By Caitilin McAdoo

April 30, 2008

If California converted its annual 5.9 million tons of food waste into biogas, it could produce enough energy to supply nearly 75 percent of Bay Area homes with electricity for a year, according to results from a new study conducted by the East Bay Municipal Utility District.

The study, funded by a 2006 grant from the U.S. Environmental Protection Agency, looked at the effectiveness of using anaerobic digesters to process food waste and convert it into gases to be used for renewable electricity generation.

Anaerobic digestion is a well-known technology that uses naturally occurring bacteria to break down solid organic waste into carbon dioxide and methane gas, known as biogas.

During the study, EBMUD used about 40 tons of food waste collected from restaurants, grocery stores and other food handling facilities and fed it to its anaerobic digesters under a variety of conditions.

Data from that study found that anaerobic digestion of food waste yields about three times more usable energy than digestion of wastewater solids.

According to EPA Life Scientist Cara Peck, food waste is the largest category of municipal solid waste in California, accounting for about 12 percent of overall municipal solid waste. If the state were to run all its food waste through anaerobic digesters, it could produce enough energy to supply nearly 5 million homes with electricity each year, according to Peck.

“We have this resource that’s already there that we’re not using,” Peck said.

There are other advantages to diverting food waste out of landfills as well.

According to Peck, food waste degrades quickly in landfills, creating large amounts of greenhouse gas emissions.

Composting is an effective way to treat food waste, but most large-scale composting plants are in rural areas where land is available, and the waste has to be brought to them in trucks.

If food waste were to be digested locally and converted into biogas before being trucked to composting areas, its volume would be reduced by about 80 percent, Peck said.

It could then be taken to composting sites more efficiently, where it could be further broken down into an important soil nutrient, Peck said.

According to the study, about 137 wastewater treatment plants in the state have anaerobic digesters, which currently have about 15 to 30 percent unused capacity.

There are, however, several hurdles to get past before biogas could be used to supply regional power grids.

While many wastewater treatment facilities already use biogas created on site to power their own plants, they are not currently able to sell it back to power companies, according to Peck.

“At this point they don’t have the safety standards, but we’re pushing for that,” Peck said.

Plants that have their own generators and are able to convert biogas into electricity can sell that electricity back to the power grid, but current regulations only allow them to sell back as much as they use, another problem the EPA is working to address, Peck said.