In the midwestern part of the U.S., farms produce a third of the world’s corn and soybean supply annually. Large agricultural fields like these require a huge volume of fertilizer to keep their crops healthy. Biosolids Fertilizer that is recovered from sewage water is composed of nitrogen and phosphorus that plants need to grow. The ability to recycle sewage water and recover valuable nutrients can make benefit many farmers by bringing down the cost of fertilizing farmlands.
Nitrogen is easier to recycle because it could literally be pulled out of the air. Phosphorus, however, is more difficult to source because it needs to be mined then treated to be safely used on plants. Since these phosphate rocks are a finite resource, finding ways of recycling phosphorus effectively is the challenge that needs to be addressed.
Phosphorus used on agricultural lands are usually lost through erosion and then subsequently carried into waterways. This joins the phosphorus effluent that comes from sewage water treatment plants, livestock farms, and other sources. Phosphorus levels then build up in the water which can degrade water quality and affect aquatic life, creating “dead zones.”
Andrew Margenot, an assistant professor in the Department of Crop Sciences at the University of Illinois, is authoring a report that details the feasibility of recycling phosphorus on a regional scale in the Midwest. His aim is to solve the problem of uncertain phosphorus global supplies and minimizing phosphorus losses from agricultural fields.
“There’s a lot of phosphorus being moved around, from fertilizer all the way to sewage. Right now it’s mostly a one-way street. It ultimately ends up in waste streams, like septic tanks or effluent from point sources like wastewater treatment plants. But there’s a lot of value in that phosphorus. We can capture it and reuse it as a fertilizer,” says Margenot. “We’re trying to make circular something that is currently a one-way trajectory.”
Phosphorus is commonly found in the form of phosphate, which is soluble in water. Although chemical agents can be used to separate the phosphate from the water, it also renders it insoluble for plants to absorb.
Margenot says that “There’s an inherent tension between the ability to recover phosphorus from wastewater, which renders it insoluble, and its utility as a fertilizer.” Insoluble forms cannot dissolve as easily, so they can’t be utilized as effectively. While there is an agronomic explanation to why 100 percent of sewage water cannot be recycled, experts can still experiment with the chemical composition of wastewater to fit the right soil type and crop type. Understanding this will be vital to overcoming the challenges of recovering phosphorus.
Margenot believes that there is a strong potential in using struvite, an inorganic, granular, slow-release form of phosphorus that is recoverable from multiple waste streams. Its low water solubility makes it unlikely to move from the field into surrounding waterways.
Phosphorus recycling in the Midwest is feasible if the right investments in infrastructure and research into untapped forms of phosphorus are explored. Margenot’s research may pave the way for new sources of recyclable phosphorus that would not only benefit farmers, but also improve the environment in the process.
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