Sewage sludge as a source of phosphorus

New process ensures raw material savings and environmental protection

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As phosphorus levels are on the decline worldwide, phosphate residues in the water cause major environmental damage. Scientists have now developed a new method to recover phosphorus from sewage sludge. As a result, they receive magnesium ammonium phosphate, which can be used as a fertilizer in agriculture.

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Every day about two grams of phosphorus are released into the wastewater via detergents, human waste and the like, which have to be filtered out to prevent environmental damage in the sewage treatment plants. On the other hand, experts estimate that the known and economically degradable geogenic phosphorus deposits in the earth are already exhausted in less than 100 years. So it makes sense to look for ways to recycle phosphorus and use it as a fertilizer. The source in Germany is the large amount of sewage sludge available.

Fouled sludge as starting material

When recovering, the scientists of the Institute of Hydraulic Engineering, Water Quality and Waste Management of the University of Stuttgart around Professor Heidrun Steinmetz, Jörg Krampe and Alexander Weidelener on the digested sludge on. Here, with about 90 percent of the infeed freight, most phosphates are produced, which makes the Stuttgart approach particularly efficient.

At the newly developed process in the university's own sewage treatment plant in Stuttgart-Büsnau, the phosphates are first dissolved with sulfuric acid and separated. To prevent the process being disturbed by metal ions, the enriched-phase scientists are adding sodium citrate. After the addition of magnesium oxide, the phosphate precipitates as magnesium ammonium phosphate (MAP). display

Same effect, less heavy metals

This compound has a comparable fertilising effect as common commercial fertilizer, but contains significantly less heavy metals. However, the phosphor fertilizer obtained in the recycling process is still more expensive than that from geogenic deposits. The Stuttgart scientists are counting on MAP to be included in the next amendment to the Fertilizers Ordinance as mineral multi-substance fertilizer.

"If a separate market for MAP is established, the fertilizer could be produced more efficiently and sold with proceeds, " says Weidelener. Then the process, which has been developed under laboratory conditions and has been tested on a semi-industrial scale since May, will soon also find its way into the sewage treatment plants in the country.

(idw - University of Stuttgart, 27.07.2007 - DLO)