“A Major Finding” For Biosolids: Gasification Removes All Detectable PFAS from Finished Solids
The full-scale study also identified how to remove or destroy more than 98% of the chemicals in the air and water as well as the solids exiting the system.
A process called gasification can turn municipal sludge into dried solids that contain no detectable PFAS chemicals, according to a pioneering Hazen-led study.
The study
also found that of the total load of PFAS measured in the incoming sludge, only 5% remained in the air and 15% was transferred to the water that came out of the gasification system. And it could achieve upwards of 98% removal or destruction efficiency (RDE) with some process adjustments.
Those results could offer much-needed solutions for the biosolids industry—which, despite not producing any new sources of PFAS, faces the looming prospect of PFAS regulations. (The term “biosolids” means sludge that has been stabilized through some form of treatment. Since the material entering the gasification facility in Edmonds has not been treated yet, in that stage it’s referred to as “municipal sludge” or “municipal solids.”)
The study used samples collected at a gasification facility in Edmonds, Washington—one of the first such facilities running at full scale.
The study used samples of air, water, and solids taken at a gasification facility designed by Ecoremedy for the City of Edmonds, Washington. Hazen led the project with the help of Ecoremedy, students and faculty from Manhattan College and the University of Buffalo, and air sampling company Mostardi Platt. The research received a grant from the Water Research Foundation and support from utility partners across the country, including Portland Water District (the lead sponsor), AlexRenew, Massachusetts Water Resources Authority, and Middlesex County Utilities Authority.
Through a technique called mass balancing, the team compared the amount of PFAS in the sludge entering the gasification system against the amount in the solids, air, and water coming out. Of the PFAS in the incoming sludge, only about 5% was released into the air emitted from the facility’s flue stack. About 15% ended up in the water collected by a scrubber, a device used to remove contaminants from air, and none was detected in the finished product, a sandy material Ecoremedy calls FLG Sand™.
“That’s a major finding,” said Mohammad Abu-Orf, who leads Hazen’s biosolids practice group and co-led the study with Hazen’s Micah Blate and Kamruzzaman Khan.“It’s the first full-scale gasification system to show no PFAS in the end product, and also the first study to measure PFAS in the air emissions from this kind of system.”
“It shows that really very little goes into the air,” Abu-Orf added. “And we’re confident that we can get to 98%, because we know where the rest of the PFAS goes and how to remove it.”
Ecoremedy President and Chief Technology Officer Dave Mooney said the company is happy with the results.
“But we’re even happier with the fact that we now finally have data—that the industry has data they can build from, and there are identifiable steps to make further improvements,” Mooney said.
Farmland in Monterey County, California. Farms currently use more than half of all biosolids produced in the U.S. as fertilizer, but concerns about PFAS could threaten that practice.
An Industry Under Pressure
Wastewater utilities currently send more than 50% of their biosolids to farms, which spread them on croplands as fertilizer—a practice called land application. The rest mostly goes to landfills and incinerators. But concerns about “forever chemicals,” as PFAS are often called, have caused growing scrutiny of those practices.
The study’s findings were announced just after the Environmental Protection Agency published a draft report on the health risks of PFAS in biosolids. The report states that two types of PFAS, PFOA and PFOS, can each pose health risks when they occur in land-applied biosolids at levels as low as 1 part per billion. There are multiple caveats to those findings, but they could lead to an eventual limit for PFAS in municipal sludge. Meanwhile, in September 2024, several U.S. states asked the federal government to classify four types of PFAS as hazardous air pollutants.
Prior research on gasification, and another technology called pyrolysis, suggests that each could be a promising way to remove PFAS from biosolids. But small amounts of PFAS were recently detected in the biochar (a term for dried solids) produced by another full-scale pyrolysis facility. And there’s little data in general—particularly data on air emissions—from such facilities.
The gasification chamber (in green) inside the Ecoremedy-designed gasification facility in Edmonds.
How to Get to 98%
The Ecoremedy Fluid Lift ™ gasification facility in Edmonds is one of the first in the U.S. that exclusively treats municipal solids and is operating at full scale. Last spring, the Hazen-led research team took samples of the solids, liquids, and air at various points in the gasification system.
The results of the mass balancing done with those samples showed that roughly 5% of the PFAS in the incoming sludge escaped into the air via a flue stack atop the facility. Another 15% was concentrated in the water used by an air scrubber to clean the air before it exited the flue stack. It wasn’t the 99% removal or destruction efficiency (RDE) that the group had hoped for, but the team quickly figured out where some adjustments in the system’s configuration could be made to achieve a higher RDE.
Gasification is actually just one step in a multi-step treatment process; the same goes for pyrolysis. In each system, after the dried solids are heated in the gasifier or pyrolizer, they’re put through a thermal oxidizer, which burns up gases released from the solids during the previous step. Earlier studies have established that at high enough temperatures, thermal oxidizers used in gasification and pyrolysis systems can achieve more than 99% RDE for PFAS.
But all the air exiting Ecoremedy’s thermal oxidizer is recycled back to another stage of the process, to dry incoming solids. After leaving the dryer, only some of that air is recycled back through the gasifier and thermal oxidizer, while the rest is sent directly to the air scrubber and flue stack. That means that air in the dryer can pick up PFAS from the incoming solids, and some of that air—the portion of it that bypasses the thermal oxidizer—can transfer PFAS to the scrubber water and flue stack air.
“This is a valuable finding because multiple companies use this kind of configuration,” Abu-Orf said.
Abu-Orf explained that a few configuration changes could achieve more than 98% RDE for PFAS: for example, treating the water from the scrubber with granular activated carbon (GAC) and using a process called regenerative thermal oxidation (RTO) to remove the PFAS from the dryer air.
More Benefits Than PFAS Removal
Abu-Orf said that while this is the first full-scale study of its kind, more will be needed to confirm gasification and pyrolysis as viable options for managing PFAS in biosolids.
Even without PFAS regulations, both technologies are attracting interest from wastewater utilities for their other benefits. Mooney said Ecoremedy’s gasification system was optimized for renewable energy use. After startup, with certain types of sludge, the process can power itself solely with heat captured from the solids themselves. That auto-thermal capacity, as it’s called, lowers energy costs as well as greenhouse gas emissions. Mooney said that renewable energy capacity remains Ecoremedy’s primary focus after more than 25 years of designing gasification systems for agricultural- and municipal-sector clients.
Dried solids from gasification and pyrolysis (like the kind produced at the Edmonds facility, which is called FLG Sand™, left) has a much lower mass and volume than the sludge it started out as.
Depending on how much carbon is left in it, the biochar produced by gasification and pyrolysis can be a sand-like material (which is useful for construction) or act as a carbon sink (useful for utilities seeking carbon removal credits).
Hazen is in the middle of designing a pyrolysis facility for the Town of Windsor, California, and Abu-Orf said he expects the interest to keep growing.
“We’ve done half a dozen biosolids master planning studies and most of them point to this technology being favored,” Abu-Orf said, “because of the energy benefits, the benefits of the biochar, the cost reduction, and the carbon footprint reduction.”
Mooney said he’s glad to add PFAS removal to the list of benefits Ecoremedy’s systems provide and hopes the study’s findings help the whole industry tackle the problem of PFAS, at least as much as it can from its place at the end of the materials chain.
“Ultimately, I’d love to think that the Ecoremedy process and other processes are a temporary need—that we will learn to make our products without these dangerous chemicals,” Mooney said.
“Because really, what we’re dealing with, what a wastewater treatment plant is dealing with, is what they receive," he added. "They don’t generate PFAS. But they have to process it. And if Ecoremedy and others can assist in the processing of the wastewater and the solids generated from wastewater and reduce the PFAS, then that’s fantastic. That’s what we need to be doing.”
Abu-Orf said collaborative research with partners like Ecoremedy, the Water Research Foundation, and partnering utilities unlocks the real-world data utilities need to make informed treatment decisions.
"You don't base a biosolids master plan on technology that's just been tested in the lab," he said.
“You have to know how it works in an actual, full-scale facility, with all of the complexities that come with that. That’s what turns something from a concept into the kind of real, practical solution we need right now.”
