Two birds, one stone? Growing sector turns to biogas systems for fuel, waste reduction
A seven-foot flare outside the Mount Pleasant Wastewater Treatment Plant serves as a constant reminder of wasted potential.
This flame is burning excess methane that's released from the plant as it processes sewage waste. But instead of burning the methane, the gas could be captured and harnessed as a form of renewable natural gas.
“This discussion has happened a thousand times,” said Tim Middleton, the Deputy Public Works Director for the City of Mount Pleasant. “Every operator that's drove by that flame, every contractor that's been in here, every student that's come through has said, ‘wow, you got that flame burning every day. It's just going to waste.’”
Middleton said it’s been a long dream at the plant to expand the methane-capture system, but until recently, that hasn’t been possible.
“How do you go to a citizen and say, ‘I would like to do this, but it's really expensive, it’s never going to be pay for itself, and it's kind of cool,’” he said.
But now sheer convenient timing and a collaboration with Central Michigan University – are turning the tide.
“We're just at a point in our infrastructure life where we have to take action," Middleton said. "But since we're doing that, this is the time to say, if we just build this like this, we've built the base for these other projects to take place.”
The city is updating most of its wastewater infrastructure, but the methane-capture expansion would focus on the plant’s two anaerobic digesters. Both are more than 40 years old.
In wastewater treatment facilities, anaerobic digesters take in organic leftover waste - or “sewage solids” - and use heat and living microbes to speed up decomposition.
The main goal of digesters in wastewater facilities is to kill pathogens and reduce the volume of sewage solids, so they can be more manageable. The remaining organic waste that isn’t broken down by microbes - around half of what was initially put in – can then be made into a nutrient-rich fertilizer known as biosolids.
“It is digestion not radically different than what you and I do," Middleton said. "The only difference is, it's making use of organic waste. Otherwise, you would be burying organics in a landfill.”
But here’s what wasn’t a target in the initial design of digesters: as microbes chow down on the sludge, they release methane. Enter the flame, wasted energy, and greenhouse gas emissions.
The city currently uses the digesters to heat the building, but any excess energy is burned off. Now, the infrastructure updates would improve methane-capture, increase the digester’s efficiency, and expand energy storage at the plant.
To make the most of this improved biogas system, Mount Pleasant wants to take in more organic material. And they hope to eventually get that in the form of food waste from local restaurants, businesses, and from CMU’s dining halls (which produces 330 tons of food waste every year that are currently composted.)
Middleton said the project is a win-win for everyone: the community cuts down on how much food it sends to landfills, and long-term, the plant could be more energy self-sufficient, no longer needing to rely on external utilities.
“You take the product from the farm," Middleton said, "the community sends it back to me as either waste or food waste. That goes through the digesters, we make heat, we make energy, we send the biosolids back to the fields to grow more food, which returns to the community. And it ends up back here again, it really is full circle.”
There are nearly 150 biogas systems in Michigan. The majority are at wastewater treatment plants, but the biggest sector that could expand its use of digesters is agriculture.
The American Biogas Council estimates that Michigan could triple the number of biogas systems. And with those systems, capture over 32 billion cubic feet of renewable methane.
“I don't think as a society we've evolved to recognize the fact that we have to recycle more than glass, metal, paper, and plastic,” said Patrick Serfass, the executive director of the American Biogas Council.
Serfass said many anaerobic digesters weren’t typically installed on farms or wastewater treatment plants with the intention of generating energy, but the opportunity to diversify streams of revenue, particularly for farmers, has driven a rise in biogas systems.
“It's the evolution of our society's desire to recycle more, use our domestic resources better, produce energy domestically," Serfass said. "More people have been realizing, ‘hey, there's this opportunity that's been sitting in front of us the entire time.’”
Serfass said it doesn’t take an advanced degree to run a biogas system, but anaerobic digesters are living systems, which can “go sour” if not taken care of properly. Other caveats include low processing capacity and the challenge of setting up systems in good, central locations, near people or animals.
“Every biogas system is a little bit different because the mix of inputs and outputs is different everywhere you go," Serfass said. "And that uniqueness creates a little bit of complexity... but it's not insurmountable.”
Back at the Mount Pleasant Wastewater Treatment Plant, Tim Middleton walks me through some of the facility’s planned updates, including the potential of adding a third digester.
We finish the tour where we started – at the flame. In just a couple of years, Middleton said he hopes the plant can extinguish the flame.
It’s one of a growing number of wastewater treatment plants that are trying to do the same.