Scientists in Germany have developed a chemical-free means of transforming Sewage sludge, wastewater and liquid manure into readily useable fertiliser for food production. Researchers at the Fraunhofer Institute claim their eco-friendly process will enable the recovered salts to be converted directly into organic food for crop plants.

The researchers have said their discovery has implications for food security, coming at a time when finite resources of materials such as phosphorous, vital to current growing methods, are beginning to bite and rising prices of raw materials risk severely affecting production and consequently reducing access to food for the world’s poorest.

Scientists at the Stuttgart-based institute, which they said makes the most of locally available resources which are abundantly available in the wastewater from sewage treatment plants and in the fermentation residues from biogas plants. The new process was developed by a team of scientists led by Jennifer Bilbao, who manages the nutrient management research group at the IGB. Bilbao said, “Our process precipitates out the nutrients in a form that enables them to be directly applied as fertiliser”.

The process

In the words of its designers, the electrochemical process separates magnesium-ammonium phosphate – also known as struvite – using electrolysis from a solution containing nitrogen and phosphorus. The struvite forms tiny crystals that can be used directly as fertilizer, without any further processing. The innovative aspect of this method is that, unlike conventional processes, it does not require the addition of synthetic salts or bases.

A two meter high electrolytic cell forms the centrepiece of the test installation, containing a magnesium anode and a metallic cathode; through this, wastewater is directed and magnesium ions created by the electrolysis react with the phosphate and ammonium molecules in the solution to form struvite.

Energy-saving, chemical-free process

Because the magnesium ions in the process water are highly reactive, the new method requires very little energy. The developers have said that, in all types of wastewater so far tested the electrochemical process has been shown to consume less electricity than conventional methods.

Moreover, IGB researchers are examining the other eco-friendly aspects of the process; according to Bilbao, long-duration tests conducted using the pilot plan’s reactor showed the concentration of phosphorus in the reactor was reduced by 99.7 percent to less than 2 milligrams per litre. This is lower than the maximum concentration permitted by the German Waste Water Ordinance (AbwV) for treatment plants serving communities of up to 100,000 inhabitants.

Bilbao elaborated, “This means that operators of such plants could generate additional revenue from the production of fertilizer as a sideline to the treatment of wastewater.” The struvite produced by the process is an attractive product for farmers, because it is valued as a high-quality, slow-release fertiliser.

The scientists intend to spend the next few months testing the mobile pilot plant at a variety of wastewater treatment plants before starting to commercialise the process in collaboration with industrial partners early next year. “Our process is also suitable for wastewaters from the food-industry and from the production of biogas from agricultural wastes.” Bilbao added.

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