SAN ANTONIO, Texas (AP) -- The city plans to turn the stench of its residents' waste into sweet green cash and renewable energy. Cities often sell water from wastewater treatment plants after it's cleaned. The San Antonio Water System will sell captured methane gas generated from the utility's treatment of 140,000 tons of biosolids, or sewage, from customers each year. The city-owned utility's board of trustees approved a contract Tuesday to provide at least 900,000 cubic feet of natural gas daily for the next 20 years to Ameresco Inc., a Framingham, Massachusetts-based energy services company. "Treating these biosolids generates an average of 1.5 million cubic feet of gas a day," said Steve Clouse, the water system's chief operating officer. "That's enough gas to fill seven commercial blimps or 1,250 tanker trucks each day." The utility already sells for reuse a portion of the water that's cleaned at its wastewater treatment plants. It also converts some biosolids into compost that's sold for use in yards and gardens. "As far as we know, SAWS is the only city in the United States that has completed the renewable recyclable trifecta," Clouse said. The water system will receive up to $250,000 a year for the methane, which will be drawn from the utility's Dos Rios Water Recycling Center. Clouse said it will take 18 to 24 months for construction of facilities needed for the contract. "We're very pleased that we can capture and sell this gas, which is good for San Antonio's air quality and puts this renewable energy resource to work for San Antonio," he said. Copyright 2008 The Associated Press. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.