5.5 Composting

Category 1936

5.5.1 Introduction

Category 1936 is an area source that accounts for organic emissions from the composting sector.

Composting is a biological process where organic materials, such as leaves and grasses, are decomposed by microorganisms to create a nutrient-rich, soil-like material. Composting most often takes place in aerobic conditions, however, under certain conditions, composting can occur under anaerobic conditions. These conditions include compost pile structure and density, particle size, oxygen supply (aeration), moisture content. Methane (CH₄) emissions increase under anaerobic conditions. Therefore, proper monitoring and management are very important to maintain aerobic conditions for efficient composting operation.

The composting process includes the following steps:

• Feedstock Preparation
• Composting
• Curing
• Screening
• Storage

In the feedstock preparation step, the composted organic materials are screened and, if required, processed in a grinder to increase surface area exposed to achieve the best composting conditions. This mixture is then placed in rows or piles where the decomposition process begins. The initial phase of composting, lasting for about one month is very “active” and generates temperatures high enough to kill weed, seeds and pathogens. After this active phase, the material is cured at a slower rate and temperature. During this phase, the moisture content is reduced and a more stable product is produced. Once the compost is cured, the material is screened to produce the desired product and then stored prior to final distribution and use.

In 2011, there were approximately 32 composting facilities in the San Francisco Bay Area (SFBA). Twelve of these facilities were permitted by the District and considered point sources (formerly, Category 1709). Reactive Organic Gases (ROG) emissions were calculated by using emission factors (generalized or specific) and control factors, if any, for a particular source operation supplied by the company which was recorded in Air District’s permit database. Some related composting activity (i.e. windrow and storage activities) were also permitted and particulate emissions were calculated. In the early 2010s, reporting requirement of the Air District as well as compliance by the facilities was not comprehensive.

The other approximate 20 facilities were considered area sources (Category 1936) where annual throughput, criteria pollutant emission factors (ROG and Total Organic Gases or TOG), and control factors (if any) were estimated to calculate emissions. The TOG and ROG emissions were calculated by multiplying the throughput by the emission factor and, if applicable, the control factor. Similarly, Greenhouse Gas (GHG) emissions, namely methane (CH₄) and nitrous oxide (N₂O), are also associated with composting activity. The CH₄ and N₂O emissions were calculated similarly to the criteria pollutant emissions mentioned above.

For the this base year inventory, the Air District’s permit database has been determined to be insufficient to account for all permitted / un-permitted composting facilities in the SFBA. Hence a custom approach has been developed here to account for ROG, TOG and Particulate Matter (PM) emissions from SFBA composting facilities. This custom approach includes information obtained from both District’s permit database as well as state’s solid waste industry database¹⁴⁵. The point source Category 1709 is no longer used to estimate emissions for this sector. All ROG/TOG and PM emissions for this sector are accounted for within Category 1936 as an area source.

5.5.2 Methodology

Emissions are now calculated using a custom approach by designating this category as a ‘Special Case’ and developing its activity/throughput and emission factor archive internally. The throughput and emissions factors (TOG, ROG, and GHG) estimates were developed based on the following approach:

(a) Activity Data / Throughput

For all composting facilities permitted by the District (point sources) as well as un-permitted (area sources), whether it’s in operation or not in base year 2015, maximum allowable throughput data is obtained from CalRecycle’s Solid Waste Information System (SWIS) database¹. The actual throughput is then estimated to be 60% of the SWIS database value for year 2015-16, 70% for year 2017-18, and 80% for year 2019 and later.

The units for throughput information should be in tons per year, however, they may appear in other units, such as cubic yards per year (yd³/yr), cubic yards per day (yd³/day), tons per day etc. For those throughputs that need conversion to tons, the following bulk density values¹⁴⁶ are used:

• Compost (greenwaste, food waste, etc.) - 2.24 yd³/ton
• Mulch (wood waste, agricultural waste, etc.) – 3.54 yd³/ton
• Mixed waste - 2.89 yd³/ton (derived from averaging compost and mulch densities)

To convert daily throughput to annual, it’s assumed that most facilities operate 260 days per year. However, for some facilities, this value may vary slightly.

(b) County Distribution / Fractions

The county distribution is derived based on the locations of composting facilities as found in the District permit database or SWIS database mentioned above.

(c) Emission Factors

Composite emission factors (EFs) are developed for ROG emissions from composting operations using information from California Air Resources Board (CARB) methodology¹⁴⁷. Alternate methodologies from San Joaquin Valley’s area source methods¹⁴⁸ are also used to develop EFs but are not considered to be representative of the SFBA. The EFs used in this chapter are as listed below:

• ROG (greenwaste, foodwaste, pomace) – 4.34 lb/wet ton
• ROG (biosolids, manure, poultry litter) – 2.54 lb/wet ton

These EFs include emissions from stock piles as well as windrow piles. They replace older ROG and TOG EFs developed from a 2007 California Integrated Waste Management Board study¹⁴⁹.

A methane (CH₄) emission factor of 3.92 lb/ton is generated¹⁵⁰. In the previous inventory methodology, CH₄ emission factors were assumed to be the difference between the TOG and ROG emission factors and were quite small in comparison (0.13 lb/ton for greenwaste and 0.29 lb/ton for mixed waste).

The Nitrous Oxide (N₂O) emission factors varies from 0.12 to 1.1997 lb/ton and is obtained from the IPCC Report on GHG Inventories¹⁵¹. The N₂O emission factors used are listed below:

• N₂O (greenwaste) – 0.12 lb/ton
• N₂O (greenwaste, including food) – 0.6599 lb/ton
• N₂O (mixed greenwaste, manure, etc.) – 1.1997 lb/ton

PM EFs are developed for SFBA composting facilities based on AP-42 emission factors. For Chip and Grind facilities, PM EFs are developed from log debarking of wood grinding¹⁵². For PM EFs of composting operations, AP-42 emission factors of stone crushing are used¹⁵³. The appropriate drop points (number of times material is picked and dropped at location reputing in emissions) are accounted for to estimate the following effective PM factor:

• PM₁₀ (uncontrolled; composting) - 0.01 lb/ton
• PM₁₀ (controlled; composting) - 0.003 lb/ton
• PM₁₀ (uncontrolled; chip and grind) - 0.024 lb/ton
• PM₁₀ (controlled; chip and grind) - 0.012 lb/ton

(d) Control Factors

Currently, there is no District rule regulating emissions from compost facilities. However, several facilities in the Bay Area have in-vessel composting which is assumed to have an overall VOC control efficiency of 90%. Any in-built controls are reflected in the EFs used in this methodology.

(e) Speciation

The PM_2.5/PM and the PM₁₀/PM ratios for this category are consistent with size fractions of speciation profiles made available by CARB on their emissions inventory web-page¹⁵⁴. For this category, CARB profile number 421 assigned to landfill dust is applied. PM_2.5 constitutes 7% of the total PM for this profile, while PM₁₀ constitutes 49% of the total PM.

The ROG / TOG ratio is not based on a specific profile but developed internally from the EFs in the previous section using the following formula:

ROG / TOG ratio = ROG / (CH4 + ROG)

For this category, ROG ranges from 39% to 53% of TOG, depending on the composting materials.

5.5.3 Changes in Methodology

Major changes have been applied to the methodology to estimate emissions from composting operations. These include:

• The District’s permit database is not directly used to estimate activity data or emission factors for emissions calculations due to data incompleteness.

• Facility-level activity data is now obtained from CalReycle¹ and a time-varying proportion of maximum allowable throughput is used to estimate yearly activity.

• All emission factors have been completely overhauled and re-developed from recent and relevant data sources.

• Previously, the growth of this sector was based on Association of Bay Area Government’s (ABAG’s) 2009 population growth profile. In this inventory iteration, growth is based on trend in the overall statewide feedstock mass being composted (see section on ‘Trends’ below).

5.5.4 Emissions

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

The TOG, ROG, CH₄, and N₂O annual emissions for the composting special case (Category 1936) were calculated by multiplying the estimated throughput by the respective pollutant emission factor. While PM emissions are minor to negligible in magnitude compared to other SFBA sources, composting operations are a considerable source of TOG emissions at ~700 tons/year.

5.5.5 Trends

(a) Historical Emissions / History

Historical emissions for Category 1936 are developed in two steps -

• The statewide composting throughputs are determined from the CARB inventory tool¹⁵⁵ between years 2000-2017 and used to create a growth profile. Emissions from year 2000-2017 are calculated based on this growth profile with the assumption that the growth of composting sector in SFBA is the same as the overall trend in the state.

• The throughput data from year 2000-03 is used to backcast throughput and extend the growth profile back to year 1990. The choice of a shorter time period to extrapolate the trend back in time (before year 2000) is assumed to be a better fit.

(b) Future Projections / Growth

The statewide composting feedstock throughput data for the most recent years (2013-17) is used to extrapolate and develop a forecast growth profile up to year 2040. Emissions are projected to double in the year 2040 as compared to year 2015 if no controls and regulations are applied.

5.5.6 Uncertainties

The largest source of uncertainties in this revised methodology, in spite of a significant update to the development of pollutant EFs, is the relative representation of the EFs. The composting EFs in existing literature are 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 composting operations (primarily due to existing anaerobic conditions) in the SFBA and indicated there could be an order of magnitude underestimation of CH₄ emissions using bottom-up inventory approaches¹⁵⁶.

5.5.7 Contact

Author: Abhinav Guha

Reviewers: Tan M. Dinh and Yuan Du

Last Update: November 06, 2023

5.5.8 References & Footnotes