Energy-from-waste project at Santa Rosa treatment facility poised to slash power costs, reduce greenhouse gas emissions
By KEVIN McCALLUM
THE PRESS DEMOCRAT
Cleaning wastewater requires huge amounts of energy.
It has to be moved with pumps, blasted with pressurized air and zapped with ultraviolet lights.
Even after it’s cleaned, it has to be pumped to local farms and soccer fields or piped 40 miles to be injected into the Geysers steam fields.
All that makes the Llano Road wastewater treatment plant, operated by the City of Santa Rosa, one of the largest energy users in Sonoma County, absorbing between 3 and 7 megawatts of electricity daily.
But an $11.3 million upgrade now under way promises to slash the plant’s power costs, reduce its greenhouse gas emissions and set the stage for other energy efficiency projects in the pipeline.
“It would insane not to do this,” said Mike Prinz, supervising engineer in Santa Rosa’s Transportation and Public Works Department.
The plant, which opened in 1968, can treat up to 21 million gallons per day of sewage from the cities of Santa Rosa, Rohnert Park, Sebastopol and Cotati. About 60 percent of its power comes from PG&E, with the plant running up a $2 million annual power bill.
The remaining 40 percent is generated on site by three massive generators that burn a mixture of natural gas and methane captured from the breakdown of human waste in silo-shaped vessels called digesters.
The heat produced from those engines is used to warm the digesters to 98 degrees, further reducing energy costs, optimizing the breakdown of the waste and maximizing methane production.
It’s an energy loop called cogeneration that gets engineers like Prinz pretty excited about their job.
“I think it’s amazing that we can treat a waste stream and harvest energy out of it to improve the quality of water and generate electricity,” Prinz said.
But the current generators are getting old, new regulations are restricting the use of natural gas in such engines, and the building that houses them is hot, poorly ventilated and never designed for the purpose.
So the plant is midway through a major upgrade of its “combined heat and power” systems that has been in the planning stages since 2006.
A new building has been constructed, four new 40,000-pound generators have been installed, and workers are hooking up the pipes and other system components for testing in the next few months.
“We’ll be running by the end of the year, for sure,” said Terry Schimmel, mechanical superintendent at the plant.
The new Cummings engines are larger than those being replaced. Each is the size of a large SUV, has 16 valves operating at up to to 4,160 rpm, cranking out 1,500 horsepower and producing 1.1 megawatt. More importantly, they are 33 percent more efficient than the old ones.
That alone will reduce the city’s PG&E bill by about $575,000 a year immediately, Prinz said.
In addition, $100,000 annually will be saved because that extra muscle means the plant won’t have to rent two massive generators for three months every winter as backup generators.
The plant needs to be able to produce enough power to operate if the power from PG&E goes out. During the rainy months, the existing generators couldn’t handle the plant’s entire load, requiring the rental of the two backups.
The savings will lead to an expected payback period of 14 years on the project, which Prinz said is excellent given that the generators — technically advanced reciprocating engine systems — are expected to last 50 years.
Other improvements include the ability to capture more heat from the engines through a two-step process that heats water from the engines themselves and from their exhaust, which is discharged at 1,200 degrees.
Ratepayers, who have seen rates more than double in the past decade, are funding the upgrades. The cash was raised through a $49 million bond issue in 2008 that has also funded part of the new Utilities Field Operations building in Santa Rosa, water conservation and urban reuse projects, and storage improvements.
To get the most out of its new investment, the plant is exploring how to generate additional methane from its four digesters. The more methane that can be produced and harvested, the lower the plant’s PG&E bills and emissions. One idea is to add other kinds of waste — food scraps, animal manure or even pumice from the winemaking process — into the waste stream to increase the production of what is known as bio-gas.
It’s a complicated decision, taking into account factors such the energy used to gather the additional material and how it would affect the reuse of what is left behind, most of which is turned into compost, said David Guhin, deputy director of operations for the plant.
Ideally, when combined with other renewable energy projects, increased use of bio-gas will be a key piece of the plant’s long-term goal to get off the grid entirely and keep down costs for ratepayers.
“Zero net energy use. That’s the holy grail,” Guhin said.
(You can reach Staff Writer Kevin McCallum at 521-5207 or firstname.lastname@example.org.)
Project: Upgrades to power plant and heat-capture system at wastewater treatment plant.
Cost: $11.3 million
Reason: Aging generators, new regulations and increasing future use of bio-gas.
Power: Four 1,500-horsepower, piston-driven gas generators each producing 1.1 megawatts.
Energy efficiency: 33% increase.
Savings: Reduction in operating costs by $675,000 per year.
Includes: New building, improved heat-capture system, gas-cleaning system, testing.
Contractor: Overaa Construction, Richmond