For nearly a decade, EBMUD has been bravely converting sewage solids to energy at its Oakland wastewater treatment plant. Today it dedicated the addition of the system's newest component, a $19 million, 4.6-megawatt turbine similar to a jet engine that will be used to convert methane gas produced by decaying organic wastes into electricity. In conjunction with a trio of older engines from the 1980s with a total capacity of 6.2 megawatts, the new turbine allows EBMUD to become the first wastewater treatment facility in the nation to produce enough energy from organic waste to sell some back to the grid.
The system is fed by organic solids filtered from the 75 million gallons of wastewater the facility processes every day, plus frequent truckloads of such goodies as restaurant grease, cheese waste, chicken blood, and winery wastewater. Together with solids separated from municipal wastewater, these highly concentrated wastes are stored in one of twelve two-million gallon digesters on site, at a temperature of about 100 degrees, for two to three weeks. As bacteria eat up the organic material, a number of byproducts are released, about 65 percent of which is methane gas, which is then captured and fed into the turbine or engines to create electricity. Prior to the installation of the new turbine, the system was producing so much methane that some of it had to be burned off.
After the digestion process is complete, the biosolids that are left over are run through a centrifuge to remove liquids, then shipped to one of two places: farms growing non-food crops in Merced County, for use as a soil amendment; or landfills in Livermore and Fairfield, for daily cover of stinking piles of trash.
EBMUD Associate Engineer Alice Towey said that the energy production of the system varies; this morning it was producing six megawatts, five of which were fulfilling all of the plant's energy needs and the last of which was being sold back to PG&E. On weekends, when the volume of available organic waste is less, the plant may need to import some electricity. "We'd like to be a net-energy producer on a yearly basis," Towey said.
Towey noted that other wastewater treatment facilities in Sacramento, San Francisco, and Watsonville are exploring similar waste-to-energy systems. EBMUD's Oakland plant is likewise looking to significantly expand its collection of organic waste, increase its energy output, and potentially replace its three older engines with another high-efficiency turbine.