Sparing Sewers All That Restaurant Grease

Every hour of every day, plants across the United States treat the waste that flows through the nation’s sewers. The process is energy-intensive. Wastewater treatment plants often consume more electricity than any other service a city provides — sometimes even as much as 30 to 40 percent of its overall energy consumption, according to the Environmental Protection Agency.

In a push to produce more of its own energy, a wastewater treatment plant in Gresham, Ore., is turning to an unlikely source to generate power: the gray-white grease-filled wastewater that flows out of restaurants and other places where food is prepared. This month the treatment plant aims to begin using restaurant wastewater to make heat and electricity in a process dubbed “fats, oils and grease” for its contents.

The method makes use of anaerobic digesters that the facility already deploys to process solid waste. As the digesters’ microbes break down the waste, they release a mix of methane and carbon dioxide similar to natural gas. Since the 1930’s, some wastewater treatment plants have used this byproduct of breaking down sludge to generate heat and electricity for their operations, said Charlotte Ely, who works on water and energy efficiency issues for the E.P.A. in the Pacific Southwest.

Gresham’s plant, which serves about 120,000 people in three cities east of Portland, has been making use of the digesters’ gas byproduct since 1990. That generates all of the heat for the plant’s buildings and about half of the electricity that the facility needs each year, said Alan Johnston, a senior engineer for the city of Gresham who oversees the treatment plant.

In January 2010, the city installed an array of solar panels that currently supply about 450,000 kilowatts hours annually, or about 8 percent of the power that the plant needs each year, he said.

Now the Gresham plant is ready to feed greasy wastewater brought in from restaurants all over the Pacific Northwest by three hauling companies to its digesters, Mr. Johnston said. He expects that adding the energy-rich restaurant wastewater to the mix — a common culprit for backed-up sewers — will increase the amount of gas that the digesters release by 40 percent.

The city began considering adapting its plant’s digesters to take in restaurant waste about five years ago, Mr. Johnston said. Then, in 2009, the city of Gresham received a grant from the state’s economic development department to study how feasible such a project would be. When city officials saw that the project would generate enough revenue to pay for itself in two and a half years, they gave it the green light, Mr. Johnston said.

The city constructed a tank to take in the grease-filled water and a pump to inject it into one of the plant’s digesters, a capital project with a price tag of $750,000. Gresham’s plant will be able to take in about 10,000 gallons each day, and haulers will pay eight cents a gallon to unload the wastewater at the plant instead of at a landfill, Mr. Johnston said.

That alone should cover the cost of building the tank and pump to take in the grease-filled water by the end of 2014. The city also expects to save 15 to 20 percent on the plant’s electricity bill, which amounts to a total of about $230,000 each year, Mr. Johnston said.

The Gresham plant, like many others around the United States, has ambitions to become increasingly self-sufficient. The city, which plans to add another engine to turn the digester gas into heat and electricity, hopes that in two years its plant will be able to produce all of the energy it needs, Mr. Johnston said.

Ms. Ely of the E.P.A. said: “It’s kind of a no-brainer for a lot of facilities. The money that you save, you can then invest in capital improvements.”

An earlier version of this post used the wrong unit in referring to the electricity generated by a solar array at the wastewater plant in Gresham, Ore. It produces 450,000 kilowatt-hours a year, not 450,000 kilowatts.