A Look Inside Maryland’s New Bioenergy Facility
WSSC Water needed a more sustainable way to manage biosolids. Its solution is a plant that generates renewable energy and includes one of the country’s first thermal hydrolysis process (THP) units.
Almost directly across the Potomac from George Washington’s famous estate, Mount Vernon—which includes two prim, bell-roofed “necessaries,” or outhouses—sits a sewage sludge treatment plant that looks like it’s from Star Wars.
The new Piscataway Bioenergy Facility ↑ turns biosolids (treated sewage sludge) from Maryland’s Montgomery County and Prince George’s County into renewable fuel and high-quality fertilizer. WSSC Water owns and operates the facility, along with the larger Piscataway Water Resource Recovery Facility it sits within. The utility officially cut the ribbon on the $271 million bioenergy system on October 30.
“This is the biggest, most complicated and technically challenging project in the 106-year history of WSSC. It introduced six new processes that were unfamiliar territory for us.” - Stanley Dabek, WSSC Water Project Manager
While the facility is generally hidden from Mount Vernon’s tourists, it has attracted biosolids experts from around the country and the world. A small but growing number of U.S. wastewater utilities are launching bioenergy projects like the one at Piscataway as a solution to multiple industry challenges, from pressure to lower the carbon footprint of biosolids management to the rising cost of trucking biosolids offsite. The Piscataway Bioenergy Facility will be able to:
- Produce renewable natural gas
- Sell that gas to a gas utility under the Renewable Fuel Standard Program, offsetting operational costs
- Produce Class A biosolids, which have a higher quality and more potential uses than the Class B biosolids WSSC previously produced
- Produce fewer biosolids, lowering the amount of money and fossil fuel needed to haul them offsite
The Bioenergy Facility processes biosolids from five of WSSC Water’s wastewater resource recovery facilities (WRRFs) and can treat up to 92 dry tons of biosolids per day. It’s one of only seven facilities in the country to adopt a technique called thermal hydrolysis to break down those solids more efficiently. And it treats wastewater captured from the facility’s treatment processes using a cutting-edge process called deammonification.
WSSC Water also used a progressive design-build project delivery approach that allowed PC Construction, Stantec, and Hazen to design and build the whole facility faster than they could have with a traditional design-bid-build process.
“With this new facility, WSSC Water is producing a very beneficial product—safe, organic solids that will basically go to farmlands as fertilizer,” said Hazen’s Matt Van Horne, a biosolids expert who led Hazen’s portion of the project. “And they’re also making renewable natural gas, which is effectively offsetting natural gas use from other, less green sources.”
Here’s a look at the different processes and machines that are now turning “poop to power” for WSSC Water and its customers in Prince George’s and Montgomery Counties.
Sludge from Piscataway WRRF enters the Bioenergy Facility and passes through sludge screens ↖ to remove debris and belt filter presses ↗ to remove excess water.
Hazen led the design of both systems, as well as:
- The facility’s anaerobic digesters and digester gas storage
- Various systems for pumping, conveying, and storing the biosolids
- The sidestream deammonification system
- A UV disinfection system for utility water (treated wastewater from the Piscataway WRRF that’s used onsite) to maintain the Class A status of the biosolids
- Utility water pumping and odor control systems
Cake bins ↑ then combine the Piscataway biosolids with biosolids from four of WSSC Water’s other WRRFs before they’re fed into the facility’s thermal hydrolysis process (THP) unit.
The THP system ↑ (cylindrical structures in foreground) uses high heat and pressure to break down the biosolids and make them more digestible for beneficial bacteria in the anaerobic digesters.
“We looked at about a dozen different technologies and finally centered on the combined thermal hydrolysis and anaerobic digestion process as being core to the project,” said Theon Grojean, who manages WSSC Water’s Facility Design and Construction Division.
Piscataway’s THP system was designed by the Norway-based company Cambi. Van Horne and Dabek both noted that it includes two extra THP vessels that can be used during annual inspections, so that the plant never has to go offline.
Heat generated from burning natural gas in the Bioenergy Facility’s engines is recycled to preheat boilers, which create steam used to power the THP unit.
A cooling heat exchanger brings the temperature of the THP-treated biosolids down before sending them into the anaerobic digesters. The exchanger uses treated effluent from the water resource recovery facility as its cooling fluid.
Two 1.5 million-gallon anaerobic digesters then use beneficial bacteria to eat up and further break down the biosolids. Van Horne said the project team designed them to be low enough that they couldn’t easily be seen from Mount Vernon.
Pipes connected to the digesters ↓ carry gas from the digesters to the gas treatment portion of the facility.
The white bubble ⮤ can store raw gas from the digesters to maximize the overall production of renewable natural gas and minimize flaring. The gas cleaning system (⮤ back row of machines just in front of the storage bubble and ⮥ a portion of the system) then strips all molecules but methane from the gas, essentially converting it to natural gas. It’s sent to a nearby pipeline operated by Washington Gas.
Meanwhile, the Class A biosolids remaining after anaerobic digestion are processed with more belt filter presses to remove excess water before they're sent to a ↑ loading station or covered storage area.
“This facility greatly reduces the amount of treated biosolids that need to be hauled away, which means fewer trucks and less fuel,” said Hazen Senior Associate Amy Hanna, who led multiple components of Hazen’s design work. “And it produces renewable natural gas that can be used as vehicle fuel. All of that helps reduce WSSC Water’s carbon footprint.”
The liquid from final dewatering, or filtrate, has high concentrations of ammonia—a form of nitrogen. It would be difficult for Piscataway’s main wastewater treatment process to remove that much ammonia while meeting its required nitrogen limits. So the filtrate is first sent to a state-of-the-art sidestream treatment system ↑ before it’s sent back to the Piscataway WRRF.
The Hazen-designed sidestream treatment system uses a novel technique called deammonification. The process removes ammonia by using special bacteria ↑ grown on plastic media roughly the shape and size of wagon wheel pasta.
Treating the post-THP filtrate with sidestream deammonification uses 60% less air than biological treatment and requires no supplemental carbon, and it’s essential to helping the Piscataway WRRF meet its effluent permit limits while operating the Bioenergy Facility.
↑ A Hazen engineer inspects instruments atop one of the anaerobic digesters.
“I’m proud of the fact that it’s been successful,” Grojean said of the Piscataway Bioenergy Facility. “The fact that we’ve got five plants all coming together at one location to treat their biosolids—that’s not really been done before. We’ve had interest groups visit from Pakistan, New Zealand, Canada. Utilities in the U.S., too.”
Hazen President Alan Stone said that the Piscataway Bioenergy Facility embodies the kind of innovation that Hazen exists for.
“That’s an energy stream we helped them recover," Stone said. "Hazen is all about helping our clients find cutting-edge solutions that deliver positive outcomes for the environment."
