Phosphorus Recovery from Sewage Sludge Ash – A Wet Chemical Approach

Abstract

Abstract: In laboratory-scale experiments with ashes from different full-scale sludge incineration plants the elution behaviour of phosphorus and metals at different pH-values were examined. The elution of ashes with water alone did not cause any significant release of phosphorus, while depending on the ash origin and formation, a phosphorus release of maximum 30 % was measured when using sodium hydroxide solution. With sulphuric acid, it was possible to release phosphorus and (heavy) metals quantitatively at pH-values less than 1.5. In addition to the elution experiments, precipitation and nanofiltration were investigated as methods for the separation of phosphorus and metals from the liquid phase. Sequential precipitation of phosphorus (SEPHOS Process) seems to be promising. The generated product, an “aluminium phosphate”, is a valuable raw material for the phosphorus industry. After alkaline treatment of the “aluminium phosphate”, it is possible to precipitate phosphorus as calcium phosphate (advanced SEPHOS Process). Following acidic elution of the ash, nanofiltration can also be used to separate phosphorus.