APPLETON — A new boiler system installed in 2011 at Appleton’s wastewater treatment plant is paying dividends for taxpayers, yielding $300,000 in energy savings annually.
The $1.2 million methane system captures the byproduct of the bacterial breakdown of sludge to operate the boiler and heat the rest of the plant.
The plant’s natural gas bill — which previously averaged $453,511 a year — dropped to $140,938 in 2012 and the savings are expected to continue annually.
“Overall our billings for natural gas have been reduced by two-thirds,” said Chris Shaw, Appleton’s utilities director. “It was a good project and the payback is less than four years. It’s a very good deal for Appleton residents.”
The methane boiler system heats the 38-acre facility’s 19 buildings and tunnel system. On very cold winter nights, the plant relies on the old natural gas system to supplement the methane boilers. While natural gas has an energy value of 1,000 Btu per cubic foot, methane produces about 650 Btu, Shaw said.
The wastewater treatment plant has two giant egg-shaped digesters. The tan Teflon and fiberglass tanks — which are easily recognizable along the Fox River — each hold 2.2 million gallons of sludge that go through an anaerobic bacterial breakdown.
“They both have to be heated up to 95 degrees so regardless of time of year, we’re constantly heating those vessels and the material,” Shaw said. “In the winter we’re fortunate to have some residual gas to heat the facilities as well.”
The “waste solids” start as a thick cottage cheese consistency before they are broken down over 40 days to the gas and tomato juice-like liquid.
The methane gets piped into a boiler room, where it’s burned. That heats water that is circulated throughout the facility. Excess methane is flared off in two stacks that can be seen from the river.
After the full process is complete, the plant returns more than 5.8 billion gallons of treated water per year to the Fox River — enough to fill a 20-foot deep lake covering 900 acres, or just about fill Little Lake Butte des Morts.
“Most people flush the toilet and think it ends there but there’s a big process down the line,” said Parks, Recreation and Facilities Director Dean Gazza.
Storing 500,000 gallons of methane yields energy savings, but creates some safety concerns.
Strict no smoking policies went into effect at the plant after the boiler system was installed to prevent a fire.
If a fire breaks out at the plant, firefighters would be kept out until the methane could be vented below a certain “explosive limit.”
“We’re obviously very careful and have a lot of monitoring sites that would alert us to any elevated levels or leaks,” Shaw said.