Much like a giant stockpot, an anaerobic phased solid digester has been converting restaurant food waste into an energy soup since October 24, 2006. Approximately eight tons of food waste from the San Francisco area’s finest restaurants has been processed weekly through the Biogas Energy Project at the University of California, Davis. The project, being developed for commercial use, is billed as the first large-scale demonstration of the new technology in the United States.
Ruihong Zhang, professor of biological and agricultural engineering at the university, has been developing the process for the past eight years. The digester has been licensed from the university and adapted for commercial use by Onsite Power Systems Inc., Davis, Calif.
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The objective of the public-private alliance is to divert organic waste away from landfills and get it into the energy grid. Each ton of fruit rinds, vegetable bits and fish bones delivered from the restaurants and stuffed into the digester can be turned into enough energy to provide electricity to power 10 average California homes for a day. The process also reduces greenhouse gas emissions from landfills and turns trash into a substantial source of clean energy.
“We know what happens with bacteria in 10 to 5,000 gallons of water and waste. Now we expect to see those bacteria perform as well, if not better, when they are in 50,000 to 300,000 gallons,” says Zhang.
While there are other anaerobic digesters primarily used on municipal wastewater treatment plants and livestock farms, Zhang’s system is different in three essential ways. It processes a wider variety of both solid and liquid wastes such as food scraps, yard trimmings, animal manure and rice straw. It turns waste into energy in half the time of other digesters. And it produces two clean energy gases — hydrogen and methane (others just produce methane), which can be burned to produce electricity and heat, or to propel vehicles.
In the laboratory, Zhang has proven on a small scale that in anaerobic, or oxygen-free, conditions, naturally occurring bacteria can quickly convert food and green wastes into hydrogen and methane gases. Now the challenge is to make the gases in consistently high quality and large volumes over the long term.
Dave Konwinski, CEO of Onsite Power Systems (www.onsitepowersystems.com), is so convinced the system will work commercially that his company has invested $2 million in the project. The company hopes to refine the technology and prepare it for commercial use. Konwinski wants to sell similar power-production facilities to waste-generating businesses such as food processors, farms and dairies, and municipal green-waste collection programs.
“Onsite will scale the digester to fit the customer's operations, then build it on their property,” he explains. “We will take the customer's waste stream in and send the energy it produces right back out to their plant.
“This technology will make a substantial dent in both our landfill needs and our use of petroleum and coal for fuels and electricity. It also will reduce our greenhouse gas emissions,” Konwinski explains.
The California Energy Commission’s Public Interest Energy Research (PIER) program (www.energy.ca.gov/pier/) has awarded almost $1 million in grants to the university. PIER supports innovative energy research, development and demonstration projects that improve the quality of life in California by bringing environmentally safe, affordable and reliable energy services and products to the marketplace.
Norcal Waste Systems (www.norcalwaste.com) of San Francisco is supplying the waste, valued at $50,000, for the project because it already collects restaurant leftovers for its composting operation near Vacaville. Every day, Norcal collects 300 tons of food scraps from 2,000 restaurants in San Francisco and 150 in Oakland, says Chris Choate, the firm's vice president of sustainability.
“New technology like UC Davis' offers California opportunities to harvest energy out of approximately 50% of the waste material that the state currently sends to landfills and to significantly reduce landfill disposal,” explains Choate. Norcal pioneered collecting restaurant food waste separately from other garbage and turning it into nutrient-rich compost for vineyards and farms as an alternative to chemical fertilizers.
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