Sewage Is A Biological Necessity, And A Methane Minefield

12:08 minutes

A view between two wastewater treatment tanks the size of swimming pools, on the left one is churning and brown and on the right is a tank that is glassy and still. Both tanks are the size of swimming pools.
Wastewater treatment facility. Credit: Shutterstock

In most cities, once you flush a toilet, the water and waste flows through the sewage system to a water treatment plant. Once it’s there, it goes through a series of chemical and biological processes which clean it up and make the water safe to drink again. But a recent paper in the journal Environmental Science & Technology finds that some of those sewage plants may be having a greater impact on the climate than previously thought.

The anaerobic decomposition of organic material in the waste stream at sewage plants produces methane, a potent greenhouse gas. The researchers used an electric car fitted with a suite of atmospheric gas sensors to sniff the emissions downwind of 63 sewage treatment plants at different times and during different seasons. They found that the wastewater treatment process may release amounts of methane nearly twice that estimated by the Intergovernmental Panel on Climate Change and the the U.S. Environmental Protection Agency. In a related study, other researchers analyzed data from published monitoring of wastewater treatment facilities around the globe—and arrived at a similar estimate of the methane production.

Mark Zondlo, a professor of civil and environmental engineering at Princeton University, and one of the authors of the methane-sniffing research, talks with guest host Shahla Farzan about the studies, and about what might be done to mitigate the methane impact of treating our cities’ sewage.

Further Reading

Segment Guests

Mark Zondlo

Dr. Mark Zondlo is a professor of Civil and Environmental Engineering at Princeton University in Princeton, New Jersey.

Heard on the Air

KATHLEEN DAVIS: This is Science Friday. I’m Kathleen Davis.

SHAHLA FARZAN: And I’m Shahla Farzan. We’re sitting in for Ira Flatow this week.

KATHLEEN DAVIS: Later in the hour, how an activist is challenging what climate education should look like. And the National Audubon Society announced it’ll be keeping its name despite it being named after an enslaver. What that means for the birding community and what local chapters are doing about it. But first, we recently told you about researchers working to extract health data from your toilet. Right. But now it’s time to talk toilets a bit further downstream. Specifically, what happens after you flush the toilet. In most cities, once you flush, that water and whatever else is in it flows to a water treatment plant. Once it’s there, it goes through a bunch of different chemical and biological processes which clean it up and make it safe to drink again.

But a recent paper in the journal Environmental Science and Technology finds that some of those sewage plants may be having a greater impact on the climate than we thought. Joining me to talk about that now is Mark Zondlo. He’s a professor of civil and environmental engineering at Princeton University and one of the authors of that report. Welcome to the program.

MARK ZONDLO: Thank you.

KATHLEEN DAVIS: All right, let’s start with just some of the basics here. Why do sewage plants produce methane?

MARK ZONDLO: So there’s a lot of organic matter in waste, as one can imagine, and that gets collected by the sewage treatment plant. And the goal of the plant is to clean the water to put it back into the waterways to make a healthy environment. But as part of that process, that organic material, that waste that comes into the plant, that carbon has to go somewhere, and anaerobic conditions produce methane. And that’s where the methane is coming from and can happen in many different sectors of plant. It can be clarifiers, it can be the sludge tanks, it can be the digesters. It can come from so many different components. Any inefficiency in the plant that creates these anaerobic conditions leads to methane emissions.

SHAHLA FARZAN: Now when we’re talking about anaerobic digestion here, we’re talking about bacteria specifically, right, munching down that organic matter that’s sewage?


SHAHLA FARZAN: And then the reason why we’re talking about methane here is that it’s a really big contributor to climate change.

MARK ZONDLO: Correct. The reason methane is important, it’s the second-most important anthropogenic greenhouse gas– CO2 being the most important. But what makes methane really interesting and from a mitigation perspective, it has a lifetime in the atmosphere of about 10 years. So if we can cut methane emissions now, we see a really big impact and it buys us a little bit of time to the climate effects from global warming. On a 20-year horizon, methane is about 80 times more potent than a molecule of carbon dioxide.

SHAHLA FARZAN: So when we’re talking about kind of the methods for this particular study, your team basically sniffed the air downwind of dozens of different sewage plants. What does that look like basically? what? Kinds of instruments are you using?

MARK ZONDLO: So many times people see us on the side of the highway or driving around, and they’re like, oh, it’s the Google car. I personally think it’s much nicer and much more sophisticated than the Google car, but we have a bunch of laser sensors that can measure methane, carbon dioxide, nitrous oxide, ammonia, and a suite of other gases and weather conditions– winds, wind speed, wind direction. So with this suite of gases, we can fingerprint, hey, this is coming from the wastewater treatment plant, or this is coming from this part even of the wastewater treatment plant and avoid, say, the car exhaust that’s around us. And the advantage of our approach is we get the facility integrated emissions.

So one of the problems with previous studies of wastewater treatment plants is they look at lagoons or they look at a clarifying tank. They look at individual components of the plant. And that’s great and it gives us process-level understanding. But it’s also a little bit biased because it assumes that everything is working perfectly and it ignores the rest of the plant. Hey, we know methane is coming from here. Let’s take a couple of samples and then we can extrapolate that to the whole plant.

But unfortunately, methane is generated in a lot of different parts of the plant. And we see methane where we don’t expect it to be. And that’s the reason why I think we’re seeing higher levels of methane is because we can’t take individual components of plants, and a very small subset of plants, taken also at a very limited amount of time, and extrapolate that to the entire sector.

SHAHLA FARZAN: Right. So when you’re talking about collecting these samples from the plumes, how close exactly do you have to get to the plant? Like do you need to get permission from plant operators to get those samples?

MARK ZONDLO: Well some operators we do go on site and we have permission to go on site. But generally speaking, we went just downwind, maybe a couple of blocks away, no more than a quarter of a mile. We go on public roads. We sample downwind of the plant, and again we can fingerprint what other sources of methane are around. Now the further you get away from the plant, our sensors are sensitive enough to do that, but then you get in other sources of methane that complicate the analyses.

For example, if there are natural gas leaks and you’re a half mile away, it becomes very difficult to say, yes, this is from the wastewater treatment plant. So typically we’re on adjacent roads or within a couple of blocks. And we can, by location as well as by the fingerprint that we see of the gases, we can say this is coming from this plant.

SHAHLA FARZAN: So there are satellite maps of methane emissions out there from natural gas facilities and fossil fuel production. Why can’t we use the same kinds of things to look at sewage plants?

MARK ZONDLO: So wastewater treatment plants, although they seem large at some level, they’re still much smaller than the kind of resolution, the pixel size, of satellite. There’s some amazing satellite measurements that are both available now and are upcoming and different missions. But they’re going to have trouble resolving the scale of a wastewater treatment plant.

On top of that, most wastewater treatment plants are in cities. So you have this soup of other sources of methane. There could be a landfill upwind. So satellite pixels are typically on the order of a couple of kilometers at the best, and that becomes really hard in a city environment to say, OK, this plume is coming from the plant. So satellite has great applications, and the technologies are improving, so maybe in 10 years we can individually pick out within part of a city these are the methane emissions from the wastewater treatment plant. But right now we just don’t have that capability.

KATHLEEN DAVIS: OK. So the big reveal here. You collected all this data. What did you find?

MARK ZONDLO: We found that the methane emissions are underestimated by about a factor of two compared to the IPCC estimates, which is what the EPA uses. And the reason this is concerning is because, as we increase urbanization, we’re going to have more and more centralized treatment. This is a good thing. However, we need to be very cognizant of how these plants operate, make sure they’re operating really efficiently, and therefore, to cut down the methane emissions. Cities are looking at net zero plans. You cannot do net zero unless you can look at wastewater treatment.

SHAHLA FARZAN: So you had mentioned these results show that sewage plants are releasing about twice as much methane as we thought, so basically the equivalent of 5.3 million metric tons of CO2. It’s about the same amount as just over a million cars per year. But help us put this into context here. Like how big of a contribution to methane emissions are we talking about overall here, globally?

MARK ZONDLO: Globally, the wastewater sector is about 6% to 9% of total methane emissions. But it’s really uncertain. Why? Because we’re using formulations from a limited number of plants, from a limited parts of the plant, at limited times. And so it’s a really uncertain number. And if we really want to understand where we can have mitigation efforts to reduce greenhouse gases– and like I said methane is a big one because we can see immediate impacts right away.

If we want to do that, we need to start understanding one of the main sectors in cities, and also a non-trivial source kind of globally in the US. I mean, 6% to 9% or even 3% to 5% in the US it says, well gee, that’s not that much. But when you think about it, one, it’s under reported at those levels. And two, with rapid urbanization, with more centralized treatment, these are going to become a bigger problem in the future. We need to pay attention to the sector. And it really hasn’t gotten that. They’re public utilities. They don’t get the extra scrutiny. It’s not sexy. Wastewater treatment plants, like you started off the show, toilets, right?

So we need to start looking at this. They’re highly engineered facilities. They’re really actually quite complex and interesting to understand. But they haven’t gotten the attention that say gas and oil wells have, even though from one very narrowly focused sector we’re talking 5% to 10% of the total global emissions roughly.

KATHLEEN DAVIS: Mm-hmm. I mean obviously, there are different types of sewage treatment plants. Are there some plants that are bigger contributors to this methane problem than others?

MARK ZONDLO: So it appears that the treatment plants with anaerobic digesters, so where they put all the– after initial treatment they put all the sludge in this container and anaerobic bacteria work on it and produce methane, that’s how it’s designed– those seem to be leaking more than other types of plants. Now that’s a problem and an opportunity. One, it’s an opportunity because if we can capture this methane we can use it for a circular economy. We can use it to power the plant, for example, and that methane doesn’t escape to the atmosphere.

So there are opportunities with this, but these digesters leak. And it’s very common with the gas and oil supply chain any time you have methane infrastructure, it leaks through valves, it leaks through seals or o-rings. I mean, it’s hard to capture methane. And I think the gas and oil industry has seen that and I think we can take advantage of some of the lessons learned there. Let’s pay closer attention to these anaerobic digesters. Let’s try and identify the leaks. And let’s cut this out. So really it is a win-win for the environment.

SHAHLA FARZAN: Obviously, we’re not going to be stopping flushing our toilets anytime soon and reducing this waste. But it sounds like there are some things that we can do to try to prevent these leaks and try to stop them at the source then.

MARK ZONDLO: Correct. We need a greater focus on trying to identify where the leaks are. We need greater monitoring both on site as well as the, hey, let’s look at facility level. Where is it leaking that we didn’t think it was? But also we need more information about the plants themselves. Because these are typically funded by public entities, there’s surprisingly not as much information, what’s happening, what was built in the 1960s still may be used today. But there are newer reactors. There are reporting requirements. But typically the reporting requirements are for water, which is a good thing.

But we also need to look at the air side of things, and that has kind of been neglected. And what our study shows, along with a study by my colleagues Cuihong Song and Jason Ren, they looked at the same sector that we did from looking at thousands of papers in the literature and came to the same conclusion. Methane is underestimated by about a factor of two. So both studies coming from independent methods show that we need to pay attention to methane.

But a lot of this is we don’t know where in the plants it’s leaking from. That could be parts of reactors that are not being well aerated, or parts of reactors that aren’t being well mixed. It could be leaks from the digester, anaerobic digesters. We need to look at the system with the level of scrutiny to understand and address the magnitude of the problem that we’re facing.

SHAHLA FARZAN: Mark Zondlo is a professor of civil and environmental engineering at Princeton University. Thanks so much for taking the time to talk with me today.

MARK ZONDLO: Thank you Shahla, it was a pleasure to be on.

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About Charles Bergquist

As Science Friday’s director and senior producer, Charles Bergquist channels the chaos of a live production studio into something sounding like a radio program. Favorite topics include planetary sciences, chemistry, materials, and shiny things with blinking lights.

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Shahla Farzan is a science journalist, PhD ecologist, and editor with American Public Media, where she helps produce science podcasts for kids. She loves showcasing the many weird and wonderful aspects of science—and encouraging young, curious thinkers to question and explore the world around them.

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