Targeting phosphorus recovery strategies for activated sludge: DoE and multi-objective optimisation to analyse key process factors and associated costs

Sewage sludge-derived matrices have been shown to be promising sources of phosphorus (P) to alleviate pressure on natural reserves. However, the diverse forms of P to be extracted from activated sludge make recovery challenging, where P release can limit P recovery. This study applies response surface methodology to evaluate the effects of pH, temperature, acetate, and their interactions, on P release, followed by multi-objective optimisation to identify the process conditions that achieve the optimal trade-off between minimising extraction costs and maximising P release. Results indicated that the most significant factors impacting P release were pH, followed by the interaction between temperature and acetate, and temperature alone. Acidification at pH 4 and room temperature achieved the lowest operational costs (∼ 1 US$/kg P_ext) for effective P extraction (44%), as compared to bioacidification or mild heating. Following extraction, P was successfully precipitated as ACP (∼ 100% precipitation efficiency at pH 9), enabling a potential recovery of 15-20 kg P per tonne of sludge (dry matter).