BioCycle March 2010, Vol. 51, No. 3, p. 18
Ongoing study indicates less than five percent of VOCs in compost emissions profile are considered “strong” ozone precursors.
Web Extra: Video of footage from the sampling process
BioCycle West Coast Conference 2010 Related Session:
VOC Emissions Research And Management
Tuesday, April 13, 2010
Ozone Formation Potential From Composting
Peter Green, University of California, Davis
FOUR simple compounds that do not contribute much toward ground-level ozone formation comprise more than 75 percent of compost pile Volatile Organic Compound (VOC) emissions, according to preliminary results of research conducted by the University of California, Davis (UC Davis). The study, made possible with support from the California Department of Resources Recycling and Recovery (CalRecycle) and the Alameda County Waste Management Authority (StopWaste.Org), indicates that commercial composting operations do not contribute as much to regional ozone problems as previously thought. It suggests that regulations attempting to limit the amount of VOCs emitted from green waste composting facilities are likely to be more effective at reaching clean air goals if targeted at the most reactive compounds.
“It looks like the overall reactivity of VOC emissions from green waste composting operations ranks relatively low on the scale of sources that have been studied,” says Dr. Peter Green of UC Davis, principal investigator on the project. “Emissions reductions always help, but these sources cannot be considered ‘high-value targets.’”
Green’s research project – one of the first attempts to identify all of the different VOCs found in composting emissions – indicates that compost piles are an extremely complex source, emitting more than 100 VOCs, mostly in very low quantities. Less than 5 percent of the compounds in the emissions profile are considered “strong” ozone precursors, and less than 10 percent are considered “medium.” The exact proportions of the compounds varied with the age of the pile.
District air quality regulators in California have been concerned about compost pile emissions for a decade. They know that VOCs mix with nitrogen oxides (from cars and trucks) in the presence of strong sunlight to create ozone. They also know that ozone harms human respiratory health; days with high ozone levels are associated with higher school absences, increased hospital admissions, and possibly, even an increase in deaths. Ozone also interferes with photosynthesis, reducing crop yields.
Some regions of the U.S., such as the heavily populated San Joaquin Valley in California’s agricultural heartland, are designated as being in “extreme nonattainment” with ozone standards set by the U.S. Environmental Protection Agency, even after regulating all major stationary air pollution sources. Local districts do not have control over mobile sources, such as cars and trucks, which contribute the nitrogen oxide (NOx) portion of the ozone recipe and are regulated instead by the California Air Resources Board.
To protect the health of residents, the federal Clean Air Act requires San Joaquin Valley officials to reduce ozone-forming emissions from all known stationary VOC sources. This has led to rules requiring biosolids cocomposters located in the San Joaquin Valley to reduce VOC emissions by 80 percent, which is accomplished through costly aeration and biofiltration systems. Similar rules apply in the Los Angeles air basin, and could spread elsewhere. Green waste composters could be facing similar rules in the near future, and are less able to afford these costly controls because of lower tipping fees and more competition for feedstocks.
EVALUATING INDIVIDUAL COMPOUNDS
Research on compost emissions to date has focused on how much total VOCs come from a pound of feedstock. However, bulk measurements of VOCs from any source ignore one key fact: not all VOCs are equal when it comes to their propensity to form ozone.
Some VOCs, like pinene and limonene, are moderately potent ozone-formers. These compounds make up around four percent of compost emissions, but also are emitted by live trees. High ozone levels measured over relatively pristine experimental pine forests east of Sacramento are a prime example of the reactivity of these compounds. Other VOCs, like acetone, are so benign they are actually exempt from the Clean Air Act.
To compare the relative reactivity of different VOCs, researchers have created scales such as Maximum Incremental Reactivity (MIR). For the purposes of the UC Davis study, a strong ozone forming compound is classified as one with an MIR value greater than 5, a medium means an MIR between 2 and 5, and low reactivity means an MIR less than 2. The typical urban VOC mix has an MIR of around 3.7, according to Green. Table 1 provides a preliminary summary of compounds making up 95 percent of compost emissions VOCs. Figure 1 shows the relative reactivity of green waste compost VOC emissions at three points in the composting process; fresh tip; three to five day pile; and two to three week pile.
In the fall of 2009, Green’s research team visited two commercial composting sites in the San Joaquin Valley, spending a week at each facility sampling fresh tipping piles, newly formed compost windrows, and windrows a few weeks old. This testing was conducted using funds donated by several composters and StopWaste.org. This spring, the team will visit two more sites to fill data gaps, add new data points, and to test potential mitigation measures. This portion of the study will be funded by CalRecycle.
To determine the individual components of the gas mix, cylinders of gas extracted from the compost pile are evaporated in a chromatograph, which identifies all of the VOCs in the sample based on their individual structure. Because the reactivity of individual gases is known, the research team can use models to determine how much ozone will be formed.
The method employed by Green has a built-in self-checking mechanism; the research team also brings a portable ozone-formation chamber, known as the MOChA (Mobile Ozone Chamber Assay), to each sampling site. After the small cylinders are filled, a much larger sample is pulled from the same location, and routed to a 1,000-liter Teflon bag where it is mixed with air from a tank specially formulated to resemble the background atmosphere on a typical California summer day. Research team member Dr. Anuj Kumar then turns on the lights in the MOChA chamber and, over a period of hours, cooks up some ozone.
The amount of ozone measured in the chamber should match the ozone formation calculated based on the gases detected in the cylinders. If it does, then everything was done correctly; if not, the team knows it has missed some VOCs, or made a calculation error.
Green’s team has been using the MOChA method at agricultural sources throughout the San Joaquin Valley. At dairies, researchers determined that the open face of a silage pile was much more potent in terms of ozone formation potential than either the animals or their manure. When testing the ozone-forming potential of pesticide formulations in orchards, the team learned that the slow-moving, gas-powered spray truck was a bigger problem than the spray. Therefore, if a particular spray must be replaced by a less polluting substitute that is less effective, then the truck would have to be used more frequently, actually making air pollution worse.
EMISSION OF REACTIVE COMPOUNDS
A surprise finding of the preliminary research on compost is that, while windrows are known to emit the most emissions over their first week, the emissions from piles that are two to three weeks old contained a higher percentage of reactive compounds. Additional sampling this spring will test that finding, attempt to determine how long that condition persists, and then test a Best Management Practice to limit these more reactive emissions.
Previous research indicates that a blanket of finished compost on top of an active compost windrow acts like a biofilter and significantly decreases VOC emissions. However, the “pseudo-biofilter compost cap” can be costly and energy intensive to use when the pile is being turned every few days. This current research may show that use of the compost cap on older (i.e., 2 to 4 weeks old) compost piles, which have completed the mandatory pathogen reduction routine of five turns in 15 days at 131°F, may be a larger benefit than previously thought because these piles are not turned as frequently.
These are the kind of real-world, common sense insights Green hopes to bring to regulators, and one of the reasons he was so keen to work with compost. “Although I don’t do much gardening, composting strikes me as having a clear benefit to society,” he says. “My research goal is simple: find practical, cost-effective ways to improve air quality. Any new rule imposes a cost on society. Like the catalytic converter, which has greatly improved car emissions, changes must be worth it.”
The Clean Air Act does not differentiate between VOCs. But the USEPA, in a 2005 guidance document to states and air districts, encouraged air agencies to take advantage of the new science of reactivity when updating their clean air plans, revising their VOC inventories, and when considering regulations.
Some agencies are already using reactivity data to craft more effective regulations. The California Air Resources Board pioneered the use of reactivity-based regulations in 2000 when it replaced a mass-limit based regulation for spray paint with regulations based on reactivity. The most successful widespread use of VOC reactivity data, according to Green, has been the reformulation of gasoline to remove ozone-forming alkenes. More recently, two other California state agencies, the Department of Food and Agriculture and the Department of Pesticide Regulation, have launched a pilot program to assess how the inclusion of reactivity would impact their emissions inventory reduction plans.
Green, who has degrees in chemistry from Stanford and MIT, says the top three compost emission gases are simple alcohols that don’t make very much ozone, and also are unlikely to be a factor in forming particulates. “These compounds are relatively well behaved,” he explains. “They stick to trees, rinse out in the rain, and oxidize to CO2. It’s the normal atmospheric process. It happens all day long, all year long, all over the globe.”
Robert Horowitz is a Senior Integrated Waste Management Specialist in the Statewide Technical & Analytical Resources Division of the California Department of Resources Recycling and Recovery (CalRecycle). The author and Dr. Green would like to thank Dr. Anuj Kumar, research assistant, Chris Alaimo, colleagues at UC Davis and Cal State Fresno who provided equipment and advice, Grover Landscaping Inc. and the Merced County Solid Waste Management Agency for the use of their sites, and all funding sponsors. Dr. Green will give a presentation on the VOCs emissions research at the BioCycle West Coast Conference on April 13, 2010.