Converting cooking grease to methane gas

Restaurants across America generate a massive amount of waste cooking grease, oils and fats on a daily basis, much of which either ends up in landfills or is poured down drains where the substances are either processed in wastewater treatment plants, coat and clog sewers or contaminate the eco-system.

Efforts to re-use this resource include using grease as a transportation fuel source for vehicles with engines modified to burn it, but this idea still has limited application.

Disposing of this grease is becoming harder as more landfills no longer accept it and cities that formerly processed grease from outside cities and counties, now only accept locally produced grease. Hence, disposal fees are increasing, which impacts cities and restaurants.

However, Riverside, California, with nearly 300,000 residents, is utilizing the grease collected from restaurants to increase production of methane gas, which is used to power the city’s water treatment plant.

A local waste management company collects the grease from area restaurants, pre-treats it (rendering and screening the grease) and then transports it to the City’s wastewater treatment plant for injection into the plant’s anaerobic digesters. Riverside, like most major metropolitan areas, requires restaurants to properly and environmentally dispose of the grease they generate. Thus the collection service is something restaurants would be in need of regardless of the program.

“Restaurant grease is an untapped source of energy that is not being harvested,” said David Wright, general manager of Riverside Public Utilities.

Wright’s department allocated more than $16,000 to the public works department, which in April 2005, initiated the pilot phase of the grease-to-gas-to-power program to increase production of a highly renewable fuel. The program was developed by Riverside’s wastewater staff that learned about the possibility of using grease from industry publications that were following ongoing experiments at the time.

“Following a test period that lasted for several months, analysis of the data supported the economic viability of the program and it was continued on a full-time basis in April 2006,” said Richard Pallante, Riverside’s Wastewater Systems manager and one of the project designers.

“It has been very valuable to the community,” said Kevin Street, Riverside’s regulatory compliance manager. “We are not the only city doing this. There are some industries that have already implemented similar technology, but we are one of the few cities in the country to have such a grease-to-gas-to-power program.”

Street stressed that pre-treatment of the grease is essential, as it removes some of the larger debris which could negatively impact the injection process if it enters the system.

“It also provides a mechanism to ensure consistency with the quality of product that we are getting,” he said. “In the wastewater treatment business, anaerobic digestion of solid waste is common practice. The breakdown of solids is enhanced with the addition of restaurant grease as is methane gas production. For us, adding grease to the digesters not only increased the amount of methane we produce, but also the quality.”

The additional methane gas produced from restaurant grease now allows the city to run two of its three cogeneration engines at the plant around the clock, producing an average of 1.5 to 1.7 megawatts of power. Through cogeneration, the waste heat produced by combusting methane in engines producing electricity is recovered, which allows the city to reduce its purchase of natural gas.

Riverside anticipates the grease-to-gas program will save the city millions of dollars by reducing the amounts of electricity and natural gas purchased to run the wastewater treatment plant. In terms of actual savings, studies showed that between May and November 2006, the city saved $85,000 a month in natural gas expenses by using restaurant grease as a cogeneration fuel.

In-house staff was responsible for the design, construction and launch of the program. Developing the program cost the city about $100,000.

Thus far more than 40 cities across the United States have contacted city officials to learn more about the program.

Riverside’s program has been recognized and this has translated into prestigious awards such as the American Public Power Association Energy Innovation Award (2007), the California Utilities Association Efficiency Award (2007), The League of California Cities Helen Putnam Award for Planning and Environmental Quality (2007) and the Southwest Coast Air Quality Management District’s 20th Anniversary Clean Air Award (2008).

“In October we had folks from other agencies on-site taking pictures and being briefed on our program,” said Street. “One of the major attractions of what we have done here is that with a relatively small budget, a little investment on-site with the staff at the plant and the resources that we had available, we were able to put up a functioning system for a relatively low cost. Our system may have some limitations, but as far as starting out as a public pilot project and moving to full production, it has been extremely successful.”

Street’s explanation matches the view that many advocates of alternative energy stress – that the equipment and technology needed to generate such energy is currently available.

Street believes that Riverside’s success can be replicated in many cities, provided the proper planning and infrastructure to match the annual grease supply are put in place.

Riverside operates two 1.3 million gallon digesters in conjunction with its grease injection system. The two digesters collectively consume an average of 32,000 gallons of grease per day.

To further enhance energy production at the plant, the city recently installed a fuel cell which uses digester produced methane as its primary fuel. The fuel cell has increased on-site power production to up to 3 MW.

“This will allow the wastewater treatment plant to be even closer to energy independence,” said Street. “Even though we are only using grease at this time, it’s important to note that there are other waste streams that can be injected into the digesters which may have similar benefits.”

Such sources include materials high in sugar and grease content.

Riverside, which in 2005 adopted a Green Action Plan in regards to everything from power generation to tree planting, has kept its citizens informed about the success of the grease-to-gas program through the city’s website, public forums and council meetings.