6.2 Wastewater Treatment
| Category ID | Description | EIC |
|---|---|---|
| 50 | Domestic Wastewater Treatment - POTW | Various |
| 2261 | Domestic Wastewater Treatment - Anaerobic Digesters | 11011000100000 |
| 2262 | Domestic Wastewater Treatment - Septic Systems | 11099503000000 |
| 2263 | Domestic Wastewater Treatment - POTW N2O Effluent Emissions | 11011003000000 |
| 2264 | Domestic Wastewater Treatment - POTW N2O Plant Emissions | 11011003000000 |
| 2265 | Industrial Wastewater Treatment - Pulp and Paper | 49934003000000 |
| 2266 | Industrial Wastewater Treatment - Red Meat | 49934003000000 |
| 2267 | Industrial Wastewater Treatment - Poultry | 49934003000000 |
| 2268 | Industrial Wastewater Treatment - Non-Citrus Fruits | 49934003000000 |
| 2269 | Industrial Wastewater Treatment - Citrus Fruits | 49934003000000 |
| 2270 | Industrial Wastewater Treatment - Apples | 49934003000000 |
| 2271 | Industrial Wastewater Treatment - Wine Grapes | 49934003000000 |
| 2272 | Industrial Wastewater Treatment - Other Vegetables | 49934003000000 |
| 2273 | Industrial Wastewater Treatment - Potatoes | 49934003000000 |
| 2274 | Industrial Wastewater Treatment - Petroleum Refining | 32034000100000 |
| 2542 | Domestic Wastewater Treatment - Centralized Anaerobic | 11011003000000 |
Introduction
This document outlines the methodology for estimating greenhouse gas (GHG) emissions from wastewater treatment activities that are designed to collect and treat contaminated water prior to discharging back into the environment or for reuse for agricultural purposes. The wastewater treatment process is a source of biologically produced methane (CH4) and nitrous oxide (N2O) emissions. Combustion-related GHG emissions from wastewater treatment plants (WWTPs) are covered within other fuel-combustion categories across the GHG inventory.
The Bay Area Air District (BAAD or Air District) classifies wastewater into two distinct groups. The first group relates to domestic wastewater generated from household activities such as bathing, cooking, laundry, etc., that is collected and treated at offsite centralized wastewater treatment facilities (WWTPs) such as publicly own treatment works (POTWs ) or onsite in standalone septic systems. The second group is wastewater produced at agricultural and industrial facilities that produce crops, consumer products (e.g. paper, petroleum-based products, etc.) and food products (e.g. poultry, beef, etc.) for human consumption. Both types of wastewater groups are a source of GHG emissions that are typically emitted during the biological processing of organic matter under anaerobic conditions, which results in substantial CH4 releases. In addition to methane, domestic wastewater treatment produces N2O as a byproduct of the nitrification and denitrification treatment processes that are applied to remove ammonia (a product found in human waste) from the wastewater to meet regulatory safety standards prior to discharge to the environment. No N2O emissions are produced during treatment of industrial wastewater as the industrial effluent is generally free of ammonia and therefore not treated using nitrification or denitrification. Carbon dioxide (CO2) is also produced from the treatment of wastewater under aerobic decomposition pathways. Because CO₂ originates from an engrained process in the natural carbon cycle (which is decomposition), it is considered biogenic and are therefore excluded from the regional anthropogenic GHG inventory, in accordance with California Air Resources Board (CARB) protocols. In addition to these fugitive sources of emissions, the Air District permits POTW operations and includes some minor emissions of methane, carbon dioxide, and dichloromethane that are reported by (but not required under permit condition) select facilities.
Domestic Wastewater Treatment
The following six source categories define specific treatment processes that produce either CH4 or N2O as a byproduct of domestic wastewater systems:
- Category 50 (General Overall Processes) includes minor to negligible reported emissions of CH4, CO2, and dichloromethane from point sources at municipal wastewater treatment plants that are permitted by the Air District.
- Category 2542 (Centralized Anaerobic) accounts for direct CH4 emissions into the atmosphere, when organic materials decompose in the wastewater stream as it is handled and transported from homes and businesses to a POTW facility for further treatment.
- Category 2261 (Anaerobic Digester) includes CH4 emissions from fugitive losses and leaks from bio-digesters at POTW facilities, as bacteria in oxygen-deficient environment break down organic matter in wastewater sludge and biosolids, producing biogas (mixture of methane, CO2 and other minor constituents) and digestate.
- Category 2262 (Septic Systems) accounts for CH4 emissions from decomposing organic matter in septic tanks located on properties that are not connected to a municipal POTW facility.
- Category 2263 (N2O Effluent) includes N2O emissions associated with treated wastewater from POTW that are then discharged to a waterbody (typically a river, lake, bay, or ocean). During wastewater treatment when nitrification and denitrification processes are used for nitrogen/ammonia removal, some dissolved nitrogen compounds such as ammonia, nitrate, and nitrite stay in the treated effluent and are released at the final discharge point into the body of water. N2O can form through microbial activity with oxygen and stay dissolved until it off-gases directly from the effluent as it is discharged to the receiving waters. N2O emissions released from the waterbody during/after the effluent discharge are generally much higher than N2O emissions that occur during the wastewater treatment process at the POTW facility (CARB. 2016).
- Category 2264 (N2O Plant) accounts for N2O emissions that are produced as a byproduct of the biological nitrogen removal processes involving nitrification and denitrification at a POTW facility. Microbes are used to break down ammonia in the wastewater treatment process from nitrogen into relatively inert nitrogen gas (N2). If the breakdown is incomplete, then N2O is formed instead of N2, some of which bubbles up from the treatment tanks into the atmosphere. Most POTW release considerably less N2O (approximately 19 times less) from this process as compared to effluent discharge into a waterbody, as much of the N2O stays dissolved in the wastewater until it is discharged into a waterbody.
Industrial Wastewater Treatment
Industries in the San Francisco Bay Area (SFBA) may release treated wastewater effluent at designated discharge points into a water body if it meets rigorous standards established by the water board. Methane is the primary GHG pollutant emitted, resulting from decomposition of organic matter in effluent stored in holding ponds and tanks prior to chemical treatment. Industries included in this sector are those identified as generating both a high volume of wastewater and those containing high organic content (or load) in their wastewater effluent. Ten industries including: (1) pulp and paper, (2) red meat, (3) poultry, (4) non-citrus fruits, (5) citrus fruits, (6) apples, (7) wine grapes, (8) other vegetables, (9) potatoes and (10) petroleum refining, are identified in the CARB GHG inventory. Although some of these processes (i.e., petroleum refining) are regulated by the Air District, their permit conditions do not set emission limits for GHGs and therefore, emissions from these industries are estimated as area sources.
Methodology
This section describes the methodology used to estimate GHG emissions from both domestic and industrial wastewater treatment processes. These source categories are considered area source categories (except for Category 50) since they cover emission sources that are not directly permitted by the Air District, so emissions are not systematically or annually reported. Category 50 accounts for emissions from the general processes at POTWs that are permitted by the Air District but do not have any reporting requirements for process-level GHGs.
The methodology used to calculate emissions for the reported base years from domestic and industrial wastewater area sources is as follows:
Base Year(s) Emissions county,pollutant =
Activity Data × Emission Factorpollutant × Control Factorpollutant × Fractioncounty × Fractionin District
× GWP pollutant
Where:
- Base Years: are years for which activity/throughput data are available in order to calculate emissions.
- Activity Data: is the total regional throughput or activity data for applicable reported base years.
- Emission Factorpollutant: is a factor that allocates a mass amount of emissions of a particular pollutant per unit of activity.
- Control Factorpollutant : is a fractional ratio (between 0 and 1) that captures the estimated reduction in emissions as a result of Air District rules and regulations.
- Fractioncounty : is the fraction of total regional emissions (between 0 and 1) estimated to be allocated to a particular county.
- Fractionin District: The Air District jurisdiction covers only a portion of Solano and Sonoma County and, therefore, an additional allocation is applied to these counties that proportions each county’s emissions that are within Air District’s boundary.
- GWPpollutant is the Global Warming Potential of a particular GHG pollutant. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO2), 34 for methane (CH4), and 298 for nitrous oxide (N2O), when calculated on a 100-year basis with climate-carbon feedback included.
Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are conducted using growth profiles as follows:
Current Year Emissions county = Base Year(s) Emission county × Growth Factor
Where:
- Growth Factor: is a scaling factor that is used to derive historical emissions estimates for years for which activity data and/or emissions are not available, and to forecast emissions for future years, using surrogates that are assumed to be representative of activity and/or emissions trends
More details on activity data/throughput, county distribution, emission factors and controls are provided in the following subsections.
Activity Data / Throughput
Activity data for each wastewater treatment source category is determined by the paired emissions factors (EFs) used by CARB in their statewide GHG inventory methodology corresponding to each category (CARB 2016; CARB, 2023). Either county level population data or national or state process throughputs serves as the basis for the activity data for most categories. For categories 50 and 2261, the facility reported throughput listed in Air District’s permit are used. When the applicable emission factors are on a per capita basis, the activity data is derived from available state-issued population statistics such as the California Department of Finance reports (CADOF, 2022). Similarly, when emissions factors are based on goods produced (e.g., per ton of apples, per ton of non-citrus fruits, etc.), the throughputs are from agricultural databases specific to that produce or meat and sorted by county. For example, census data from United States Department of Agriculture (USDA, 2022a; USDA, 2022b) reports county-level produce processed that is used as activity data for several industrial wastewater categories (e.g., red-meat, poultry, apples etc.). For category 2261, where the emission factor for anaerobic digester is based on the volume of biogas generated, direct biogas throughput is obtained from the reported volume in the Air District’s facility permits. In some cases, for example the paper and pulp industry, activity data is derived from national level reports and/or summaries (UN, 2015). For highly regulated industries like petroleum refining, the activity data is based on each refinery’s production capacity (in barrels of oil), as published by the California Energy Commission (CEC, 2024).
A breakdown of each wastewater category activity collection is summarized in the table below.
Category Number | Category Description | Basis of Activity | Basis of County Split | References |
50 | Process | Air District Point Source Data | Aggregated Point Source Facility Data at County-level | BAAQMD, 2022 |
2542 | Centralized Anaerobic | Regional Population Data | County Population Data | CADOF, 2022 |
2261 | Anaerobic Digester | Air District Point Source Data | Aggregated Point Source Facility Data at County-level | BAAQMD, 2022 |
2262 | Septic Systems | Regional Population Data & Onsite Wastewater Survey | County Population Data | CADOF, 2022 |
2263 | N2O Effluent | Regional Population Data | County Population Data | CADOF, 2022 |
2264 | N2O Plant | Regional Population Data | County Population Data | CADOF, 2022 |
2265 | Pulp and Paper | United Nations Food and Agriculture Organization (UNFAO) Pulp and Paper Capacities Reports – US Production Pulp, Paper and Paper Board Capacity Survey | NAICS 3221 Regional Fraction | UN, 2015 |
2266 | Red Meat | 2022 USDA Census of Agriculture - CA State and County Data | County-level USDA Beef Data | USDA, 2022a |
2267 | Poultry | 2022 USDA Census of Agriculture - CA State and County Data | County-level USDA Poultry Data | USDA, 2022a |
2268 | Non-Citrus Fruits (includes apples) | 2022 USDA Census of Agriculture - CA State and County Data | County-level USDA Non-Citrus Fruit Data | USDA, 2022a |
2269 | Citrus Fruits | 2022 USDA Census of Agriculture - CA State and County Data | County-level USDA Citrus Fruit Data | USDA, 2022a |
2270 | Apples | 2022 USDA Census of Agriculture - CA State and County Data | County-level USDA Apple Data | USDA, 2022a |
2271 | Wine Grapes | 2022 CA Department of Agriculture (DOA) – Errata to The Grape Crush Report | County-level Zone Data on Grape Pricing Districts (Air District Zone 3-6) | USDA, 2022b |
2272 | Other Vegetables | 2022 USDA Census of Agriculture - CA State and County Data | County-level USDA Other Vegetable Data | USDA, 2022a |
2273 | Potatoes | 2022 USDA Census of Agriculture - CA State and County Data | County-level USDA Potato Data | USDA, 2022a |
2274 | Petroleum Refining | 2024 CEC Data – CA Refinery Locations and Installed Capacity (barrels/day) | Aggregated facility installed capacity (barrels/day) at county-level | CEC, 2024 |
County Distribution / Fractions
The county distribution is based on a fraction of the total throughput for all wastewater categories that are associated with each county. Throughput from permitted source category 50 and area source category 2261 are assigned to the county where the POTW facilities operate. Similarly, throughput for category 2274 is assigned based on the location of the permitted refinery. For the remaining area source categories, county-level throughputs are available directly from the CADOF and USDA databases for each reported base year. The table below shows the county distribution for domestic and industrial wastewater treatment source categories for the year 2022.
| ID | Description | ALA | CC | MAR | NAP | SF | SM | SNC | SOL | SON |
|---|---|---|---|---|---|---|---|---|---|---|
| 2261 | Domestic Wastewater Treatment - Anaerobic Digesters | 0.51 | 0.04 | 0.04 | 0.02 | 0.15 | 0.10 | 0.06 | 0.02 | 0.06 |
| 2262 | Domestic Wastewater Treatment - Septic Systems | 0.04 | 0.11 | 0.08 | 0.09 | 0.00 | 0.06 | 0.19 | 0.06 | 0.38 |
| 2263 | Domestic Wastewater Treatment - POTW N2O Effluent Emissions | 0.22 | 0.16 | 0.03 | 0.02 | 0.11 | 0.10 | 0.26 | 0.04 | 0.06 |
| 2264 | Domestic Wastewater Treatment - POTW N2O Plant Emissions | 0.22 | 0.16 | 0.03 | 0.02 | 0.11 | 0.10 | 0.26 | 0.04 | 0.06 |
| 2265 | Industrial Wastewater Treatment - Pulp and Paper | 0.25 | 0.06 | 0.01 | 0.03 | 0.05 | 0.08 | 0.45 | 0.03 | 0.05 |
| 2266 | Industrial Wastewater Treatment - Red Meat | 0.04 | 0.22 | 0.22 | 0.02 | 0.00 | 0.03 | 0.17 | 0.20 | 0.10 |
| 2267 | Industrial Wastewater Treatment - Poultry | 0.00 | 0.01 | 0.06 | 0.01 | 0.00 | 0.00 | 0.02 | 0.02 | 0.88 |
| 2268 | Industrial Wastewater Treatment - Non-Citrus Fruits | 0.03 | 0.03 | 0.00 | 0.60 | 0.00 | 0.00 | 0.04 | 0.03 | 0.26 |
| 2269 | Industrial Wastewater Treatment - Citrus Fruits | 0.02 | 0.02 | 0.01 | 0.11 | 0.00 | 0.00 | 0.17 | 0.60 | 0.07 |
| 2270 | Industrial Wastewater Treatment - Apples | 0.00 | 0.01 | 0.02 | 0.01 | 0.00 | 0.03 | 0.03 | 0.00 | 0.91 |
| 2271 | Industrial Wastewater Treatment - Wine Grapes | 0.04 | 0.03 | 0.00 | 0.56 | 0.00 | 0.00 | 0.04 | 0.03 | 0.31 |
| 2272 | Industrial Wastewater Treatment - Other Vegetables | 0.00 | 0.16 | 0.01 | 0.00 | 0.00 | 0.04 | 0.56 | 0.22 | 0.01 |
| 2273 | Industrial Wastewater Treatment - Potatoes | 0.00 | 0.00 | 0.73 | 0.00 | 0.00 | 0.08 | 0.03 | 0.06 | 0.10 |
| 2274 | Industrial Wastewater Treatment - Petroleum Refining | 0.00 | 0.83 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.17 | 0.00 |
| 2542 | Domestic Wastewater Treatment - Centralized Anaerobic | 0.22 | 0.16 | 0.03 | 0.02 | 0.11 | 0.10 | 0.26 | 0.04 | 0.06 |
| 50 | Domestic Wastewater Treatment - POTW | 0.43 | 0.08 | 0.09 | 0.00 | 0.01 | 0.12 | 0.13 | 0.12 | 0.01 |
Emission Factors
Emission Factors (EF) are obtained from CARB’s GHG Inventory Query Tool (CARB, 2023) and are in units specific to each wastewater category. For category 50, the EFs are listed in the Air District permits, although reported GHG emissions are negligible. CARB EFs are taken directly from US Environmental Protection Agency’s (USEPA) Inventory of U.S. Greenhouse Gas Emissions and Sinks (USEPA, 2015). The EF for domestic and industrial wastewater remain constant year to year except for centralized anaerobic (category 2542), N2O effluent (category 2263) and plant (category 2264) categories where available EFs vary by year based on available research studies.
The table below presents category specific EF used to derive the 2022 GHG emissions estimates.
Category Number | Category Description | Emission Factor – Year 2022 |
50 | Process | General EFs from permit database |
2542 | Centralized anaerobic | 0.89 lbs CH4 per person (varies year to year) |
2261 | Anaerobic digesters | 268.96 lbs CH4 per cubic feet of biogas |
2262 | Septic systems | 8.62 lbs CH4 per person |
2263 | N2O effluent emissions | |
2264 | N2O plant emissions | 0.008 lbs N2O per person (varies year to year) |
2265 | pulp and paper | 3.15 lbs CH4 /ton pulp and paper |
2266 | red meat | 6.06 lbs CH4 /ton red meat |
2267 | poultry | 5.79 lbs CH4 /ton poultry |
2268 | non-citrus fruit (includes apples) | 0.494 lbs CH4 /ton non-citrus fruit |
2269 | citrus fruit | 0.106 lbs CH4 /ton citrus fruit |
2270 | apples | 0.166 lbs CH4 /ton apples |
2271 | wine grapes | 0.16 lbs CH4 /ton wine grapes |
2272 | other vegetables | 0.238 lbs CH4 /ton of other vegetables |
2273 | potatoes | 0.600 lbs CH4 /ton potatoes |
2274 | petroleum refining | 0.075 lbs CH4 /cubic meters of wastewater flow |
Control Factors / Emission Controls
For both the domestic and industrial wastewater groups, there are no current controls for GHGs. Future controls, if applicable, are incorporated into the projections and growth of emissions for individual categories. Emission reductions from the 2004 amendments to Air District Regulation 8, Rule 8 (BAAQMD, 2023) which required 65% reduction of volatile organic compounds (VOC) by 2007 from wastewater collection and separation systems at refineries have already been achieved and reported in the permitted emissions. A co-benefit of this rule is that a corresponding reduction in CH4 emissions of 65% is assumed and already applied in the estimation of category 2274 emissions. The 2023 amendments to Regulation 8, Rule 8 changed the terminology in the regulation to indicate that emissions limits applied to total organic compounds (that includes methane) but did not impose a stricter (or different) emissions limit than what was adopted in the 2004 amendments.
For category 50, archived emissions, throughputs, and emissions factors for historical and base years were taken from their Air District permit. Extrapolation techniques are used to account for missing data for the years 1991-1992. For the domestic area septic systems (category 2262), N2O effluent (category 2263) and N2O plant (category 2264), historical emissions align with population as the backcast relies on the California Department of Finance population data (CADOF, 2022). Centralized anaerobic (category 2542) emissions are estimated using a historic profile provided by CARB (CARB, 2017) for the Bay Area. The remaining anaerobic digester (category 2261) category uses a historic profile based on Association of Bay Area Governments (ABAG) household population projections (ABAG, 2014).
For the industrial wastewater categories, historical emissions follow profile trends based on ABAG historical agricultural employment trends (ABAG, 2014) for the Bay Area. This is because these categories are related to production of perishable goods, which are generally assumed to be consumed locally and therefore the activity tracks agricultural demand and employment growth trend. Since California is a major wine producer that is made from cultivated grapes, industrial wastewater treatment of wine grapes (category 2271) uses California’s historical grape crush data archives (USDA, 2022b) to backcast historical emissions. The backcasting of refinery wastewater (category 2274) source category emissions is based on US Energy Information Administration (USEIA)’s California Refinery Operable Atmospheric Crude Oil Distillation Capacity dataset (EIA, 2022).
For all source categories within the domestic and industrial wastewater group, the growth profiles have been verified and updated to represent the most likely surrogate for projecting emissions up to the year 2050. Forecasted emissions for point source categories only includes reductions from existing regulations but does not address future policy emission targets or proposed regulation and legislation.
For process emissions (category 50), centralized anaerobic (category 2542) and anaerobic digester (category 2261) categories, growth profiles were derived specifically by Lawrence Berkeley National Laboratory, through a 2016 contract with the Air District (Greenblatt, 2017). These growth profiles are based on adopted state policies and regulations impacting the wastewater sector in California. For septic systems, N2O effluent, and N2O plant source (categories 2262-2264, respectively), the growth is based on projected population growth in the SFBA.
For industrial wastewater categories, the agricultural and natural resources employment forecasts from ABAG (ABAG, 2021) are considered to best represent the projected growth for these categories (categories 2265-2270; 2272-2273). For wine grapes (category 2270), the growth is based on projected quantity of grapes crushed from USDA (USDA, 2022b). Projected growth for all refinery related processes, including petroleum refining wastewater (category 2274), is based on a linear extrapolation of California Annual Operable Atmospheric Crude Oil Distillation Capacity from 2010 to 2020 (EIA, 2022) which results in a 0.67% decrease annually from 2021 to 2050.
Sample Calculations
The table below shows an example calculation to estimate base year 2022 CH4 emissions in units of metric tons of CO2 equivalents (MTCO2eq) from non-citrus fruit wastewater (category 2268) in Napa County.
Step 1 | Obtain statewide throughput non-citrus fruit (ton) from USDA Census of Agriculture for 2022 (USDA, 2022a) | 4,389,384 tons non-citrus fruit/year |
Use CH4 emission factor (lbs of CH4/ton) for non-citrus fruits from USEPA, 2015. | 0.494 lbs. CH4/ton | |
Step 3 | Calculate non-citrus fruit CH4 emissions (ton) for California | = 4,389,384 tons non-citrus fruit/year × 0.494 lbs CH4 /ton non-citrus fruit × 1 ton / 2000 lbs = 1084.18 tons CH4/year |
Step 4 | Obtain California and Air District non-citrus fruit acreage (acre) from USDA | State = 1,219,235 acres Air District = 96,663 acres |
Step 5 | Calculate the percent of statewide non-citrus acreage that is within the Air District | = 96,663/1,219,235 = 7.93 % or 0.0793 |
Step 6 | Estimate the CH4 emissions (tons/year) for the Air District | = 1084.18 tons/year × 0.0793% = 85.98 tons/year |
Step 7 | Obtain the Napa County non-citrus fruit acreage (acre) from USDA for 2022 | 58,051 acres |
Step 8 | Obtain the Napa County apple acreage (acre) from USDA for 2022 | 7 acres |
Step 9 | Estimate the non-citrus fruit acreage for Napa county by subtracting the apples acreage from the non-citrus acreage (acre) to avoid double counting | = 58,051 – 7 = 58,044 acres |
Step 10 | Estimate the fraction of non-citrus fruit acreage in Napa County by dividing by the total acreage in the Bay Area | = 58,044/96,663 = 60.05% or 0.6005 |
Step 11 | Calculate CH4 emissions (tons/year) for Napa County from non-citrus fruit by multiplying the CH4 emissions for the Air District by the fraction of land acreage in Napa County used to grow non-citrus fruit | = 85.98 tons/year x 0.6005 = 51.63 tons/year |
Step 12 | Use the Global Warming Potential for CH4 based on IPCC, 2014 | 34 |
Step 13 | Calculate the total CO2 equivalent emissions using GWP (MTCO2eq/year) | = 51.63 tons/year × 34 × 0.907 MT/tons = 1,592 MT CO2eq/year |
Updates to the wastewater categories for the GHG inventory include incorporating the most recent population data, updating the emission factors, and using the latest GWPs from the IPCC assessment report.
Base Year | Revision | Reference |
2022 |
|
CADOF, 2022 UN, 2015 USDA, 2022a USDA, 2022b CEC, 2024
|
2015 |
|
|
Emissions
The table below shows the total GHG emissions by pollutant in MTCO2eq for domestic and industrial wastewater categories.
| ID | Description | CH2Cl2 | CH4 | CO2 | N2O | Total |
|---|---|---|---|---|---|---|
| 2263 | Domestic Wastewater Treatment - POTW N2O Effluent Emissions | 0.0 | 0.0 | 0.0 | 150243.9 | 150243.9 |
| 2542 | Domestic Wastewater Treatment - Centralized Anaerobic | 0.0 | 101247.7 | 0.0 | 0.0 | 101247.7 |
| 2262 | Domestic Wastewater Treatment - Septic Systems | 0.0 | 43785.9 | 0.0 | 0.0 | 43785.9 |
| 2265 | Industrial Wastewater Treatment - Pulp and Paper | 0.0 | 19810.5 | 0.0 | 0.0 | 19810.5 |
| 2261 | Domestic Wastewater Treatment - Anaerobic Digesters | 0.0 | 15134.6 | 0.0 | 0.0 | 15134.6 |
| 2274 | Industrial Wastewater Treatment - Petroleum Refining | 0.0 | 11565.8 | 0.0 | 0.0 | 11565.8 |
| 2264 | Domestic Wastewater Treatment - POTW N2O Plant Emissions | 0.0 | 0.0 | 0.0 | 8017.2 | 8017.2 |
| 2266 | Industrial Wastewater Treatment - Red Meat | 0.0 | 5438.5 | 0.0 | 0.0 | 5438.5 |
| 2268 | Industrial Wastewater Treatment - Non-Citrus Fruits | 0.0 | 2599.0 | 0.0 | 0.0 | 2599.0 |
| 2272 | Industrial Wastewater Treatment - Other Vegetables | 0.0 | 2021.7 | 0.0 | 0.0 | 2021.7 |
| 2271 | Industrial Wastewater Treatment - Wine Grapes | 0.0 | 599.8 | 0.0 | 0.0 | 599.8 |
| 2267 | Industrial Wastewater Treatment - Poultry | 0.0 | 406.6 | 0.0 | 0.0 | 406.6 |
| 50 | Domestic Wastewater Treatment - POTW | 75.6 | 53.5 | 23.6 | 0.0 | 152.7 |
| 2270 | Industrial Wastewater Treatment - Apples | 0.0 | 19.5 | 0.0 | 0.0 | 19.5 |
| 2273 | Industrial Wastewater Treatment - Potatoes | 0.0 | 17.5 | 0.0 | 0.0 | 17.5 |
| 2269 | Industrial Wastewater Treatment - Citrus Fruits | 0.0 | 2.3 | 0.0 | 0.0 | 2.3 |
Summary of Base Year 2022 Emissions
Domestic and industrial wastewater treatment produces CH4 and N2O emissions due to biological decomposition of waste in oxygen-deficient conditions. Emissions are initially calculated in pounds and then converted to MTCO2eq.
The tables below show the contribution of domestic and industrial wastewater GHG emissions to the overall SFBA total and to the Waste Management sector. Unlike other sectors, the wastewater sectors only include emissions from CH4 and N2O, as CO2 emissions are considered biogenic and not included in these summaries. Combined, the domestic and industrial wastewater groups account for approximately 14% of the GHG emissions in the Waste Management sector and nearly 0.6% of the regional GHG inventory for the year 2022. In terms of individual GHG pollutants, CH4 emissions from domestic and industrial wastewater groups constitute approximately 9% of the Waste Management sector and nearly 5% of the regional CH4 inventory. Similarly, for N2O, the domestic and industrial wastewater groups constitute around 84% of the Waste Management sector and nearly 22% of the regional N2O inventory.
Contribution of Wastewater Treatment Emissions by Sector| Subsector | Sector | Subsector GHG Emissions (MMTCO2eq) | Sector GHG Emissions (MMTCO2eq) | % of Sector |
|---|---|---|---|---|
| Wastewater Treatment | Waste Management | 0.36 | 2.61 | 13.85% |
Contribution of Wastewater Treatment Emissions to Regional Total
| Subsector | Subsector GHG Emissions (MMTCO2eq) | Regional Total GHG Emissions (MMTCO2eq) | % of Regional Total |
|---|---|---|---|
| Wastewater Treatment | 0.36 | 65.68 | 0.55% |
Trends
The time series chart below shows the emission trends for domestic and industrial wastewater categories.
Summary of Trends
Domestic wastewater activity is closely tied to population trends and has generally increased over time, reflecting the steady population growth in the Bay Area. The only significant exception occurred during the COVID-19 pandemic, when the region experienced a temporary population decline as people moved to more affordable areas outside the Bay Area. Despite the long-term rise in activity, this has not translated into higher greenhouse gas (GHG) emissions. This is primarily due to the decline in emission factors for the major GHG category 2542 (centralized anaerobic systems). Between 2000 and 2021, the emission factor decreased from 1.78 to 0.89 pounds of CH₄ per person, representing a reduction of approximately 50%. Over the longer term (1990–2022), GHG emissions from domestic wastewater have been decreasing slowly, at a rate of less than 1% per year.
In contrast, industrial wastewater activity is influenced by various factors, including agricultural output, pulp and paper production, meat processing, and refinery operations. In recent decades, this activity has gradually declined, driven largely by the SFBA’s growing demand for real estate to support the expanding technology sector. As a result, land available for agriculture and livestock production has diminished (ABAG, 2021). From 1990 to 2022, industrial wastewater GHG emissions have remained relatively stable, showing minimal year-to-year variation. Looking ahead, a continued slow decline in agricultural and livestock activity in the region is expected, which will likely result in a modest reduction in industrial wastewater throughput and associated emissions.
Uncertainties
The primary uncertainty in the updated methodology is from the use of emission factors that have remained unchanged over several decades despite potential implementation of technological controls at facilities. Additionally, these emission factors are rarely verified and validated against actual measurements at POTWs, largely due to the challenges with accurately measuring fugitive emissions from diffused area sources. The emission factors used in this inventory are from national-level literature and have not been calibrated or validated against local operating conditions or practices.
Additionally, uncertainties in future projection of emissions arise from the use of surrogates such as trends in agricultural sector employment for projections, which may not be the most representative indicator of regional activity growth, especially if a significant portion of regional agricultural production is exported for consumption outside of the Bay Area.
Category 2542 is an all-inclusive alternate to permitted source category 50 for estimating biological CH4 emissions, arising from anaerobic decomposition pathways, from POTWs (domestic wastewater treatment). As such any GHG emissions (even minor and negligible) reported by operators in source category 50 may represent a double count. In future updates, GHG emissions found in source category 50 will be assumed to have been already accounted for in source category 2542 and programmatically removed (voided) from the inventory accounting pipeline.
Contact
Author: Tan Dinh
Reviewer: Abhinav Guha
Last Update: 07/03/2025
References
ABAG. 2014. ABAG 2040 Projections. https://abag.ca.gov/our-work/land-use/forecasts-projections
ABAG. 2021. ABAG Planned Area 2050 Growth Pattern. https://abag.ca.gov/our-work/land-use/forecasts-projections
BAAQMD. 2015. Bay Area Emissions Inventory Summary Report: Greenhouse Gases. Base Year 2011. http://www.baaqmd.gov/~/media/Files/Planning%20and%20Research/Emission%20In ventory/BY2011_GHGSummary.ashx?la=en
BAAQMD. 2022. BAAQMD Internal Database System. Emission Year 2022.
BAAQMD. 2023. BAAQMD Regulation 8, Rule 8: Wastewater Collection and Separation Systems. https://www.baaqmd.gov/rules-and-compliance/rules/reg-8-rule-8-wastewater-oilwater-separators?rule_version=2023%20Amendment
BAAQMD. 2024. BAAQMD Base Year 2022 GHG Emission Inventory.
CADOF. 2022. California Department of Finance Report E-4. Population estimates for Cities, Counties, and State. Table 1 County State. https://dof.ca.gov/forecasting/demographics/estimates/e-4-population-estimates-for-cities-counties-and-the-state-2021-2024-with-2020-census-benchmark/
CARB. 2016. CARB California’s 2000-2014 Greenhouse Gas Emission Inventory Technical Support Document (2016 Edition). https://ww2.arb.ca.gov/sites/default/files/classic/cc/inventory/pubs/reports/2000_2014/ghg_inventory_00-14_technical_support_document.pdf
CARB. 2017. BAAQMD staff email correspondence in 2017 on POTWs historical growth data.
CARB. 2023. CARB Greenhouse Gas Emission Inventory Query Tool for Years 2000-2021 (2023 Edition). https://ww2.arb.ca.gov/applications/greenhouse-gas-emission-inventory-0
CEC. 2024. California Energy Commission data on California’s Oil Refineries. https://www.energy.ca.gov/data-reports/energy-almanac/californias-petroleum-market/californias-oil-refineries
EIA. 2022. United Stets Energy Information Administration 2022 Oil Refinery Distillation Capacity. https://www.eia.gov/dnav/pet/pet_pnp_cap1_dcu_SCA_a.htm
Greenblatt. 2017. Jeffery B. Greenblatt. BA-CALCAPS: GHG Inventory Model (historical and projected GHG emissions) for the Bay Area. March 2017.
IPCC. 2014. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II, and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyers (eds.)]. IPCC, Geneva, Switzerland, 151 pp. https://www.ipcc.ch/site/assets/uploads/2018/02/SYR_AR5_FINAL_full.pdf
Onsite. 2003. Onsite Wastewater Treatment Systems in California 2001/2002 Survey of Septic Systems, August 2003.
UN. 2015. Data request between CARB and Aselia Urmanbetova of the Center of Paper Business and Industry Studies (CPBIS) at Georgia Institute of Technology. December 2015.
USDA, 2022a. California Census of Agriculture Volume 1, Chapter 1: 2022 State Level Data, United States Department of Agriculture. https://www.nass.usda.gov/Publications/AgCensus/2022/Full_Report/Volume_1,_Chapter_1_state_Level/California/
USDA. 2022b. California Errata to the Grape Crush Report, 2022, California Department of Agriculture.. https://www.nass.usda.gov/Statistics_by_State/California/Publications/Specialty_and_Other_Releases/Grapes/Crush/Errata/2022/2022_errata.pdf
USEPA. 2015. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013. U.S. Environmental Protection Agency. Chapter 7: Waste. http://www.epa.gov/climatechange/emissions/usinventoryreport.html