Phosphate fertilizer value of heat treated sewage sludge ash

This study focuses on the question whether heat treated sewage sludge ashes are more favourable as fertilizers than untreated sewage sludge ashes (USSA) and whether their fertilization effects are comparable with commercial triple superphosphate (TSP). In a pot experiment, maize was fertilized either with one of three heat treated and Na-, Ca-and Si-compounds amended sewage sludge ashes (two glown phosphates, steel mill slag + sewage sludge ash) or USSA or TSP as control. Fertilization with USSA did not increase the biomass yield and the P uptake of maize in comparison to the P0 treatment (7.25 resp. 8.35 g dry matter/pot). Fertilization with heat treated sewage sludge ashes and TSP resulted in significantly higher yields and plant P uptakes which are on average eight times higher than treatment with USSA and P0 control. Biomass yields and P uptake of maize after fertilization with heat treated sewage sludge ashes are not significantly different from those after TSP fertilization. The main P compound in USSA is Ca-3(PO4)(2). By heat treatment and amendment with different sodium, calcium, sulphur and silicon containing additives or steel mill converter slag, Ca-3(PO4)(2) is converted to Ca- and Na-silico-phosphates, which have a higher water solubility than Ca-3(PO4)(2). This increased solubility is responsible for the high plant availability of this phosphates.