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Caterpillar's innovative cogeneration solution helps Gresham WWTP achieve energy net zero goals

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Caterpillar helps municipalities around the world achieve their sustainability goals through innovative solutions that deliver higher energy efficiency while reducing energy costs.

Caterpillar is a leading supplier of cogeneration technologies and has expertise in designing, installing, and maintaining power plants that convert waste gas from wastewater plant processes into electric power and heat.

A prime example of Caterpillar’s successful support for wastewater treatment facilities is the installation at Gresham Wastewater Treatment Plant (WWTP), a basic secondary-activated sludge facility located near the Columbia River northeast of Portland, Oregon, U.S., that receives an average of 10 million gallons/day (mgd) of wastewater.

Dating back more than 30 years, efforts have been made to curb the plant’s big appetite for power, according to Alan Johnston, senior engineer with the City of Gresham Wastewater Treatment Plant. Anaerobic digesters were installed at the plant in 1990 in an effort to make the plant more efficient by deriving biogas from the solid waste matter that settles out during the treatment process. Raw, untreated biogas derived from the digesters fed a 250 kW generator that helped provide power to about one quarter of the plant.

A prime example of Caterpillar’s successful support for wastewater treatment facilities is the installation at Gresham WWTP

In 2005, after receiving grants from the Energy Trust of Oregon and the Oregon Department of Energy, a Cat® G3508 gas generator set was installed that produces 400 kW in a combined heat and power (CHP) application. The cogeneration system includes a modern biogas scrubbing system that removes moisture, hydrogen sulfide and siloxanes. The Cat generator set produces power and heats buildings with jacket water heat. It has been supplying 50% of the treatment plant’s power needs since 2005, saving about $250,000 in annual avoided electrical costs.

Even with the addition of this unit, excess biogas had to be flared, Johnston says. In 2007, Gresham’s mayor, Shane Bemis, signed a climate action agreement with the U.S. Conference of Mayors, which ultimately led Johnston to examine ways to make the treatment plant even more energy efficient.

After further analysis, Gresham decided to shift its focus in 2008-09 to reduce power consumption while increasing biogas production.

Raw, untreated biogas derived from the digesters fed a 250 kW generator that helped provide power to about one quarter of the plant

The journey to energy independence began in 2009 when the Gresham WWTP received a grant from the Oregon Economic Development Commission to study ways for increasing the environmental and operational efficiency of the treatment plant. One outcome was a study on the benefits of accepting fats, oils, and grease (FOG) from restaurants to remove them from the waste stream and instead use them to boost electrical generation.

Based on revenues generated by FOG tipping fees at the facility and avoided electrical utility fees, the conclusion was that it would be cost-effective with an ROI of seven years.

Gresham acted on the recommendation, taking in about 15,000 gallons per day of FOG. The product is slowly injected into the digesters, and according to Johnston, the organic matter has nearly doubled biogas production.

The journey to energy independence began in 2009 when the Gresham WWTP received a grant from the Oregon Economic Development Commission

“FOG has a lot of energy stored in it; about 12 cubic feet of biogas produced for every gallon injected into the digesters,” Johnston says. “The study concluded that adding FOG receiving facilities is economically viable, and it turns out that there is a market for this service in our area.”

For Gresham, net zero became a real goal in 2010 when investments were made as part of the capital improvement cycle. A formal energy management team was created, and Gresham established a goal of achieving energy net zero at the WWTP by 2015.

The increase in biogas made possible the installation of a second Cat G3508 gas genset in 2015 to increase power output.

Today, the WWTP generates about 10% more electricity than it needs via a combination of energy efficiency, codigestion and cogeneration

With support from management and staff, five major capital upgrades were phased in over a five-year span. In February 2015, on schedule and on budget, the first energy net zero month occurred. The WWTP generated more electrical energy on site from renewable biogas cogeneration and solar power than it consumed. Today, the Gresham WWTP generates about 10% more electricity than it needs via a combination of energy efficiency, codigestion and cogeneration, and a solar array.

Over the 10-year journey to energy net zero, the Energy Trust of Oregon supported the City of Gresham with technical assistance, project development support, and cash incentives that lowered the costs of the energy efficiency measures — as well as the two CHP generator sets and the facility’s solar array. Business Energy Tax credits and biomass incentive funds from the State of Oregon were also essential in making net zero a reality.

The net result is about $1 million in annual savings to the ratepayers of Gresham, which includes $500,000 in avoided utility costs, $350,000 from FOG tipping fees, and the balance in avoided costs by using the jacket water heat from the generator sets to heat buildings. As an added benefit, Class B biosolids that are the by-product from the anaerobic digesters are applied to agricultural fields as fertilizer at no cost to area farmers.

Achieving net zero status is made possible by the high uptime of the Cat generator sets, Johnston says. To maintain continuous operation, Gresham relies on its Cat dealer, Peterson Power Systems, which provides service through a Customer Support Agreement.

“One of the big reasons why we have a successful program here is because of the relationship we have with our local Cat dealer, Peterson,” he says. “We try to average over 90% run time. That includes all the overhauls, top end work, oil changes, and unrelated construction issues that take those engines down.”