Alkaline recovery of phosphorous from sewage sludge and stabilisation of sewage sludge residue.

The alkaline leaching of Phosphorous (P) from sewage sludge (SS) using either NaOH or KOH was investigated. The effect of type and concentration of alkali, solid liquid loading and leaching temperature were also investigated. The possibility to stabilise the leached SS residue via geopolymerisation was also explored. The results showed that KOH leached more P than NaOH. The leaching conditions yielding the highest P recovery were 8 M KOH, 4% m/v solid to liquid loading, a leaching time of 2 h and a leaching temperature of 95 °C resulting in a 85% P leached from SS. The leaching kinetics followed modified Jander kinetics with the leaching process at 95 °C being controlled by diffusion of reactants through a porous layer of reaction products, whilst at 80 °C and 70 °C was controlled by diffusion of reactants through a dense layer of reaction products. A 10% replacement of gold mine tailings (GMT) with the leached SS residue yielded a geopolymer which met the minimum requirements for construction materials with regards to ASTM C67/14 and C129/14a. The geopolymer was cured at 100 °C for 5 days yielding a geopolymer with strength of 14.2 MPa and an open porosity of 0.38. The geopolymer also met the minimum statutory requirements for metal leachability. This study presents an opportunity for the increased alkaline recovery of P from SS and the stabilisation of the SS residue thereby creating a no solid waste process.