6.1 Landfills

    Category ID Description EIC
    1157 Waste Management - Landfills, Fugitive Point Sources Various
    1158 Waste Management - Landfills, Fugitive Area Sources 12012202420000
    1686 Landfills - Flares 13013201300000, 13013201100000
    1687 Landfills - Internal Combustion Engines Various

    Introduction

    This document describes the methodology used to estimate greenhouse gas (GHG) emissions from landfill operations in the San Francisco Bay Area (SFBA). Landfills are engineered facilities designed for the long-term disposal of municipal solid waste. They must comply with stringent standards and requirements for design, operation, monitoring, and closure. Although modern landfills are regulated to minimize environmental impact, their operations remain significant sources of GHG emissions, primarily methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2).

    GHG emissions from landfills arise through a combination of biological and combustion processes. When organic waste (e.g. food scraps, paper, yard trimmings) decomposes anaerobically – without oxygen – within the landfill, it produces landfill gas (LFG), consisting of a mixture of CH4, CO2, and other non-methane organic compounds. If not captured and combusted, CH4 from landfills can escape into the atmosphere. N2O emissions are also produced in less quantities than CH4 through nitrification and denitrification of nitrogen-containing compounds under varying aerobic and anaerobic conditions.

    Each of the different processes that produces landfill gases are organized in the following categories:

    • Category 1157 (Landfill Fugitive Permitted Sources): This category includes fugitive emissions resulting from biodegradation of waste at all active landfills and large, recently closed landfills within the Bay Area Air Quality Management District (Air District). Inactive landfills that are less than 30 years old and contain more than one million tons of waste are included in this category. These landfills are subject to permitted requirements and report their annual throughputs directly to the Air District.
    • Category 1158 (Landfill Fugitive Area Sources): This category covers fugitive emissions from inactive or closed landfills with less than one million tons of waste in place. These landfills are no longer under an active permit by the Air District and are considered area sources due to the diffuse nature of their emissions and limited monitoring requirements.
    • Category 1686 (Landfill Combustion Flares): This category includes emissions from the combustion of collected landfill gases (LFGs) through flaring. Active landfills are required to install landfill gas collection and control systems which rely on a network of vertical and horizontal pipes under vacuum to capture LFG. The collected gases are often flared, a process in which the gases are burned in open or enclosed flames to convert methane to carbon dioxide and water vapor. These systems are permitted by the Air District.
    • Categories 1687 (Landfill Combustion Internal Combustion Engines): Emissions in this category result from the combustion of collected LFGs used to fuel on-site internal combustion engines (ICEs) for electricity generation. The LFGs are collected and transported through the network of pipes and, unlike flaring, combusted on-site in reciprocating engines that convert the thermal energy into mechanical or electrical energy. These systems are regulated and permitted by the Air District. Combustion of LFG at offsite locations are covered in categories 303 (Reciprocating Engines—Gas Fuels) and 305 (Turbines—Gas Fuels) in the Fuel Combustion - Turbines & Engines methodology.

    Methodology

    The following methodology describes how GHG emissions are determined from landfill operations depending on whether the source is permitted such as for categories 1157, 1686, and 1687 or is a diffused non-permitted area source as described for category 1158. Methodologies for both permitted and area source categories are described separately in the subsections below.

    Permitted Sources (Categories 1157, 1686 & 1687)

    Categories 1157, 1686, and 1687 are permitted source categories that are subject to the Air District permitting requirements whereby they are required to report their annual throughput. The Air District relies on either facility provided process information or published sources for general factors like United States Environmental Protection Agency’s (USEPA) AP-42 (USEPA, 2024) to derive pollutant emission rates. The reported throughput, combined with the emission rates and any control factors to reduce the source emissions, is used to estimate the annual source-level emissions. These source level emissions are then aggregated by year, county, and category.

    Emission Rates

    For the landfill permitted sources, a two-step methodology is used to estimate the base years’ emissions as follows: (1) derive base year’s emission rates, and (2) multiply the emission rate by the LFG throughput to produce base year emissions. Key factors for estimating the emission rates are:

    • Volumetric measurements based on the ideal gas law,
    • Molecular weights of each GHG pollutant,
    • Landfill gas composition from California Air Resources Board (CARB) and USEPA sources,
    • Methane oxidation factors to carbon dioxide from IPCC sources,
    • Combustion destruction efficiencies, and
    • Pollutant formation/generation rates.

    Landfill Fugitive Permitted Source (Category 1157)

    Landfill operators report their landfill gas (LFG) volume from gas collection systems as part of the permitting process to address potential flaring and use of ICEs to generate electricity. The reported LFG represents a portion of the total LFG generated from the landfill. The remaining LFG is assumed to be uncollected gas released as fugitive emissions through the landfill surface and categorized under category 1157. The base year emission rate for category 1157 is calculated using equations 1.a. and 1.b. as described below:

    1.a. Uncollected Landfill Gas Ratesource category, pollutant, base year = Collected Landfill Gas Ratesource category, pollutant, base year x % Pollutant gas x (pollutant molecular weight/ideal gas molar volume) x [(1-Collection Efficiency) / Collection Efficiency]

    1.b. Base Year Emissions Ratesource category, pollutant = Uncollected Landfill Gas Ratesource category, pollutant – Destruction Ratepollutant + Generation Ratepollutant

    Where:

    • Uncollected Landfill Gas Rate is the mass rate (per unit volume) of the landfill gas (LFG) that is not collected by the LFG collection system and moves up through the buried waste mass.
    • Collected Landfill Gas Rate is the single unit volume of collected LFG expressed as 1000 (M) standard cubic feet (SCF) or 1 M SCF.
    • % Pollutant gas is the percentage by volume of the pollutant gas (i.e., CH4, CO2, N2O) in the gas stream.
    • Ideal gas molar volume is the standard molar volume occupied by one mole of gas at standard temperature (70 F) and pressure (14.7 PSI) in SCF/lbmol. It is equal to 386.7 SCF/lbmol.
    • Pollutant molecular weight is the molecular weight of a mole of pollutant gas (lbs/lbmol).
    • Collection Efficiency is the proportion between 0 and 1 that represents the percentage volume of LFG generated by microbes that is captured by the LFG collection system. It measures effectively the efficiency of gas control systems, system design, maintenance, and operational practices to capture methane and other gases.
    • Fugitive Emission Rate is a fraction (between 0 and 1) representing the percentage volume of LFG generated by microbes that is not collected by the LFG collection system. It is represented by 1-Collection Efficiency in equation 1.a.
    • Base year refer to the year in which the facility reported their actual throughput to the Air District.
    • Destruction Ratepollutant is the mass rate of oxidation of a pollutant as the uncollected LFG moves through soil cover material. For example, Destruction RateCH4 is the relative efficiency of oxidation of methane by methanotrophs present in the soil cover material.
    • Generation Ratepollutant is the mass-based generation rate of a pollutant as the uncollected LFG moves through soil cover material. For example, Generation RateCO2 represents the production of carbon dioxide due to methanotrophic oxidation in soil cover material. Generation RateCO2 is equal to Destruction RatCH4 x (CO2 molar weight/CH4 molar weight).

    A summary of the relevant assumptions and sources used to develop the GHG emissions for permitted sources under category 1157 are as follows:

    1. Based on internal source test measurements and other literature, LFG, on average, contains by volume 55% CH4, 40% CO2, 0.001% N2O, with the remaining 5% nitrogen, oxygen, non-methane organic compounds, etc. (USEPA, 2020 and IPCC, 2007). These percentages are used to represent the % Pollutant gas.
    2. The LFG collection system captures 75% of the total amount of LFG generated, characterized as Collection Efficiency while the remaining 25% of LFG are fugitives that remain uncollected and is the Fugitive Emission Rate based on USEPA’s recommendations (USEPA, 2024). This 25% of the total LFG represents the Uncollected Landfill Gas Rate in the base year emissions calculations.
    3. In accordance with CARB GHG inventory procedures (CARB, 2016), 10% of the CH4 in fugitive LFG is oxidized to CO2 prior to surface release or collection. Hence, the Destruction RateCH4 is 10% of the Uncollected Landfill Gas Rate.
    4. No CO2 is destroyed in the soil cover and/or converted into CH4, so Destruction RateCO2 and Generation RateCH4 are both assumed to be zero.
    5. Based on averaging of collected landfill gas rate for active dry and wet areas from permitted landfills, a default generation rate of 75 standard cubic feet (SCF) per ton of decomposable waste-in-place is used to calculate CH4, CO2 and N2O emission rates, only for instances where the operation has not reported the collected LFG rate.
    6. LFG production rates are highly variable and largely a function of the biodegradation of the current and past deposited waste. It is generally assumed that maximum biodegradation occurs within 15 years, but the process continues at a reduced rate beyond 35 years. For the purpose of calculating throughput for landfill, a biodegradation time frame of 40 years is used, and the default generation rate is kept steady at 75 SCF per ton .

    Landfill Combustion - Flares & Internal Combustion Engine (Categories 1686-1687)

    Unlike fugitive LFG emissions, which escape directly into the atmosphere, collected LFG is not vented but instead used in combustion processes. Combustion involves mixing collected LFG gas with air and igniting it using a heat source (e.g., pilot light or actuator), which initiates a chemical reaction that converts CH4 into CO2, a less potent GHG. This is the primary way to mitigate methane in flaring systems, where the heat generated during combustion is not utilized but is released as waste energy. If an ICE is utilized as a combustion device, the heat generated from combustion is used to produce mechanical energy for electricity generation.

    The emission byproducts of the collected and combusted LFG are categorized under category 1686 (flaring) and category 1687 (ICE). The base year emission rate for categories 1686 and 1687 are calculated using equations 2.a. and 2.b. Some of the terms in the following equations have already been defined above:

    2.a. Inlet Landfill Gas Ratesource category, pollutant, base year = Collected Landfill Gas Ratesource category, pollutant, base year x % Pollutant gas x (pollutant molecular weight/ideal gas molar volume)

    2.b. Base Year Emissions Ratesource category, pollutant = Inlet Landfill Gas Ratesource category, pollutant – Removal Ratepollutant + Production Ratepollutant

    Where:

    • Inlet Landfill Gas Rate is the mass rate (per unit volume) of the landfill gas (LFG) that is captured by the LFG collection system and is combusted.
    • Base year refer to the year in which the facility reported their actual throughput to the Air District.
    • Removal Ratepollutant is the mass rate of a pollutant in the inlet LFG stream that is combusted and destroyed using control technologies such as flaring systems or energy recovery systems (like engines and turbines). For example, Removal RateCH4 is based on the efficiency of combustion systems in converting methane to CO2
    • Production Ratepollutant is the percentage of a pollutant generated in the collected LFG stream due to control technologies such as combustion in flaring systems or energy recovery systems (like engines and turbines). For example, Production RateCO2 includes generation of carbon dioxide due to combustion of CH4 in flaring and combustion systems. Production Rate CO2 is equal to Removal RateCH4 x (CO2 molar weight/CH4 molar weight).

    Some additional assumptions used to develop the GHG emissions for permitted sources under categories 1686 and 1687 are as follows:

    1. District Regulation 8, Rule 34 (BAAQMD, 2005) mandates that effective July 1, 2002, collected gases from landfills that are processed as flares or in an energy recovery device are required to achieve an organic compound and methane minimal destruction efficiency of 98 and 97 percent, respectively. For purposes of emission calculations, the Air District use a slightly higher removal rate of 99% (a value reflective of most controlled flare systems) for total organic compounds (TOG) including CH4 for flaring operations, while an efficiency of 97%, as specified in the Rule, is used for LFG combustion in ICEs. Thus, the Removal RateCH4 for categories 1686 and 1687 is 0.99 and 0.97 times the Inlet Landfill Gas Rate, respectively.
    2. No CO2 is removed and/or reduced to CH4 during the controlled combustion of LFG, so Removal RateCO2 and Generation RateCH4 are both assumed to be zero.
    3. The origin of CO2 is microbially produced LFG and considered biogenic (CO2_bio) and hence base year emissions rate of CO2 is not included in the GHG inventory totals.

    The base year emission rate is then converted into category-specific emissions using the volume of collected LFG reported by the landfill operator and as shown in equation 3. Equation 4 is then used to aggregate emissions from each landfill within a given county to derive annual county-specific landfill subsector emissions.

    3. Base Year(s) Emissionssource category, pollutant = Base Year(s) Emissions Ratesource category, pollutant x Volume of landfill gas reported × GWPpollutant × Control Factorpollutant

    4. Base Year Emissionscounty = ∑ Ni=1 Emissionsi

    Where:

    • Volume of Landfill Gas is the actual landfill gas collected (in units of M SCF) by operators and reported annually to the Air District.
    • Control Factorpollutant is a fraction (between 0 and 1) that captures the estimated reduction in emissions from Air District rules and regulations.
    • GWPpollutant is the Global Warming Potential (GWP) which is based on 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.
    • N is the number of permitted landfills in the county

    The emissions from these permitted facilities are allocated to each county based on the location of the landfill. San Francisco is the only county where there are no active landfills within its boundaries as all their refuse is shipped to other counties for processing, waste disposal, or recycling.

    Local Controls 

    Upon review of Air District regulations, only one rule applies to solid waste disposal sites:

    • Regulation 8, Rule 34 (BAAQMD, 2005) limits the emissions of non-methane organic compounds (NMOC) and methane from the waste decomposition process at solid waste disposal sites. Landfill operators routinely submit quarterly landfill emissions monitoring reports and annual reports to the Air District that include the design capacity of the landfill and estimated NMOC emission rates. The landfill operators can adjust their operations to comply with the NMOC emission rate standards that will correspondingly impact GHG emissions from ancillary processes, not subject to permit conditions.

    The requirements of Regulation 8, Rule 34 have been fully implemented and achieved by all permitted facilities by 2022 and therefore, the forecasted emissions do not include any additional reductions from regulatory controls.

    Historical and Future Projections 

    The forecast and backcast of emissions are performed by applying historical and future projection profiles based on the earliest and latest base year emissions.

    Designated Year Emissionscounty = Base Year(s) Emissioncounty × Backcast or 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

    Categories 1157, 1686, and 1687 use the methodology outlined above to estimate backcast and forecast emission:

    • Historical Backcast (1990-2006): Linear extrapolation is applied to the 2007 data backfill historical throughput information missing from the Air District permitting database for years 1990 to 2006.
    • Base Years (2007-2022): Calculated annual emissions from Air District permits based on actual throughputs provided by the operators.
    • Growth Factor (2023-2050): Applied CARB’s Scoping Plan (CARB, 2020) growth profile (Appendix H) on non-energy organic waste growth profile to account for per capita waste disposal trend, instituted controls, efficiency improvements and projected population growth (CADOF, 2022). This scenario also assumes a gradual decline in organic waste disposal due to increasing rates of organic waste recycling.

    Permitted Source Sample Calculations

    An example calculation is provided below to estimate methane emissions for base year 2022 from fugitive permitted sources at a single facility (category 1157):

    Step 1

    Collect factors needed to estimate the LFG emission rate:

        1. LFG unit rate
        2. Pollutant gas from USEPA, 2020 and IPCC, 2007
        3. Ideal gas molar volume
        4. Methane gas molecular weight
        5. Collection efficiency from USEPA, 2024
        6. Fugitive emission rate from USEPA, 2024
        7. Destruction rate based on oxidation %
        8. Generation rate in soil cover
    1. 1 M scf or 1000 scf
    2. 55% or 0.55 of LFG is CH4; 40% is CO2
    3. 387 scf/lbs mol
    4. 16 lbs/lbs mol
    5. 75% or 0.75
    6. 25% or 0.25
    7. 10% of Uncollected Landfill Gas Rate
    8. 0

    Step 2

    Estimate the Uncollected LFG rate

    (1 M scf/1000 scf)

    × 0.55 × (16 lbs/lbs mol / 387 scf/lbs mol)

    × (0.25/0.75)

    = 7.60 lbs/ 1000 scf

    Step 3

    Estimate the destruction rate for methane by multiplying the destruction factor of 10% by the Uncollected LFG emission rate from Step 1

    7.60 lbs/ 1000 scf × 0.10

    = 0.76 lbs/ 1000 scf

    Step 4

    Estimate the generation rate for CO2 by multiplying the destruction rate by the molar weight of CO2 divided by the molar weight of methane (for demonstration purposes; not used in methane emissions calculations)

    Generation rate of CH4 in soil cover is 0 lbs / 1000 scf

    CO2 molar weight = 44 lbs/lbs mol

    CH4 molar weight = 16 lbs/lbs mol

    0.76 × 44 ÷ 16

    = 2.08 lbs/ 1000 scf

    Step 5

    Estimate the base year emission rate

    7.60 – 0.76 + 0

    = 6.84 lbs/ 1000 scf

    Step 6

    Reported volume of LFG collected from operator for a single facility

    96,200 M scf or 96,200,000 scf

    Step 7

    Estimate the methane emissions from a single landfill by multiplying the base year emission rate by the volume of LFG collected

    96,200,000 scf

    × 6.84 lbs/ 1000 scf

    = 658,008 lbs of CH4

    Step 8

    Convert emissions to metric tons of CO2 equivalents (MTCO2eq) by applying GWP of 34 for methane

    658,008 lbs

    × 1 ton/2000 lbs

    × 0.907185 tonne/ton

    × 34

    = 10,147.9 MTCO2eq

    Assessment of Methodology for Permitted Sources

    The general methodology for determining emissions from the permitted sources has not changed, since the previous inventory (base year 2015), although most of the data inputs have been updated.

    Base Year

    Revision

    Reference

    2022

    1. Updated pollutant gas components in LFG
    2. Updated collection efficiency of LFG collection system based on USEPA AP-42
    3. Updated destruction and generation factors, and removal and production factors for LFG gas
    4. Updated volume of collected LFG reported to the Air District by operators
    5. Updated the GWP based on IPCC Fifth Assessment Report
    6. Revised backcast using linear extrapolation to fill missing reported throughput data from 1990 to 2006
    7. Updated growth projection using CARB’s Scoping Plan, Appendix H
    1. USEPA, 2020; IPCC, 2007
    2. USEPA, 2024a
    3. CARB, 2016; BAAQMD, 2005
    4. BAAQMD, 2022
    5. IPCC, 2014
    6. Air District calculation
    7. CARB, 2022

    2015

    1. Updated pollutant gas components in LFG
    2. Updated collection efficiency of LFG collection system based on USEPA AP-42
    3. Updated destruction and generation factors, and removal and production factors for LFG gas
    4. Updated volume of collected LFG reported to the Air District by operators
    5. Updated the GWP based on IPCC Fifth Assessment Report
    6. Revised backcast using linear extrapolation to fill missing reported throughput data from 1990 to 2006
    7. Updated growth projection using Bay Area-specific growth profiles derived from LBNL’s CALGAPS model
    1. USEPA, 2020; IPCC, 2007
    2. USEPA, 2024a
    3. CARB, 2016; BAAQMD, 2005
    4. BAAQMD, 2022
    5. IPCC, 2014
    6. Air District calculation
    7. BAAQMD, 2017

    Area Sources (Category 1158)

    Category 1158 is an area source category that covers fugitive emissions from inactive or closed landfills that are no longer under an active permit by the Air District. The methodology used to calculate emissions for the base year(s) is as follows:

    Base Year(s) Emissionscounty,pollutant =

    Activity Data × Emission Factorpollutant × Control Factorpollutant × Fractioncounty × Fractionin District

    × GWPpollutant

    Where:

    • Base Year is a year for which activity/throughput data are available in order to calculate emissions.
    • Activity Data is the total regional throughput or activity data for applicable base year(s).
    • Emission Factorpollutant is a factor that allocates a mass of emissions of a particular pollutant by unit of activity data.
    • Control Factorpollutant is a fraction between 0 and 1 that captures the estimated reduction in emissions based on Air District rules and regulations.
    • Fractioncounty is the fraction between 0 and 1 of total regional emissions allocated to a particular county.
    • GWPpollutant is the Global Warming Potential of a particular GHG pollutant
    • Fractionin District is the portion of Sonoma and Solano counties where emissions are occurring within the Air District’s jurisdiction.

    Historical backcast and forecast relative to the base year emissions are estimated using the same methodology that is applied to the permitted sources above.

    Activity Data

    For landfill fugitive area sources (category 1158), throughput estimates are based on a time-varying statewide per capita waste disposal rate provided by CARB through CalRecycle (CalRecycle, 2021). This rate is combined with population data from California Department of Finance (CADOF, 2022) to estimate the total waste-in-place specific to the Bay Area. These estimates are further refined using linear growth assumptions that differentiate between permitted and non-permitted source contributions, allowing for the development of throughput projections for historical, current and future years. A key assumption in this methodology is that area source throughputs decrease annually by 0.25% relative to point source throughput, eventually reaching 0% by the year 2040. For the base year 2022, this area-to-point source throughput ratio is assumed to be at 6%.

    County Distribution / Fractions

    For landfill fugitive area source (category 1158), the county fractions are based on the number of closed and non-permitted landfills in each county that is provided in the Solid Waste Information System (SWIS) database (CalRecycle, 2021) and confirmed through the Air District’s permit database (BAAQMD, 2022).

    ID Description ALA CC MAR NAP SF SM SNC SOL SON
    1157 Waste Management - Landfills, Fugitive Point Sources 0.21 0.12 0.05 0.02 0.00 0.22 0.24 0.07 0.06
    1158 Waste Management - Landfills, Fugitive Area Sources 0.27 0.10 0.15 0.06 0.04 0.10 0.14 0.05 0.11
    1686 Landfills - Flares 0.15 0.17 0.05 0.02 0.00 0.10 0.43 0.04 0.04
    1687 Landfills - Internal Combustion Engines 0.42 0.00 0.11 0.03 0.00 0.00 0.00 0.31 0.13

    Emission Factors

    There is no source test data available to support development of specific emission factors for Bay Area landfills as they vary significantly depending on the waste type, waste treatment, operational conditions, and moisture content. For closed landfills, CH4 and N2O emission factors are based on a default generation rate of 30 SCF per ton of decomposable waste-in-place, derived from information in USEPA’s AP-42 (BAAQMD, 2022 ; USEPA, 2024a). This default factor applies to all landfills that have been closed for more than 30 years or have been inactive. The LFG composition for area source landfills without gas collection systems is assumed to be 50% CH4 and 50% CO2 (USEPA, 2024b) similar to permitted landfills lacking such systems.

    Control Factors and Emission Controls

    The Air District’s Regulation 8, Rule 34 -Solid Waste Disposal Sites (BAAQMD, 2005) mandates that most landfills must process LFGs through a gas collection and emission control system that meets the following requirements:

    • No component or connector of the LFG collection system may exhibit leaks exceeding 1000 parts per million by volume measured as methane.
    • Collected LFG must be processed in an enclosed ground-level flare with a total organic gas (TOG) destruction efficiency of at least 98% by weight; or
    • Alternatively, LFG may be treated in an energy recovery device or emission control system that achieves a minimum 97% TOG reduction by weight.

    While there are no formal requirements to control GHG emissions from ancillary landfill processes, these operations may be indirectly affected if the facility adjusts its operations to meet regulatory thresholds or emissions standards. Complying with the organic waste thresholds set in the regulation ensures efficient combustion of methane, converting it to less harmful CO2.

    In addition, California's Senate Bill (SB) 1383 - Short-Lived Climate Pollutants mandates statewide efforts to divert organic waste from landfill. SB 1383 sets targets to reduce organic waste disposal by 50% from 2014 level by 2020 and by 75% by 2025. This legislation is expected to significantly reduce the volume of organic waste sent to landfills, leading to lower LFG generation and consequently, a reduction in associated GHG emissions. The mandate combined with existing rules has led to significant decline in per capita waste generation in the Bay Area (see future projections). It should be noted that California has missed on achieving its 2020 goal for landfill organics diversion (LHC, 2023).

    Historical Emissions and Future Projections 

    In previous inventories, 90% of the cumulative waste generation in the Bay Area was attributed to permitted sources, with the remaining 10% attributed to closed landfills. Since the State of California passed a mandate that requires a reduction in waste generation, per capita waste generation in the Air District has decreased. The principal factors affecting historical emissions and future projections from landfills include:

    • Association of Bay Area Government’s (ABAG’s) population growth profile (ABAG, 2019) and population growth estimates of California Department of Finance (CADOF, 2022), which indicates that Bay Area’s population has steadily increased in the past and will in the future,
    • The relative distribution of cumulative waste generation between permitted and closed landfills as less waste is being diverted to non-permitted landfills,
    • Statewide trends predict lower per capita waste generation over time, and,
    • The overall improvement in LFG collection efficiency across Bay Area landfills.

    In contrast to permitted sources, emissions from closed landfills are expected to decline over time, reflecting a reduction in cumulative waste attributed to these sources. As noted above, area sources accounted for 10% of cumulative waste generation in 1999 (BAAQMD, 2015). This share is expected to decline linearly to 2.25% by 2030 and reach 0% by 2040, in line with the anticipated phaseout of landfills without gas collection systems.

    Area Source Sample Calculations

    An example calculation is provided below to estimate CH4 and N2O emissions in Sonoma County for the year 2022 for landfill fugitive area sources (category 1158).

    Step 1

    Obtain the per capita waste disposal rate for California from CalRecycle (2021)

    6.3 lbs/person/day

    Step 2

    Gather the population data for the Bay Area based on CADOF data for year 2022

    7,842,508 people

    Step 3

    Estimate the total waste-in-place for the Bay Area by multiplying the per capita waste disposal rate by the Bay Area population

    6.3 lbs/person/day

    × 7,842,508 people

    × 365 day/year

    ÷ 2000 lbs/ton

    = 9,016,924 tons

    Step 4

    Estimate the fraction of closed and inactive landfills located in Sonoma County based on SWIS database (CalRecycle, 2021)

    10.6% or 0.106

    Step 5

    Estimate the waste throughput for Sonoma County by multiplying the Bay Area waste-in-place rate for 2022 by the fraction of closed and inactive landfills in this county

    9,016,924 tons

    × 0.106

    = 955,794 tons

    CH4

    N2O

    Step 6

    Obtain Emission Factor

    for each pollutant based on default generation rate of 30 SCF per ton of decomposable waste-in-place

    0.5596 lbs/tons

    3.07 × 10-5 lbs/tons

    Step 7

    Calculate the pollutant emissions from fugitive area sources in Sonoma County (metric tons/year)

    0.5596 lbs CH4/ton

    × 955,794 tons

    × 1 ton/2000 lbs

    × 0.9072 tonne/ton

    = 242.61 metric tons

    3.07×10-5 lbs N2O /ton

    × 995,794 tons

    × 1 ton/2000 lbs

    × 0.9072 tonne/ton

    = 0.0133 metric tons

    Step 8

    GWP from IPCC’s Fifth Assessment report (IPCC, 2014).

    34

    298

    Step 9

    Estimate the pollutant emissions by applying the GWP and convert the emissions to metric tons of CO2 equivalents

    242.61

    × 34

    = 8248.74 MTCO2eq/yr

    0.0133

    × 298

    = 3.96 MTCO2eq/yr

    8248.74

    + 3.96

    = 8,253 MTCO2eq/yr

    Assessment of Methodology

    The general methodology for determining emissions for these categories has not changed, although all of the data inputs have been updated.

    Base Year

    Revision

    Reference

    2022

    1. Updated per capita waste disposal activity data based on CalRecycle data
    2. Continued use of population data based on CADOF projections
    3. Updated county fractions based on the number of closed and inactive landfills in each county
    4. Continued to use GWP based on IPCC’s Fifth Assessment Report
    5. Emissions factors were reassessed
    6. Updated growth profile showing a phaseout of closed landfills assuming an annual decline of 0.25% up to year 2030

    1. CalRecycle, 2021

    2. CADOF, 2022

    3. CalRecycle, 2021; BAAQMD, 2022

    4. IPCC, 2014

    5. BAAQMD, 2022

    6. BAAQMD, 2022

    2015

    1. Updated per capita waste disposal activity data from CalRecycle
    2. Updated population data for the Bay Area
    3. Updated county fractions based on the number of closed and inactive landfills in each county
    4. Updated GWP from IPCC’s Fifth Assessment Report
    5. Updated emission factors
    6. Updated growth profile based on ABAG population growth profile
    7. Developed backcast using CalRecycle data
    1. CalRecycle, 2014
    2. ABAG, 2019
    3. CalRecycle, 2014; BAAQMD, 2015
    4. IPCC, 2014
    5. BAAQMD, 2015
    6. ABAG, 2019
    7. BAAQMD, 2015

    Emissions

    The table below shows the total GHG emissions by pollutant in MTCO2eq by landfill category.

    ID Description CFC-11 CFC-113 CFC-12 CH2Cl2 CH4 CO2 CO2_bio HCFC-21 HCFC-22 N2O Total
    1157 Waste Management - Landfills, Fugitive Point Sources 2892.1 0.0 75165.6 80.2 1924400.7 0.0 151084.0 1.2 100.8 884.5 2154609.1
    1686 Landfills - Flares 87.1 1.0 0.0 6.4 28565.6 3.1 446894.9 0.0 0.0 279.7 475837.8
    1687 Landfills - Internal Combustion Engines 34.4 0.0 0.0 1.1 22475.0 360.6 204448.4 0.0 0.0 86.4 227405.9
    1158 Waste Management - Landfills, Fugitive Area Sources 0.0 0.0 0.0 0.0 77818.3 0.0 7674.4 0.0 0.0 37.5 85530.2

    Summary of Base Year 2022 Emissions

    The tables below show the contribution of landfill processes GHG emissions to the overall regional total and to the Waste Management sector. The landfill subsector, in terms of CO2 equivalents, is the primary contributor of emissions to the Waste Management Sector, accounting for over 80% of the emissions and 3% of the regional GHG inventory for the year 2022. In terms of GHG pollutants, CH4 emissions from landfills constitute nearly 50% of the regional CH4 inventory while N2O emissions are statistically insignificant contributors to both the Waste Management sector and total emissions for the Bay Area totals. Unlike other industries and subsectors, where CO2 from combustion processes is typically the dominant GHG, landfills primarily emit methane making it the principal GHG of concern in this subsector. CO2 emissions from this subsector are largely considered biogenic.

    Contribution of Landfills Emissions by Sector
    Subsector Sector Subsector GHG Emissions (MMTCO2eq) Sector GHG Emissions (MMTCO2eq) % of Sector
    Landfills Waste Management 2.13 2.61 81.83%

    Contribution of Landfills Emissions to Regional Total
    Subsector Subsector GHG Emissions (MMTCO2eq) Regional Total GHG Emissions (MMTCO2eq) % of Regional Total
    Landfills 2.13 65.68 3.25%

    Trends

    The time series chart below shows the emission trends for landfill categories.

    Summary of Trends

    The emissions of GHG from Categories 1157 (fugitive permitted sources) and 1158 (fugitive area sources) have remained relatively stable over the past decade, largely due to the balance between decreasing per capita waste generation and a growing regional population. A notable deviation is during the COVID-19 pandemic, when the Bay Area experienced a slight population decline. Looking ahead, emissions from these categories are projected to rise slightly in line with anticipated population growth in the Bay Area.

    In contrast, emissions from the categories 1686 and 1687 peaked in the late 2000s and early 2010s. Since then, improved flare management and increased efficiency of LFG combustion has resulted in a decline in emissions over the past decade.

    Uncertainties

    The largest source of uncertainties in the landfill methodology arises from the representativeness and accuracy of the assumed methane emission factors (developed from the default LFG generation rate). These uncertainties arise from assumptions made for general pollutant composition in LFG, collection efficiency, destruction rate, generation rate, combustion efficiency, and those for waste generated, growth projection, etc. The LFG generation data in existing literature is based on very limited number of field studies conducted in a suite of widely varying conditions. Measurement studies in the region and statewide have found higher emissions of methane occurring from landfill operations (e.g., Guha et al., 2020) which may indicate that the collection efficiencies are much lower than what is used in this methodology report. Future inventory updates will need to determine the validity of the emission factors used to estimate emissions.

    Contact

    Author: Tan Dinh

    Reviewer: Abhinav Guha

    Last Update: 06/05/2025

    References

    ABAG. 2019. Plan Bay Area 2040, Association of Bay Area Government. http://2040.planbayarea.org/reports↩︎

    BAAQMD. 2005. Regulation 8 Rule 34: Solid Waste Disposal Sites, Bay Area Air Quality Management District. https://www.baaqmd.gov/en/rules-and-compliance/rules/reg-8-rule-34-solid-waste-disposal-sites?rule_version=Adopted

    BAAQMD. 2015. Permit and Compliance Database, Bay Area Air Quality Management District. Accessed Year 2017.

    BAAQMD, 2017. Greenhouse Gas Emissions Estimates and Draft Forecasts. https://www.baaqmd.gov/~/media/files/planning-and-research/plans/2017-clean-air-plan/ghg_emissions_and_forecasts_draft.pdf

    BAAQMD. 2022. Permit and Compliance Database, Bay Area Air Quality Management. Accessed Year 2024.

    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/

    CalRecycle. 2014. 2014 Disposal-Facility-Based Characterization of Solid Waste in California, California Department of Resources Recycling and Recovery. https://www2.calrecycle.ca.gov/WasteCharacterization/Study

    CalRecycle. 2021. 2021 Disposal-Facility-Based Characterization of Solid Waste in California, California Department of Resources Recycling and Recovery. https://www2.calrecycle.ca.gov/WasteCharacterization/Study

    CARB. 2016. California’s 2000-2014 Greenhouse Gas Emission Inventory, 2016 Edition – Technical Support Document, California Air Resources Board. https://ww2.arb.ca.gov/sites/default/files/classic/cc/inventory/pubs/reports/2000_2014/ghg_inventory_00-14_technical_support_document.pdf

    Board. https://ww3.arb.ca.gov/cc/inventory/pubs/reports/2000_2014/ghg_inventory_00-14_technical_support_document.pdf↩︎

    CARB. 2020. Category: Managed waste Disposal Sites, Sector: Industrial: Landfills, California Air Resources Board. https://ww3.arb.ca.gov/cc/inventory/pubs/reports/2000_2014/ghg_inventory_00-14_technical_support_document.pdf↩︎

    CARB. 2022. 2022 Scoping Plan for Achieving Carbon Neutrality, California Air Resources Board. Appendix H: AB 32 GHG Inventory Sector Modeling

    Guha et al. 2020. Assessment of Regional Methane Emission Inventories through Airborne Quantification in the San Francisco Bay Area. Environmental Science & Technology. https://pubs.acs.org/doi/abs/10.1021/acs.est.0c01212↩︎

    IPCC. 2007. AR4 Synthesis Report, Intergovernmental Panel for Climate Change. https://www.ipcc.ch/report/ar4/syr/↩︎

    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.)]. Intergovernmental Panel for Climate Change, Geneva, Switzerland, 151 pp. Available here: https://www.ipcc.ch/site/assets/uploads/2018/02/SYR_AR5_FINAL_full.pdf

    LHC. 2023. Little Hoover Commission, Report # 274. https://lhc.ca.gov/report/reducing-californias-landfill-methane-emissions-sb-1383-implementation/

    USEPA. 2024a. AP-42: Compilation of Air Emissions Factors from Stationary Sources, United States Environmental Protection Agency. https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors-stationary-sources.

    USEPA. 2024b. Basic Information About Landfill Gas, United States Environmental Protection Agency. Updated September 2024. https://www.epa.gov/lmop/basic-information-about-landfill-gas