Will Recycling Phosphorus Help Stop Algae Blooms?

Excess phosphorus in lakes can cause destructive algae blooms big enough to be seen from satellites. (Courtesy of NOAA)

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.

A gravity-belt thickener separates a sludgy solid full of plant nutrients from phosphorus-rich water. Image courtesy of Michael Mucha.
A gravity-belt thickener separates a sludgy solid full of plant nutrients from phosphorus-rich water. Image courtesy of Michael Mucha.

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.

The liquid is combined with two other chemicals that pull that phosphorus out of the water to form tiny particles of a mineral called struvite. These tiny particles are built up layer by layer, like a pearl. The end product is more than a ton of smooth, cream-colored struvite pellets–a ton and a half of them every day. Struvite contains nitrogen and magnesium as well as phosphorus. All three are important nutrients for plants, so a company called Ostara (which helped develop the recycling process) buys back the pellets from the Madison Metropolitan Sewerage District (MMSD) and turns them into a special plant-activated fertilizer that releases phosphorus only when it is near growing roots. In other words, it won’t feed algae blooms in lakes.

The wastewater-derived fertilizer is applied to thousands of acres in Wisconsin. Photo courtesy of Michael Mucha.
The wastewater-derived fertilizer is applied to thousands of acres in Wisconsin. Photo courtesy of Michael Mucha.

The gray, smelly, sludgy solid left behind at the treatment facility is trucked to farms where it’s used as fertilizer. Despite thoughtfully designed application methods that keep much of it below ground, sooner or later that fertilizer will reach Madison’s lakes. Since the implementation of the phosphorus recovery program, the amount of phosphorus in the fertilizer has been reduced by 85 percent. That means that the thousands of acres where this fertilizer is applied will be sending much less phosphorus into the watershed than before.

In addition to cleaning up the water and providing a new income stream, deliberately recycling phosphorus into struvite pellets also keeps the struvite from building up in pipes and tanks, where it had been a perpetual nuisance. MMSD Director Michael Mucha explains that this is becoming a common theme: the environmentally responsible thing to do turns out to be cost-effective, too.

And Mucha isn’t stopping there. He is also working with farmers to help develop methods that will keep phosphorus out of the lakes. Reducing phosphorus in fertilizer isn’t the only issue: manure is a major piece of the puzzle. Mucha is advocating that farmers use holding tanks and manure digesters. “Our industry is changing,” he said. “As engineers, the way we would always solve a problem is build a bigger treatment plant, [but] actually, the better solution many times is not building at all. It’s working with people to change behavior.” He admits, though, that it typically takes longer, and, he laughs, “There’s no spec book.”

So, in the meantime, a ton and a half a day of phosphorus pellets is a good place to start.

For more on how water affects communities, and where we might be 50 years from now, check out the QUEST video “Picturing a Future Wrought by Climate Change.”

Will Recycling Phosphorus Help Stop Algae Blooms? 18 September,2015Eleanor Nelsen

Author

Eleanor Nelsen

Eleanor Nelsen is a graduate student in chemistry at the University of Wisconsin-Madison. When she's not studying rhodium chemistry, Eleanor enjoys reading and writing about science. She lives in Madison with her husband Luke and their growing collection of livestock.

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