Electrochemical Phosphorus Recovery from Anaerobically Digested Sludge: Improving Product Purity and Concentration

Recovering high-concentration and high-quality phosphorus (P) from municipal sludge presents significant technical challenges. Herein, an electrochemical phosphorus recovery system (EPRS) was developed to treat the anaerobically digested sludge (ADS), featuring a leaching unit for P leaching and a recovery unit for P separation. The leaching unit consistently reduced the ADS pH from 7.5 to 3.3 and elevated the dissolved ortho-P concentration from 65.7 ± 19.7 to an average of 215.2 ± 44.6 mg L^-1. The recovery unit achieved a P recovery efficiency of 74.8 ± 7.5% until reaching a maximum ortho-P concentration of ∼4040 mg L^-1 after 33 cycles. This maximum concentration could potentially be increased by adjusting the anolyte pH to 3-4 as predicted by a mechanistic model. Mass distribution analysis revealed that 54% of total P input was in the final anolyte of the recovery unit, which contained less than 1% of Mn, Al, Zn, Cu, Pb, Cd, and Ni that were in the ADS. However, 10% of total As was detected in the recovery unit anolyte, likely because of chemical speciation. The solid product from the EPRS consisted of struvite and magnesium phosphate. Although the leaching unit was the main energy and chemical consumer, it significantly reduced the total coliform levels that satisfied the USEPA Class A pathogen standards.