The Greening of Commercial Compost
Compost happens, as they say in the biz. It’s true that, given enough time, most natural materials will decompose. The whole idea behind “composting” is to optimize nature’s process by providing the right mix of carbon, nitrogen, water and air. Most commercial-scale composting in California is done in long, narrow piles called windrows, which are mixed regularly by a specialized piece of equipment called a windrow turner. These enormous, diesel-powered critters range from 200 to 600 horsepower, but you’ll be traveling in hours per mile not miles per hour. What if there was a way to replace some of that diesel using the sun?
CalRecycle was involved in in a project, funded by the San Joaquin Valley Air Pollution Control District, involving the Association of Compost Producers, private consultants and local partners, to see whether large piles of green materials could be composted using small “bounce house” blowers powered entirely by the sun to pump air into the pile instead of being mechanically turned. The point of the project was so see whether air emissions could be reduced during the first three weeks of composting, which is the time when most emissions of volatile organic compounds—aka VOCs—occur. VOCs are important to the air district because they mix with oxides of nitrogen (NOx) from vehicle tailpipes to form ozone, a very dangerous air pollutant at ground level. Summer ozone levels in the Central Valley are some of the highest in the country, and per the federal Clean Air Act must be reduced.
The same sunlight that helps VOCs and NOx turn into ozone can provide more than enough electricity to power a 1.5-horsepower electric blower using only one solar panel, and that energy can be stored in batteries so that pile aeration occurs day and night. A series of pipes laid under the composting material provide a path to inject air into the bottom of the pile, and it filters up to the top, keeping things happily aerobic. To trap more emissions, the tops of the piles are covered with a layer of finished, unscreened compost, and kept damp with sprinklers, again sparing the air because the diesel-powered water trucks so common at windrow facilities get a little rest.
VOC emissions were reduced in this pilot project by 98 percent, diesel use was reduced by about 87 percent during those critical first few weeks of composting, GHG emissions were reduced by about half, and the amount of water used to keep the pile moist was reduced by 20 percent. That’s quite a savings.
To reach California’s goal of 75 percent recycling and composting by 2020, and ARBs draft goal to get 90 percent of organics out of landfills by 2025, many new compost facilities will be needed. No one type of facility will fit all communities, but all new organic materials handling sites will need to have 21st century infrastructure that protects air and water quality, and they will have to be good neighbors. The aeration system for the pilot project cost less than $15,000 per zone, which is pretty affordable considering a new diesel windrow turner would likely set you back half a mil.
Solar panel prices keep coming down, and efficiency is still going up, so it looks like solar-powered aerated static piles are a good option for community-sized compost sites, and probably could be scaled up quite large. Already, several compost operators are looking at adopting this technology.
The full report on the project can be found linked to the very bottom of this page: http://valleyair.org/grants/technologyadvancement.htm
Robert Horowitz is a Supervising Environmental Scientist at CalRecycle.