5.2 Landfills

Categories 1157, 1158, 1686, and 1687

5.2.1 Introduction

Categories 1157, 1158, 1686, and 1687 account for process and combustion emissions at landfills in the San Francisco Bay Area (SFBA). Categories 1157 and 1158 account for surface release of organic and greenhouse gas (GHG) emissions including biogenic carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from point and area source landfills, respectively. Category 1157 is a point source category where the emissions are calculated from information reported by facilities as part of the District’s permitting process. Category 1157 also accounts for particulate matter (PM) emissions from activities such as vehicle traffic on unpaved roads, bulldozing, and scraping.

Category 1158 is considered an area source category and includes emissions from inactive and closed landfills that have less than one million ton of refuse in place. These landfills typically have closed many years ago and current data for these facilities is very limited. In 1999, it was estimated that area source landfills comprised 10% of the total waste in place in SFBA. In 2008, it was estimated area source landfills comprised 7.75% of the total waste in place in the region. In future years, this value is expected to decrease linearly because all new waste ends up in permitted/point source landfills, and the share of waste-in-place at closed landfills is decreasing.

Categories 1686 and 1687 account for point source criteria pollutant emissions including those of PM, total organic gases (TOG), nitrogen oxides (NOx), sulfur oxides (SOx), and carbon monoxide (CO), as well as GHG emissions of CO2, biogenic CO2, CH4, and N2O from the combustion of collected landfill gases (LFG) at/in flares and internal combustion engines (ICE), respectively. LFG may also be combusted in ICEs outside landfills; however, they are covered in categories 303 (Reciprocating Engines—Gas Fuels) and 305 (Turbines—Gas Fuels). The following table summarizes the category designation of landfill-related categories.

Category # Description Classification
1157 Landfills - Fugitive Point Sources Point
1158 Landfills - Fugitive Area Sources Area
1686 Landfill Combustion - Flares Point
1687 Landfill Combustion - IC Engines Point

LFG production rates and compositions vary greatly, even within a single landfill. There are no comprehensive data leading to the development of specific emission factors for the SFBA. Biodegradation takes place over an extended period of time, therefore, waste generated in prior years must be considered for emissions calculations. It is assumed it takes 15 years for maximum biodegradation to take place. However, biodegradation occurs, to a lesser extent, 35 years and beyond. Prior to the mid-1960’s, it was assumed that 60% of the waste was buried in landfills and 40% of the waste was burned on-site. After that it is assumed the waste that was not burned also got diverted to the landfills.

5.2.2 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.

A comprehensive methodology has been developed within the Air District’s Engineering Division to estimate pollutant emissions from point / permitted landfill sources. The main effort principally involves the development of representative pollutant emission factors. The documentation in this Chapter largely reflects the salient features of that internal methodology protocol125.

It is assumed all active landfills within the District are permitted sources. Inactive landfills that are less than 30 years old and have greater than one million tons of refuse in place are also required to be permitted. These permitted landfills are considered to be point sources; their throughput and emission information has been inventoried in the District’s database since the early 1980s. All active point source landfills have collection systems associated with them, whereas older closed landfills may not have a collection system. The District’s internal permit system contains information on the county location of each landfill; hence, emissions are distributed to the counties accordingly. It is assumed that San Francisco county has had no active landfills within their boundaries; therefore, their refuse is allocated to other counties.

Landfills with Collection Systems

For those landfills with active collection systems, it is assumed that 25% of the emissions escape as fugitive LFG, while 75% of the LFG are captured and are either destroyed by flaring or burned in an ICE or turbine. The captured LFG that are flared or burned in an ICE or turbine are accounted for and reported to the California Air Resources Board (CARB) under our combustion source codes and summarized in Categories 1686 and 1687 respectively. The following are a list of source codes associated with the criteria pollutant and GHG emissions from landfills with collection systems:

G7145511. The throughput associated with this source code represents the amount of LFG not collected for a particular year, and that which is lost to the atmosphere. The District uses the throughput and emission factor data to calculate the fugitive methane and GHG emissions from this source based on the following assumptions:

  • The LFG collection system captures 75% of the total amount of LFG generated at site according to US Environmental Protection Agency’s methodology recommendations126; the remaining 25% of LFG is assumed to be fugitive emissions.

  • Based on internal source test measurements and other literature2,3, LFG, on average, contains 55% CH4, 40% CO2, 0.001% N2O, with the remaining 5% nitrogen, oxygen, non-methane organic compounds, etc.

  • In accordance with the Intergovernmental Panel on Climate Change (IPCC) and EPA GHG inventory procedures127, 10% of the CH4 in fugitive LFG is oxidized into CO2 prior to surface release or collection.

  • Based on averaging of emission factors for active dry and wet areas within a landfill, a default generation rate of 75 scf (standard cubic feet) per ton of decomposable waste-in-place is computed and used to calculate CH4 and N2O emission factors. The N2O emission factor is also provided by CARB in their Documentation of California’s Greenhouse Gas Inventory128.

  • The CO2 emissions from this source are considered to be biogenic and not reported in anthropogenic inventories.

C8540511 Combustion Sources Using LFG. This is the principal combustion source code although there are others with minor contributions. The assumed 75% LFG captured are combusted in these sources (source codes starting with “C”) by flaring or used as fuel in turbines or IC engines. A 99% destruction efficiency of TOG is assumed for flaring operations while 97% is assumed when LFG is burned in an IC engine. A slightly higher TOG destruction efficiency, 99.5% or more, is achieved in turbines. Particulates, NOx, SOx, and CO emissions are considered as products of combustion. GHG (CO2, CH4, and N2O) emissions are also calculated from these combustion sources. The CH4 and CO2 emission factors were derived from both EPA AP-42129 and District sources. The CO2 emissions emitted from these combustion sources are considered to be biogenic.

G7145580. This source code represents the fugitive reactive organic gas (ROG) emissions with throughput given as the amount of refuse in tons-in-place. There are no methane or GHG emissions associated with this source code.

G7145540. This source code represents fugitive ROG emissions from the on-site handling of contaminated soils. This soil is usually contaminated by gasoline or diesel, and the units associated with this material code are in tons/yr. There are no methane or GHG emissions associated with this source.

G7145466. This source code represents PM10 emissions from landfill activity that includes vehicle traffic on unpaved roads, bulldozing, scraping and dumping activities, wind erosion from land cover. Throughput is in tons of incoming waste for a particular year.

Landfills without Collection Systems

For those few landfills without collection systems, the following is the source code associated with this type of landfill:

G7159580. The throughput from this source code represents the total tons-in-place of refuse. The District uses the throughput and emission factor data to calculate the fugitive organic and GHG emissions from this type of landfill based on the following assumptions:

  • The LFG generation rate is dependent upon the age of landfill, moisture content, type of refuse, etc. and will vary from landfill to landfill.

  • The LFG composition for these types of landfills is approximately 50% CH4 and 50% CO2. There is additionally a small amount of ROG, but for calculation purposes the equal percentage split is assumed.

  • In accordance with the IPCC and EPA GHG inventory procedures3, 10% of the CH4 in fugitive LFG is oxidized into CO2. The N2O emission factor was provided by CARB in their Documentation of California’s Greenhouse Gas Inventory4.

  • The CO2 emissions from this source are considered to be biogenic.

Area Sources

Category 1158 is considered an area source category since it covers 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. Category 1158 accounts for emissions from surfaces of closed and inactive landfills. The salient features of the inventory methodology include the following key elements:

(a) Activity Data / Throughput

The base throughput is time-varying statewide per capita waste disposal data obtained form CARB through CalRecycle130. This data is combined with population data131 to generate SFBA-specific refuse estimates (tons of waste-in-place). These estimates are then combined with linear growth assumptions of point versus area source attribution to generate throughputs for Category 1158. The assumption is 0.25% decrease in area source throughput relative to point source throughputs each year, reducing to 0% in year 2040. The ratio is 6% for base year 2015. Estimated daily emissions are assumed to be uniform for all months of the year. This is due to burial/insulation of the refuse that reduces impact of ambient temperature changes, allowing somewhat uniform biodegradation throughout the year.

(b) County Distribution / Fractions

The county fractions are based on the number of closed and unpermitted landfills (not accounted for by the District) in each county. This data was obtained from the Solid Waste Information System (SWIS) database132 and also confirmed with the District’s internal permit database.

(c) Emission Factors

For closed landfills, a generation rate of 30 scf (standard cubic feet) per ton of decomposable waste-in-place is used to calculate CH4 and N2O emission factors. This factor is the default for landfills that have been closed for more than 30 years but is applied to all inactive facilities in SFBA. The LFG composition for area source landfills is similar to point source landfills without collection systems, namely 50% CH4 and 50% CO2. In accordance with the IPCC and EPA GHG inventory procedures3, 10% of the CH4 in fugitive LFG is oxidized into CO2. The N2O emission factor was provided by CARB in their Documentation of California’s Greenhouse Gas Inventory4. The CO2 emissions from this source are considered to be biogenic. Closed landfills do not have any vehicle activity and hence no PM emissions at these facilities.

(d) Control Factors

There are no regulatory controls or emissions abatement devices that are assumed to be in place for closed landfills. Prior to 1984 when District’s Rule 8-34133 was passed, it was assumed all LFG was emitted as fugitive emissions. By 1996, the overall collection efficiency at landfills was assumed to be 75%, with 25% of the LFG emitted as fugitive emissions. During that intermediate period, the collection efficiency is interpolated (reflected in growth curve).

Currently, District’s Rule 8-34 requires that most landfills process LFG through a gas collection and emission control system such that:

  1. There are no leaks that exceed 1000 PPM (part per million) by volume measured as methane at any component or connector of the LFG collection system; and

  2. The collected gases are process in an enclosed ground type flare with a TOG destruction efficiency of at least 98% by weight; or

  3. The collected gases are processed in an energy recovery device or emission control system that reduces the amount of TOG by at least 97% by weight.

(e) Speciation

In addition to CH4 and CO2, there is a very small amount of ROG in LFG, and for calculation purposes the percentage is derived from internal speciation ratios used for point source permitting. The ROG factor for area source landfills is held at 0.0105 (~1%) of total TOG (sum of CH4 and ROG).

5.2.3 Changes in Methodology

This Chapter has seen minor changes to the methodology for Category 1158 since the last version of emissions inventory (with a base year of 2011). The modified methodology is based on a top-down technique that combines a statewide per capita waste generated metric with SFBA population and a time-varying point to area source ratio to estimate the total throughput for the region. Emissions factors for TOG now include CH4 emissions which may have been missing in the historical data.

5.2.4 Emissions

A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

Category 1157 is the largest source of TOG emissions in the SFBA accounting for ~60,000 to 65,000 tons of emissions, mostly as CH4 each year. Category 1158 is another major source contributing an additional 5,000 tons or so of TOG emissions each year. Category 1157 is also a significant contributor to PM inventory accounting for 2,000 to 3,500 tons of PM10 emissions each year in the 2010s. The combustion categories 1686 and 1687 contribute 1,000-2,000 and 300-500 tons/year of TOG emissions (mostly as CH4) through the 2010s.

5.2.6 Uncertainties

The largest source of uncertainties in the landfill methodology arises from the completeness and accuracy of the assumed methane emission factors (developed from the default LFG generation rate). The LFG generation data in existing literature is based on very limited number of field studies conducted in a suite of widely varying conditions. Recent measurement studies have found significant emissions of methane occurring from landfill operations in the SFBA which indicate that landfill CH4 emissions are being underestimated by about two times135.

5.2.7 Contact

Author: Abhinav Guha

Reviewers: Tan M. Dinh and Yuan Du

Last Update: November 06, 2023

5.2.8 References & Footnotes

  1. BAAQMD. 2008. General Emission Factors for Determining Fugitive and Combustion Greenhouse Gas Emissions from Landfills. available on request.↩︎

  2. USEPA. 2020. US Environmental Protection Agency, AP-42, Chapter 2.4. https://www3.epa.gov/ttnchie1/ap42/ch02/final/c02s04.pdf↩︎

  3. IPCC. 2007. AR4 Synthesis Report. https://www.ipcc.ch/report/ar4/syr/↩︎

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

  5. USEPA. 2008. AP-42. Chapter 2.4 Municipal Solid Waste Landills. https://www3.epa.gov/ttn/chief/ap42/ch02/draft/d02s04.pdf↩︎

  6. CalRecycle. 2014. 2014 Disposal-Facility-Based Characterization of Solid Waste in California. https://www2.calrecycle.ca.gov/WasteCharacterization/Study↩︎

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

  8. CalRecycle. 2021. SWIS Facility Search database. https://www2.calrecycle.ca.gov/SolidWaste/Site/Search↩︎

  9. BAAQMD. 2022. Rule Development, Rule 8-34. https://www.baaqmd.gov/rules-and-compliance/rules/reg-8-rule-34-solid-waste-disposal-sites?rule_version=Under%20Development↩︎

  10. AB939. 1989. California Assembly Bill 939. https://calrecycle.ca.gov/lgcentral/enforcement/↩︎

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