Compost Emissions – More Than Just a Matter of Smell
Posted by Karen Hills | October 20, 2020
This is part of a series highlighting work by Washington State University (WSU) researchers through the Waste to Fuels Technology Partnership between the Department of Ecology and WSU during the 2017-2019 biennium. This partnership advances targeted applied research and extension on emerging technologies for managing residual organic matter.
Composting organic waste is, in many ways, a win-win scenario. It diverts waste from the landfill, while creating a valuable soil amendment (explored in my recent post on the value of compost). However, even composting is not without its share of environmental impacts. Large commercial composters know that emissions of smelly compounds can occur and cause unhappy neighbors. But little attention has been paid to less noticeable compounds which could have climate and air quality impacts. But how much is known about the emissions of these compounds from composting operations? Reading a recently published report by Tom Jobson and Neda Khosravi of WSU’s Laboratory for Atmospheric Research helped me to better grasp the state of the science on this question.
Composting results in emission of many different types of gases as carbon-containing compounds are transformed through biological activity. Some of these compounds are greenhouse gases. Other gases emitted are volatile organic compounds (VOCs), some of which are problematic. Sulfur-containing VOCs are the sources of unpleasant odors that can be associated with compost. Other chemically reactive VOCs affect the formation of ozone and particulate matter, and others are listed as air toxics by the EPA, and directly impact human health. Here’s what Jobson’s team had to say about these emissions.
Emissions of Greenhouse Gases
Jobson and Khosravi used existing modeling tools to compare greenhouse gas emissions from composting to disposal of organic waste through landfilling (assuming 20% degradable organic carbon, a level similar to what composters receive) by modelling nitrous oxide (N2O) and methane (CH4) production in each situation. They found that composting as an alternate waste management strategy will likely decrease greenhouse gas emissions from organic waste compared to landfilling, with most of the reduction attributed to two factors: removing food waste emissions of methane from landfills and the soil carbon storage benefit of applying compost to soils. However, two factors cause uncertainty in these calculations: the N20 and CH4 emissions factors from compost vary widely in the literature, and the fraction of carbon that is stored in agricultural soils in stable form varies depending on soil texture, environmental conditions, and management. Further refining these estimates based on Washington conditions would be needed to accurately quantify greenhouse gas reduction benefits of composting, important for informing policy on diverting organic waste from landfills.
Emissions of Volatile Organic Compounds
In understanding Washington compost emissions, one logical first step is to see what other relevant studies may already exist. The State of California has led the nation in studies and regulations focused on reducing VOC emissions from compost facilities. In an effort to understand the applicability of these California studies to Washington composters, Jobson and Khosravi reviewed existing data collected by the Department of Ecology at six Washington commercial composting facilities. What they found was perhaps surprising, in that emissions measured at these Washington facilities varied significantly from those found in a California study, that was used to define the EPA Speciate profile for green waste compost. While isopropanol (i-propanol), a light alcohol, made up 42% of emissions in the California study, it was rarely detected in the six Washington facilities that were sampled. On the other hand, monoterpenes, a chemically reactive VOC that can affect particulate matter and the formation of ozone, comprised the largest fraction of VOCs in some of the Washington facilities, but made up only 6% of emissions in the California data. Because of these differences, which are likely due to differences in feedstocks, using California data from the EPA Speciate profile when considering Washington compost facilities could be misleading. Work is currently underway by Jobson’s team to develop methodologies suited for quantifying Washington compost emissions.
What This Means for Composters
The bottom line is that, from a climate perspective, composting is still a beneficial practice when compared to landfilling, but some further understanding of VOC emissions and mitigation strategies may preserve air quality. The good news for composters and the environment is that emission of VOCs can be substantially reduced by strategies already in use in most large composting facilities, including covering a windrow with a cap of finished compost, using membrane covers (such as the Gore® cover which is used to control odors and emissions), or venting emission for aerated static piles through a biofilter. For example, the first and simplest of these strategies results in an estimated 75% reduction in VOC emissions.