Strategies for improving economic and environmental sustainability of wet chemical phosphorus recovery from sewage sludge ash

Phosphorus (P) recovery from sewage sludge ash (SSA) represents a promising solution to P supply concerns, with a key challenge consisting of the poor circularity of current technologies. This research explores the potential of an innovative two-step (TS) wet chemical P recovery process from SSA, assessing the use of industrial by-products as secondary reagents and investigating the reuse of the leachate from a conventional one-step (OS) process as an extractant for a subsequent P extraction stage. Lab-scale TS tests were conducted by simultaneously employing secondary extractants (H2SO4 and HCl) and precipitants (lime milk), comparing process performances to a conventional OS process performed with primary reagents. Secondary reagents were effective in recovering P, with average extraction and precipitation efficiencies of 86% and 99%, respectively. The second extraction step successfully increased P concentration in the leachate by 33% to 43%, indicating its potential for reuse. The TS process generated high-value P-rich products, with a P content up to 18% when using H2SO4. However, the overall recovery efficiency of the TS process was up to 27% lower than that of the OS process, indicating the need for further optimization of the second extraction step.