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Towards Sustainable Struvite Production

Struvite may not be a household word, but it is all too familiar to the operators of wastewater treatment plants.

When it crystallizes on equipment surfaces in treatment plants, says Jonas Baltrusaitis, struvite can clog pipes, requiring them to be chemically cleaned or replaced and, in some cases, forcing a plant to be shut down.

But there is a brighter side to the picture, says Baltrusaitis, an assistant professor of chemical and biomolecular engineering.

Struvite contains three nutrients vital to plant growth—nitrogen, phosphorus and magnesium. Treatment plant operators aim to recover these nutrients early in the wastewater treatment process and convert them to fertilizer. This must be done before the nutrients harm pipes and equipment and before they are discharged into the environment, where they pollute streams, rivers and lakes.

Baltrusaitis and his group have used advanced microscopy and spectroscopy techniques to study the formation of struvite crystals at the molecular level. Their work promises to lead to a cheaper, less energy-intensive conversion method and eventually to greater sustainability in wastewater treatment and in agriculture.

Most wastewater treatment plants, says Baltrusaitis, use insoluble iron or aluminum salts to recover phosphorus (phosphates) from sludge and then dispose of the nutrients in landfills. Water-soluble magnesium salts such as magnesium chloride (MgCl2) are also used to recover the nutrients via struvite formation.

“The traditional way of making struvite is not very sustainable,” says Baltrusaitis. “Soluble magnesium salts are typically made from seawater or brine. Seawater has to be evaporated in order to recover struvite, and this requires a lot of energy.”

Baltrusaitis and his group are proposing to form struvite by using magnesium oxide (MgO), dolomite (MgCO3*CaCO3) or magnesium carbonate (magnesite, or MgCO3) instead of magnesium obtained from seawater or magnesium chloride containing salt brines. MgO and MgCO3 are naturally occurring abundant minerals.

The work by his group, says Baltrusaitis, will lead not only to greater sustainability in the recovery of nutrients from wastewater but potentially to greener agricultural practices as well.

“Waste from wastewater treatment plants is going to grow proportionally as population grows,” he says. “The current technology, taking MgCl2 from seawater, is less sustainable and is going to require more and more energy to recover nitrogen and phosphorus from animal waste and other waste.

“We believe we have devised a low-energy, low-environmental-impact technology that can potentially break this spiral.”