Cogeneration, FOG injection to digesters, solar power and conversation help the Gresham treatment plant meet its goal of energy self-sufficiency


What used to be the City of Gresham’s largest energy user is now its biggest energy producer. The community celebrated the achievement on Earth Day 2014, capping off a decade of work by Gresham Wastewater Treatment Plant staff.

Alan Johnston, P.E., senior engineer, says the plant met its goal to achieve net zero energy consumption right on schedule.

Gresham, in Oregon just east of Portland, began using biogas-fueled cogeneration in 1989 but installed a modern cogeneration engine in 2005.

Related: Sheboygan WWTP Achieves Net Zero Energy With Biogas

“That was the start of a 10-year process of looking at energy efficiency and energy production projects,” says Johnston. “In 2010, we formed an Energy Management Team and set the goal of being energy net zero within five years.”

In January 2015, the second of two 400 kW Caterpillar 3508 lean-burn engine-generators went online, helping the plant generate 92 percent of its daily electricity demand of 15,000 kWh. The rest comes from a 420 kW fixed solar array with 1,902 panels, installed in 2010 at no cost to the city under a 20-year power purchase agreement at about two-thirds of the utility rate.

All about budgeting
The city has invested about $5 million over 10 years in energy projects, aided by $4 million in grants.

Related: Greening the Plant: Solar Proves its Power

“For us, there were two main sources of grants: the Energy Trust of Oregon and the state’s Department of Energy,” says Johnston. “It’s important that you constantly monitor grant opportunities and talk to those people to find out exactly what’s available.”

Rather than hiring consultants, city staff wrote all the grant proposals. 

“We became expert grant writers," he says. "We slowly met all the people in those agencies and got on their email list for grant opportunities. That really helped.”

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Having a zero monthly electric bill helps keep rates low for the 114,000 customers of the 13 mgd plant. The city saves about $500,000 a year in avoided utility costs while bringing in about $250,000 from a fats, oils and grease (FOG) tipping fee of 8 cents per gallon.

Before building the FOG receiving station in 2012, the plant produced about 180,000 cubic feet of biogas per day. Injection of FOG into the digesters boosted production to about 280,000 cubic feet. After tripling FOG storage capacity in 2014, the plant increased gas production to an average of more than 300,000 cubic feet per day.

Running full time
“We haven’t had to turn off either of the cogeneration engines for lack of available biogas since we started them,” says Johnston. “We chose not to spend another million dollars to put in more gas storage – we couldn’t justify it.”

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Any excess biogas is flared, though that doesn’t happen often. It did happen one day in May 2015 when biogas production reached 530,000 cubic feet because a hauler delivered an especially high-strength load of organic waste.

Other plants use extra biogas to generate power for sale back to the grid, but that’s not the case for Gresham. While about 5 percent of its generation may get exported to the grid in a typical month, the net metering agreement with Portland General Electric does not result in revenue – only kilowatt-hour credits on future bills. The plant could sell the power to PGE, but only at wholesale rates, much lower than the retail rate.

Johnston expects to use up most of the credits for times when the engines are down for maintenance or if electricity demand exceeds production. The PGE program includes a provision to help the community: Any credits not used by the plant by March 1 of each year are transferred to a fund to help families who get energy assistance from PGE.

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Driven by a plan
Gresham’s Energy Management Team meets monthly to review progress on written goals involving power generation, conservation and consumption. Operated by Veolia Water NA, the plant has cut is electrical demand by about 17 percent over the past five years. “The Energy Management Team is a big part of why we’re where we are at today,” says Johnston. “It forces us once a month to talk about nothing but energy.” Most of the energy savings came from three projects:

  • Upgrading the digester mixing process by replacing three 40 hp gas compressors with a pair of 5 hp linear motion mixers (Ovivo)
  • Replacing two centrifugal blowers for aeration with APG-Neuros turbo blowers
  • Switching to fine-bubble diffusers (Sanitaire)

There were many other smaller energy efficiency projects, such as an automated system to capture heat from the engines, which are cooled with plant effluent rather than potable water, for building and digester heating.

The plant team took a creative approach to converting all exterior lighting to LEDs. “I didn’t want to spend a lot of money on it because the math didn’t show that it was all that helpful,” says Johnston. “We partnered with a contractor doing a major LED street lighting project for the city. We allowed them to stage their equipment on 25 acres of our property for a year for free if they replaced all our exterior lighting for free. Those types of public-private partnerships are out there. You just have to look for them.”

Not done yet
The net zero goal is accomplished, but that doesn’t mean the end of the effort. “We’ll continue meeting monthly with the Energy Management Team to look at ideas,” says Johnston. “There are still things out there, like micro-hydro in-line technology for our effluent. I’d still like to see windmills. They aren’t cost-effective yet, but we’ll continue to look for grants and other things to see if we can make that happen someday.”

To help promote energy efficiency, the plant has partnered with the local Reynolds School District, Pamplin Media Group, and MetroEast Community Media to produce elementary classroom materials, a booklet and a video about the energy-producing processes at the plant. It will be given free to schools and to student who visit the plant.


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