East Bay Utility Digests Food Waste to Produce Energy, Compost

The utility takes food waste from San Francisco and Contra Costa County restaurants and commercial food processors and uses the waste to produce green renewable energy and compost through anaerobic digestion.

On June 23, San Francisco Mayor Gavin Newsom signed the nation's first mandatory composting law. "It's the most comprehensive recycling and composting legislation in the country and the first to require residents and businesses to compost food scraps," Newsom said.

Now, residents and businesses must begin increasing their composting efforts and finding new ways to keep food scraps out of the landfill.

EBMUD's anaerobic digester, in operation since 2004, is leading the way, and currently processes 90 tons per week of post-consumer food waste from restaurants and food processing facilities.

Now, the facility plans on more than doubling the amount of food waste processed to its maximum capacity of 200 tons per day.

A truck offloads food waste to be processed in EBMUD’s dome-shaped anaerobic digester. (Photo courtesy U.S. EPA)

The East Bay Municipal Utility District's patented food waste processing system removes contaminants from food waste and creates a homogenous, energy-rich slurry, which is anaerobically digested to produce biogas and a soil amendment or fertilizer.

Anaerobic digestion, which means without oxygen, works by using bacteria inside the digester to decompose the food. It converts organic carbon in the food into biogas, which contains both carbon dioxide and methane.

The process begins when food waste is separated for disposal and pick up. At the EBMUD's main wastewater treatment plant, the food waste is broken down in large containers - the anaerobic digesters.

As the food decomposes, the digester captures the methane to power the wastewater treatment plant. Material remaining after the digestion process can be composted and used as a natural fertilizer to help grow food.

To further study this technology, in 2006 the EPA awarded a $50,000 grant to East Bay Municipal District. EBMUD processed 40 tons of post-consumer food waste per day in anaerobic digesters that break down sewage sludge, and in turn reduced greenhouse gas emissions, generated renewable electric power, produced a soil amendment, and recycled the largest single component of urban municipal solid waste - organic waste.

In the United States, more than 30 million tons of food waste are sent to landfills annually, and less than three percent of food waste is diverted from landfills, according to the U.S. EPA.

Food waste is the second largest category of municipal solid waste in the United States, accounting for 18 percent of the waste stream.

If half of the food waste produced in the United States was anaerobically digested, the U.S. EPA says, enough electricity would be generated to power about 2.5 million homes for a year.

Landfills are the second largest source of human-caused methane in the United States, and food waste contributes significantly to landfill methane production, the EPA says.

In California, there are 137 wastewater treatment plants that have anaerobic digesters for sludge, with an estimated excess capacity of 15-30 percent. This excess capacity could provide a potent recycling opportunity for post-consumer food waste in California, EPA officials suggest.

Copyright Environment News Service, ENS, 2009. All rights reserved.