Matt Richtel’s NY Times article (25 March 2016, “San Francisco, the Silicon Valley of Recycling”) concludes with the spokesman for Recology’s recycling facility in San Francisco wishing that his visitors would also have the chance to tour the composting facility. Last Monday, I had the chance to do just that.
At 9 am on a cool, sunny day, I met Greg Pryor at Recology’s composting facility in Vacaville, CA. Greg, whose title is Organics Branch Manager, generously spent a good part of his morning with me, answering my questions and showing me around the impressive facility. He is a wealth of knowledge and has a deep understanding of all aspects of the business, having overseen the facility for twenty years after starting his career in construction management. We spent the morning discussing everything from business models to odor control, microbial decomposition to government regulations, and high-speed grinders versus low-speed shredders.
What is compost? Essentially, it is the soil organic matter that is left behind after an organic waste has been decomposed. When added to soil, compost increases aeration, water retention, and creates a healthy microbial community (think of a rich, black, earthy-smelling soil versus a beach sand).
The Vacaville site accepts yard and food waste from San Francisco, Vallejo, Dixon, Vacaville, and other communities and processes it into compost that is sold to farmers and landscapers. Food wastes are processed first thing in the morning (and, here, first thing means 4 am). The organic waste is loaded into a low-speed shredder which tears apart large pieces and rips open the plastic bags into which many consumers put their food scraps before tossing in their green curbside bucket. The waste then moves through a rotating drum with four-inch openings. Bits that are smaller pass through, while the over-sized pieces (or, “overs”, in composting parlance) continue on. The overs travel to a sorting tent where employees pick through the waste to remove contaminants: plastics, glass, metal – usually items that should have been tossed in the recycling bin rather than the compost bin. The sorted overs are ground up and mixed back with the waste that originally passed through the four-inch screen.
Four-inch screen at the Recology composting facility. (Photo credit: Andrew Guswa)
Then the process really begins. To ensure an appropriate carbon-to-nitrogren ratio for the microbes and a bulk density that allows sufficient air to pass through the waste, one bucket of food waste is mixed with two to three buckets of yard waste (think grass clippings and brush cuttings). The combined organic wastes are then placed on top of four perforated pipes that draw air down through the pile to ensure that the microbial decomposition remains aerobic and at the right temperature. After thirty days “on air”, the waste is moved into rows, where it cures for another thirty days. The finished compost is then sieved through a quarter-inch screen to remove any stray bits of plastic and glass before being sold.
Organic waste “on air” – note the pipes along the bottom that pull air down through the pile. (Photo credit: Andrew Guswa)
As Greg points out, more and more farmers and landscapers are recognizing the value of organic matter as a contributor to healthy soils, and the composting business is transitioning from a focus on waste-management to a focus on creating a valuable product. When I asked Greg to identify his biggest challenge, he indicated the contamination of their feedstocks, e.g., that stray plastic bottle that ends up in the compost bin rather than the recycling bucket. As you might imagine, farmers and landscapers do not want compost riddled with shards of plastic and glass. With better quality control on their inputs, i.e. with all of us doing a better job of sorting our wastes, this circular economy has the potential to really take off.
– Andrew Guswa, Professor of Engineering, is currently on sabbatical in California.