The Durham Treatment Facility has used cogeneration since 1993, but the 500 kW system was too small and getting old. Serving 250,000 people around Tigard, just south of Portland, Oregon, the 20 mgd plant was flaring excess biogas and the engine wasn’t as clean or efficient as today’s technology.

“It was an old rich-burn system with no gas treatment, and we were producing about 30 percent more gas than the engine could use,” says Bruce Cordon, business opportunities manager for Clean Water Services, the regional wastewater management utility. “That’s wasteful.”

In its place, the plant opened a 1.7 MW cogeneration system in April 2015 that uses all available biogas and has capacity to use gas produced in a fats, oil and grease (FOG) receiving facility. The old engine was replaced with two 848 kW gas engines (Jenbacher). Unison Solutions provided the gas treatment system.

Between the new cogeneration system and a 2-year-old 403 kW fixed solar array, about 60 percent of the plant’s power comes from renewable sources. That’s about 12,800 MWh per year, enough electricity to power 1,100 homes and avoid producing 6,000 tons of carbon dioxide.

The $16.8 million project, which included some smaller plant renovations, received $3 million in cash incentives from Energy Trust of Oregon and $2.8 million in tax credits from the Oregon Department of Energy. It saves $800,000 per year in energy costs, including $100,000 for heating, and will produce some $340,000 in tipping fees.

Lessons learned

As much as FOG has helped, it also brings challenges. “The FOG does increase your capital costs because you have to build the infrastructure,” Cordon says. “And there’s always the question of what it’s going to do to the digesters. We had some nervousness about that, but we knew of some places that had done this. Our engineers went to some of those and talked to the plant staffs.”

The introduction of FOG was slowly ramped up and caused no digester issues, but the material produced more energy than expected. “Originally, we were thinking we’d need 100,000 gallons of FOG per week,” says Cordon. “We’re getting about 70,000 gallons, and that’s enough to run our engines at full capacity.”

That doesn’t mean the going has been completely smooth. “We have four contractors that deliver to us,” Cordon says. “Sometimes they don’t show up when they’re supposed to, or they give us more FOG than we bargained for. The scheduling part can be a bit of a challenge. And we’ve had a few problems with contaminants like chunks of concrete.

We do have a rock trap that helps clean out that material before it gets too far into our system, but it’s not foolproof.”

For a while, the plant also accepted waste from a food processer. “It turned out the FOG was much better, probably twice as good when it comes to gas production,” says Cordon. “We told the food processer we’d have to charge a higher tipping fee, but they balked at that and we parted company. It took several months to run through that experiment, and we switched to FOG exclusively in the fall of 2015.”

Solar helps

The solar array on plant property, owned and operated by SolarCity, provides 450,000 kWh per year to the plant through a power purchase agreement. That’s about 2 percent of what the plant needs. Cordon would like to expand solar resources, but there are some barriers.

“We have space for it and are going to be replacing roofs on some buildings, so it makes sense to bring in more solar,” he says. “But we’re up against a problem I think is something you can encounter in other states. For net metering (with local electrical utilities) in Oregon, you can’t have more than 2 MW of generation on your site. We’re already up against that at Durham.”

So Cordon continues to explore options for increasing sustainability at Durham and other Clean Water Services facilities.

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Market forces hamper sustainable solutions

Clean Water Services operates four wastewater treatment plants serving 560,000 customers around Washington County, south of Portland, Oregon. Since 2004, the agency has worked with Energy Trust of Oregon on more than 100 improvements, creating more than 9 million kWh in annual electricity savings.

But not everything has worked out as planned. A proposed vehicle biogas project at the 30 mgd Rock Creek Advanced Wastewater Treatment Facility has been held up by low fossil fuel prices and a lack of a market for its excess biogas.

Rock Creek’s old cogeneration plant provides 30 percent of its electricity with two 500 kW Superior engines; a 65 kW SolarCity system also supplies power. There was a plan to invest $3.5 million to capture its surplus methane and compress it into vehicle fuel using $1.2 million in federal grants through the Oregon Department of Transportation and another $880,000 in tax credits from the Oregon Department of Energy. The project would have produced the equivalent of about 600 gallons of diesel fuel per day.

“Compressed natural gas vehicles haven’t really taken off in Oregon,” says Bruce Cordon, business opportunities manager for Clean Water Services. “We don’t have a regulatory mandate like California. We have people willing to buy our gas and ship it to California and sell it. The problem is how to put the gas into the pipeline. Our local gas utility has impossibly high quality standards that have to be met.”

One company approached Cordon about transporting the gas by tanker to a nearby interstate pipeline, but hasn’t made a firm offer. For lack of a market, Cordon had to turn back the grant and the tax incentives.

Another firm broached the topic of gasifying the plant’s biosolids. “We want to look at this hard from a feasibility standpoint,” Cordon says. “They tell me it’s a low-energy gas so I’m not sure what we would do with it. One thing is certain: We’d end up with a lot less solids to manage.”

The plant ships about 11,000 dry tons of biosolids over long distances annually, mostly to eastern Oregon farms. That adds up to about 500,000 miles per year and 100,000 gallons of diesel fuel.

A startup company approached Cordon about a new process to convert biogas into a biodegradable plastic using microbes. “I don’t think they’re quite ready for prime time,” he says. “But it’s interesting to think about.” In the meantime, Rock Creek continues to flare biogas. Says Cordon, “We’d like to change that situation.”