Composting Emissions and Air Permits

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

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%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% compared to typical windrows. Water savings from the aerated static pile system averaged around 20%, and the footprint needed for the ASP system is some 55% 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.

Read the full report PDF download.

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.

• A peer-reviewed article in Atmospheric Environment: “Volatile Organic Compound Emissions from Green Waste Composting: Characterization and Ozone Formation“
• CalRecycle publication: An Investigation of the Potential for Ground Level Ozone Formation Resulting from Compost Facility Emissions

Subsequent work to investigate the ozone formation potential of biosolids co-compost PDF download was funded by members of the California Association of Sanitation Agencies. 

The most cost-effective way to reduce emissions from compost operations may be to carefully control operational variables to provide the best possible environment for the aerobic organisms that power composting. Poorly managed piles smell bad and attract bugs, and likely produce more pollution than well-managed piles, and in doing so may lose some of the nutrients that benefit the compost end user.

Even well-managed piles can have sections that 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 that have too much nitrogen may lose nitrogen to the atmosphere in gaseous form. Excessive carbon may slow or halt decomposition.
• Moisture content: Piles that are too wet may go anaerobic. Piles that 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 that 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 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.