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March 2007
Anaerobic digestion technology  

Should federal, state and municipal governments embrace anaerobic digestion technologies, these systems could play a major role in reducing the amount of trash going into landfills and green house gas (GHG) emissions, as well as producing biogas that can help reduce America’s dependence on foreign oil and gas.

Currently at the University of California in Davis a small-scale commercial anaerobic phased solids digester, built by Onsite Power Systems Inc., has been converting table scraps from San Francisco restaurants, grasses and other yard trimmings into clean renewable energy.

It is estimated that one ton of degradable waste can produce electricity to power 10 average California homes for 1 day.

The Biogas Energy Project is the first large-scale demonstration in the United States of a new technology developed in the past eight years by Ruihong Zhang, a UC Davis professor of biological and agricultural engineering.

Professor Ruihong Zhang of UC Davis deposits table scraps collected from San Francisco restaurants into the university's biogas energy plant.

The goal of this innovative public-private alliance is to harvest energy from degradable waste to preserve landfill space.

Unlike most anaerobic digesters, which are in use on municipal wastewater treatment plants and livestock farms, Zhang’s system utilizes both solid and liquid material, including food scraps, yard trimmings, animal manure and rice straw.

More than five million tons of food scraps go into California landfills annually.

According to Zhang, her system turns waste into energy in half the time of other digesters and produces two clean energy gases - hydrogen and methane. While other digesters produce only methane, the UC Davis system provides a fuel that can be burned to produce electricity and heat or to propel cars, trucks and buses.

Moreover, Zhang has proved in the laboratory on a small scale that in oxygen-free conditions, naturally occurring bacteria can quickly convert food and green wastes into hydrogen and methane gases.

Zhang believes that she can meet the challenge of making the gases in consistently high quality and large volumes over the long term.

“My UC Davis students and I have determined the efficient bacterial species and their favorite environmental conditions for turning various wastes into gases,” she says. “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.”

Onsite has invested almost $2.5 million in helping Zhang refine the technology and prepare it for transfer to the commercial market, including two pilot projects and the latest production facility at the university.

“The model we have at UC Davis demonstrates pumps, valves, motors and a computer control system that have commercial applications,” said Dave Konwinski, Onsite’s CEO.

The California Energy Commission's Public Interest Energy Research (PIER) program has also awarded the university nearly $1 million in grants to support Zhang’s research.

Norcal Waste Systems Inc., which already collects restaurant leftovers for its composting operation near Vacaville, supplies restaurant leftovers and yard clippings to UC Davis.

On a daily basis, Norcal collects 300 tons of food scraps from 2,000 restaurants in San Francisco and 150 in Oakland and from thousands of city homes.

Statewide, the California Integrated Waste Management Board believes that 22 million tons of wet organic waste is produced annually; which if harvested, could generate 895,000 kilograms of hydrogen gas per day, which translates into 1,363,000 gallons of gasoline. According to Zhang, the digesting process converts between 60 and 90 percent of organic solids to biogas.

“Zhang and other researchers are testing different types of waste – cannery waste, different grasses and various food scraps,” says Robert Reed, Norcal’s director of corporate communications.

Reed stresses that anaerobic digestion provides four benefits: harvesting energy, producing high-quality compost as a by-product, preservation of landfill space, and reducing GHG

“Compost is highly desired by the agricultural community and it’s an alternative to chemical and petroleum-based fertilizers,” he says. “We make a lot of compost. Most of it goes to agriculture professional farmers and vineyard managers who know how to apply it. The finished compost is very rich in nutrients. We do not bag it. Instead we deliver truckloads to farms, vineyards and landscape supply yards.”

Norcal is currently selling its basic compost for about $10 a cubic yard. Two years ago the price was $6 and the cost of transportation exceeded the cost of the compost. At $10, the two costs are about equal.

Standard compost production takes about 60 days.

“We can do that in 10 days in the digester,” says Konwinski. “When you do grass clippings and collected food waste, you get about a 60 to 65 percent reduction in weight and that gets converted into energy and water. The nutrients pass through the remaining solid material. That can be converted into compost. With various combinations of straw, animal bedding and manures, we are doing horticultural studies to produce growing mediums and potting soils for greenhouses.”

Moreover, using compost reduces GHG. The revenue from compost sales helps to offset the cost of its production.

“More than half the garbage that this country currently sends to landfill could go into anaerobic digestion and you control GHG because you don’t have methane and other gases emitting into the atmosphere,” says Reed. “Degradable waste – food scraps and yard trimmings - can be collected separately from other waste materials. We’ve been doing it in San Francisco for 10 years.”

By reducing the amount of trash deposited in landfills, Reed says that municipalities extend landfill life and reduce closure and post-closure expenses.

“You have to monitor and maintain a landfill in perpetuity,” he says.

California legislation requires that its 400 municipalities must divert 50 percent of their waste from landfills into re-use, source reduction, recycling and other alternative programs.

Reed says that UC Davis expects that six months of experimentation should answer the questions of which feedstock’s are the most efficient and how much energy each feedstock can produce.

There are between 16 and 30 digesters in the United States.

Norcal has been aiding other researchers in the anaerobic digestion field.

“When you send material to a digester, you have to prepare it – it has to go through pre-digestion,” says Reed. “We do that here and we have been designing and testing equipment to prepare waste material. We have patent applications on those systems.

“We screen 80 tons of food scraps several days a week,” he adds. “Then it goes to a grinder and we produce a product that we call manufactured biomass. It looks like mud pies and then we send it to a digester.”

Food scraps are also sent to the compost facility, where they are screened and processed into a size called four inch minus.

Both materials have been tested at UC Davis.

“Zhang likes the four inch minus material better than the manufactured biomass,” says Reed. “It’s more solid and she found that she is able to produce the biogas from that material in a measure that she prefers.”

Waste materials must also be collected separately so they can be processed, a general rule of thumb for recycling.

“You have to screen it [waste material] because there are often contaminants,” says Reed. “There will be a plastic bag or something else to be removed. You don’t want to put anything in a digester that is going to plug it or disrupt the microorganisms that are inside the digester – you don’t want to give your digester a stomach ache.”

Last September Chevron Corp. announced it would fund up to $25 million in research at UC Davis in the next 5 years to develop affordable, renewable transportation biofuels from farm and forest residues, urban wastes and crops grown specifically for energy.

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