SAN ANTONIO (AP) — The city plans to turn the stench of its residents' waste into sweet green cash and renewable energy.

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, Mass.-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."