Plant to increase efficiency
CdA upgrade to tap more methane gas for energy
Each day, digesters at the city of Coeur d’Alene’s sewage treatment plant belch out 96,000 cubic feet of methane gas.
Some of the flammable gas is captured and used as a heating source. But most is burned off as waste.
After a major upgrade at the plant, the city will be able to tap more of the methane’s energy potential.
“We’re very definitely underutilizing it,” said Sid Frederickson, the city’s wastewater superintendent. “If I had to take a wild guess, I’d say we’re using less than 25 percent of the methane gas we produce. After the upgrades, it will be closer to 75 percent.”
Methane is one of the heat-trapping gases associated with global warming. Better known as a component of cow belches, methane gas also is released during the sewage treatment plant’s anaerobic digestion process.
Solid waste is pumped into large tanks, where microbes break it down. Water vapor, carbon dioxide, methane gas and hydrogen sulfide are byproducts of the 21-day sewage digestion process. (Hydrogen sulfide, not methane, is responsible for the rotten egg smell. “It’s the stinky stuff,” Frederickson said.)
Since the late 1990s, the Coeur d’Alene treatment plant has burned methane gas in its boilers. The energy is used to heat three digestion tanks, which must be kept at 100 degrees year-round. In addition, two buildings at the treatment plant are heated with hot water warmed in methane-fired boilers.
“We rarely use natural gas,” Frederickson said.
In March, the city will embark on an $11.5 million plant upgrade, which includes adding a fourth digester that will hold 250,000 gallons of solid waste. As part of the plant upgrade, nearly 15,000 square feet of office and equipment area will be heated with hot water from the methane-fired boilers.
At Spokane’s sewage treatment plant, methane gas also heats digesters and buildings. But there’s potential for more, said Tim Pelton, the plant’s administrative superintendent. Within five years, methane gas could be running a co-generation facility at the treatment plant that produces both heat and electricity, he said.