Electricity from Wastewater

The world's most productive geothermal electrical generation site, Geysers geothermal field in Lake County, California, has been running out of steam. And Lake County Sanitation officials were facing expensive options for disposing of their partially treated wastewater. Now it appears that they have solved both problems with one elegant solution. In what is believed to be a first, partially treated wastewater will be used to recharge underground steam chambers feeding the Geysers geothermal field. Though water has been injected into the underground chambers in the past, water supply has been a problem recently in drought-stricken California, as has wastewater disposal. Together the two problems may literally solve each other.

Pipelines and injection systems for the project will cost $29 million, paid for jointly by utilities, the county, and state and federal government. Wastewater will be injected at a rate of 3,500 gallons per minute, to a depth of about 7,000 feet. At that depth a 400°F to 500°F layer of rock sits, heated by a magma chamber 30,000 feet down. Mark Dellinger, energy and resource manager for the Lake County Sanitation District, estimates that five times as much water could be injected without noticeably cooling the rock. The project is expected to generate 50 to 70 megawatts of power according to Dellinger, and will consume about 3 megawatts for water transportation and injection.