Fraunhofer Institute researchers announced they have developed a chemical-free, green process that enables the recovered salts from sewage sludge, wastewater, and liquid manure be transformed into organic fertilizer.
Researchers from Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart developed a method that transforms wastewater from sewage treatment plants or fermentation residues from biogas plants into a phosphate-based fertilizer. Jennifer Bilbao, who manages the nutrient management research group, said their method “precipitates out the nutrients in a form that enables them to be directly applied as fertilizer.”
That form is struvite, or magnesium ammonium phosphate, a phosphate mineral discovered by Heinrich Christian Gottfried von Struve, coincidentally, in 1845 in German medieval sewer systems. It was first used as a fertilizer in 1857.
Its presence in sewage and wastewater treatment can cause expensive operational problems. It forms scale and reportedly clogs pipes, typically in the stage following anaerobic digestion.
Phosphorus, which is an essential mineral for plants and other living organisms, is in increasingly shorter supply. The substance used in fertilizers is derived from rock phosphate, a mineral mined in China, Morocco, and the United States. Its price has also been steadily increasing, which means alternatives sources are needed.
The process created by the Fraunhofer researchers is electrochemical-based and reportedly uses electrolysis to precipitate struvite from a solution of nitrogen and phosphorus. The primary component in the system is a 2-meter tall electrolytic cell that splits the water molecules, ultimately causing magnesium ions in the water to react with phosphate and ammonium, forming struvite. No chemicals are required, and the process also reportedly consumes very little energy — no more than 70 watt-hours per cubic meter. The small crystals formed can be used directly on food crops as fertilizer.
In testing, the researchers found that the wastewater’s phosphorus concentration was reduced to less than 2 milligrams per liter, which Bilbao says means wastewater treatment plant operators “could generate additional revenue from the production of fertilizer as a sideline to the treatment of wastewater.”
Fraunhofer is not the only organization working on struvite recovery technologies. University of British Columbia developed a struvite reactor, “so cost effective that within five years a sewage treatment plant can pay for the system by selling the fertilizer it produces,” according to The Globe and Mail. As of 2009, there were six struvite reactors in operation, one of which is able to precipitate 300 kilograms of struvite per day from wastewater. The technology has also been licensed by Ostara Nutrient Recovery Technologies Inc., which uses it in its Crystal Green fertilizer.
Dana Cordell, while a doctoral candidate at Linköping University in Sweden, explored the world’s phosphorous use and the challenges associated with ensuring its future availability. The results were published in “The story of phosphorus: Global food security and food for thought,” in the journal Global Environmental Change (PDF). Recovering phosphorous from wastewater, she said, could assist in stemming an impending global shortage:
Every year, the global population excretes around 3 million tonnes of phosphorus in urine and faeces. Given that more than half the world’s population now lives in urban centres, and urbanization is set to increase, cities are becoming phosphorus ‘hotspots’ and urine is the largest single source of phosphorus emerging from cities. While nutrient flows from food via human excreta typically found their way back to land in the past, today they more often end up in waterways via wastewater from urban centres or as sludge in landfills. […] It is estimated that on average, around 10% of human excreta is currently recirculated, either intentionally or unintentionally, back to agriculture or aquaculture. Examples of how this occurs include poor urban farmers in Pakistan diverting the city’s untreated wastewater to irrigate and fertilize the crops, and pit or composting toilets in rural China, Africa and other parts of the world. Recirculating urban nutrients such as urine back to agriculture therefore presents an enormous opportunity for the future.
Cordell specifically notes that struvite recovery is but one of these opportunities. She says:
Struvite crystalisation and recovery is a promising technological process that has the potential to both remove phosphorus from wastewater byproducts more efficiently, and, provide an alternative source of phosphate fertilizer.
United States farmers typically have a difficult row to hoe when use of excreta as fertilizer enters discussions with government organizations, whether the waste is animal or human. The idea of using human waste, or sludge, as an agricultural fertilizer tends to attract opposition — even when the more palatable term “biosolids” are used to describe it.
The process created by the German researchers is not solely for use with municipal wastewater, says Bilbao. “Our process is also suitable for wastewaters from the food-industry and from the production of biogas from agricultural wastes,” she notes, but the feedwater used needs to be ammonium- and phosphate-rich.
The Fraunhofer researchers will be testing their mobile pilot plant at various wastewater treatment plants. They intend to start commercialization of the process starting in early 2013.
