The sewer collection system has experienced sanitary sewer overflows (SSOs) due to grease discharges from restaurants. Problems have arisen with the disposal of the grease interceptor wastewater. Many sanitation agencies are refusing to accept out-of-area grease interceptor waste. Landfills have banned the disposal of grease interceptor wastewater.
The city researched the use of grease wastewater additions to the anaerobic digestion process to increase gas production and discovered this was a viable solution to dispose of restaurant grease wastewater. On April 27, 2005, the city began the Grease to Gas to Power project. The Grease to Gas to Power project uses grease wastewater collected from restaurants and adds it to the anaerobic digestion process.
The anaerobic digestion process is sort of like a stomach. We feed the anaerobic digesters food (sewer waste and grease wastewater) and let methane creating microbes feast on the solids to create a gas. This gas can be treated and used for power.
By adding grease wastewater to the anaerobic digestion process the RWQCP was able to increase methane gas concentration from 53.0 % to 67.0 % and the average BTU value of the methane from 536 to 626. Pure natural gas is 96% methane with a BTU value of 906. The volume per day of methane gas produced in the grease digester increased by 133% with a peak of 493,000 cubic feet.
The increase in daily methane gas production is enough gas in one day to heat 1,213 homes for one month in Riverside. The electrical power generation of about 1.6 megawatts per day is enough electrical power in one day to provide the electrical needs of 1,203 homes for one month in Riverside. The effects on the sewer system were equally favorable. The sanitary sewer overflows caused by restaurant grease blockages were reduced from 30% of all calls to less than 1%.
The project currently receives about 30,000 gallons per day of grease wastewater from restaurants throughout southern California.
One of the goals of this project is to achieve energy independence from natural gas. The project has reduced the natural gas requirements of the cogeneration power plant by 80%. This yielded a monthly savings from $80,000 to $85,000 per month. The energy costs savings created by this project have saved the city almost $1,000,000 in one year.
Grease to Gas Awards
The City's Public Works Sewer Division Grease to Gas to Power project has achieved the following: