Using table scraps from restaurants
To make any headway in replacing more polluting fuels, however, waste-devouring bioreactors would need to demonstrate their efficiency on a much larger scale, a challenge being taken up across the Pacific by researchers at the University of California at Davis.

Since October 2006, the university-led Biogas Energy Project has converted table scraps from some of the region’s top-tier restaurants, vegetable waste, grass clippings and cow manure into methane and hydrogen. The demonstration reactor processes between three and eight tons of the organic waste daily, according to Ruihong Zhang, a professor of biological and agricultural engineering. At that rate, the reactor’s daily output yields enough energy in the form of electricity to power up to 80 homes for a day.

There’s no shortage of starter fuel, with an estimated 5 million tons of food scraps dumped into California landfills every year. Once it’s had its fill, the multi-tank Davis reactor relies on a two-step anaerobic process in which hardy microbes turn the ingredients of, say, organic lemongrass tofu or endive salad into a less appetizing glop of acids and water. A second phase uses a separate bacteria mix to convert those acids into biogas.

If Onodera’s tack isn’t entirely new, Zhang said his use of spoiled milk has hit upon an “excellent” sugar and protein-rich environment for bacteria. Some dairy processing plants, in fact, have built digesters within their own facilities, though Zhang said the sizable reactors are designed to treat milk like wastewater and yield methane gas rather than hydrogen.

Like Onodera, Zhang said her process yields both gases, though the California project has arguably proved its mettle on a wider range of waste and on a much larger scale. “The conditions for treating different types of material can be different, but technically speaking, an anaerobic process can be used to convert anything that is biodegradable,” she said.

Building a commercial system
Zhang’s method is now licensed to Davis-based Onsite Power Systems, Inc., where she serves as the research and technology development director. With the prototype reactor as a guide, the company is building a commercial system that can handle up to 250 tons of waste per day. Within a year, Zhang said, the first unit could be ready for the City of Industry, a community west of Los Angeles that, as its name suggests, is overwhelmingly industrial. The reactor would consolidate and convert mostly food waste, but also some grass clippings and cheese processing leftovers. The biofuel, in turn, could power the city’s fleet of garbage trucks, offsetting the cost and power needed to transport the reactor’s raw materials.

“The reaction from the commercial businesses and even cities has been very positive,” Zhang said, noting that the technology offers not only biogas but also an environmentally-friendly way of reducing solid waste. “The leftover sludge will be processed into compost and also organic fertilizer,” she said. More surprisingly, the sludge’s undigested fibers could provide an excellent source of particle board.

In their own search for industrial-scale waste streams, researchers at the University of Birmingham in the United Kingdom found a willing partner in Cadbury Schweppes, of Creme Egg fame. A university spin-off called Biowaste2energy, or BW2E, has since taken the reins, with plans to build a mobile demonstration unit of its three-step system.

Like its counterpart in California, the BW2E method begins with a fermentation step that breaks foodstuffs down into organic acids, removing about 40 percent of the waste in the process, according to company CEO David Anthony. A purification step removes another 40 percent and a photobioreactor uses both light and bacteria to convert much of the remaining sludge to hydrogen, carbon dioxide and water. “I think where we fit into the niche is that we are reducing waste volume significantly more than what folks are doing by a single-step process,” Anthony said.

Beyond helping Cadbury cut down on a sea of caramel, BW2E is fielding calls from companies looking for better ways to dispose of fruit drink waste and spoiled fruit. All of which goes to show that where biogas is concerned, there’s no such thing as a bad apple.

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