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Wastewater treatment

Food waste is one of the least recycled materials in municipal solid waste systems, according to the Environmental Protection Agency. But at least one organization in the San Francisco Bay Area is trying to change that.

The East Bay Municipal Utility District is experimenting with innovative techniques to convert raw food waste into usable energy, taking some of the massive amounts of food waste generated by local restaurants and using it to power its operations in Oakland, Calif.

In 2007, EBMUD was awarded a $50,000 grant from the EPA as part of the Resource Recovery Program to explore new ways of digesting food waste to produce methane gas.

Today, the facility is home to a million-dollar facility that is generating usable methane and producing nearly 100 percent of the power needed to operate the regional wastewater treatment operation.

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East Bay

The main wastewater treatment plant sits at the base of the San Francisco-Oakland Bay Bridge. It was the first sewage treatment facility in the nation to convert post-consumer food scraps to energy via anaerobic digestion. EBMUD is adapting some of the equipment that's been used for decades in water treatment. That, coupled with new ideas and patented processes, enable EBMUD to produce nearly 100 percent of the energy needed to power the facility.

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Pizzaiolo

The process all starts at restaurants like this one, Pizzaiolo in Oakland. Currently, a few hundred restaurants contribute to the program, amounting to 100 tons of compostable food waste that gets delivered to the EBMUD facility every week to be anaerobically digested to produce methane.

Restaurants in nearby Alameda, Contra Costa, and San Francisco counties generate 1,700 tons of food waste each day, giving the program a lot of potential for expansion, according to Dave Williams, the director of wastewater at EBMUD.

A bill signed in 2006 by California Governor Arnold Schwarzenegger requires the state to limit greenhouse gas emissions to 1990 levels by the year 2020. Processes like the one EBMUD is testing could help the state achieve that goal, said Williams.

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Compostables

Roger Taylor, an employee at Pizzaiolo, says the restaurant generates around 100 gallons of food waste a day, more compostable waste than any restaurant he has worked at before. Pizzaiolo sends all of it to the EBMUD processing facility to be turned into usable energy.

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Dumping

A truck arrives with a load of commercial restaurant food waste and dumps it into a holding tank below. The scraps from restaurants typically contain all sorts of foreign material like plates, knives, forks, chopsticks, and other plastics. These items must be filtered out before the food makes its way into the digestor to produce methane.

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Dumping waste

Technicians at the processing facility watch as a load of food waste is added to the tanks below.

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After a delivery

After delivering a truckload of waste into the holding tanks, water is added and the mixture is stirred to give the solution the proper consistency for processing.

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Infrastructure

The processes EBMUD employs to convert the food scraps into methane are done using existing waste treatment infrastructure. As it has developed techniques, several of the processes have been patented.

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Removing materials

Before the pulp can be processed, it goes through the paddle finisher, which screens and filters unwanted fibrous materials and bits of paper and plastics. This rejected material, the pumice, is sent over to the landfill to be composted, Williams said.

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Pure sludge

With the pumice removed, the end product is a smooth liquid that flows down a trough and is then pumped into anaerobic digestors, where it will break down in an oxygen-free environment, producing methane gas.

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Oakland

The methane gas flows out of this huge tube and is sent toward the three co-generation engines that power the wastewater treatment operations. In this photo, shot from the top of digestor No. 12, you can see a view of downtown Oakland.

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Co-generation engines

Using organic solids to produce methane is not uncommon across the country, but an average facility would probably only make about 35 percent of the energy necessary to power the wastewater facilities. By implementing the food scrap program, EBMUD is now generating nearly 100 percent of the energy it needs, and hopes to someday produce more, which could then be sold back to PG&E.

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