Whether it’s a combined heat and power system, fat, oil and grease (FOG) management, solar energy or even hydropower, the Gresham (Ore.) Wastewater Treatment Plant leaves no stone unturned in its quest for energy independence.
The plant and its operating staff are well on the way toward that goal. This 20-mgd activated sludge facility has been operated since 2005 by Veolia Water North America. Sixteen Veolia employees work there. Alan Johnston, senior engineer with the City of Gresham Wastewater Services Division, oversees the contract with Veolia.
Energy savings have always been a priority for the city. “The ultimate goal is to make sure we’re doing what we can to manage costs,” Johnston says. “Keeping our rates low and doing everything we can to ensure long-term sustainability is a benefit to everyone.”
The plant got a jump on energy savings in 1989 when it installed a 250-kW combined heat and power (CHP) system using digester gas for fuel. The system supplies a portion of the plant’s electric power and provides process heat for two digesters, which produce gas containing 60 percent methane.
The plant also initiated a number of energy-efficiency measures (EEMs) in the mid-1990s. These include such activities as using motion sensors for lighting and replacing inefficient motors with energy-efficient units on a routine basis.
By 2003, the CHP system was nearing the end of its useful life and operating inefficiently. The facility was flaring 41 percent of its digester gas. In late 2005, the city installed a more efficient 395-kW CHP system. The unit routinely operates near full capacity to supply nearly half of the plant’s 800-kW peak load.
It also can be manually switched to backup mode to keep critical components online during a utility outage. The system includes:
â¢ An eight-cylinder Caterpillar G3508 engine-generator set specifically designed to use low-pressure digester gas.
â¢ A fuel treatment system that removes moisture and contaminants. It consists of a chiller to condense moisture, a packed bed reactor to remove hydrogen sulfide, and two packed bed reactors to remove siloxanes.
â¢ A heat recovery system that produces water at 180 degrees F from the engine coolant and exhaust. The hot water heats the plant’s two digesters to 91 degrees F. In cold weather, some of the hot water heats the facility’s administration building.
â¢ A sophisticated control system that provides multiple operating modes, depending on the system conditions and facility loads.
Back on track
Since its startup in 2005, the CHP system has performed to expectations, and staff members credit it with helping the plant get back on track with its energy- and cost-management goals. To date, the system has achieved 95 percent availability and produces electric power savings of $18,000 to $20,000 per month.
Johnston also says the ability to tap into the unit’s hot water reduces the amount of natural gas and electricity the plant would otherwise need to purchase to heat the digesters and the administration building. Savings are significant, although he has yet to calculate them. Flaring of digester gas has been reduced to zero. The payback for the system is projected at 5.9 years.
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“The numbers show that it’s a very productive and very successful machine,” says Veolia project manager Paul Proctor, who oversees daily treatment plant operations. He says the plant has only encountered relatively minor issues with the system.
“If we run into an issue, we’ll troubleshoot it ourselves to find out if it’s something we can address ourselves,” Proctor says. “If it gets into the machine itself, we’ll call in the local Caterpillar dealer.”
On a daily basis, Veolia Water’s crew checks the system for oil and water leaks. The team also pays close attention to the fuel treatment system to ensure it’s delivering contaminant-free fuel.
“You have to give credit to a good machine to achieve high uptime, but I also think a real key is providing clean gas to engine,” he says. “In terms of the bang for the buck, we get a lot of value out of the unit for the investment and the effort we put into it. It’s a real win for the city and the plant.”
Johnston couldn’t agree more. A side benefit of the CHP system, he says, is that it helped stimulate a stream of fresh ideas for gaining energy independence. “If the CHP system is producing 55 to 60 percent of the energy to power the plant, we figure we’re close to being energy-independent,” Johnston says. “So we’ve asked ourselves, ‘What else can we be doing?’ There’s got to be better ways to manage our power.”
After studying the issue, officials learned of a number of ways to better manage power and become totally energy-independent. The solution will take the shape of alternative energy sources, third-party purchase agreements and best industry practices.
One alternative energy source that shows promise is solar power. A recent study funded with help from Energy Trust of Oregon Inc. demonstrated that the plant could produce all the power it needs by installing a large solar power system on 20 acres of unused land. But rather than jump in with both feet, the city has taken a conservative approach. Toward that end, it let bids to companies willing to build a small-scale solar system on the land and sell the power back to the plant.
And the idea is starting to take hold. Gresham is negotiating an agreement for a 400-kW solar power system that would produce about 5 percent of the plant’s power for six cents per kWh — two cents less than it pays for utility power. The goal was to have the system up and running in January 2009.
In 2007, the plant also began an aggressive FOG outreach program. The ultimate goal is to inject FOG directly into the plant’s two digesters and capitalize on the additional methane produced. “Based on what we’ve seen in the industry, FOG produces something like three times as much methane as regular wastewater, and to us, that’s a tremendous source of energy,” Johnston says.
The city has been working diligently to assess the volume of FOG it can capture and put to good use. To do so, the city assigned an employee to visit food service establishments and accurately record the amount of FOG produced by each throughout the course of a year. The city is also exploring grants to study the concept further. That includes finding a way to get the FOG to the plant and process it.
Yet another concept on the table is what Gresham calls “micro-hydro power” — installing a turbine at the plant’s outfall. “It would be on a very small scale with a peak output of something like 40 kW,” Johnston says. “But that pipe is pushing out water, and it’s got energy in it, so we want to know what we can do to harness that energy. It would be another piece of low-cost energy in our pocket.”
Moving toward success
Johnston says energy independence is definitely in the plant’s future. It’s just a matter of creating its own sources of power for the right cost. “We’re already saying we can achieve energy independence just with solar,” Johnston says. “But the capital costs of solar are tremendous, so that’s why we’re working to find another way around it.
“I think it’s great to say you’re energy-independent and you’re using green energy, but you have to be careful. You have to understand lifecycle costs, crunch the numbers and look at payback.”
Johnston says it’s not a matter of whether the plant will succeed but when. He said officials are most excited about the potential of a FOG program. “If you implement third-party agreements like we’re doing with solar, and combine it with other kinds of power production, energy independence is absolutely possible,” he says. “And each one of these initiatives gets us closer to our goal. Ultimately, it’s about saving money and sustainability, which benefits everyone.”
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