Small-Town Phoenixville Launches a Big Innovation in Beneficial Use of Biosolids

A Pennsylvania community tests a new hydrothermal carbonization conversion technology to bypass digestion and turn biosolids into an energy-dense hydrochar fuel.

Small-Town Phoenixville Launches a Big Innovation in Beneficial Use of Biosolids

The Bucher filter press, shown being moved into the wastewater treatment plant, will dry biosolids processed by hydrothermal carbonization to 50-60% solids.

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The Borough of Phoenixville is a small municipality in southeastern Pennsylvania that rises to the challenges of being first.

It was first in the state to pledge a transition to 100% renewable energy for electricity by 2035 for all municipal operations. Now it is first to install a new technology that could be a giant step toward that goal.

Phoenixville’s hydrothermal carbonization system, the first large-scale system of its kind in the United States, transforms biosolids into hydrochar, a coal-like substance. When at capacity and integrated with a cogeneration system, it is expected to produce enough hydrochar to generate a surplus of electricity to operate the wastewater treatment plant.

Since early this year, the HTC process has been running alongside the anaerobic digesters. If the technology proves itself, all the biosolids will eventually go through the system, which occupies a much smaller footprint than a digester, yields a drier end product, and breaks downs PFAS and pharmaceuticals in just a few hours.

“We’re excited about it,” says E. Jean Krack, borough manager. “We’re at the embryonic stage right now. The good thing is we’ve got leadership that sees the borough as an innovator and a leader in getting to net zero. This will help us get there very quickly.” 

First in the nation

Phoenixville’s partner is SoMax BioEnergy, which has tested the HTC technology over the last eight years on waste streams including wastewater biosolids, food waste and industrial waste through a pilot project in Switzerland.

The company has sponsored master’s level laboratory research at Villanova University. The installation at the Phoenixville Wastewater Treatment Plant (4 mgd design, 1.5 mgd average) will be the first commercial-scale and first municipally owned HTC system in the nation. 

The process can be “bolted on” to a treatment plant ahead of an anaerobic digester, after a digester or, as at Phoenixville, in place of a digester. The reactor and heat exchanger for the HTC process fit in two shipping containers, which become part of the installation.

Sludge at about 15% solids flows into the reactor, where they are broken down by heat (375-420 degrees F) and pressure (150-290 psi). “The water acts as a catalyst, and it causes chemical reactions to occur,” says Dan Spracklin, SoMax CEO. “In layman’s terms, it takes compounds in the waste, whether undigested food in the fecal matter or food waste or pharmaceuticals, and breaks those long-chain compounds into smaller compounds.”

 Super dry

After the reactor and heat exchanger, the biosolids go through a filter press (Bucher). “The material comes out 50-60% dry matter through those presses,” Spracklin says. “Most presses for biosolids have a limit of 25-30% dry matter.”

The press can squeeze more water out because the HTC process weakens the cell walls of microorganisms and because the hydrochar is hydrophobic. “It actually repels water, which allows us to achieve that high level of dewatering,” Spracklin says. “If we include an acid treatment in the filter press, we can achieve up to 70%.”

The water pressed out of the hydrochar gets pumped back to treatment plant headworks, just as the water removed from anaerobically digested solids would be.

Mimicking nature

Spracklin describes the HTC process as similar to the way fossil fuels are produced: biomass is compressed and subjected to heat over a long time. The difference is that HTC acts much faster: “Essentially we mimic the way that Mother Nature creates coal. We take that same approach by applying mechanical and chemical engineering to create a modern-day biofuel.”

While a typical anaerobic digester processes solids for 14-30 days, the solids usually stay in the HTC reactor for just three hours. “Anaerobic digestion is a biological process,” Spracklin says. “Biology is slow. Chemistry is quick. The minimum time to complete the reaction is 30 minutes. The reason we do it for three hours is that it makes a much better carbon product with more energy density.”

The hydrochar can be used in a variety of ways. It has an energy profile similar to anthracite and so it can fuel a cogeneration system. It can also be used to create activated carbon for filtering water or gas, or to sequester carbon in building products like roof shingles or concrete. “We believe we’ve solved the fundamental issue that faces all municipal wastewater treatment plants: What do we do with our biosolids?” Spracklin says.

Surplus energy

Phoenixville hopes to use its hydrochar for cogeneration. The treatment plant has used biogas for heating only. The hydrochar can be heated and gasified, and the gas then can fuel a generator. Phoenixville expects to produce enough hydrochar to generate 153% of the treatment plant’s energy demand, which would mean selling a lot of power back to the utility grid. 

That won’t happen right away. “We’re years away from that,” Krack says. “If we’re the first ones doing this, there’s nobody making that type of boiler. We’ve got to show everybody that this is all working.”

The project will cost about $5 million, but the borough received more than $1 million in state and county grants. The project also won a $250,000 U.S. Department of Energy Water Resource Recovery Prize for SoMax, one of two winners in 2021.

The project has the aura of a startup company. The process is so new that Phoenixville has to run it in parallel with its anaerobic digestion process to generate data and demonstrate to regulators that it is effective.

“We had to figure out financially how this could work, but equally, if not more important, how do you get a permit to do it?” Krack says. “Nobody has done it in the United States. The way the rules are, we couldn’t use the empirical data that was being provided in Europe. We had to do it here. We had to scale it down to do it at Villanova in a laboratory environment. Once they showed that it could be done, then the next step was to scale it back up.”

Biosolids savings

Phoenixville’s biosolids traditionally have been distributed to farms, but that has become more difficult as regulators recognize risks from PFAS and pharmaceuticals.

“With this HTC process, we’ll virtually eliminate most of those things that are still getting through in anaerobic digestion,” Krack says. “We’ve got to show that we’re removing PFAS and biopharmaceuticals and thereby putting cleaner water back into the river.”

Phoenixville plans to keep its anaerobic digesters operational to enable storage of solids if the HTC process is down for maintenance. Until the plant has the equipment and permits for cogeneration using hydrochar, the borough will continue with land application, although Krack expects considerable hauling cost savings since the product is so much drier.

He also envisions eventually processing food waste from restaurants and grocery stores through the HTC system. Initially, operating costs will be higher while running both HTC anaerobic digestion, but the long-term prospects are for enormous energy cost savings.

Krack says, “First, we have to show that it works.”   


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