One fascinating thing about wastewater treatment is that while plants are filled with big tanks, motors, pumps, mixers, blowers and valves, the real work gets done at the microscopic level — by the bacteria that consume the waste.

In recent years, a new family of microorganisms has come into focus: anammox bacteria, which can turn ammonium into nitrogen gas anaerobically. The obvious advantage over conventional treatment is that anammox bacteria require far less aeration and thus far less energy to remove nitrogen from the wastewater.

So theoretically, that’s great. The question is how to make the anammox bacteria function most efficiently in the real-world treatment plant environment. It turns out that they grow slowly, and their effectiveness requires close control over oxygen and temperature. This is where science comes in. Researchers at the University of Wisconsin-Madison are studying how anammox bacteria interact with conventional nitrifying bacteria in the wastewater treatment ecosystem. It’s a project that’s funded in part by the National Science Foundation, the UW-Madison Microbiome Initiative, and a training fellowship from the Natural Sciences and Engineering Research Council of Canada.

The research may provide clues to unlocking the full potential of the anammox organisms, first discovered in the 1990s. Then comes the work of deploying and testing the anammox-based processes in the field, and that’s the province of operators. Supervising the UW-Madison research are Katherine “Trina” McMahon and Daniel Noguera, professors of civil and environmental engineering. Executing the day-to-day work is Christopher Lawson, a graduate student in civil and environmental engineering. McMahon talked about the research in an interview with Treatment Plant Operator.

TPO: How is your research team connected to the wastewater treatment industry?

McMahon: I studied wastewater treatment as a Ph.D. student before I came here 14 years ago. I focused on phosphorus cycling in wastewater. I am really interested in how microorganisms interact and how those interactions lead to positive things for humans, like treating wastewater and keeping lakes clean. Noguera has studied nitrogen cycling for almost all of the 20 years he has been here, and I’ve collaborated with him on this work for the past five years. Lawson did his undergraduate and master’s degrees in civil and environmental engineering at the University of British Columbia. For his master’s thesis he worked on phosphorus removal, but he got hooked on microbiology and the principles of interactions between types of bacteria.

TPO: What is it that makes anammox bacteria so intriguing?

McMahon: Plant operators know about nitrification — the conversion of ammonia into nitrate. That’s critical in wastewater treatment, and the organisms that are really important for that in most systems are traditional nitrifiers, which are aerobic. Oxygen is a huge part of BOD removal; estimates are that aeration accounts for about 50 percent of the energy use at many activated sludge treatment plants. The benefit of anammox bacteria is that they can take ammonia and oxidize it to nitrogen gas using nitrite, and do it without oxygen. That means you’re able to get more conversion of ammonia to harmless nitrogen gas with less aeration, so you save money.

TPO: In working with the anammox bacteria, why is it important to understand the interactions they have with other organisms?

McMahon: Anammox organisms are autotrophs, which means they take carbon dioxide in order to make their cells, like plants do. It’s a very difficult way to make a living. It’s easier to take organic matter, such as the carbon compounds that are in the wastewater, in order to grow. And so Lawson was interested in interactions between the anammox and heterotrophic bacteria, the ones that use the organic carbon that’s already there. He had ideas about how these heterotrophs could be providing the anammox bacteria with important molecules that allow them to grow more efficiently or in a more stable way.

Many researchers are so laser-focused on the anammox bacteria that they neglect the principle of ecology that says organisms never live in isolation and always depend on other organisms. If you respect that and use it as a principle to design a system in a way that leverages those interactions, then you can come up with much better designs for wastewater treatment. What we learn about the interactions can help us better engineer systems that use anammox bacteria.

TPO: What is the challenge around making the use of anammox bacteria more widespread?

McMahon: On paper, these organisms look like magic. They do this job for us that we used to have to coax the traditional nitrifiers into doing, but they do it with less oxygen. However, we haven’t figured out the best way to grow the anammox organisms in a wastewater treatment plant in a stable way and in a way that integrates into existing plant infrastructure.

TPO: How are anammox bacteria being applied today in actual plant operations?

McMahon: We have activated sludge plants that are not going away anytime soon, and modifying them to include anammox for the whole stream is not easy. The key is finding creative ways to modify existing infrastructure. Often that involves sidestream treatment. Someday, if we get a whole lot of money to rebuild our wastewater treatment infrastructure, then maybe we can imagine designing an anammox system from scratch. But right now, it seems the major focus is on sidestreams.

With sidestreams, you’re able to get more nitrification in systems that you can control independently from the main activated sludge treatment train. A sidestream gives more bang for the buck in terms of the ability to control it. A lot of research still needs to be done to make it as easy as it should be to justify any treatment plant installing these kinds of systems.

TPO: What is an example of a sidestream that can be treated with anammox organisms?

McMahon: There is a great deal of ammonia in digestate from anaerobic digesters, and that’s where a lot of people have focused their early work. Anammox treatment greatly reduces the ammonia load at the headworks; it’s ammonia that has gone through the full process already. It’s kind of a futile cycle in that during aeration the organisms turn some of the nitrogen into proteins that end up in the digester and turn back into ammonia, which then has to be converted back again into proteins. If you divert that ammonia from the digester to a sidestream where you can turn it into nitrogen gas using less oxygen, you’re greatly reducing the total oxygen load on the plant.

TPO: From the operators’ perspective, is there some resistance to or hesitancy about treatment processes using anammox bacteria?

McMahon: I think there is still a perception that they’re fussy and tricky to work with. Researchers need to come up with strategies to make the systems less fussy. Lawson is working on exactly that. If you can engineer an interdependency of the organisms, you can make the system more stable and robust. That way if something happens, like a toxic shock or some process operations failure, the organisms can recover quickly.

TPO: Are there challenges to culturing and growing the anammox bacteria?

McMahon: Yes, and that is one of the barriers to wider adoption. The companies that install anammox systems have to provide the biomass because you can’t just enrich for anammox bacteria from regular activated sludge. You have to get a pretty dense culture of the organisms and use them to inoculate and jump-start the system. There are only a few of those cultures in the world.
I see a lot of opportunity for someone who is entrepreneurial to go out and find other cultures of these organisms from the environment and enrich them. It could be that if we found slightly different versions of the organisms, they might be more robust and have better interactions with the local bacteria.

TPO: What needs to be done to improve anammox processes?

McMahon: I believe having more innovative designs of the reactors they grow in, the recirculation systems, and the process controls could get them to perform even better. Production of nitrous oxide is something people need to focus on. Nitrous oxide is a very potent greenhouse gas and is something nitrogen-cycling bacteria produce when they’re not working 100 percent efficiently. You have to get them really humming to avoid production of that gas. That’s another research opportunity. If we can get the system to be more stable and operate exactly how we want it to operate, we can avoid nitrous oxide production.

TPO: What research approaches are being used to help make progress toward more widespread uses of anammox organisms?

McMahon: The research and practitioner communities are going to come at it from different angles. There will be people doing more empirical work — “I’m going to try a new reactor configuration and see if that helps.” Then people like Lawson will say, “I’m going to understand the biology of the organisms, and that’s going to tell me what kind of reactor to design.”

Both of those approaches are valuable, and the history of wastewater treatment research is all about that: trying the empirical, practical approach, but also the basic science. Somewhere along the way they meet, and we get great advances in how we treat wastewater.