We depend on big farms for our food. For crops, that means a lot of fertilizer; for animals, that means a lot of waste. For the lakes near these farms, that means a lot of phosphorus.
Phosphorus washes into lakes with manure and fertilizer and the erosion of phosphorus-rich, fertilized soil. Cyanobacteria feast on that glut of nutrients and their populations explode, with dramatic consequences for the aquatic life in the lake and the people who depend on it. The toxic bloom of cyanobacteria that made Toledo’s water undrinkable in the summer of 2014 is just one example of what can happen when the biochemistry of a lake drifts out of balance.
Steve Carpenter, a professor at the University of Wisconsin-Madison and the director of its Center for Limnology, describes phosphorus management as the “keystone” issue for healthy lakes. “If we can get phosphorus under control,” he said, “we have a much better shot at dealing with all of the other problems that the lakes have,” like invasive species, which can swoop in when a lake’s nutrient levels are unbalanced. There are ways to slow the gush of phosphorus into nearby lakes, such as contour plowing and winter cover crops, but Carpenter explains that the phosphorus load has gotten so high that those kinds of strategies “almost don’t matter anymore.” Instead, we have to remove phosphorus from the system entirely.
This summer Madison, Wisconsin, unveiled its new phosphorus recycling facility, the state’s first. It’s part of a strategy by the city’s wastewater-treatment utility to reduce the amount of phosphorus that ends up in the lakes. The recycling program takes most of the phosphorus out of the agricultural fertilizer that the plant produces and puts it in a lake-friendly fertilizer designed to be used on urban lawns and gardens.
Steve Reusser, an operations engineer who helped develop the phosphorus harvesting process, explains that it relies on a careful combination of engineering and biology. Certain species of bacteria, which, like plants, also need phosphorus to survive, either absorb or release phosphorus depending on whether there is oxygen present or not. Wastewater is full of phosphorus from human waste. At the treatment facility, it’s stocked with what Reusser characterizes as “a zoo of different kinds of bacteria.” As it wends its way through the facility’s tanks and filters, the oxygen concentration and filtration systems are manipulated in concert to yield two separate products: a sludgy solid that contains a lot of bacteria and not much phosphorus, and water that is phosphorus rich.