Turning Kitchen Grease into Biogas

More than 3,000 gallons of restaurant grease -- the kind washed from grills and pans -- will be delivered to the plant each day by grease-hauling companies, which otherwise would pay a city fee for its disposal. Microorganisms in the plant's digester tanks eat the grease and other organic matter, naturally producing methane gas to fuel the plant's new 250-kilowatt microturbine cogenerator to produce electricity for wastewater treatment.

Meanwhile, excess heat produced by the cogenerator warms the digester tanks to their optimum temperature for methane production.

The grease and other organic matter will produce enough biogas at the plant to generate about 1.7 million kilowatt hours annually, which will meet 80 percent of the plant's power needs and reduce its electricity purchases significantly.

"This project clearly demonstrates that cities can develop renewable energy economically, with multiple benefits to urban communities," said Jim Davis, president of Chevron Energy Solutions. "By applying proven technologies and looking at the entire waste stream in new ways, the City of Millbrae has cost effectively upgraded its facilities, reduced its operating costs, created new revenue and solved environmental challenges all at the same time."

Restaurants produce an average of 14 pounds of inedible grease per capita annually -- a total of nearly 4.2 billion pounds each year in the United States alone. Much of this grease is disposed of in landfills, where it releases methane -- a potent greenhouse gas -- as it decomposes, sometimes directly to the atmosphere.

"This innovative project brings new meaning to the term 'sustainable development,'" said Millbrae Mayor Robert Gottschalk. "Through our partnership with Chevron Energy Solutions, we're taking an urban waste and turning it into an asset for the city and the environment."

The total cost of the project, $5.5 million, was reduced by about $200,000 with a rebate awarded through the state of California's Self-Generation Incentive Program administered by Pacific Gas and Electric Company. The net amount, along with maintenance costs, is being funded entirely by savings from the new system and, therefore, will have no effect on the city's wastewater treatment rates.

"This is the only wastewater treatment plant in the U.S. to receive and process inedible grease in a self-funding, purpose-built system that successfully addresses so many challenges simultaneously," said Dick York, superintendent of the Millbrae plant. "It's a complete solution that could be adopted in many cities around the country."

Since 2000, Chevron Corporation, through its various subsidiaries, has spent more than $1.5 billion on renewable energy projects including geothermal, hydrogen, biofuels and advanced batteries as well as wind and solar technologies.