Nutritional value of struvite deriving from sewage sludge

Since natural phosphorus sources are limited, recycling processes are becoming important for agriculture. A great nutrient source is given by wastewater and sewage sludge. However, besides potential recyclables, sewage sludges also often contain harmful substances. One product of sewage sludge to be evaluated is struvite, which is recovered during the treatment of municipal wastewater and sludge production. This study aims to present the groundwork for developing a new fertilizer from upgraded sewage sludge, including an evaluation of contents of nutrients and heavy metals. The struvite under study contains 10.9% total phosphorus, but only 0.3% of the material was found to be water-soluble phosphorus. The amount of phosphorus available after extraction in citric acid was much higher, reflecting a fertilizing potential within the range of other legally certified fertilizers. In plantpot experiments a high plant availability of the struvite phosphorus under study was confirmed. In terms of heavy metal contents struvite showed contaminant levels manifold lower than the limiting values of the German Sewage Sludge Ordinance. Thus struvite phosphorus recovered from the wastewater path may substitute existing mineral fertilizers.