Closing the loop: Turning city food and garden waste into fertilizer
During a break between meetings at the office, one of my coworkers asked, "So, Marc: got any vacations planned?"
"Just a little one. I'm going to the Vacaville landfill next Friday," I replied.
His eyebrows raised a little bit, and he responded with a quizzical tone, "Hmm...that's an interesting choice. I suppose it's a lot less crowded than Yosemite or Mendocino."
I didn't get a chance to explain that this wasn't just any landfill — it's the place where food waste from some of the finest restaurants and home kitchens in San Francisco and Berkeley are composted into organic matter that then fertilizes some of Northern California's best small farms and vineyards.
The tour of Jepson Prairie Organics facility in Vacaville was organized by the Center for Urban Education about Sustainable Agriculture (CUESA), the non-profit that runs the San Francisco Ferry Plaza farmers market. For the past few years, CUESA has been operating farm tours, but this was the first visit to a site specializing in decomposition instead of growth. It fits nicely with CUESA's new waste-reduction program at its farmers markets, the Waste Wise Initiative. The initiative aims to reduce the use of non-recyclable and non-compostable items at the markets, promote reuse, and educate the customers and vendors about waste reduction. So far the results are impressive: they have reduced the Saturday market garbage output from 4,500 gallons to 250 gallons per week, with most of that difference due to collection of recyclables and compostable material.
Let's start at the beginning, with the food scraps. Many cities in Northern California provide special bins for residents and businesses for yard trimmings. Some cities, such as San Francisco, Oakland, and Berkeley, allow food waste to be mixed in with the yard debris or have special food waste bins for restaurants. Residents can toss in vegetable trimmings, bones, unused leftovers, the mysterious stuff from the back of the refrigerator, and even paper towels and milk cartons. Periodically, a special garbage truck picks up the material and takes it to the composting site in Vacaville (probably after some repacking into a big transfer truck). The motivation for the special treatment of "green waste" was a state law (AB 939) that mandated that cities reduce the amount of material they send to the landfill by 50%. For a city like San Francisco, it's a sizable portion, about 300 tons per day, according to SF Environment, compared with about 1,800 tons per day sent to the landfill.
Many facilities, like the farmers markets at the San Francisco Ferry Plaza and in Berkeley, the Ferry Building Marketplace, and conscientious restaurants provide a special bin for compostables next to the trash and recycling container.
Grind it, pile it, turn it, sell it
At first glance, the overall process seems fairly simple: grind it, pile it, and turn it. But there are many of clever techniques employed by the facility operators to speed up the process, make it more efficient, or produce a better product. For example, the type of covering on the windrows (the rows of composting material) and even the type of microbes mixed into the compost can have a significant effect.
The first step is to shred the material, which often includes large objects like tree branches. The shredded material is then screened to remove pieces that are too large. The yellow object in the photo below is the screener. Raw material enters on the upper left side and exits on the right, with the too-big pieces dropping to the bottom of the machine to be carried away by a conveyor belt to the pile on the left side of the photo. The contents of that pile — the "overs," in composting lingo — are either sent back through the shredder or used as cover material in the neighboring sanitary landfill.
Being pestered by plastic
Not surprisingly, the material that comes to the facility often contains non-organic contaminants like plastic bags, energy bar wrappers, baseballs, and various other junk. It's a big problem for the company, so they have four employees working on a picking line to remove as much as possible. The photo below shows their working area.
Jepson Prairie removes over 60 tons of non-green material from the green waste stream each month (mostly plastic), which is about 1 to 3 percent of the total material received (by weight), according to an article by Deborah Rich in the San Francisco Chronicle. It does so because California requires compost to have a contaminant level below 2 percent by weight. Fortunately for the users of the compost and those who eat the crops grown using the material, the current thinking is that toxic chemicals in plastics like phthalates esters (used to make plastic soft) are unlikely to be found in the plastics that show up in the green waste stream.
The waiting game
After removing as much plastic as possible, the material goes through a high speed grinder to further reduce its size. The company then blends various batches of ground material with wood shavings and then piles them into windrows. If the windrow contains food scraps, it is covered with a special fabric to help the process along and increase the temperature to kill pathogens. If it is a windrow with only yard trimmings (some municipalities do not allow food scraps in the green waste bins), no cover is needed.
The windrows are left undisturbed for 30 days, then a specially designed machine passes over them, turning the material to expose new surfaces to air and spraying water containing a special blend of microbes to help the degradation process. Over the next month, the rows are turned and sprayed every few days.
After roughly 60 days, the company passes the compost through a screen to remove any remaining large pieces. The material is then ready to sell to farmers and wholesalers, or give away to the public at special events.
On its surface, the solid waste industry is dusty, noisy, and smelly. But dig a little deeper and you'll find a lot of innovation. During the introductory talk by company representative Greg Pryor, we learned about some of the recent advances in composting and got a preview of some future improvements. An example of a recent advance was the covers used for the food-waste windrows. For a long time, they packed the compost into very long plastic tubes. It did the job, but because they were single use, hard to manage, and didn't fit the image of a composting business, finding a replacement material was always a high priority. The company eventually found a fabric cover that is easier to handle and can be used multiple times.
The company has been collaborating with the East Bay Municipal Water District (EBMUD, supplier of water to most of Alameda and Contra Costa Counties in California) in a project to convert food scraps into energy. At its facility in Oakland, EBMUD has a "bio-reactor" that allows food scraps to anaerobically (i.e., without oxygen) decompose under controlled conditions. The bacteria that work under anaerobic conditions release methane as they eat the waste. The methane is then burned in an engine or turbine to produce electricity, thereby creating energy with reduced greenhouse gas emissions.
Norcal Waste Systems (the owner of Jepson Prairie Organics) is interested in this process because it could reduce fuel consumption by their trucks, reduce methane and other pollutant emissions during composting, and also improve the composting process. Much of food waste is water, which is expensive to transport by a truck; anaerobic pre-digestion cuts the water content in half. In the long term, the company sees benefits in performing the food-scrap to energy process close to the collection point (i.e., in San Francisco or Oakland), and then trucking the lighter material to the composting site in Vacaville.
Many others are looking at food-waste to energy processes, such as Professor Ruihong Zhang and her collaborators at the University of California, Davis. Using grants from various private and public agencies, they have built a biogas reactor that will convert food waste, yard trimmings, animal manure, and rice into methane and hydrogen, which then can be burned to produce electricity (and perhaps the hydrogen can even be separated to power fuel-cell cars like the Honda FCX Clarity).
At the end of June, Tom Philpott wrote a long piece at Grist that touched on several important issues facing agriculture and the organic movement. Writing that developing a soil fertility strategy is the No. 1 challenge for new organic farms, Tom asked, "Why not champion a national composting policy, one that compels municipalities to transform food waste into high-quality, crop-grade compost? And why not then give it away to farmers — the ones who grow food for their nearby communities? That's an agricultural subsidy that makes all kinds of sense."
I agree. Why shouldn't the Farm Bill or the annual USDA appropriation help cities and counties set up municipal composting facilities like the one used in San Francisco, Oakland, and other California cities? It's probably not efficient for waste collection trucks to pick up food scraps from rural and exurban households — for those households we could provide subsidized composting equipment and give classes, like they do in Alameda County. For dense urban areas and suburbs, however, the added costs are probably minor. Whatever the case, we're wasting a tremendous resource when we send vegetable and fruit scraps to the landfill for entombment, instead of composting them and closing a loop in our food system.
- A detailed photo summary of the composting process from EatsForOne
- A video segment on biofuels from KQED's Quest
- A 2001 article in the San Francisco Chronicle about San Francisco's early food waste collection program.
- More of my photos of the tour on
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