6.3 Composting
| Category ID | Description | EIC |
|---|---|---|
| 1936 | Composting (Area) | 19917002600000 |
Introduction
Category 1936 is an area source that accounts for organic emissions from the composting sector in the San Francisco Bay Area (SFBA).
Composting is a controlled biological process in which microorganisms decompose organic materials—such as yard trimmings, food waste, and agricultural residues—into a stabilized soil amendment. This process primarily occurs under aerobic (oxygen-rich) conditions, which promote efficient breakdown of organic matter and help minimize the formation of methane (CH₄). However, when oxygen availability is limited—due to factors such as pile compaction, excessive moisture content, fine particle size, or inadequate aeration—anaerobic (oxygen-deficient) zones may form within the compost pile. Under these conditions, anaerobic microbial activity can lead to elevated emissions of CH₄, a potent greenhouse gas (GHG).
In addition to CH₄, composting operations also produce nitrous oxide (N₂O), primarily through microbial processes involving nitrogen compounds in the feedstock. N₂O is typically emitted during both aerobic and anaerobic phases, especially in areas with fluctuating oxygen levels and high nitrogen content. Carbon dioxide (CO₂) is also released during composting as a byproduct of organic matter degradation. Because this CO₂ originates from the natural carbon cycle (i.e., it is biogenic), it is reported for completeness but excluded from total anthropogenic GHG emissions inventories, in accordance with California Air Resources Board (CARB) protocols.
GHG emissions from composting arise across several key process steps, each of which contributes differently to overall emissions:
- Feedstock Preparation:
During this initial stage, organic materials are sorted, screened, and mechanically reduced in size (e.g., via grinding or shredding) to promote uniformity, increase surface area, and enhance microbial breakdown. These steps optimize the composting process by enhancing microbial access to organic matter. During this phase, CO2 and CH4 emissions may occur from the handling and disturbance of organic materials, as decomposable material is exposed to air. - Active Composting:
The prepared organic mixture is formed into windrows, static piles, or in-vessel systems, where microbial activity rapidly decomposes organic matter. This stage is thermophilic, generating high temperatures (above 55°C), which are essential for pathogen and weed seed destruction. Aerobic microbial respiration produces CO₂ as a primary emission, while anaerobic zones within the pile may emit CH₄. N₂O can also be generated due to nitrogen transformations during this highly active phase. - Curing:
Following active composting, the material is transferred to a curing area where biological activity slows, and temperature gradually decreases. The curing stage allows for further stabilization and moisture reduction. Emissions of CH₄ and N₂O may still occur during this stage, particularly if piles remain compacted or retain excess moisture. - Screening:
Cured compost is screened to remove oversized materials and produce a consistent final product. This process may release small amounts of CO2 and CH4 as compost is disturbed, and fine particles are exposed. - Storage:
Finished compost is stored on-site prior to distribution and use. If stored for extended periods, especially under wet or poorly aerated conditions, the compost may continue to emit CH₄ and N₂O.
To more comprehensively estimate GHG emissions from composting operations in the SFBA, a custom methodology has been developed that integrates data from the California Solid Waste Information System (CalSWIS). This approach was adopted because the Air District’s permit database does not capture all permitted and unpermitted composting facilities operating in the region. As a result, GHG emissions from all composting facilities—regardless of permit status—are now included under Category 1936 (area source), and Category 1709 (point source) is no longer used for this sector.
As of 2024, CalSWIS reported 48 composting facilities in the SFBA. Of these, 12 are permitted by the Bay Area Air Quality Management District (BAAQMD or the Air District) and were previously treated as point sources. The remaining 36 were treated as non-permitted area sources. Under the revised approach, GHG emissions are estimated by multiplying facility-specific throughput by pollutant-specific emission factors and applying control factors where applicable. This approach ensures more complete accounting of GHG emissions from composting across the region.
Methodology
This methodology describes how GHG emissions are estimated for the area source category 1936. This category is considered an area source category since it includes facilities/emission sources that are not directly permitted by the Air District, and hence emissions are not systematically or annually catalogued. The methodology uses activity data that are multiplied by appropriate emission factors to estimate emissions for each composting facility. These emissions are then summed up to represent county level emissions.
The methodology used to calculate emissions for the reported base year(s) of these area source categories is as follows:
Base Year(s) Emissionscounty,pollutant =
Activity Data × Emission Factorpollutant × Control Factorpollutant × Fractioncounty × GWPpollutant × Fractionin District
Where:
- Base Year: is a year for which activity / throughput data is available (or can be derived or estimated)
- Activity Data: is the throughput or activity data for applicable base year(s).
- Emission Factorpollutant: is a factor that expresses how the amount, in mass, of a particular pollutant is emitted per unit of activity data. Examples include tons of CO2 per gallon of gasoline burned and pounds of N2O per million standard cubic feet of natural gas combusted. This factor generally comes from a published literature source such as EPA’s AP-42 or CARB’s Mandatory Reporting Requirement (MRR) for Greenhouse Gases.
- Control Factorpollutant: is a fractional ratio (between 0 and 1) that captures the estimated reduction in emissions due to 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. It is typically derived from regional socioeconomic metrics and/or actual county-level throughput data.
- GWP is the Global Warming Potential. 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.
- Fractionin District: The BAAQMD jurisdiction covers only a portion of Solano and Sonoma County. For this reason, additional allocation must be done for these counties to determine the proportion of the county’s emissions occurring within BAAQMD’s jurisdiction.
Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:
Current Year Emissionscounty = Base Year(s) Emissioncounty x 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 throughput, county distribution, emission factors and controls are provided in the following subsections:
Activity Data / Throughput
The activity data for composting operations is obtained from CalSWIS (2024). The CalSWIS database provides facility-level information for each county, including:
- Facility name and location
- Types of waste accepted
- Facility start-up and (if applicable) closure year
- Regulatory status
- Reported throughput and permitted maximum capacity
To ensure consistency and accuracy in activity data reporting, the lower of the reported throughput or permitted maximum capacity is used for each facility. Throughput is typically reported in tons per year, although some facilities report in other units such as cubic yards per day (yd³/day) or cubic yards per year (yd³/year). To convert volumetric measurements to tons, standardized bulk density values from CalRecycle (2010) are applied:
- Compost (green waste, food waste, etc.): 2.24 yd³/ton
- Mulch (wood waste, agricultural waste, etc.): 3.54 yd³/ton
- Mixed waste: 2.89 yd³/ton (average of compost and mulch densities)
It is assumed that composting facilities operate an average of 288 days per year, based on operational data compiled from the Air District’s internal permit database. This figure accounts for non-operational days due to weekends, holidays, and maintenance periods.
Throughput utilization is not assumed to be at full capacity immediately upon a facility’s start-up. Instead, a ramp-up schedule is applied to reflect the typical growth in operational scale over time. Based on expert input from an Air District permit engineer familiar with composting operations in the SFBA, the following utilization factors are applied to scale the estimated annual throughput for each facility depending on its operational age:
Years Since Start-Up | Utilization Factor |
0 | 0.60 |
1 | 0.70 |
2 | 0.80 |
3 | 0.85 |
4 | 0.90 |
5+ | 0.90 |
When a facility ceases operations, emissions are assumed to phase out over a two-year period, rather than stopping abruptly. The following post-closure utilization assumptions are applied:
- Year of closure: Utilization reduced to 0.80
- First full year after closure: Utilization reduced further to 0.60
- Second year after closure onward: Utilization assumed to be 0 (i.e., no activity)
This gradual phase-out approach accounts for the likelihood that facilities may continue to process residual materials or wind down operations during and shortly after the year of official closure.
BAAQMD Jurisdiction Fraction and County Distribution
The distribution of composting activity across counties in the SFBA is determined based on facility-specific data from the CalSWIS database. Each facility's location, name, and ownership details were reviewed to confirm geographic placement within the jurisdictional boundaries of the Air District. This is particularly important for Solano and Sonoma Counties, which are only partially within Air District’s jurisdiction. Only the composting facilities located within the BAAQMD portion of these counties are included in the emissions inventory. Facilities located outside the BAAQMD boundary are excluded to ensure accurate attribution of regional emissions.
The table below summarizes the distribution of composting throughput and the number of active facilities by county in 2022:
County | Fraction of Regional Throughput (2022) | Number of Facilities (2022) |
Alameda | 0.124 | 6 |
Contra Costa | 0.269 | 9 |
Marin | 0.107 | 5 |
Napa | 0.038 | 6 |
San Francisco | 0.004 | 2 |
San Mateo | 0.014 | 4 |
Santa Clara | 0.311 | 10 |
Solano | 0.054 | 3 |
Sonoma | 0.078 | 9 |
Emission Factors
Emission factors for GHGs used in this inventory are sourced from Nordahl et al. (2023). This study presents a comprehensive meta-analysis of 46 peer-reviewed publications related to composting emissions, resulting in a dataset of 388 emission factors across multiple pollutants. The authors evaluated emissions based on waste feedstock type, composting method, and key operational factors—such as aeration, moisture content, and temperature—that influence emission rates. The objective of the study was to provide a consistent and scientifically robust foundation for estimating emissions from composting systems under varying conditions.
All emission factors are expressed in pounds (lb) of pollutant per ton of wet feedstock, to align with available activity data.
To apply these emission factors to composting activity reported in the California Solid Waste Information System (CalSWIS), each material type listed in the CalSWIS database is mapped to a corresponding feedstock category defined in Nordahl et al. (2023). This mapping ensures the emission factors accurately reflect the composition of waste materials processed at each facility.
The table below presents the assigned emission factors by pollutant and waste category:
CalSWIS Material(s) | Nordahl et al. (2023) Feedstock | Pollutant | Emission Factor |
Manure | Manure | CH4 | 5.64 |
N2O | 0.708 | ||
CO2 | 280 | ||
Mixed Municipal | Organic Fraction of Municipal Solid Waste (OFMSW) | CH4 | 1.76 |
N2O | 0.136 | ||
CO2 | 112.6 | ||
Sludge (BioSolids) | Sludge | CH4 | 0.468 |
N2O | 0.167 | ||
CO2 | 35 | ||
Agricultural | Yard Waste | CH4 | 4.12 |
N2O | 0.091 | ||
CO2 | 342 |
Control Factors
Since there are no current Air District rules that directly regulate emissions or activity associated with composting operations, no control factors are formally applied in the emissions calculations at this time. However, three proposed rules under Regulation 13 are in various stages of development and may influence future composting practices and associated emissions if adopted:
- Regulation 13, Rule 2: Organic Material Handling Operations (BAAQMD, 2024a; development currently suspended)
This rule was initially proposed to address emissions from facilities engaged in the transfer, processing, or storage of organic materials. It aimed to reduce emissions of reactive organic gases (ROG), ammonia (NH₃), and particulate matter (PM) by requiring best management practices, such as prompt material covering, moisture content maintenance, and operational limits. Although development of Rule 13-2 is currently suspended, its adoption could affect GHG emissions indirectly by altering the timing or conditions under which organic material is handled prior to composting, thereby influencing the formation of methane (CH₄) or nitrous oxide (N₂O) under anaerobic or suboptimal composting conditions. - Regulation 13, Rule 3: Composting Operations (BAAQMD, 2024b; development currently suspended)
Rule 13-3 was designed to specifically regulate composting facilities and reduce emissions associated with the biological decomposition of organic materials. The rule would have included requirements for aeration, pile management, curing protocols, and potentially the use of emission control systems such as biofilters or covers. If adopted, Rule 13-3 could lead to a measurable reduction in CH₄ and N₂O emissions from composting piles by promoting more consistently aerobic conditions and mitigating fugitive emissions. Development of this rule is also currently suspended. - Regulation 13, Rule 4: Sewage Treatment and Anaerobic Digestion (BAAQMD, 2024c; currently in development)
Although primarily focused on emissions from sewage treatment plants and anaerobic digestion operations, Rule 13-4 may have ancillary impacts on composting activity, particularly for facilities that co-process biosolids or digestate as part of their feedstock. The rule is under active development and may introduce requirements that affect the type and volume of materials available for composting, as well as the design and operation of co-located composting systems. These changes could influence overall composting throughput and the corresponding GHG emissions.
Until any of these rules are finalized and adopted, emissions from composting operations continue to be estimated without applying control factors.
Historical Backcast
Emissions from composting activities prior to 2011 are estimated using a linear regression based on total county-level emissions from the 2011–2015 period, as reported in the previous inventory (CalSWIS, 2020). These estimates are not based on facility-level data from the CalSWIS database due to limited availability of historical records for earlier years.
This method aligns well with the statewide composting emissions trend observed in the CARB Greenhouse Gas (GHG) Inventory from 2000 to 2010, which also shows a relatively steady increase in emissions over time. Given the consistency between regional and statewide patterns, the regression approach is considered a reasonable and representative method for estimating historical emissions in the Bay Area.
Future Projections
Future emissions from composting are projected under the assumption that facility throughput remains constant, and changes in emissions are driven solely by increasing capacity utilization rates as facilities mature.
An alternative method—based on the 2022 CARB Scoping Plan modeling results from Energy + Environmental Economics (E3) for the “Non-Energy: Other – Recycling and Waste” category (CARB, 2022; Appendix H)—was considered. However, this approach projects a sharp increase in emissions, with a 47.6% rise between 2022 and 2025, which is not reflected in facility data from the CalSWIS database.
Because the CalSWIS records do not indicate any major recent surge in composting activity or throughput, staff determined that the more gradual, utilization-based projection approach is more appropriate for this inventory cycle.
Sample Calculations
The table below shows the step-by-step approach to calculating base year 2022 composting emissions for three counties and pollutants. Emissions are reported in million metric tons of CO2 equivalents (MMTCO2eq) per year.
Pollutant CH4; Alameda County; year 2024 | Pollutant N2O; Solano County; year 2024 | Pollutant CO2_bio; County Contra Costa; year 2024 | ||
Step 1 | CalSWIS Throughput Data (tons/year) | 294,921 | 88,959 | 485,628 |
Step 2 | Derive county-specific emission factor based on waste mixture (lbs pollutant/ton feedstock | 3.22 | 0.0908 | 316 |
Step 3 | Derive county-specific utilization factor based on facility ages | 0.87 | 0.9 | 0.89 |
Step 4 | Global Warming Potential | 34 | 298 | 1 |
Step 5 | Derive Base Year 2022 Emissions (MMTCO2eq) | 294,921 tons/year × 3.22 lbs/ton × 0.87 × 1/2000 ton/lbs × 0.907185 MT/ton × 10-6 MMT/MT = 0.0127 MMTCO2eq | 88,959 tons/year × 0.091 lbs/ton × 0.9 × 1/2000 ton/lbs × 0.907185 MT/ton × 10-6 MMT/MT = 0.001 MMTCO2eq | 485,628 tons/year × 316 lbs/ton × 0.89 × 1/2000 ton/lbs × 0.907185 MT/ton × 10-6 MMT/MT = 0.062 MMTCO2eq |
Assessment of Methodology
The approach for calculating greenhouse gas (GHG) emissions from composting operations in the Bay Area has been revised from the previous base year inventory. In this updated methodology, all emissions are treated as area sources, rather than splitting sources between permitted point sources and unpermitted area sources. This change was made because the Air District’s permit database does not capture the full range of composting activity, particularly from smaller or unpermitted facilitis, whereas the California Solid Waste Information System (CalSWIS) provides more comprehensive facility coverage.
Base Year | Revision | Reference |
2022 |
|
|
2015 |
|
|
Emissions
The table below summarizes greenhouse gas emissions from SFBA’s composting operations for the base year 2022 in metric tons of CO2 equivalents (MTCO2eq).
| ID | Description | CH4 | CO2_bio | N2O | Total |
|---|---|---|---|---|---|
| 1936 | Composting (Area) | 75444.1 | 171365.4 | 28793.9 | 275603.4 |
Summary of Base Year 2022 Emissions
GHG emissions from the Waste Management sector represent a small portion of the Bay Area’s total regional emissions—approximately 4% in 2022. Within this sector, emissions from composting operations make up about 4% of the Waste Management total.
The dominant GHG emitted from composting is CO₂_bio, estimated at 0.171 MMTCO₂eq. However, CO₂₋bio is not considered an anthropogenic GHG because it would be released through natural decomposition even without human intervention. Methane (CH₄), which is a potent GHG, accounts for 0.075 MMTCO₂eq, or roughly 3% of the CH₄ emissions within the Waste Management sector. Composting is also the most significant source of nitrous oxide (N₂O) emissions in this sector, contributing 0.029 MMTCO₂eq—or about 15% of sector-wide N₂O emissions—and approximately 4% of all N₂O emissions in the Bay Area’s regional GHG inventory.
While composting is currently a relatively small source of GHGs in the Bay Area, its contribution may grow as more facilities are developed in response to Senate Bill 1383 (SB 1383, 2016). This statewide mandate requires significant diversion of organic materials from landfills to reduce methane emissions. By shifting organic waste to composting facilities that promote aerobic decomposition, CH₄ generation is minimized and more of the carbon is converted to CO₂, which is approximately 34 times less potent than CH₄ over a 100-year time horizon.
The Air District plays a role in regulating composting operations through its permitting authority and may implement additional controls to help ensure methane emissions are minimized as composting capacity expands.
Contribution of Composting Emissions by Sector| Subsector | Sector | Subsector GHG Emissions (MMTCO2eq) | Sector GHG Emissions (MMTCO2eq) | % of Sector |
|---|---|---|---|---|
| Composting | Waste Management | 0.10 | 2.61 | 4.00% |
Contribution of Composting Emissions to Regional Total
| Subsector | Subsector GHG Emissions (MMTCO2eq) | Regional Total GHG Emissions (MMTCO2eq) | % of Regional Total |
|---|---|---|---|
| Composting | 0.10 | 65.68 | 0.16% |
Trends
The time series chart below shows the emission trends for category 1936.
Summary of Trends
Composting activity in the SFBA has steadily increased over the past several decades, as reflected in the rising number of operating facilities and the associated emissions. This growth is expected to continue in the coming years, particularly as state policies like SB 1383 drive further diversion of organic materials from landfills.
Over the long term, composting has the potential to remain a relatively low source of CH₄ emissions, provided that facilities are well-managed to maintain aerobic conditions. Methane emissions are highly sensitive to composting practices, and improper management can lead to anaerobic conditions that promote CH₄ formation.
CO₂₋bio emissions may increase over time, due to two main factors: (1) population growth leading to greater volumes of organic waste, and (2) improved composting practices that oxidize a greater share of methane to CO₂. However, these trends have not yet been quantitatively assessed in this inventory.
Ultimately, while composting emissions are expected to rise with activity, the overall climate impact can remain low if facilities are effectively designed and operated to minimize methane and nitrous oxide emissions.
Uncertainties
Several factors contribute to the uncertainty in emissions estimates for composting. These uncertainties stem from variability in input data, assumptions made in the methodology, and limitations in available emissions research. The primary sources of uncertainty are outlined below:
- Throughput Data: The throughput values used in this analysis are primarily self-reported by composting facilities through the California Solid Waste Information System (CalSWIS). While these data provide a useful starting point, they are not always independently verified and may not reflect actual material volumes processed. Facilities may over- or under-report throughput due to inconsistencies in measurement practices, regulatory reporting requirements, or estimation errors.
- Emission Factors: The emission factors applied in this inventory are based on published literature, including Nordahl et al. (2023), which compiled data from multiple studies. However, emission factors vary widely depending on feedstock composition, composting methods (e.g., windrows vs. aerated static piles), moisture content, temperature, and site-specific management practices. The inherent variability in these factors means that emission estimates may not fully capture real-world conditions at individual facilities.
- Waste Stream Composition: The composition of waste streams processed at composting facilities changes over time due to shifts in municipal waste management policies, organic waste diversion efforts (e.g., SB 1383), seasonal variations in feedstock availability, and operational decisions at individual facilities. Because emission factors are specific to different feedstock types, changes in waste composition introduce uncertainty into emissions calculations.
- Double counting: An assumption is made that all animal manure accepted for composting goes through the decomposition process at the composting facility and not at origin at a dairy/animal farm, or during transit. There is some likelihood of facilities accepting animal manure for composting to partially double count emissions from anaerobic decomposition activity that may have already begun to occur at the source due to in-place animal manure management processes. It is beyond the scope of this inventory estimation process to accurately understand and split those minor animal manure decomposition emissions across two distinct emissions source categories (composting versus animal manure management).
- Utilization Factors: The methodology applies utilization factors to account for how composting facilities ramp up operations over time or reduce throughput when nearing closure. These estimates are based on internal discussions with Air District permit engineers but may not reflect actual operational patterns.
- Growth Assumptions: Future emissions projections assume constant throughput levels with changes only in capacity utilization rates. This approach does not account for potential expansions of composting infrastructure, regulatory changes, economic factors affecting waste diversion, or shifts in public policy that may accelerate or slow the growth of composting operations.
- Control Measures: The current methodology assumes no emissions controls are in place for composting operations, as no specific Air District regulations currently apply. However, individual facilities may implement best management practices (e.g., biofilters, covered aerated static piles) to reduce emissions. The impact of such practices is not explicitly accounted for, which could lead to overestimation of emissions at facilities employing mitigation measures.
Contact
Author: Sally Newman
Reviewer: Abhinav Guha
Last Update: 07/21/2025
References
BAAQMD. 2024a. Regulation 13 Rule 2: Organic Material Handling Operations, Bay Area Air Quality Management District. https://www.baaqmd.gov/en/rules-and-compliance/rules/regulation-13-rule-2-organic-material-handling-operations
BAAQMD. 2024b. Regulation 13 Rule 3: Composting Operations, Bay Area Air Quality Management District. https://www.baaqmd.gov/en/rules-and-compliance/rules/regulation-13-rule-3-composting-operations
BAAQMD. 2024c. Regulation 13 Rule 4: Sewage Treatment and Anaerobic Digestion, Bay Area Air Quality Management District. https://www.baaqmd.gov/en/rules-and-compliance/rules/regulation-13-rule-4-sewage-treatment-and-anaerobic-digestion
CARB. 2016. California’s 2000-2014 GHG Inventory - Technical Support Document, California Air Resources Board. https://ww2.arb.ca.gov/sites/default/files/classic/cc/inventory/ghg_inventory_tsd_00-14.pdf
CARB. 2017. Method for estimating GHG emission reductions from diversion of organic waste from landfills to compost facilities, California Air Resources Board.
https://ww2.arb.ca.gov/sites/default/files/classic/cc/waste/cerffinal.pdf
CARB. 2022. 2022 Scoping Plan Documents, California Air Resources Board. https://ww2.arb.ca.gov/our-work/programs/ab-32-climate-change-scoping-plan/2022-scoping-plan-documents
CALSWIS. 2020. Facility Database, California Solid Waste Information System. https://www2.calrecycle.ca.gov/SolidWaste/Site/DataExport. Accessed 2020.
CALSWIS. 2024. Facility Database, California Solid Waste Information System. https://www2.calrecycle.ca.gov/SolidWaste/Site/DataExport. Accessed 2024.