FORT WAYNE – Theres precious little good that can come from the river of raw sewage that flows into the sewage treatment plant.
But officials hope that, at least, one byproduct will be able to make the job of dealing with the sewage slightly less expensive.
While the water pollution control plant uses massive pipes, pumps, valves and tanks to move the citys wastewater through the various processes, the bulk of the work is done by bacteria. One of the byproducts of the bacterias work is methane gas.
That flammable gas is captured by the lids over the plants digesters and some of it is used for boilers that can heat the plants buildings and keep the digesters at a balmy 95 degrees so the bacteria are most efficient. Much of the gas produced, however, is simply burned off.
Now, however, officials plan to make the most of it by using the gas to generate electricity.
Were going to make the methane regardless, said the citys Zach Schortgen. We might as well get all the use out of it we can.
Officials said they dont expect the savings from their plans to be tremendous – about $50,000 a year – but if they could also use it for heating it could double that figure. Even better, if the generator can also use natural gas, it would provide a backup source of power for the plant in case of a blackout.
A decade ago, the city studied using the methane to power the aerators, which pump air into the secondary clarification tanks. Adding oxygen to those tanks helps those bacteria break down solids in the wastewater, and officials hoped that by not using electricity to run the aerators they could save up to $400,000 a year.
But the numbers didnt pan out, said Doug Fasick, a senior program manager for City Utilities. At the time, electricity was about 3.5 cents per kilowatt-hour, he said, but capturing and using methane would have cost up to 13 cents per kilowatt-hour.
Since then, however, electric rates have gone up, the processes at the plant have changed somewhat and the capacity of the plant has dramatically increased. More waste processed means more gas produced.
The plant is also undergoing $32 million in upgrades, recently approved by the City Council, which will overhaul the digesters and increase capacity again, to 80 million gallons a day. So officials have budgeted an additional $3 million to make better use of the methane.
Instead of burning the gas in a boiler, plans call for burning it in an engine that turns a generator. The heat from the engine will boil the water to warm the digesters, while the generator could churn out up to 560 kilowatts of electricity, about one-sixth of what the plant uses.
Matt Wirtz, deputy director of engineering for Fort Wayne City Utilities, said the project is designed to be scaleable, so as opportunities – and funding – appear, more options could be added. For example, a gas absorption chiller could be added that would use the heat from the generators for air conditioning in the plant.
Recreational vehicles often use gas absorption refrigerators burning propane or liquid petroleum.
We have to do the work there anyway, Schortgen said. We have to do the upgrades, so its a great time to incorporate the backbone of this system and build it with lots of room for expansion in the future.
One challenge in using the methane is that since its source is decomposing sewage, its not pure.
Its not nice, clear, beautiful gas, Wirtz said. They say theres up to 10 times the energy in wastewater as it takes to treat it. The challenge is the cost of getting that energy.
Using methane from decomposition is hardly new. The National Serv-All landfill captures methane created by rotting trash. The gas is then piped to the General Motors assembly plant nine miles away and used to heat boilers.
Unlike 10 years ago when the city studied using more of its methane, the engines that burn it can now handle the impurities without having to filter them out first. They are also much smaller, meaning equipment can be better sized to the plants needs, theyre more efficient and they create less pollution from the burning process.