 kg CH4/kg COD (IPCC 2006). 

For each industry, the percent of plants in the industry that treat wastewater on site, the percent of plants that have a 

primary treatment step prior to biological treatment, and the percent of plants that treat wastewater anaerobically 

were defined. The percent of wastewater treated anaerobically onsite (TA) was estimated for both primary treatment 

(%TAp) and secondary treatment (%TAs). For plants that have primary treatment in place, an estimate of COD that 

is removed prior to wastewater treatment in the anaerobic treatment units was incorporated.

The methodological equations are: 

CH4 (industrial wastewater) = [P  W  COD  %TAp Bo  MCF] + [P  W  COD  %TAs Bo  MCF]

%TAp = [%Plantso  %WWa,p  %CODp]

%TAs = [%Plantsa  %WWa,s  %CODs] + [%Plantst  %WWa,t  %CODs]

where,

CH4 (industrial wastewater)= Total CH4 emissions from industrial wastewater (kg/year)

P = Industry output (metric tons/year)

W = Wastewater generated (m3/metric ton of product)

COD = Organics loading in wastewater (kg/m3)

%TAp = Percent of wastewater treated anaerobically on site in primary treatment

%TAs = Percent of wastewater treated anaerobically on site in secondary treatment

%Plantso = Percent of plants with onsite treatment

%WWa,p = Percent of wastewater treated anaerobically in primary treatment

%CODp = Percent of COD entering primary treatment

%Plantsa = Percent of plants with anaerobic secondary treatment

%Plantst = Percent of plants with other secondary treatment

%WWa,s = Percent of wastewater treated anaerobically in anaerobic secondary treatment

%WWa,t = percent of wastewater treated anaerobically in other secondary treatment 

%CODs = percent of COD entering secondary treatment

Bo = Maximum CH4 producing potential of industrial wastewater (default value of 

0.25 kg CH4/kg COD)

MCF = CH4 correction factor, indicating the extent to which the organic content 

(measured as COD) degrades anaerobically

As described below, the values presented in Table 8-13 were used in the emission calculations and are described in 

detail in Aguiar and Bartram (2008).

Table 8-13: Variables Used to Calculate Percent Wastewater Treated Anaerobically by 

Industry (%)

Variable

Industry

Pulp 

and 

Paper

Meat 

Processing

Poultry 

Processing

Fruit/ 

Vegetable 

Processing

Ethanol 

Production 

– Wet Mill

Ethanol 

Production 

– Dry Mill

Petroleum 

Refining

%TAp 0 0 0 0 0 0 0

%TAs 10.5 33 25 4.2 33.3 75 100

%Plantso 60 100 100 11 100 100 100

%Plantsa 25 33 25 5.5 33.3 75 100

%Plantst 35 67 75 5.5 66.7 25 0

%WWa,p 0 0 0 0 0 0 0

%WWa,s 100 100 100 100 100 100 100

%WWa,t 0 0 0 0 0 0 0

%CODp 100 100 100 100 100 100 100

%CODs 42 100 100 77 100 100 100

Source: Aguiar and Bartram (2008) Planned Revisions of the Industrial Wastewater Inventory Emission Estimates for the 

1990-2007 Inventory. August 10, 2008.
Waste 8-21

Pulp and Paper. Wastewater treatment for the pulp and paper industry typically includes neutralization, screening, 

sedimentation, and flotation/hydrocycloning to remove solids (World Bank 1999, Nemerow and Dasgupta 1991). 

Secondary treatment (storage, settling, and biological treatment) mainly consists of lagooning. In determining the 

percent that degrades anaerobically, both primary and secondary treatment were considered. In the United States, 

primary treatment is focused on solids removal, equalization, neutralization, and color reduction (EPA 1993). The

vast majority of pulp and paper mills with on-site treatment systems use mechanical clarifiers to remove suspended 

solids from the wastewater. About 10 percent of pulp and paper mills with treatment systems use settling ponds for 

primary treatment and these are more likely to be located at mills that do not perform secondary treatment (EPA 

1993). However, because the vast majority of primary treatment operations at U.S. pulp and paper mills use 

mechanical clarifiers, and less than 10 percent of pulp and paper wastewater is managed in primary settling ponds 

that are not expected to have anaerobic conditions, negligible emissions are assumed to occur during primary 

treatment.

Approximately 42 percent of the BOD passes on to secondary treatment, which consists of activated sludge, aerated 

stabilization basins, or non-aerated stabilization basins. No anaerobic activity is assumed to occur in activated 

sludge systems or aerated stabilization basins (note: although IPCC recognizes that some CH4 can be emitted from 

anaerobic pockets, they recommend an MCF of zero). However, about 25 percent of the wastewater treatment 

systems used in the United States are non-aerated stabilization basins. These basins are typically 10 to 25 feet deep. 

These systems are classified as anaerobic deep lagoons (MCF = 0.8). 

A time series of CH4 emissions for 1990 through 2001 was developed based on production figures reported in the 

Lockwood-Post Directory (Lockwood-Post 2002). Published data from the American Forest and Paper Association, 

data published by Paper Loop, and other published statistics were used to estimate production for 2002 through 2011 

(Pulp and Paper 2005, 2006, and monthly reports from 2003 through 2008; Paper 360◦ 2007). The overall 

wastewater outflow was estimated to be 85 m3/metric ton, and the average BOD concentrations in raw wastewater 

was estimated to be 0.4 gram BOD/liter (EPA 1997b, EPA 1993, World Bank 1999). The COD:BOD ratio used to 

convert the organic loading to COD for pulp and paper facilities was 2 (EPA 1997a).

Meat and Poultry Processing. The meat and poultry processing industry makes extensive use of anaerobic lagoons 

in sequence with screening, fat traps and dissolved air flotation when treating wastewater on site. About 33 percent 

of meat processing operations (EPA 2002) and 25 percent of poultry processing operations (U.S. Poultry 2006) 

perform on-site treatment in anaerobic lagoons. The IPCC default Bo of 0.25 kg CH4/kg COD and default MCF of 

0.8 for anaerobic lagoons were used to estimate the CH4 produced from these on-site treatment systems. Production 

data, in carcass weight and live weight killed for the meat and poultry industry, were obtained from the USDA 

Agricultural Statistics Database and the Agricultural Statistics Annual Reports (USDA 2012). Data collected by 

EPA’s Office of Water provided estimates for wastewater flows into anaerobic lagoons: 5.3 and 12.5 m3/metric ton 

for meat and poultry production (live weight killed), respectively (EPA 2002). The loadings are 2.8 and 1.5 g 

BOD/liter for meat and poultry, respectively. The COD:BOD ratio used to convert the organic loading to COD for 

both meat and poultry facilities was 3 (EPA 1997a).

Vegetables, Fruits, and Juices Processing. Treatment of wastewater from fruits, vegetables, and juices processing 

includes screening, coagulation/settling, and biological treatment (lagooning). The flows are frequently seasonal, 

and robust treatment systems are preferred for on-site treatment. Effluent is suitable for discharge to the sewer. 

This industry is likely to use lagoons intended for aerobic operation, but the large seasonal loadings may develop 

limited anaerobic zones. In addition, some anaerobic lagoons may also be used (Nemerow and Dasgupta 1991). 

Consequently, 4.2 percent of these wastewater organics are assumed to degrade anaerobically. The IPCC default Bo

of 0.25 kg CH4/kg COD and default MCF of 0.8 for anaerobic treatment were used to estimate the CH4 produced 

from these on-site treatment systems. The USDA National Agricultural Statistics Service (USDA 2012) provided 

production data for potatoes, other vegetables, citrus fruit, non-citrus fruit, and grapes processed for wine. Outflow 

and BOD data, presented in Table 8-14, were obtained from EPA (1974) for potato, citrus fruit, and apple 

processing, and from EPA (1975) for all other sectors. The COD:BOD ratio used to convert the organic loading to 

COD for all fruit, vegetable, and juice facilities was 1.5 (EPA 1997a).

Table 8-14: Wastewater Flow (m3/ton) and BOD Production (g/L) for U.S. Vegetables, Fruits, 

and Juices Production

Commodity Wastewater Outflow (m3

/ton) BOD (g/L)
8-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

Vegetables

Potatoes 10.27 1.765

Other Vegetables 8.69 0.794

Fruit

Apples 3.66 1.371

Citrus 10.11 0.317

Non-citrus 12.42 1.204

Grapes (for wine) 2.78 1.831

Ethanol Production. Ethanol, or ethyl alcohol, is produced primarily for use as a fuel component, but is also used in 

industrial applications and in the manufacture of beverage alcohol. Ethanol can be produced from the fermentation 

of sugar-based feedstocks (e.g., molasses and beets), starch- or grain-based feedstocks (e.g., corn, sorghum, and 

beverage waste), and cellulosic biomass feedstocks (e.g., agricultural wastes, wood, and bagasse). Ethanol can also 

be produced synthetically from ethylene or hydrogen and carbon monoxide. However, synthetic ethanol comprises 

only about 2 percent of ethanol production, and although the Department of Energy predicts cellulosic ethanol to 

greatly increase in the coming years, currently it is only in an experimental stage in the United States. According to 

the Renewable Fuels Association, 82 percent of ethanol production facilities use corn as the sole feedstock and 7 

percent of facilities use a combination of corn and another starch-based feedstock. The fermentation of corn is the 

principal ethanol production process in the United States and is expected to increase through 2012, and potentially 

more; therefore, emissions associated with wastewater treatment at starch-based ethanol production facilities were 

estimated (ERG 2006).

Ethanol is produced from corn (or other starch-based feedstocks) primarily by two methods: wet milling and dry 

milling. Historically, the majority of ethanol was produced by the wet milling process, but now the majority is 

produced by the dry milling process. The wastewater generated at ethanol production facilities is handled in a 

variety of ways. Dry milling facilities often combine the resulting evaporator condensate with other process 

wastewaters, such as equipment wash water, scrubber water, and boiler blowdown and anaerobically treat this 

wastewater using various types of digesters. Wet milling facilities often treat their steepwater condensate in 

anaerobic systems followed by aerobic polishing systems. Wet milling facilities may treat the stillage (or processed 

stillage) from the ethanol fermentation/distillation process separately or together with steepwater and/or wash water. 

CH4 generated in anaerobic digesters is commonly collected and either flared or used as fuel in the ethanol 

production process (ERG 2006).

Available information was compiled from the industry on wastewater generation rates, which ranged from 1.25 

gallons per gallon ethanol produced (for dry milling) to 10 gallons per gallon ethanol produced (for wet milling) 

(Ruocco 2006a,b; Merrick 1998; Donovan 1996; and NRBP 2001). COD concentrations were also found to be 

about 3 g/L (Ruocco 2006a; Merrick 1998; White and Johnson 2003). The amount of wastewater treated 

anaerobically was estimated, along with how much of the CH4 is recovered through the use of biomethanators (ERG 

2006). Methane emissions were then estimated as follows:

Methane = [Production × Flow × COD × 3.785 × ([%Plantso × %WWa,p × %CODp] + [%Plantsa × %WWa,s × %CODs] + 

[%Plantst × %WWa,t × %CODs]) × Bo × MCF × % Not Recovered] + [Production × Flow × 3.785 × COD × ([%Plantso ×

%WWa,p × %CODp] + [%Plantsa × %WWa,s × %CODs] + [%Plantst × %WWa,t × %CODs]) × Bo × MCF × (% Recovered) × (1-

DE)] × 1/10^9

where,

Production = gallons ethanol produced (wet milling or dry milling)

Flow = gallons wastewater generated per gallon ethanol produced (1.25 dry milling, 10 wet milling)

COD = COD concentration in influent (3 g/l)

3.785 = conversion, gallons to liters

%Plantso = percent of plants with onsite treatment (100%)

%WWa,p = percent of wastewater treated anaerobically in primary treatment (0%)

%CODp = percent of COD entering primary treatment (100%)

%Plantsa = percent of plants with anaerobic secondary treatment (33.3% wet, 75% dry)

%Plantst = percent of plants with other secondary treatment (66.7% wet, 25% dry)

%WWa,s = percent of wastewater treated anaerobically in anaerobic secondary treatment (100%)
Waste 8-23

%WWa,t = percent of wastewater treated anaerobically in other secondary treatment (0%) 

%CODs = percent of COD entering secondary treatment (100%)

Bo = maximum methane producing capacity (0.25 g CH4/g COD)

MCF = methane conversion factor (0.8 for anaerobic systems)

% Recovered = percent of wastewater treated in system with emission recovery

% Not Recovered = 1 - percent of wastewater treated in system with emission recovery

DE = destruction efficiency of recovery system (99%)

1/10^9 = conversion factor, g to Gg

A time series of CH4 emissions for 1990 through 2011 was developed based on production data from the Renewable 

Fuels Association (RFA 2012).

Petroleum Refining. Petroleum refining wastewater treatment operations produce CH4 emissions from anaerobic 

wastewater treatment. The wastewater inventory section includes CH4 emissions from petroleum refining 

wastewater treated on site under intended or unintended anaerobic conditions. Most facilities use aerated biological 

systems, such as trickling filters or rotating biological contactors; these systems can also exhibit anaerobic 

conditions that can result in the production of CH4. Oil/water separators are used as a primary treatment method; 

however, it is unlikely that any COD is removed in this step.

Available information from the industry was compiled. The wastewater generation rate, from CARB (2007) and 

Timm (1985), was determined to be 35 gallons per barrel of finished product. An average COD value in the 

wastewater was estimated at 0.45 kg/m3(Benyahia et al. 2006).

The equation used to calculate CH4 generation at petroleum refining wastewater treatment systems is presented 

below:

Methane = Flow × COD × Bo × MCF

where,

Flow = Annual flow treated through anaerobic treatment system (m3/year) 

COD = COD loading in wastewater entering anaerobic treatment system (kg/m3) 

Bo = maximum methane producing potential of industrial wastewater (default value of 0.25 

kg CH4 /kg COD)

MCF = methane conversion factor (0.3)

A time series of CH4 emissions for 1990 through 2011 was developed based on production data from the Energy 

Information Association (EIA 2012).

Domestic Wastewater N2O Emission Estimates

N2O emissions from domestic wastewater (wastewater treatment) were estimated using the IPCC (2006) 

methodology, including calculations that take into account N removal with sewage sludge, non-consumption and 

industrial/commercial wastewater N, and emissions from advanced centralized wastewater treatment plants:

 In the United States, a certain amount of N is removed with sewage sludge, which is applied to land, incinerated, 

or landfilled (NSLUDGE). The N disposal into aquatic environments is reduced to account for the sewage sludge 

application. 

 The IPCC methodology uses annual, per capita protein consumption (kg protein/[person-year]). For this 

inventory, the amount of protein available to be consumed is estimated based on per capita annual food 

availability data and its protein content, and then adjusts that data using a factor to account for the fraction of 

protein actually consumed. 

 Small amounts of gaseous nitrogen oxides are formed as byproducts in the conversion of nitrate to N gas in 

anoxic biological treatment systems. Approximately 7 g N2O is generated per capita per year if wastewater 

treatment includes intentional nitrification and denitrification (Scheehle and Doorn 2001). Analysis of the 2004 

CWNS shows that plants with denitrification as one of their unit operations serve a population of 2.4 million 

people. Based on an emission factor of 7 g per capita per year, approximately 21.2 metric tons of additional N2O 

may have been emitted via denitrification in 2004. Similar analyses were completed for each year in the 
8-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

inventory using data from CWNS on the amount of wastewater in centralized systems treated in denitrification 

units. Plants without intentional nitrification/denitrification are assumed to generate 3.2 g N2O per capita per 

year. 

N2O emissions from domestic wastewater were estimated using the following methodology:

N2OTOTAL = N2OPLANT + N2OEFFLUENT

N2OPLANT = N2ONIT/DENIT + N2OWOUT NIT/DENIT

N2ONIT/DENIT = [(USPOPND) × EF2 × FIND-COM] × 1/10^9

N2OWOUT NIT/DENIT = {[(USPOP × WWTP) - USPOPND]× FIND-COM × EF1} × 1/10^9

N2OEFFLUENT = {[(((USPOP × WWTP) – (0.9 × USPOPND)) × Protein × FNPR × FNON-CON × FIND-COM) - NSLUDGE] × EF3 × 

44/28} × 1/10^6

where,

N2OTOTAL = Annual emissions of N2O (Gg)

N2OPLANT = N2O emissions from centralized wastewater treatment plants (Gg)

N2ONIT/DENIT = N2O emissions from centralized wastewater treatment plants with 

nitrification/denitrification (Gg)

N2OWOUT NIT/DENIT = N2O emissions from centralized wastewater treatment plants without 

nitrification/denitrification (Gg)

N2OEFFLUENT = N2O emissions from wastewater effluent discharged to aquatic environments (Gg)

USPOP = U.S. population

USPOPND = U.S. population that is served by biological denitrification (from CWNS)

WWTP = Fraction of population using WWTP (as opposed to septic systems)

EF1 = Emission factor (3.2 g N2O/person-year) – plant with no intentional denitrification

EF2 = Emission factor (7 g N2O/person-year) – plant with intentional denitrification

Protein = Annual per capita protein consumption (kg/person/year)

FNPR = Fraction of N in protein, default = 0.16 (kg N/kg protein)

FNON-CON = Factor for non-consumed protein added to wastewater (1.4)

FIND-COM = Factor for industrial and commercial co-discharged protein into the sewer system 

(1.25)

NSLUDGE = N removed with sludge, kg N/yr

EF3 = Emission factor (0.005 kg N2O -N/kg sewage-N produced) – from effluent

0.9 = Amount of nitrogen removed by denitrification systems (EPA 2008)

44/28 = Molecular weight ratio of N2O to N2

U.S. population data were taken from the U.S. Census Bureau International Database (U.S. Census 2012) and 

include the populations of the United States, American Samoa, Guam, Northern Mariana Islands, Puerto Rico, and 

the Virgin Islands. The fraction of the U.S. population using wastewater treatment plants is based on data from the 

1989, 1991, 1993, 1995, 1997, 1999, 2001, 2003, 2005, 2007, 2009, and 2011 American Housing Survey (U.S. 

Census 2011). Data for intervening years were obtained by linear interpolation. The emission factor (EF1) used to 

estimate emissions from wastewater treatment for plants without intentional denitrification was taken from IPCC 

(2006), while the emission factor (EF2) used to estimate emissions from wastewater treatment for plants with 

intentional denitrification was taken from Scheehle and Doorn (2001). Data on annual per capita protein intake were 

provided by U.S. Department of Agriculture Economic Research Service (USDA 2009). Protein consumption data 

for 2005 through 2011 were extrapolated from data for 1990 through 2004. An emission factor to estimate 

emissions from effluent (EF3) has not been specifically estimated for the United States, thus the default IPCC value 

(0.005 kg N2O-N/kg sewage-N produced) was applied. The fraction of N in protein (0.16 kg N/kg protein) was also 

obtained from IPCC (2006). The factor for non-consumed protein and the factor for industrial and commercial codischarged protein were obtained from IPCC (2006). Sludge generation was obtained from EPA (1999) for 1988, 

1996, and 1998 and from Beecher et al. (2007) for 2004. Intervening years were interpolated, and estimates for 

2005 through 2011 were forecasted from the rest of the time series. An estimate for the N removed as sludge 

(NSLUDGE) was obtained by determining the amount of sludge disposed by incineration, by land application 

(agriculture or other), through surface disposal, in landfills, or through ocean dumping. In 2011, 277 Gg N was 

removed with sludge. Table 8-15 presents the data for U.S. population, population served by biological 
Waste 8-25

denitrification, population served by wastewater treatment plants, available protein, protein consumed, and nitrogen 

removed with sludge.

Table 8-15: U.S. Population (Millions), Population Served by Biological Denitrification 

(Millions), Fraction of Population Served by Wastewater Treatment (%), Available Protein 

(kg/person-year), Protein Consumed (kg/person-year), and Nitrogen Removed with Sludge 

(Gg-N/year)

Year Population PopulationND WWTP Population Available Protein Protein Consumed N Removed

1990 253 2.0 75.6 38.7 29.6 215.6

2005 300 2.4 78.8 41.7 32.0 260.3

2007 305 2.3 79.4 42.1 32.3 265.9

2008 308 2.2 79.4 42.2 32.4 268.7

2009 311 2.2 79.3 42.4 32.5 271.4

2010 313 2.1 80.0 42.6 32.7 274.2

2011 316 2.1 80.6 42.8 32.8 277.0

Table 8-16: Fate of Sludge Removed by Domestic Wastewater Treatment

Disposal Practices

Distribution (1000 kg N) 1990 1995 2000 2005 2010 2011

Incineration 35,027.35 37,806.16 38,399.04 38,595.85 38,301.05 38,215.54

Land Application 77,378.34 97,230.98 113,311.73 129,196.74 144,113.04 147,054.99

Ag 52,198.15 69,001.16 83,522.63 98,080.96 112,014.99 114,778.24

Other 25,180.19 28,229.81 29,789.11 31,115.78 32,098.05 32,276.75

Surface Disposal 20,325.19 16,142.13 10,243.93 4,586.01 2,558.71 2,275.43

Landfill 72,962.21 75,945.15 74,158.54 71,407.98 67,609.40 66,790.83

Ocean Dumping 8,294.65 - - - - -

Other 1,645.76 6,353.98 11,312.32 16,478.76 21,661.26 22,702.30

Uncertainty and Time-Series Consistency

The overall uncertainty associated with both the 2011 CH4 and N2O emission estimates from wastewater treatment 

and discharge was calculated using the IPCC Good Practice Guidance Tier 2 methodology (2000). Uncertainty 

associated with the parameters used to estimate CH4 emissions include that of numerous input variables used to 

model emissions from domestic wastewater, and wastewater from pulp and paper manufacture, meat and poultry 

processing, fruits and vegetable processing, ethanol production, and petroleum refining. Uncertainty associated with 

the parameters used to estimate N2O emissions include that of sewage sludge disposal, total U.S. population, 

average protein consumed per person, fraction of N in protein, non-consumption nitrogen factor, emission factors 

per capita and per mass of sewage-N, and for the percentage of total population using centralized wastewater 

treatment plants. 

The results of this Tier 2 quantitative uncertainty analysis are summarized in Table 8-17. Methane emissions from 

wastewater treatment were estimated to be between 11.5 and 20.7 Tg CO2 Eq. at the 95 percent confidence level (or 

in 19 out of 20 Monte Carlo Stochastic Simulations). This indicates a range of approximately 29 percent below to 

28 percent above the 2011 emissions estimate of 16.2 Tg CO2 Eq. N2O emissions from wastewater treatment were 

estimated to be between 1.2 and 10.2 Tg CO2 Eq., which indicates a range of approximately 77 percent below to 97 

percent above the 2011 emissions estimate of 5.2 Tg CO2 Eq. 

Table 8-17: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Wastewater 

Treatment (Tg CO2 Eq. and Percent) 

Source Gas

2011 Emission 

Estimate

Uncertainty Range Relative to Emission 

Estimatea

(Tg CO2 Eq.) (Tg CO2 Eq.) (%)
8-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

Lower 

Bound

Upper 

Bound

Lower 

Bound

Upper 

Bound

Wastewater Treatment CH4 16.2 11.5 20.7 -29% +28%

Domestic CH4 7.6 5.6 9.6 -26% +27%

Industrial CH4 8.6 4.6 12.7 -47% +48%

Wastewater Treatment N2O 5.2 1.2 10.2 -77% +97%

a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent 

confidence interval.

Methodological recalculations were applied to the entire time-series to ensure time-series consistency from 1990 

through 2011. Details on the emission trends through time are described in more detail in the Methodology section, 

above.

QA/QC and Verification 

A QA/QC analysis was performed on activity data, documentation, and emission calculations. This effort included a 

Tier 1 analysis, including the following checks:

 Checked for transcription errors in data input;

 Ensured references were specified for all activity data used in the calculations;

 Checked a sample of each emission calculation used for the source category;

 Checked that parameter and emission units were correctly recorded and that appropriate conversion factors 

were used;

 Checked for temporal consistency in time series input data for each portion of the source category;

 Confirmed that estimates were calculated and reported for all portions of the source category and for all years;

 Investigated data gaps that affected emissions estimates trends; and

 Compared estimates to previous estimates to identify significant changes.

All transcription errors identified were corrected. The QA/QC analysis did not reveal any systemic inaccuracies or 

incorrect input values.

Recalculations Discussion

Production data were updated to reflect updated USDA NASS datasets. This resulted in minor changes to the 

emission estimates from the previous inventory. In addition, population updates from the U.S. Census resulted in 

minor changes to domestic wastewater treatment emission estimates from 2000 through 2010.

Planned Improvements

The methodology to estimate CH4 emissions from domestic wastewater treatment currently utilizes estimates for the 

percentage of centrally treated wastewater that is treated by aerobic systems and anaerobic systems. These data 

come from the 1992, 1996, 2000, and 2004 CWNS. The question of whether activity data for wastewater treatment 

systems are sufficient across the time series to further differentiate aerobic systems with the potential to generate 

small amounts of CH4 (aerobic lagoons) versus other types of aerobic systems, and to differentiate between 

anaerobic systems to allow for the use of different MCFs for different types of anaerobic treatment systems, 

continues to be explored. The CWNS data for 2008 were evaluated for incorporation into the inventory, but due to 

significant changes in format, this dataset is not sufficiently detailed for inventory calculations. However, additional 

information and other data continue to be evaluated to update future years of the inventory.

For industrial wastewater emissions, data recently collected by EPA’s Office of Air for pulp and paper mills and 

petroleum refineries is being evaluated to determine if sufficient information is available to update the estimates of 

wastewater generated per unit of production and the percent of industry wastewater treated anaerobically in these 

industries (%TA). Initial evaluations of EPA’s Office of Air data for pulp and paper manufacturing indicate there is 

sufficient information to update emission estimates in the next inventory year. Data collected in 2012 under the 

EPA’s GHGRP will also be investigated for updating this variable. In examining data from EPA’s GHGRP for use 

in improving the emission estimates for the industrial wastewater category, particular attention will be made to 
Waste 8-27

ensure time series consistency, as the facility-level reporting data from EPA’s GHGRP are not available for all 

inventory years as reported in this inventory. In implementing improvements and integration of data from EPA’s 

GHGRP, the latest guidance from the IPCC on the use of facility-level data in national inventories will be relied 

upon.255 For all industries, EPA will continue to review new research on industrial wastewater characteristics, 

utilization of treatment systems, and associated greenhouse gas emissions as it becomes available. Before the 

incorporation of any new data, EPA will ensure it is representative of industry conditions.

Currently, it is assumed that all aerobic wastewater treatment systems are well managed and produce no CH4 and 

that all anaerobic systems have an MCF of 0.8. Efforts to obtain better data reflecting emissions from various types 

of municipal treatment systems are currently being pursued.

With respect to estimating N2O emissions, the default emission factors for indirect N2O from wastewater effluent 

and direct N2O from centralized wastewater treatment facilities have a high uncertainty. Research is being 

conducted by WERF to measure N2O emissions from municipal treatment systems. In addition, a literature review 

has been conducted focused on N2O emissions from wastewater treatment to determine the state of such research 

and identify data to develop a country-specific N2O emission factor or alternate emission factor or method. Such 

data will continue to be reviewed as they are available to determine if a country-specific N2O emission factor can or 

should be developed, or if alternate emission factors should be used. 

Previously, new measurement data from WERF were used to develop U.S.-specific emission factors for CH4

emissions from septic systems and incorporated it into the inventory emissions calculation. Due to the high 

uncertainty of the measurements for N2O from septic systems, estimates of N2O emissions were not included. 

Appropriate emission factors for septic system N2O emissions will continue to be investigated as the data collected 

by WERF indicate that septic soil systems are a source of N2O emissions. 

In addition, the estimate of N entering municipal treatment systems is under review. The factor that accounts for 

non-sewage N in wastewater (bath, laundry, kitchen, industrial components) also has a high uncertainty. Obtaining 

data on the changes in average influent N concentrations to centralized treatment systems over the time series would 

improve the estimate of total N entering the system, which would reduce or eliminate the need for other factors for 

non-consumed protein or industrial flow. The dataset previously provided by the National Association of Clean 

Water Agencies (NACWA) was reviewed to determine if it was representative of the larger population of 

centralized treatment plants for potential inclusion into the inventory. However, this limited dataset was not 

representative of the number of systems by state or the service populations served in the United States, and therefore 

could not be incorporated into the inventory methodology. Additional data sources will continue to be researched 

with the goal of improving the uncertainty of the estimate of N entering municipal treatment systems.

The value used for N content of sludge continues to be investigated. This value is driving the N2O emissions for 

wastewater treatment and is static over the time series. To date, new data has not been identified that would be able 

to establish a time series for this value. The amount of sludge produced and sludge disposal practices will also be 

investigated. In addition, based on UNFCCC review comments, improving the transparency of the fate of sludge 

produced in wastewater treatment will also be investigated.

A review of other industrial wastewater treatment sources for those industries believed to discharge significant loads 

of BOD and COD has been ongoing. Food processing industries have the highest potential for CH4 generation due 

to the waste characteristics generated, and the greater likelihood to treat the wastes anaerobically. However, in all 

cases there is dated information available on U.S. treatment operations for these industries. Previously, organic 

chemicals, the seafood processing industry and coffee processing were investigated to estimate their potential to 

generate CH4. Due to the insignificant amount of CH4 estimated to be emitted and the lack of reliable, up-to-date 

data, these industries were not selected for inclusion in the industry. Preliminary analyses of the beer and malt and 

dairy products industries has been performed. These industries will continue to be investigated for incorporation. 

Other industries will be reviewed as necessary for inclusion in future years of the inventory using EPA’s Permit 

Compliance System and Toxics Release inventory.

In addition, available datasets will be reviewed to provide further information on the fates of sludge removed by 

domestic wastewater treatment in the next inventory report.

 

255 See: http://www.ipcc-nggip.iges.or.jp/meeting/pdfiles/1008_Model_and_Facility_Level_Data_Report.pdf.
8-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

8.3 Waste Incineration (IPCC Source Category 

6C)

As stated earlier in this chapter, CO2, N2O, and CH4 emissions from the incineration of waste are accounted for in 

the Energy sector rather than in the Waste sector because almost all incineration of municipal solid waste (MSW) in 

the United States occurs at waste-to-energy facilities where useful energy is recovered. Similarly, the Energy sector 

also includes an estimate of emissions from burning waste tires and hazardous industrial waste, because virtually all 

of the combustion occurs in industrial and utility boilers that recover energy. The incineration of waste in the United 

States in 2011 resulted in 12.4 Tg CO2 Eq. emissions, nearly half of which is attributable to the combustion of 

plastics. For more details on emissions from the incineration of waste, see Section 3.3 of the Energy chapter.

Additional sources of emissions from waste incineration include non-hazardous industrial waste incineration and 

medical waste incineration. As described in Annex 5 of this report, data are not readily available for these sources

and emissions estimates are not provided. Further investigations will be made, including assessing the applicability 

of state-level data collected for EPA’s National Emission Inventory (NEI)256.

8.4 Composting (IPCC Source Category 6D)

Composting of organic waste, such as food waste, garden (yard) and park waste, and sludge, is common in the 

United States. Advantages of composting include reduced volume in the waste material, stabilization of the waste, 

and destruction of pathogens in the waste material. The end products of composting, depending on its quality, can 

be recycled as fertilizer and soil amendment, or be disposed in a landfill. Approximately 400 composting facilities 

operate in the United States (WBJ 2010). 

Composting is an aerobic process and a large fraction of the degradable organic carbon in the waste material is 

converted into carbon dioxide (CO2). Methane (CH4) is formed in anaerobic sections of the compost, but it is 

oxidized to a large extent in the aerobic sections of the compost. Anaerobic sections are created in composting piles 

when there is excessive moisture or inadequate aeration (or mixing) of the compost pile. The estimated CH4

released into the atmosphere ranges from less than 1 percent to a few percent of the initial C content in the material 

(IPCC 2006). Depending on the N content of the feedstock and how well the compost pile is managed, nitrous 

oxide (N2O) emissions can be produced. The formation of N2O is complicated, but is mainly associated with 

anaerobic conditions. Emissions vary and range from less than 0.5 percent to 5 percent of the initial content of the 

material (IPCC 2006). 

From 1990 to 2011, the amount of material composted in the United States has increased from 3,810 Gg to 18,449 

Gg, an increase of approximately 384 percent. From 2000 to 2011, the amount of material composted in the United 

States has increased by approximately 24 percent. Emissions of CH4 and N2O from composting have increased by 

the same percentage. In 2011, CH4 emissions from composting (see Table 8-18 and Table 8-19) were 1.5 Tg CO2

Eq. (74 Gg), and N2O emissions from composting were 1.7 Tg CO2 Eq. (5.5 Gg). The wastes composted primarily 

include yard trimmings (grass, leaves, and tree and brush trimmings) and food scraps from residences and 

commercial establishments (such as grocery stores, restaurants, and school and factory cafeterias). The composted 

waste quantities reported here do not include backyard composting. The growth in composting since the 1990s is 

attributable to primarily two factors: (1) steady growth in population and residential housing, and (2) the enactment 

of legislation by state and local governments that discouraged the disposal of yard trimmings in landfills. In 1992, 

11 states and the District of Columbia had legislation in effect that banned or discouraged disposal of yard 

trimmings in landfills. Currently, 23 states and the District of Columbia, representing about 50 percent of the 

 

256 See < http://www.epa.gov/ttn/chief/eiinformation.html>
Waste 8-29

nation’s population, have enacted such legislation (EPA 2010). The total amount of waste composted has decreased 

slightly since 2008, by approximately 8 percent. 

Table 8-18: CH4 and N2O Emissions from Composting (Tg CO2 Eq.)

Activity 1990 2005 2007 2008 2009 2010 2011

CH4 0.3 1.6 1.7 1.7 1.6 1.5 1.5

N2O 0.4 1.7 1.8 1.9 1.8 1.7 1.7

Total 0.7 3.3 3.5 3.5 3.3 3.2 3.3

Table 8-19: CH4 and N2O Emissions from Composting (Gg)

Activity 1990 2005 2007 2008 2009 2010 2011

CH4 15 75 79 80 75 73 74

N2O 1 6 6 6 6 5 6

Methodology 

Methane and N2O emissions from composting depend on factors such as the type of waste composted, the amount 

and type of supporting material (such as wood chips and peat) used, temperature, moisture content and aeration 

during the process.

The emissions shown in Table 8-18 and Table 8-19 were estimated using the IPCC default (Tier 1) methodology 

(IPCC 2006), which is the product of an emission factor and the mass of organic waste composted (note: no CH4

recovery is expected to occur at composting operations):

Ei

 M  EFi

where,

Ei = CH4 or N2O emissions from composting, Gg CH4 or N2O,

M = mass of organic waste composted in Gg,

EFi = emission factor for composting, 4 g CH4/kg of waste treated (wet basis) and 0.3 g 

N2O/kg of waste treated (wet basis) (IPCC 2006), and

i = designates either CH4 or N2O.

Estimates of the quantity of waste composted (M) are presented in Table 8-20. Estimates of the quantity composted 

for 1990 and 1995 were taken from the Characterization of Municipal Solid Waste in the United States: 1996 

Update (Franklin Associates 1997); estimates of the quantity composted for 2000, 2005, 2006, 2007, 2008, and 2009 

were taken from EPA’s Municipal Solid Waste In The United States: 2009 Facts and Figures (EPA 2010); 

estimates of the quantity composted for 2010 were taken from EPA’s Municipal Solid Waste In The United States: 

2010 Facts and Figures (EPA 2011); estimates of the quantity composted for 2011 were calculated using the 2010 

quantity composted and a ratio of the U.S. population in 2010 and 2011 (U.S. Census Bureau 2012). The estimated 

quantity of waste composted in 2010 was revised based on updated information (EPA 2011).

Table 8-20: U.S. Waste Composted (Gg)

Activity 1990 2005 2007 2008 2009 2010 2011

Waste 

Composted 3,810 18,643 19,695 20,049 18,824 18,298 18,449

Source: EPA 2008 and EPA 2011.
8-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2011

Uncertainty and Time-Series Consistency

Little is known about the site-specific operating conditions at the composting facilities in the United States. The 

generation of CH4 and N2O emissions is highly dependent on the characteristics of the feedstock material (e.g., 

moisture content, C to N ratio, size), on the climate, and on the operating and maintenance practices (e.g., use of a 

shredder/grinder to maintain consistency in size of the feedstock material, frequency of pile rotation, addition of 

moisture, application of finished compost on the pile). The estimated uncertainty from the 2006 IPCC Guidelines is 

±50 percent for the Tier 1 methodology. Emissions from composting in 2011 were estimated to be between 1.6 and 

4.9 Tg CO2 Eq., which indicates a range of 50 percent below to 50 percent above the actual 2011 emission estimate 

of 3.3 Tg CO2 Eq. (see Table 8-21). 

Table 8-21 : Tier 1 Quantitative Uncertainty Estimates for Emissions from Composting (Tg 

CO2 Eq. and Percent)

Source Gas

2011 Emission 

Estimate Uncertainty Range Relative to Emission Estimate

(Tg CO2 Eq.) (Tg CO2 Eq.) (%)

Lower 

Bound

Upper 

Bound

Lower 

Bound

Upper 

Bound

Composting CH4, N2O 3.3 1.6 4.9 -50% +50%

Methodological recalculations were applied to the entire time series to ensure time-series consistency from 1990 

through 2011. Details on the emission trends through time are described in more detail in the Methodology section, 

above.

QA/QC and Verification

A QA/QC analysis was performed for data gathering and input, documentation, and calculation. A primary focus of 

the QA/QC checks was to ensure that the amount of waste composted annually was correct according to the latest 

EPA Municipal Solid Waste In The United States: Facts and Figures report.

Recalculations Discussion

The estimated amount of waste composted in 2010 was updated based on new data contained in EPA’s Municipal 

Solid Waste In The United States: 2010 Facts and Figures (EPA 2011). The amounts of CH4 and N2O emissions 

estimates presented in Table 8-18and Table 8-19 were revised accordingly. 

Planned Improvements

For future Inventories, additional efforts will be made to improve the estimates of CH4 and N2O emissions from 

composting. For example, a literature search may be conducted to determine if emission factors specific to various 

composting systems and composted materials are available. Further cooperation with estimating emissions in 

cooperation with the LULUCF Other section will be made. 

8.5 Waste Sources of Indirect Greenhouse 

Gases

In addition to the main greenhouse gases addressed above, waste generating and handling processes are also sources 

of indirect greenhouse gas emissions. Total emissions of NOx, CO, and NMVOCs from waste sources for the years 

1990 through 2011 are provided in Table 8-22.
Waste 8-31

Table 8-22: Emissions of NOx, CO, and NMVOC from Waste (Gg)

Gas/Source 1990 2005 2007 2008 2009 2010 2011

NOx + 2 2 2 2 2 2

Landfills + 2 2 2 2 2 2

Wastewater Treatment + + + + + + + 

Miscellaneousa + + + + + + + 

CO 1 7 7 7 7 7 7

Landfills 1 6 6 6 6 6 6

Wastewater Treatment + + + + + + + 

Miscellaneousa + + + + + + + 

NMVOCs 673 114 111 109 76 76 76

Wastewater Treatment 57 49 48 47 33 33 33

Miscellaneousa 557 43 42 41 29 29 29

Landfills 58 22 21 21 14 14 14

a Miscellaneous includes TSDFs (Treatment, Storage, and Disposal Facilities under the Resource Conservation 

and Recovery Act [42 U.S.C. § 6924, SWDA § 3004]) and other waste categories.

Note: Totals may not sum due to independent rounding.

+ Does not exceed 0.5 Gg.

Methodology 

Due to the lack of data available at the time of publication, emission estimates for 2010 and 2011 rely on 2009 data 

as a proxy. Emission estimates for 2009 were obtained from preliminary data (EPA 2010, EPA 2009), and 

disaggregated based on EPA (2003), which, in its final iteration, will be published on the National Emission 

Inventory (NEI) Air Pollutant Emission Trends web site. Due to redevelopment of the information technology 

systems for the NEI, publication of the most recent emissions for these pollutants (i.e., indirect greenhouse gases) 

was not available for this report257. Emission estimates of these gases were provided by sector, using a “top down” 

estimating procedureemissions were calculated either for individual sources or for many sources combined, using 

basic activity data (e.g., the amount of raw material processed) as an indicator of emissions. National activity data 

were collected for individual source categories from various agencies. Depending on the source category, these 

basic activity data may include data on production, fuel deliveries, raw material processed, etc.

Uncertainty and Time-Series Consistency

No quantitative estimates of uncertainty were calculated for this source category. Methodological recalculations 

were applied to the entire time-series to ensure time-series consistency from 1990 through 2011.

 

257 For an overview of the activities and the schedule for developing the 2011 National Emissions Inventory, with the goal of 

producing Version 1 in the summer of 2013, see < http://www.epa.gov/ttn/chief/eis/2011nei/2011plan.pdf>