Measuring And Controlling Composting Emissions

CIWMB-funded study on composting emissions from food and green wastes

provides valuable data.



Bob Horowitz



A NEW study funded by the California Integrated Waste Board (CIWMB) indicates that well-managed composting of green waste and food waste releases significantly less air pollution than previously thought. The study also demonstrates that a relatively inexpensive management technique – covering active compost piles with a layer of finished compost – may lower air emissions even further, by as much as 75 percent compared to uncovered piles.



These findings come at a critical time for California. Air quality districts in several regions of the state are considering regulations that require green waste composters to reduce emissions of Volatile Organic Compounds (VOCs), which react with other chemicals in the air to produce ground level ozone, a “criteria” pollutant subject to the federal Clean Air Act. These new rules could significantly impact composters.

The San Joaquin Valley Unified Air Pollution Control District (SJVUAPCD, or the Air District) covers eight agricultural counties in the southern half of California‘‘s Central Valley. The Air District has already begun its rulemaking process. Composters located in the San Joaquin Valley produce about half of all compost made in California. The largest operations in this region import curbside green waste and food waste from the Los Angeles and San Francisco/Oakland urban areas, but sell their finished product to nearby farmers. Therefore, actions taken in the San Joaquin Valley have a potentially huge impact on California‘‘s compost industry, on the solid waste diversion performance of some of California largest jurisdictions, and on the continued growth of sustainable agriculture in California‘‘s agricultural heartland.

The CIWMB conducted its new study within the San Joaquin Valley, at the City of Modesto‘‘s composting facility, located about 100 miles east of San Francisco. This facility composts about 65,000 tons of materials per year, mostly from the city‘‘s own curbside program, but also from landscapers and neighboring cities. The materials are tipped onto a concrete pad and ground before being shaped into windrows, which are then turned with a windrow turner.

The CIWMB‘‘s goals were two-fold: first, to establish realistic VOC emissions from green waste and food waste composting for a complete composting cycle of around 60 days; and second, to test the efficacy of two potential emission control practices over the most active composting period, the first two weeks of the process. “We wanted to go beyond the previous work, which looked at a point in time – a few days, or at best a couple of weeks,” says Brenda Smyth, project lead for the CIWMB. “We wanted to create a detailed emissions profile of an actual compost windrow over the entire lifecycle of the process, as well as evaluate the two potentially most cost-effective best management practices.”



MODESTO STUDY

To initiate the study, workers created four new windrows, each about 100 feet long. The windrows are described as follows: 1) Pure green waste; 2) Green waste mixed with approximately 15 percent food waste from a local cannery; 3) Green waste capped with approximately 20 tons of finished, screened compost, applied with a loader (pseudo-biofilter windrow); and 4) Green waste with two commercial additives (additive windrow). The first additive, a micronutrient to feed beneficial microbes, was mixed into the windrow with a turner; the second additive, designed to form a crust on the windrow that limits the escape of odors and emissions, was applied topically.

For the pseudo-biofilter windrow, unscreened finished compost was used to replenish the cap after windrow turnings. For the additive windrow, the topical additive was reapplied after turnings. Aside from those differences, the w