R&D PULSE
Researchers from the University of Wisconsin-Milwaukee are looking at animal waste to leverage wastewater sludge in pyrolysis applications.
Written by Kayt Sukel
ACROSS THE WORLD, water treatment plants create safe drinking water for communities, removing contaminants, pathogens, and sewage. The byproduct of this process, however, is sludge, a semi-solid material made up of organic and inorganic materials. According to the Environmental Protection Agency, wastewater treatment results in approximately 4 million dry metric tons of sludge. The question is what to do with it.
Cheikh Kada, a doctoral candidate in the mechanical engineering department at the University of Wisconsin-Milwaukee, said, historically, wastewater plants have put the sludge into landfills or incinerated it. But he said there has to be a better way to get value out of this waste product.
“Imagine in a year how much sludge is produced and how much money we are spending to dispose of it all while making sure the environment stays clean and we avoid contamination,” he said. “If we could use it to create energy or fertilizer, we could get more value out of it.”
Thermal decomposition dynamics of sludge–chicken manure blends. Enthalpy release curves at 5 °C/minute. Figure: Cheikh Kada
Thermal decomposition dynamics of sludge–cow manure blends. Enthalpy release curves at 5 °C/minute. Figure: Cheikh Kada
Kada and colleagues looked to pyrolysis, a thermal decomposition that breaks down organic materials in the absence of oxygen in the hope of transforming sludge into biochar for fertilizer or bio-oil for renewable energy production. There was only one problem. Sludge has low enthalpy.
“When you burn sludge, or treat it thermally, it doesn’t release all that much heat, which we call enthalpy,” he said. “So, we started to think about ways we could add things to it to make the enthalpy increase.”
Instead of going for diesel fuel or another fossil fuel, Kada and team looked to another source of abundant waste: animal manure. Kada said his research showed that both cow and chicken manure have quite good enthalpy. And it’s an abundant waste source in agricultural communities across the country that must also be effectively dealt with to avoid soil or water contamination. To use it to pyrolyze sludge, however, the team needed to determine the optimal “recipe” mix of manure and sludge to make biochar and other useful products. Doing so, Kada said, required a lot of trial-and-error testing.
“Imagine in a year how much sludge is produced and how much money we are spending to dispose of it all while making sure the environment stays clean and we avoid contamination. If we could use it to create energy or fertilizer, we could get more value out of it.”
—Cheikh Kada, a doctoral candidate in the Mechanical Engineering Department at the University of Wisconsin-Milwaukee
“We needed to figure out whether it should be 50 percent sludge and 50 percent manure, 70 percent and 30 percent sludge and manure—we didn’t know,” he said. “I started at 10 percent sludge, 90 percent manure, and then changing the ratios to find out.”
By going through each mixture, Kada learned that 30 percent of sludge and 70 percent of cow manure had the highest enthalpy release, increasing enthalpy by 117 percent, because, he said, it made the sludge “thermally stable,” helping to manage sludge’s natural thermal properties, elevated moisture level, and inorganic content. When it came to chicken manure mixtures, the “sweet spot” that Kada spoke of occurred at 40 percent sludge and 60 percent chicken manure, with a 60 percent increase in enthalpy release.
Kada contends that these results show that optimizing sludge-to-manure ratios will be imperative to leverage pyrolysis to manage both sludge and manure waste. In future work, he wants to look at other mixtures, including sludge mixed in with cork or wood, to see how they affect enthalpy and the properties of sludge decomposition during pyrolysis. But, in the meantime, he believes that manure, in the right ratios, can be a viable option for waste management.
“We are already spending a lot of money to dispose of both of these types of waste,” he said. “If we want to keep our environment clean, we can use sludge and manure to help us. Animal manure can act as a natural catalyst so we don’t have to get fossil fuels involved and increase pollution to get rid of all this waste.”
Kayt Sukel is a technology writer and author in Kansas City.
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