5.1 Wastewater Treatment
Categories 50, 2261, 2262, 2265-74, and 2542
This chapter contains organic emissions (TOG and ROG) from two kinds of wastewater sources. The first group of sources are emissions from wastewater originating from human use. These are referred to as Domestic Wastewater Treatment categories. The emissions for this group are covered in Chapter 5.1.1. The second group of sources are emissions from wastewater generated from various industrial and agricultural uses and operations. These are referred to as Industrial Wastewater Treatment categories and covered in Chapter 5.1.2.
5.1.1 Domestic Wastewater Treatment
5.1.2 Introduction
Categories 50, 2542, 2261 and 2262 account for emissions under the Domestic Wastewater Treatment sector. Typically, most of these emissions occur in a Publicly Owned Treatment Work (POTW) wastewater treatment plant that is owned and run by local government agencies. POTWs consist of a facility collection systems that contain a lift station, trenches, junction boxes, manholes and other emission points. Wastewater streams are introduced into the POTW collection system through individual or area drains, which can be open to the atmosphere or sealed to prevent wastewater coming into contact with air. Pollutant emissions are emitted from wastewater collection, treatment, and storage systems through volatilization of organic compounds at the liquid surface (Category 50). Methane (CH4) emissions occur at POTWs from centralized anaerobic processes (Category 2542) and from anaerobic digestion of wastewater biosolids during biogas production (Category 2261). A small fraction of homes, especially in the rural parts of the San Francisco Bay Area (SFBA), are not connected to municipal POTW systems. Such homes rely on standalone septic systems for their wastewater collection and treatment needs. Organic emissions from septic systems are estimated as CH4 emissions (Category 2262). CH4 emissions from the above-mentioned categories are reported as total organic gas (TOG) emissions.
POTWs are also a major source of nitrous oxide (N2O) emissions occurring from effluents (Category 2263) and plant-level biological denitrification processes (Category 2264). These emissions are reported and accounted for separately in the District’s Greenhouse Gas (GHG) Emissions Inventory, and are not discussed in this document. The following table summarizes the Categories covered in Chapter 5.1.1. with their classification designation.
| Category # | Description | Classification |
|---|---|---|
| 50 | Domestic Wastewater Treatment - Process | Point |
| 2542 | Domestic Wastewater Treatment - Centralized Anaerobic | Area |
| 2261 | Domestic Wastewater Treatment - Anaerobic Digesters | Area |
| 2262 | Domestic Wastewater Treatment - Septic Systems | Area |
5.1.3 Methodology
Point Sources
Point Sources are operations that emit air pollution into the atmosphere at a fixed location within a facility, for which the Air District has issued a permit to operate, e.g. refinery cooling towers. These could also be a collection of similar equipment / sources located across multiple facilities, e.g. reciprocating engines.
During the permit to operate (PTO) issuance process, the BAAQMD collects information from the operating facility and/or determines from published literature, e.g. EPA’s AP-42, characteristics of a source including maximum throughput, emission factors for emitted pollutants, and control factors associated with downstream abatement devices. These characteristics are then stored for future use in the BAAQMD’s internal database. Facilities that hold a permit to operate are required to renew this permit periodically (this period varies based on facility and source type). Upon renewal, the facilities are requested to provide any updates to source characteristics as well as the source throughput for the last 12 months. This throughput, in combination with the emission factors and controls factors stored in the internal database, are used to estimate annual emissions at the source level. These source level emissions are then sorted and aggregated into categories.
Further speciation and quality assurance of emissions are performed as a part of the inventory process. The BAAQMD staff also perform a systematic crosswalk between CEPAM’s source category classification (Emission Inventory Code - EICs) and the District’s source category classification (category identification number - cat_ids), which ensures consistency in the annual emissions reporting process (CEIDARS) to California Air Resources Board. The last part of the inventory development process includes forecasting and back casting, and aggregation into sub-sectors and sectors for documentation purposes. For those years where no data is available, emissions data are backcasted to year-1990, as well as forecasted to year-2040 using either interpolation or another mathematical approach (see Trends section). Finally, emissions trends spanning from year 1990-2040 for each category and pollutant are evaluated for anomalies that are then investigated and addressed.
Category 50 (POTW) accounts for organic emissions from point sources occurring at municipal wastewater treatment plants. All plants operating in the SFBA are permitted in the internal permit system. The internal database contains throughput information submitted by individual plants at the source-level. Emissions are then calculated by using specific emission factors as well as control factors, if any, for a particular source operation supplied by the company. Throughputs for point source Category 50 is volume of wastewater effluent (in thousand gallons) which is queried from the District’s internal database. The county location of each POTW facility is used to aggregate the calculated emissions across each county.
The GHG emissions of the permitted POTW facilities under Category 50 come from combustion sources only and are accounted for within combustion source categories (i.e. Categories 291, 292, 304, 307, 312).
Area Sources
Categories 2542, 2261 and 2262 are considered an area source category since they cover facilities / emission sources that are not directly permitted by the District, and hence not systematically cataloged. Emissions for area source categories are determined using the formula:
Current Year Emissions = Base Year Emission X Growth Profile, and,
Base Year Emission = Throughput X Control Factor X Emission Factor
where,
- throughput or activity data for applicable base year(s) is determined using a top-down approach (e.g. state-, national-level data);
- emission factor is derived from general literature, specific literature and reports, and/or source testing results provided by Air District staff;
- control factor (if applicable) is determined by District and state rules and regulations in effect;
- and, historical backcasting and forecasting of emissions is based on growth profiles as outlined in the Trends section of this chapter
Throughput information for these categories are only available at the state level and need to be scaled down using county-level proxies. The CH4 emissions from wastewater processes are estimated using the California Air Resources Board’s Documentation of California’s Greenhouse Gas Inventory117 and represented as TOG in the current base year inventory. These area source GHG emissions are estimated for the SFBA as a whole and not for a specific POTW facility.
More details on throughput, county distribution, emission factors and controls is provided in the following subsections.
(a) Activity Data / Throughput
For this base year inventory, total population (served by the municipal POTW system) of SFBA in year 2015 is used as the throughput for Category 2542 (centralized anaerobic). The throughout for Category 2261 is the volume of biogas produced (in million standard cubic feet) in SFBA in year 2015. The biogas volume information is obtained from the internal permit database. The throughput for septic systems (Category 2262) is also based on proportion of SFBA population that is served by septic systems.
(b) County Distribution / Fractions
The throughput in Category 2542 and 2262 were distributed according to each District’s county population, while POTW locations are used to distribute the throughput for Category 2261 to various counties.
(c) Emission Factors
Emission factors (often varying by year) are obtained from the CARB GHG Inventory Query Tool1. The CARB inventory, in turn, derives emissions for the wastewater treatment sector from the US EPA inventory 118.
(d) Control Factors
No separate or specific controls are applied to the emission factors, but future controls are in-built into the projections and growth of emissions for the individual categories.
(e) Speciation
For all the concerned categories, methane (CH4) is the pollutant that is being estimated and characterized as TOG. Since CH4 is considered a non-reactive organic, ROG / TOG ratio is assumed to be 0.
5.1.4 Changes in Methodology
This chapter has seen major changes to the methodology since the last version of emissions inventory release (based on a base year of 2011). The most important change is the re-designation of the former area source category 1911 which was a catch-all category for all organic emissions in the wastewater treatment sector which were not already reported within the point source permit database (Category 50). The use of Category 1911 in has now been discontinued and the category has been split into process- and source-specific categories 2542, 2261-64 (N2O-specific categories are not covered here). Emission factors and growth profiles for all of these area source categories have been updated to reflect the newest information available.
5.1.5 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
The centralized anaerobic TOG emissions are about ~3000 tons/year making this a major source of TOG emissions. Other categories like septic systems (1405 tons/year) and anaerobic digestion (460 tons/year) are also large TOG sources when compared to many non-waste sector categories.
5.1.6 Trends
The area source categories use a range of base years to determine historical and future projected emissions. For historical emissions, 2011 is used as a base year for projections. For future emissions, 2015 is used as a base year for projections.
(a) Historical Emissions / History
Historical emissions for point sources are derived from source-specific throughputs provided by the permitted facility, compiled/reported emission factors, and regulation-based control factors. This information is archived in the BAAQMD’s internal database which is queried to retrieve the data for historical and current years. Interpolation techniques to account for missing data are used when necessary, this is the case for years 1991-1992.
The historical growth profile for point source Category 50 is based on a combination of prior emissions data reported in the internal permit database (back to 1990). TOG emissions from category 50 peaked at ~200 tons/year and have steadily decreased to ~100 tons of year in base year 2015.
For the area source categories (2542, 2261-62), historical emissions co-vary with population growth as the emissions are back cast using the SFBA’s historical population curve119.
(b) Future Projections / Growth
Forecasting of point source emissions is done based on calculations as shown in the equation below using recently updated growth profiles and a base year of 2020. The growth profiles for this inventory have been verified and updated to represent the most likely surrogate for growing emissions for a given category up to year 2040. Forecasting for point source emissions includes impact of in-place regulations, but does not include estimation of controls that will theoretically be implemented as part of future policy emission targets or proposed regulation and legislation.
\[ \text{PE} = \text{Gr} * \text{Ci} * \text{Ei} \]
\(PE\) = projected emissions of pollutant i in a future year
\(Gr\) = growth rate by economic profile of industry or population
\(Ci\) = control factor of pollutant i based on adopted rules and regulations
\(Ei\) = base year emissions of pollutant i
Projections to year 2040 for category 50 is based on ABAG’s population growth profile3. For the area source categories 2542 and 2261, the District collaborated with researchers at Lawrence Berkeley National Laboratory to derive policy- and regulation-based sector specific forecasting models for Bay Area’s GHG emissions120. A growth profile developed specifically to estimate wastewater sector emission trends is then applied to estimate future emissions. For Category 2262 (septic systems), the growth is not based on POTW activity but instead on projected population growth in the region3.
5.1.7 Uncertainties
The main uncertainty in the updated methodology arises from the use of emission factors for certain categories that remain constant over several decades although technological controls may have been applied at facilities. Additionally, the emission factors themselves are seldom verified and validated against measurements at POTW facilities, in part due to the difficulty in measuring fugitive emissions from area sources.
5.1.8 Industrial Wastewater Treatment
5.1.9 Introduction
Categories 2265 through 2274 account for emissions under the Industrial Wastewater Treatment sector. A large chunk of wastewater emissions do not arise from direct human use but instead occur at agricultural and industrial facilities set up to produce crops, consumer products (e.g. paper) and food products for human consumption. These emissions occur due to accumulation of organic effluents in the waste stream that volatilize from holding ponds and tanks prior to being treated chemically. While emissions from these categories comprise of both ROG and CH4, the pollutant estimated here is methane (CH4) and is classified as TOG. All the categories mentioned below are estimated as area sources in a top-down approach.
Emissions covered in this sub-sector are classified under the following category numbers:
| Category # | Description | Classification |
|---|---|---|
| 2265 | Industrial Waste Treatment - Pulp and Paper | Area |
| 2266 | Industrial Waste Treatment - Red Meat | Area |
| 2267 | Industrial Waste Treatment - Poultry | Area |
| 2268 | Industrial Waste Treatment - Non-citrus Fruits | Area |
| 2269 | Industrial Waste Treatment - Citrus Fruits | Area |
| 2270 | Industrial Waste Treatment - Apples | Area |
| 2271 | Industrial Waste Treatment - Wine Grapes | Area |
| 2272 | Industrial Waste Treatment - Other Vegetables | Area |
| 2273 | Industrial Waste Treatment - Potatoes | Area |
| 2274 | Industrial Waste Treatment - Petroleum Refining | Area |
5.1.10 Methodology
These categories are considered an area source category since they cover facilities / emission sources that are not directly permitted by the District, and hence not systematically cataloged. Emissions for area source categories are determined using the formula:
Current Year Emissions = Base Year Emission X Growth Profile, and,
Base Year Emission = Throughput X Control Factor X Emission Factor
where,
- throughput or activity data for applicable base year(s) is determined using a top-down approach (e.g. state-, national-level data);
- emission factor is derived from general literature, specific literature and reports, and/or source testing results provided by Air District staff;
- control factor (if applicable) is determined by District and state rules and regulations in effect;
- and, historical backcasting and forecasting of emissions is based on growth profiles as outlined in the Trends section of this chapter
More details on throughput, county distribution, emission factors and controls is provided in the following subsections.
(a) Activity Data / Throughput
Activity data at the state level is made available by CARB’s GHG Inventory Tool1. Throughput for all but one category (Category 2265) is determined from annual state-level agricultural statistics data provided by USDA121. Throughput data for the paper and pulp industry (Category 2265) is obtained from the paper industry sources122.
(b) County Distribution / Fractions
The county distribution of the emissions is obtained by apportioning the throughputs to the counties whenever finer resolution data is available (for example, agricultural production data is available at the county level for each year). If only state-level data is available, the data is scaled down to the county-scale by using population surrogates3.
(c) Emission Factors
Emission factors (often varying by year) are obtained from the CARB GHG Inventory Query Tool1. The CARB inventory, in turn, derives emissions for the wastewater treatment sector from the US EPA inventory2. Methane emission factors range from 0.1 tons/day for citrus fruits (Category 2269) to 5.9 tons/day for the red meat industry (Category 2266).
(d) Control Factors
No separate or specific controls are applied to the emission factors, but future controls are in-built into the projections and growth of emissions for the individual categories.
(e) Speciation
For all the concerned categories, CH4 is the pollutant that is being estimated and characterized as TOG. Since CH4 is considered non-reactive oragnic, ROG / TOG ratio is assumed to be 0.
5.1.11 Changes in Methodology
This sub-chapter has seen major changes to the methodology since the last version of emission inventory. Previously, the former area source Category 1911 was a catch-all category for all organic emissions in the wastewater treatment sector for all non-point source emissions. However, Category 1911 only accounted for emissions from Domestic Wastewater Treatment. The new set of categories 2265 - 2274 cover a broader scope of waste sector including Industrial Wastewater Treatment emissions (that were previously not accounted for). These categories are aligned in scope and coverage with CARB’s GHG inventory classification.
5.1.12 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
In base year 2015, the largest source of TOG emissions in the industrial sector is the petroleum refining category with about 1,175 tons/year of TOG emissions followed by the paper and pulp industry (630 tons/year) and red meat industry (~150 tons/year). These emissions are substantial when compared to TOG emissions from non-waste sector categories.
5.1.13 Trends
This category uses a range of base years to determine historical and future projected emissions. For historical emissions, 2011 is used as a base year for projections. For future emissions, 2015 is used as a base year for projections.
(a) Historical Emissions / History
The historical emissions of these categories mostly trend with population growth in the SFBA. This is because these categories are related to production of goods, which generally track with consumption and demand that is driven by population growth. Since California is a major grape/wine producer, Category 2271 uses historical grape crush data for backcast of historical emissions123
(b) Future Projections / Growth
The agricultural and natural resources employment forecasts put forward by ABAG3 is considered to best represent the growth of activity within the categories under Industrial Wastewater Treatment. This employment-based growth profile is used to forecast emissions for Categories 2265-2273.
Projected growth for all refinery related categories, including Category 2274 (Petroleum Refining), are based on the even extrapolation of California Annual Operable Atmospheric Crude Oil Distillation Capacity from 2010 to 2020, i.e., it’s assumed a 0.67% decrease annually from 2021 to 2040. The data used in the extrapolation is taken from EIA website124.
5.1.14 Uncertainties
The main uncertainty in estimating emissions from the Industrial Waste Treatment categories arise from the use of EPA emission factors that are averages derived from potentially outdated national-level data sources and controlled experiments. They are seldom verified and validated against regional measurements, in part due to the difficulty in measuring fugitive emissions from area sources.
5.1.15 Contact
Author: Abhinav Guha
Reviewers: Tan M. Dinh and Yuan Du
Last Update: November 06, 2023
5.1.16 References & Footnotes
CARB. 2021. California 2000-2019 GHG Inventory, Category: Waste, IPCC: 4D1 – Wastewater Treatment and Discharge. https://ww2.arb.ca.gov/applications/california-ghg-inventory-documentation↩︎
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↩︎
Association of Bay Area Government (ABAG). 2019. Plan Bay Area 2040. http://2040.planbayarea.org/reports↩︎
LBNL. 2013. California Greenhouse Gas Inventory Spreadsheet Model. LBNL Report # 6451E. https://eta-publications.lbl.gov/sites/default/files/lbnl-6451e.pdf↩︎
USDA. 2016. U.S. Department of Agriculture, National Agricultural Statistics Service, Quick Stats. http://quickstats.nass.usda.gov/↩︎
Paper and Pulp. 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↩︎
California Agricultural Statistics Services. 2015. Grape Crush Final Report. https://www.nass.usda.gov/Statistics_by_State/California/Publications/Specialty_and_Other_Releases/Grapes/Crush/Errata/2015/201608errata.pdf↩︎
EIA. 2022. 2022 Oil Refinery Distillation Capacity. https://www.eia.gov/dnav/pet/pet_pnp_cap1_dcu_SCA_a.htm↩︎