California Department of Resources Recycling and Recovery (CalRecycle)

Organic Materials Management

Air Emissions Reduction from Composting and Related Facilities


In California, local air pollution districts set operational rules and limitations for businesses that emit significant amounts of "criteria pollutants." In many rural areas, district boundaries follow county lines. In other areas, districts are multi-county entities that address air problems regionally. Local air pollution districts are led by their own executive boards, typically comprised of county supervisors and city council members. They are supervised by the U.S. Environmental Protection Agency. Under the federal Clean Air Act, local air quality districts must produce and implement plans for cleaning up any pollutant that exceeds federal standards.

Local air districts are not able to enact rules that restrict "mobile sources" including cars, trucks, locomotives, and other vehicles. Only "stationary sources" of air pollution fall under their control. Mobile sources are regulated by the California Air Resources Board.

Stationary sources include industrial sources. As obvious sources of pollution--like power plants or refineries--reduce their emissions, local air pollution districts increasingly look toward less obvious sites--such as organic recycling operations--as potential places to reduce emissions of either criteria pollutants or their precursor chemicals, which are also subject to regulation. Actively composting piles of organic feedstocks emit volatile organic compounds (VOC), which can react in the atmosphere with oxides of nitrogen (NOx) to make ground-level ozone, a criteria pollutant. VOCs can also react with ammonia (NH3) to create fine particulates (alternatively referred to as particulate matter (PM), another criteria pollutant). VOCs are a class of more than 1,000 chemicals with greatly varying degrees of reactivity and toxicity. Some VOCs may be considered desirable, such as the ones which give off the scent of lemons or pine, or an expensive perfume. Others, like cadaverine, are extremely offensive. Common VOCs that are actually quite dangerous include benzene and formaldehyde.

VOC emissions from composting piles are dominated by small, light alcohols such as wood alcohol, isopropyl, and ethanol. None of these compounds are strong ozone-forming agents when part of a diverse atmosphere. Composting emissions do contain small amounts of stronger ozone-forming agents, such as terpenes and aldehydes. The overall ozone formation potential of composting emissions is quite low, about one-third as potent as a typical urban VOC mix.

VOCs may be man-made or naturally occurring. In the San Joaquin Valley, it is estimated that 1,000 tons of naturally occurring VOCs are created daily, compared to some 300 tons per day of man-made VOCs. The Blue Ridge Mountains of the eastern United States, and the Blue Mountains of New South Wales, Australia are named after the naturally occurring VOCs that create a blue haze in those regions. Research indicates that at least one plant, kudzu, is capable of emitting both NOxand VOCs and may significantly increase ozone in areas where it grows. Isoprene (C5H8) is the most common naturally occurring VOC and is highly reactive.

A more recent concern in compost emissions are greenhouse gases, which are implicated in global climate change. Previous assumptions that a well-managed aerobic composting process would not produce greenhouse gases, particularly methane (CH4), are being challenged. Methane has a global warming potential 25 times greater than CO2 over 100 years. In addition to methane, compost piles are now suspected of emitting nitrous oxide (N2O), which has a global warming potential 310 times higher than CO2. The Department of Resources Recycling and Recovery (CalRecycle) funded research to explore this issue is now under way; however, results are not expected until 2014.

The amount of VOCs or greenhouse gas (GHG) emissions released during composting appears to be highly variable, and is influenced by feedstocks, management practices, and even climate. The relationships between the types of gases being emitted at any one time are complicated and remain poorly understood.

New Research

Environmental Impacts of Direct Land Application of Uncomposted Green and Woody Wastes on Air and Water Quality

 This report presents the results of a year-long field and laboratory study of greenhouse gas and VOC emissions from chipped and ground uncomposted organic materials applied directly to agricultural lands.  UC Davis researchers also assessed migration of chemical constituents into the soil and soil water, and characterized the composition of these materials from seven facilities throughout the state.

Rules Listed by Air District

South Coast Air Quality Management District (AQMD)

The South Coast AQMD is comprised of all of Orange County, Los Angeles County except for the high desert, the urbanized southwest corner of San Bernardino County, and Riverside County except for the area near the Nevada border. Formed by legislative action in 1976, the district has executed one of the most aggressive anti-smog campaigns in history. Because of these actions, the air in the Los Angeles basin, particularly inland valleys directly to the east of downtown has improved dramatically. Despite these efforts, the estimated 16 million people living within the AQMD still breathe air that ranks at or near the very worst in the annual U.S. air quality rankings.

San Joaquin Valley Unified Air Pollution Control District

The district, known as Valley Air, is comprised of eight counties: San Joaquin, Stanislaus, Merced, Madera, Fresno, Kings, Tulare, and the western half of Kern. This is California's agricultural heartland, and air pollution resulting from agricultural operations has proven both difficult to quantify and difficult to regulate. The district is spanned by two major north-south highways with heavy diesel truck traffic that the district cannot regulate. Years of effort have improved air quality, but not as dramatically as the in Los Angeles basin. Theoretically, the district could shut down every stationary source within its boundaries and still not attain U.S. ozone standards. All of the district's major metropolitan areas, and several of its smaller ones, regularly rank among the U.S. cities with the worst air.

The district adopted its bio-solids co-composting rule in 2007 and its greenwaste composting rule in 2011.

Mojave Desert and Antelope Valley Air Districts

Rule 1133. Together, the Mojave Desert Air Quality Management District and the Antelope Valley Air Quality Management District cover most of the California’s High Desert. The districts have separate boards, but share a border and staff. Their rule books are nearly identical. Their rules include raw material holding-time limitations identical to South Coast Air District Rule 1133.1 and best management practices for biosolids co-composters very similar to those found in San Joaquin Valley Unified APCD Rule 4565. They also contain a contingency clause requiring biosolids co-composters to install a system to capture 80 percent of all ammonia and VOC emissions in the event that the district falls out of compliance for fine particulate matter pollution.

CalRecycle Research on Compost Emissions

Solar-Powered Aerated Static Piles (ASP) With Compost Cap

Reaching the 75 percent recycling and composting goals of California's Mandatory Commercial Recycling law will require new ways of managing organic materials, which make up the largest share of materials still going to landfills. In 2012 CalRecycle partnered with the Association of Compost Producers, the City of Bakersfield, O2 Compost and emissions consultants Chuck Schmidt and Tom Card to test the potential emissions reductions and sustainability enhancements from using a small-horsepower positive aeration system hooked to a photovoltaic power system, coupled with a biofilter compost cap for emissions control. The project was funded by the San Joaquin Valley Air Pollution Control District through its Technology Advancement Program. The project was hosted by Harvest Power at its Tulare location. An electric conveyor system was designed and built by Kevin Barnes, compost site manager for the City of Bakersfield, to move feedstocks directly from the grinder to the pile without the use of diesel power.

The project showed VOC emissions reductions of nearly 99 percent for the aerated system, along with significant reductions in ammonia (NH3) and greenhouse gas (GHG) emissions, when compared to windrows made out of the same materials on the same day (see table ES-1). Diesel use in pile construction and active-phase management was reduced by 87 percent compared to typical windrows. Water savings from the aerated static pile system averaged around 20 percent, and the footprint needed for the ASP system is some 55 percent smaller than that required by windrows. Compost produced through the ASP system was similar in quality and maturity to the product from normally managed windrows after 30 days.

Table ES-1: Project Results
    NH3 GHG
  VOC Field Lab CO2 CH4 N2O CO2e
Prototype ASP (22 days) 0.099 0.017 0.007 205.70 5.05 0.010 315
Baseline Windrow (22 days) 8.604 0.099 0.014 731.63 5.81 0.093 883
Percent Reduction From Baseline -98.8% -83.2% -53.3% -71.9% -13.0% -88.8% -64.3%

Read the full report.(PDF, 6.9 MB)

Ozone Formation Potential Study

CalRecycle partnered with the School of Civil and Environmental Engineering at UC Davis to perform the first-ever assessment of the ozone formation potential of composting emissions. The initial research was funded by a combination of composters and public agencies, and the results from that work were published in the peer-reviewed journal, Atmospheric Environment, in March 2011. The second half of the study expanded on the first round of research and also tested the impact of an emissions reduction practice on ozone formation. In both studies, traditional VOC measurement techniques were combined with the use of a mobile ozone formation chamber to ensure that no major ozone-forming emissions were ignored.

Subsequent work to investigate the ozone formation potential of biosolids co-compost (PDF, 4 MB) was funded by members of the California Association of Sanitation Agencies. Whether or not the addition of food waste to composting feedstocks would influence the types of VOCs emitted from piles is a missing piece of this puzzle. Funding is being sought to complete this work.

The Modesto Study

CalRecycle, then known as the California Integrated Waste Management Board (CIWMB), collected and analyzed 100 emissions samples from four compost windrows built and managed at the city of Modesto's composting facility. The study calculates a life-cycle VOC emissions factor for greenwaste and food waste windrows, and also tests the emissions-reducing potential of two best management practices.

Compost vs. Biogenic Emissions

CalRecycle funded research at San Diego State University to determine whether emissions from the decay of organic materials vary based on whether the materials are composted or allowed to decay in an uncontrolled environment. The study concluded that VOCs are "very likely to be biodegraded within the composting matrix, and thus result in lower emissions when composted than if the materials were to be handled differently." This research was first published in Compost Science and Utilization.

Best Management Practices

The most cost-effective way to reduce emissions from compost operations may be to carefully control operational variables in order to provide the best possible environment for the aerobic organisms which power composting. Most composters do these things purposefully; others may do it unwittingly, inconsistently, or not at all. It’s known that poorly managed piles smell bad and attract bugs; it is also likely they produce more pollution than well-managed piles, and in doing so may lose some of the nutrients which benefit the compost end user.

Even well-managed piles can have sections which go "anaerobic." That means a lack of oxygen has depleted the desirable microbes which break down organic wastes, and has allowed other microorganisms which operate without the presence of oxygen to take their place. Emissions of methane, ammonia and other VOCs like cadaverine or putrescine seem to be related to anaerobic conditions.

While the types and amounts of emissions may be closely related to actual feedstocks (which vary seasonally), it is thought that the following variables influence emissions, and that optimizing these variables may significantly reduce composting emissions.

  • Initial carbon-nitrogen ratio: Piles which have too much nitrogen may lose nitrogen to the atmosphere in gaseous form. Excessive carbon may slow or halt decomposition.
  • Moisture content: Piles which are too wet may go anaerobic. Piles which are too dry may not compost well or may get too hot. In California, composters generally add water to piles during the warm months and when aerating. In other sections of the country, piles must be protected from excessive rain.
  • Temperature: Piles which are too hot kill valuable micro-organisms and may volatize more compounds than is optimal. Cold temperatures may indicate an anaerobic pile. Composters are required to maintain pile temperatures greater than 131ºF for 15 days in order to kill pathogens.
  • Oxygen content: Lack of oxygen impedes or kills aerobic organisms, leading to anaerobic conditions. Blowers inject oxygen deep into the pile. Windrow turners fluff up the pile, allowing oxygen to penetrate. Lowering the bulk density of composting feedstocks generally improves oxygen content. Oxygen penetration into actively composting piles can be enhanced by blending in large particles, such as oversized materials screened out at the end of the compost process.

CalRecycle Research on Best Management Practices

CalRecycle (then CIWMB) completed emissions tests on greenwaste composting designed to evaluate emission reductions that could be achieved by controlling feedstock mixtures and aeration techniques. Two blends of feedstock were used: a woody blend (high carbon-to-nitrogen ratio) and a grassy blend (low carbon-to-nitrogen ratio). Two levels of aeration were evaluated as well: static windrows (natural convection only) and mechanically turned windrows. The tests were conducted at Tierra Verde Industries in Irvine, in Southern California. Test results indicated that ammonia emissions were extremely low and should not be a concern for greenwaste composting. The emissions from the woody blend were lower than the grassy blend. And, changing the aeration techniques altered the emission profiles over time. However, it is difficult to conclude whether aeration techniques lowered the total emissions.

Emissions-reducing best management practices tested at Modesto included a "pseudo-biofilter compost cap," and a pair of commercial inoculants blended into and sprayed upon a windrow. The compost cap, a 4-to-6-inch layer of finished compost covering the newly formed windrow, acts as a biofilter to destroy emissions. The inoculants stimulated beneficial microbes and helped form a crust on the active compost pile. Both practices resulted in emissions reductions during the initial two weeks of composting. However, the pseudo-biofilter was more effective, reducing emissions by about 75 percent during the first two weeks. This is significant because the Modesto study suggests that roughly 80 percent of all emissions occur during the first two weeks of composting.


CalRecycle staff periodically participate in panels and conferences where the compost emissions issue is discussed with operators and other regulatory agencies. These presentations serve as an excellent introduction to the complex regulatory dilemma:

Additional Resources

CalRecycle Contact

Last updated: August 21, 2015
Organic Materials Management