Cleantech: Tapping Wastewater for Energy

By Published On: May 13, 2011

San Diego Public Utilities’ Wastewater Branch is on verge of converting all the biogas produced from its sewage treatment operations into electricity, both for its own operations and for San Diego Gas & Electric. The city also produces electricity with photovoltaic panels and a hydropower turbine that taps the energy of water as it falls from its main treatment plant on Point Loma into the sea. All told, it soon is set to become the source of 22 MW of power. “We’ve taken all the squeal out of the pig,” laughs Tom Alspaugh, senior mechanical engineer and manager of the branch’s energy production projects. He noted the agency finally has figured out how to turn all of the gas it produces into usable energy. The agency first began producing power during the late 1970s at its Point Loma Wastewater Treatment Plant by burning digester gas in a Solar turbine then considered experimental. In 1985, it installed two reciprocating engines to turn the gas into power, as well as a hydro turbine in the outfall, which drops almost 100 feet. Combined, Alspaugh explained, the Point Loma facility produces about 5.5 MW of power. It uses 2 MW and the rest goes to SDG&E. The Point Loma plant treats about 175 million gallons/day of wastewater generated in a 450-square mile area by more than 2.2 million San Diego residents. When the agency upgraded the wastewater treatment plant’s biosolid digesters in 1998, Alspaugh explained, it wound up with surplus gas it had to flare. Now, it plans to turn that gas into usable energy under a contract with San Diego-based Biofuels Energy LLC. Under the deal announced last year, Biofuels Energy is to clean up the gas and inject it into SDG&E’s pipeline system. Then, an equivalent amount of gas is to be used to make power in fuel cells being installed at the agency’s South Bay Water Reclamation Plant (which does not produce gas onsite) and the University of California at San Diego. One fuel cell is to produce 1.4 MW of electricity to run the South Bay plant. Another larger fuel cell is to make 2.8 MW of power for the university campus. A third smaller 300 kW fuel cell is to power the digester gas purifier at Point Loma. Biofuels Energy is to pay the agency $2.6 million for the gas over the next 10 years, plus the company estimates the agency stands to save $780,000 on electricity costs over that period. “This is a model that can work for other municipalities to generate revenue and renewable electricity from their waste streams,” according to Frank Mazanec, BioFuels managing director. Meanwhile, since 2003 the agency has been meeting 20 percent of its electricity needs at its main building complex using a 30 kW photovoltaic system that it owns and operates. In addition, it has power purchase agreements entered into over the last three years with Sun Edison, which has installed two 1 MW solar systems at two of its facilities. Last, but not least, the agency makes about 6.4 MW of power for its Metro Biosolids Center with both gas from the digesters there and also gas piped in from a nearby landfill. The agency ships biosolids to the center both from its South Bay facility and nearby North City Reclamation Plant for digestion. The center uses about 30 percent of the power there and sells the remainder to the grid. Additional gas from the landfill is piped over to the North City plant to make another 3.8 MW of power, 75 percent of which is used on site with the rest going to SDG&E. It would seem that the agency has fully tapped its energy production potential, yet Alspaugh promises more coming attractions still too secret to mention. So stay tuned to this San Diego agency.

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