Q&A: The future of thermal treatment for biosolids and PFAS management

During WEFTEC 2025, Wastewater Digest VP of Content Strategy Bob Crossen spoke with Patrick McNamara, PFAS and residuals research leader for Black & Veatch, about thermal treatment technologies in wastewater systems, including the latest research on the fate of PFAS in biosolids, concerns about air emissions, and what's next for thermal treatment technologies.

Today's thermal wastewater treatment landscape

Bob Crossen: What's going on in the thermal technology space? How have those types of technologies evolved over time? And where are we in that landscape today?

Patrick McNamara: Yeah, it's a great question. There's a lot of interest on thermal technologies on the face, but they're at the early part of the S-curve in the sense that we are doing a lot of research and pilot studies. There's been some attempts at full scale operation, and we're still sort of waiting to see how they can do long term operations. But that's always part of a new technologies: figuring out the kinks in the system.

There are great efforts ongoing. They're not widespread right now in the United States as a current option for biosolids management, but there are some facilities that have them or are attempting to have them and get it going. So it's an exciting stage, and we're watching and learning as they try to evolve here to become an actual tool in the toolbox for biosolids management.

BC: When you're talking to utilities, where do their interests seem to lie? How are their views adapting to the rate of change with this technology?

PM: They have general interest in it. I think there's that old adage of "keep the end in mind." So what's the product? If you want biosolids because you want to land apply them for the value of nutrients and carbon, then thermal processes don't give you that option because they convert the biosolids to biochar, and they convert it to energy in a gas phase. 

But if a utility is saying "you know what, I don't want the biosolids. I don't want to deal with the material, I want to recover energy." Then there's more interest in considering thermal processes. So it's really about what they want to do with their biosolids, recover energy or generate a beneficial use and product.

How PFAS influence utility decisions

BC: Well, and one of the bigger concerns, I think, over the past couple of years is the PFAS equation there. The fate of PFAS in wastewater and in biosolids. The concern there oftentimes leads to the question in Maine with land application. But what are you seeing from that perspective? What are utilities thinking right now? How is that informing some of their decisions? Where do thermal processes come into the equation there?

PM: A lot of it is about having the chance to pivot. So you might have a biosolids management plan that works right now because PFAS are a regulation in your particular state. But what if they become regulated in two years or four years, are you set up in a way where you might be able to pivot?

There are processes required before thermal processes. For example, you would need to have a dryer onsite to dry your biosolids. Drying has better benefits as well because you can get a biosolids product that you can apply to land. So if you had a dryer, maybe you're done and you land apply the biosolids, or maybe you want a thermal process a couple of years later, you could add that after your dryer.

So I think it's about understanding that we might have pivot points of either what we're doing now, or do we build to the future for another option if needed? So they are thinking long term about the role of thermal processes might play in their facility planning.

BC: It's like you're taking one step at a time and then as things evolve or change, you have these moments where you can adapt without upsetting the apple cart, so to speak.

PM: And thermal processes by and large are ready to go for every utility everywhere right now, but people are planning over time, then maybe over time, thermal processes advance to the point where if they need them in a couple of years, they might have more operational reliability and stability and become part of that. So I think it's in the conversation. It's at the forefront. So it is a conversation all utilities are thinking about. Where could this fit for us?

PFAS and air emissions: What utilities should know

BC: Speaking of forefront, I think one of the things I'm hearing occasionally now from utilities when you're talking about the fate of PFAS is that thermal drying often has some type of emissions component tied to it. What is the situation there? Where's the research on PFAS and emissions? Do utility owners and leaders need to know about things currently relating to that? What should they know today, and where's the research headed tomorrow?

PM: That's a great question, one that's near and dear to my heart, and it's why I'm here at WEFTEC because we're thinking a lot about the fate of PFAS during thermal processes and drying.

We have found our data shows that dryers do remove PFAS from biosolids. About 80%. So from a biosolids management standpoint, that's great because land application thresholds in certain states are set on PFAS concentrations. So if you can go from above that threshold to below, you can now land apply your biosolids.

And then the next question is, "Well that's great, where does it go? What happens?" And so we're trying to figure that out. That's an open question we have as a field as researchers. We have some data that we showed at WEFTEC that shows it might be in that gas phase or vapor phase and that leaves with water. Is it destroyed? We don't have conclusive data to say that it's destroyed, but it could be moving with either particulate matter and/or air and water that leaves a dryer.

But often it's not the case that the air is just vented to the atmosphere. So if you're a bigger facility, if you had a drum dryer, you might have a regenerative thermal oxidizer (RTO) after it. EPA had a study this year looking at air treatment for PFAS, and it was actually at a manufacturing facility where they make PFAS, but they saw that both activated carbon and RTOs can remove a substantial amount, over 99% of PFAS.

So I think, while we don't have a lot of data specifically on what happens to PFAS in the air, we should feel good that as an industry, we have technology for air pollution control already, right? Because as a field we have to reduce odors. Incinerators can't just emit gas to the atmosphere without it.

I think the air pollution control processes we have as an industry will be applicable and work for removing PFAS that goes in the air phase, if it goes to the air phase. So it's an open question, and it's a good one, but I think it's one we're ready to handle and that air pollution control technology is further ahead than actual thermal processes.

BC: In a lot of ways it sounds like it's almost mirroring the drinking water regulation situation. We're regulating PFAS in water. We have the technologies already to deal with it. It's just a matter of getting it into the right places.

PM: Right. One thing I'll add is air sampling for PFAS is very expensive. Much more expensive than solids or water, which most people would say yes, solids or water is expensive for PFAS. But you know, rough numbers if a solid or water sample is $500. It's a lot, but it's doable. But if an air sample in triplicate at a site is $50,000, how are you going to regulate air when it's that expensive at utilities everywhere? So it's another component regulators and contract labs are all working with is how to make the gas phase more affordable to understand the fate of PFAS.

The future of thermal treatment technologies in wastewater systems

BC: So if you look at the horizon here, thinking about thermal treatment, about these PFAS issues, where where do you see us headed? What does it look like five, 10 years down the road from your vantage point? 

PM: I think there will be renewed and continued growing interest in thermal processes. I think incineration will have more interest as well, because it's a process that we do know how it works, how it operates. It has that long term operational stability, and it's been shown to remove PFAS, but there's air permitting regulatory hurdles.

But I think we might have more conversations about incineration as well as pyrolysis and gasification. So they're still going to be of interest. And we're trying to figure out how to make them work with biosolids. Sometimes these thermal processes work well with wood chips or chicken litter, but biosolids, there's a reason there's tens of thousands of people here at WEFTEC because it's tricky.

And you know, it's been around for a long time, but we still have learning opportunities with how to move different types of biosolids and sludge and mix it and get that into a thermal process that we can work longterm. So it's going to keep growing in interest because it provides another tool in the tool box for management.

I think we'll still be talking about this in five years and hopefully have more success stories of, "Oh yeah, it's working at this facility, and it's been working for a while here." And then other utilities will start to pick it up more in the future.

BC: And hopefully you'll have more research, too.

PM: Yeah! And hopefully we answer the questions and we can move on from PFAS.