The world is full of good and bad bacteria. We tend to think more about the bad types such as the MSRA now making the news as it turns up in high schools or the common strep throat. On the good side, we see the powers of bacteria that decompose kitchen scraps, cow manure, and yard clippings to become compost and replenish our soil. Whether they serve good or evil purposes, these bugs operate in a mysterious, unknown nano-world that few of us understand. But engineers like Katherine McMahon relish this.

As an assistant professor in the Department of Civil and Environmental Engineering at the University of Wisconsin-Madison, McMahon researches and teaches about the microbes used to treat wastewater in common sewage treatment plants throughout the world with an eye on improving them. She also studies lakes that have been polluted with phosphorous over many years. “When I teach my classes, I always go on and on about how exciting the microbes are just because they’re so weird,” she proclaims, showing an obvious appreciation for what they can do.

McMahon, 34, admits, “I kind of stumbled into it,” when asked how she got into the world of microbes. “I knew I wanted to do something environmental when I was a senior in high school. And my father was an electrical engineer, so I wanted to be an engineer.” Having grown up in Urbana, Illinois, McMahon got a B.S. in civil engineering from the University of Illinois at Urbana-Champaign.

As part of an undergraduate research project the summer after her freshman year, she looked at remediating groundwater contamination using bacteria. “I thought that was really fun, so when I came back to school my sophomore year, I did more undergraduate research for another professor who was doing bioremediation,” McMahon recalls. “I just sort of got into the biology side of it, and when you study biology as an environmental engineer, it usually has to do with wastewater treatment and sometimes natural things like lakes and rivers. I just sort of gravitated towards that.”

This would lead to an M.S. in environmental engineering, also from
University of Illinois, for McMahon. As a master’s student, she studied anaerobic treatment processes, which operate in the absence of oxygen, and looked at digestion of wastewater sludge and solid waste and the bacteria used for such processes.

When it came time to pursue a PhD., McMahon wanted to go to the University of California at Berkeley, and a grant opportunity came up to study phospherous accumulation in wastewater treatment and bacteria. This involved aerobic processes, which require oxygen, unlike her previous work with anaerobic bacteria. “It was really good because I got exposure to both kinds of systems. I ended up preferring the stuff that I worked on for my PhD,” she relates. The Ph.D. came in environmental engineering with minors in chemical engineering and microbial biology, a perfect combination for the work she would undertake. 

A Complex Process
Known as enhanced biological phosphorus removal (EBPR), treating wastewater with bacteria is a 3-phase process to remove phosphorus from municipal and industrial wastewaters. Bacteria appear to clean sewage by gorging themselves on phosphorous, releasing it, gorging themselves on carbon, extracting energy from it, and repeating the cycle over and over.

The first phase is anaerobic, where bugs charged with phosphorous from a previous cycle mix with the effluent (solids have settled out), sucking up organic matter and releasing the phosphorous. “The thing we think makes them store the phosphorous inside their cells is a combination of feast and famine,” as McMahon describes it. “You force them to have all this food without any oxygen for an hour or so, and then you move them into a tank where they have lots of oxygen, and now they can break down all that stored organic matter inside their cells and grow. While they’re growing, they take up the phosphorous. And because they grow, there are more cells at the end of that than there were at the beginning, so the phosphorous gets equally apportioned among all the different cells. Because there are more cells, a net gain results that gets removed from the water.”

Plant operators siphon off some of the cells full of phosphorous every cycle, sending them from the clarifier to an anaerobic digestor. They release all their phosphorous as their cells break down, along with ammonia and magnesium. That becomes part of the resulting sludge, a dirt-like material that doesn’t smell and is biologically stable and often gets spread on farm fields as fertilizer. Sludge has nitrogen in it also.

But where does the bacteria come from in the first place? It is naturally occurring and not genetically engineered. “We actually don’t know where it comes from. That’s one of the things we’re trying to study,” McMahon says. “We don’t know if they come in the rain because tanks we have them in are open to the atmosphere. Birds could bring them in because they sit on the surface of the water.” “They are very similar to bacteria you see in a lake, so they like fresh water and oxygen, they like decomposing organic matter you see in the sediments of the lake. So we think their natural habitat is lakes and rivers.”

Having explained the process, McMahon now reflects on it. “What fascinates me is how resourceful bacteria are, just how good they are at carrying out these strange reactions, catalyzing odd transformations in the environment -- things plants and animals can’t do. So we can harness that and use it to clean water.”

Scientists’ current understanding of EBPR is based only on empirical evidence from operating treatment plants. From an engineering perspective, we know how to make the bacteria remove phosphorous from wastewater, but we don’t understand why or how they do it. Microbes possess diverse and sophisticated physiologies, communication strategies, and mechanisms of evolution. Scientists and engineers are only beginning to understand and exploit the metabolic potential of these organisms. As a start, one major group of organisms was recently identified and named Accumulibacter phosphatis. McMahon says her research aims to improve our capacity to predict and model microbial behavior while searching for new biological transformations that can be harnessed to remove pollutants from water.

Different Aspect of the Same Thing
Another side of McMahon’s research focuses on eutrophied lakes, which exhibit unsightly, smelly blue-green algae blooms caused by an overload of phosphorous. These originated more than a century ago, when untreated sewage was dumped into lakes. “There is so much residual legacy phosphorous left in such a lake that isn’t going anywhere, the lake isn’t getting any cleaner,” she reports.

Scientists generally accept that microbes control phosphorous cycling in lakes in both the water column and sediment, operating much as they do in treatment plants. McMahon says a better understanding of the fundamental mechanisms involved will ultimately help predict the effects of lake management practices on water quality. “We just want to understand how phosphorous recycles in a lake, which affects the amount of phosphorous available to algae. You have to figure out what those organisms are doing and because they do it in such a strange way, it’s often tricky to figure out.”

McMahon and fellow researchers have selected three contrasting eutrophic lakes to study: Lake Mendota, a large deep lake near Madison; Lake Wingra, a small shallow lake in Madison; and Lake Taihu, a large shallow lake in China that has terrible toxic algae blooms. She will collect bacteria samples for three years from the Madison lakes while a collaborator in China will sample Lake Taihu, which supplies drinking water to a staggering 40 million people in Shanghai and surrounding cities.
As McMahon points out, wastewater formerly had higher phosphorous concentration because of the detergents used in the 1960s and 70s. Then the government clamped down on the use of phosphorous in detergent because of the problem of eutrification of lakes and rivers. Most of the phosphorous in sewage now comes from human waste because it isn’t an essential part of our cells. Phosphorus also comes from fertilizers put on lawns and farm fields that runs off into bodies of water.

Many wastewater treatment plants were built in the 1960s and 70s, and those are being upgraded and expanded to accommodate population growth. McMahon takes a historical look at our track record in the United States. “I think we’ve done a remarkably good job with a very rudimentary understanding of the underlying fundamentals. I’m always amazed that wastewater treatment plants work as well as they do, even though we don’t understand very much about the bacteria. The government invested a lot in building this infrastructure, and the problem we’re having now is that a lot of these systems were designed for a 50-year life span. People aren’t prepared to make the investment to maintain them because it’s unbelievably expensive. And now, do we tear them down and rebuild, or do we just keep adding on or patching them?”

While we may have done well for the knowledge we had of treatment processes, McMahon thinks we can do better. “The tricky part comes in when you start really clamping down on the discharges. We need to get ten times lower nitrogen and phosphorous. That’s where you have to make some of the technological advances. The technology we have to solve it is so expensive, and it’s going to be a decision the country will have to make if we want to invest in that and go through that same building out of our infrastructure like we went through 50 years ago.”

Progressive Engineer
Editor: Tom Gibson
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©2006 Progressive Engineer