From sewage sludge ash to calcium phosphate fertilizers

Our work presents the results of the research on the utilization of ashes after sewage sludge combustion comprising phosphorus recovery in the form of useful products. The investigations were divided into three parts: selecting the combustion parameters of sewage sludge, examining ash leaching with mineral acids (nitric and phosphoric) to high phosphorus selectivity assuring a low content of iron and heavy metals in the extracted solutions and precipitation of CaHPO4 .2H2O. Suitable temperature of a sewage sludge combustion enables selective extraction of phosphorus compounds from ash because of hematite phase forming, insoluble in mineral acids. The extracts from phosphoric acid leaching, where the extraction of phosphorus compounds was 96.1%, have very good properties for its further use as the initial solution for CaHPO4 .2H2O with 6% lime milk. The obtained product is characterized by high purity and phosphorus availability compatible even with the feed phosphate standard.