Hayward is turning its wastewater green.

This week, the Bay Area city unveiled a 1-megawatt (MW) solar-energy system built by REC Solar, offsetting 24 percent of Hayward’s wastewater treatment plant’s energy needs and thus saving 24 million pounds of carbon dioxide over a projected 25-year operating life. The installation covers about eight acres and will produce enough energy to power more than 153 homes.

Since the treatment plant is the biggest consumer of energy in the city of 220,000 inhabitants, the facility must take further steps to reduce greenhouse-gas emissions and energy use. The facility has been operating a co-generation plant using biogas that met about 30% of the plant’s energy needs. The plant can treat 12 million gallons of wastewater daily, consuming 8.9 million kWh of energy a year.

“Approximately 25 percent of the plant’s load will come from the solar system and the rest from the grid,” said Dave Morosoli, vice president of REC Solar’s commercial division.

Hayward’s ‘go green’ wastewater-treatment facility is part of a growing number of sites combining wastewater and solar. It makes sense: wastewater treatment plants cover a lot of ground, which gives them lots of roof real estate. Sun Power, for example, installed a 520 kW solar system at its wastewater treatment plant in Oroville, Ca in 2005 -- the first solar wastewater-treatment plant in the U.S.

In November 2010, REC Solar installed a 1.1 MW solar system at a wastewater treatment facility in the city of Madera. It will produce 2.4 million kWh of energy annually and will reduce about 47 million pounds CO2 emissions over the next 20 years.

For now, all these plants are operating on the co-generation of solar and conventional energy, but some are completely self-sustainable. “They can be completely sustainable, because they can use gas from the waste stream as a feed source. Combining that with renewable energies such as solar and wind, you could actually build a facility that operates on its own power source, completely off the grid,” explained Morosoli.