U looks to wastewater plants to recover farmland fertilizer

·4 min read

Most of the phosphorus that is used to grow Minnesota's vast corn and soybean fields finds its way to the Mississippi River. Then it travels 2,000 miles, past St. Louis and New Orleans, to the Gulf of Mexico, where it not only creates a massive dead zone that kills off ocean life, but is lost forever as a useful fertilizer.

University of Minnesota researchers believe they can help stop that loss.

They'd like to capture the phosphorus, which is spread in manure and fertilizers, before it reaches the Mississippi, keeping it out of lakes, rivers and other waters where it creates, among other problems, massive toxic algae blooms.

If they're successful, wastewater treatment plants throughout the region would also have a new way to recycle the phosphorus they remove from stormwater and sewage sludge, making the use of fertilizers in the United States more sustainable.

"We know we're running out of phosphorus, even if we don't know when," said Bo Hu, a researcher at the U and project manager. "In the meantime, we're wasting a whole lot of it in pollution that ends up in our waterways, in the Great Lakes and in the Gulf of Mexico."

While phosphorus is found everywhere, only a few parts of the world have it in dense enough concentrations for the element to be useful as a fertilizer.

Nearly all of the phosphorus used in fertilizer and in hog or animal feed comes from phosphate rock, most of which is found and mined in Morocco at sites built up over eons from mostly bird droppings.

Some estimates say the world's supply of minable phosphate rock will run out in less than a century. Other estimates say it could last hundreds of years. In either case, about 75% of the phosphate rock left is believed to be in Morocco.

"So in the U.S., almost all of the phosphorus is imported from other countries," Hu said. "It's similar to petroleum, where one or two countries really control it."

After fertilizers and manures are spread, Minnesota's soils are saturated in phosphorus. While some of it is used up and stored in corn and soybean crops, the majority washes away through runoff.

Much of that runoff eventually finds its way into a wastewater treatment plant.

Hu and researchers at the U would have treatment plants separate out the phosphorus from the rest of the sewage sludge, recover it and turn it back into a fertilizer using a combination of artificial stomach-like digesters and chemical treatments.

Just one treatment plant in Minnesota, the city of St. Cloud's, is now collecting and recycling phosphorus, using a more traditional anaerobic digestive process.

One reason there is only one treatment plant in the state working with phosphorus? The equipment to separate and recover the fertilizer is expensive, Hu said.

That makes it especially difficult for treatment plants in smaller rural areas — places with ample farm fields — to recover fertilizers, he said.

The U will test using a new, potentially cheaper method that uses bacteria that eat the phosphorus in the sludge and store it in their cells, said Jim Postiglione, lead researcher on the project.

When the bacteria die, the phosphorus dissolves in the water. The researchers will then separate solids from liquids and treat the liquids with chemicals, such as magnesium hydroxide.

Once the liquid is chemically treated, the phosphorus settles and can be easily recovered as minerals ready to be used, once again, as fertilizer.

The process could cut the time it takes to break down and separate out the phosphorus from around 30 days using a traditional anaerobic digestive process to less than one day, Postiglione said.

It would also involve much smaller facilities and could help treatment plants avoid clogs in piping systems and other common problems, he said.

"If we can make for an easier separation, we can make the process more efficient," he said.

The U is asking for $200,000 from the state's Environmental and Natural Resources Trust Fund to build and test the system.

Each process to recover phosphorus has its pros and cons, said Tracy Hodel, St. Cloud public service director.

"Each treatment facility has a variety of different wastes; some are treating a lot more municipal waste and others have a higher percentage of industrial waste," she said.

But St. Cloud has proved that the potential for phosphorus recovery is great.

"We're seeing greater than 98% recovery, so very little phosphorus is lost," Hodel said. "There is not an unlimited supply, so to be able to recover it is a great thing."

Greg Stanley • 612-673-4882

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