Phosphorus recovery from municipal sewage sludge using bio-based re-dissolution with acetate and precipitation as calcium phosphate on dolomite seed grains – A pilot-scale study under real-world conditions

The recovery of phosphorus (P) from wastewater is crucial for circularity of plant nutrients. While wastewater treatment plants (WWTPs) employing enhanced biological P removal offer potential for P recovery by biological P re-dissolution, its scalability and performance under real-world conditions remain underexplored. Here we report on a pilot-scale test (15 m 3 return sludge per day) for rapid P recovery from return sludge at a full-scale municipal WWTP induced by acetate supplementation and subsequent P precipitation as a soil amendment and fertilizer. In a total of 54 re-dissolution batches (treatment time approx. 80 min), supernatant P concentrations varied greatly. Batches were combined and used as the feed at 19.2–22.7 mg P/L for fluidized bed precipitation with dolomite seed grains. This step was highly efficient since upon milk of lime addition (pH >9.6), 99 % of the P-input load precipitated onto the dolomite, forming a calcium phosphate layer with 0.9–1.9 wt% P. Trace element levels in the product complied with the German Fertilizer Ordinance. Residual ortho-P effluent levels were ≤ 0.3 mg P/L. The precipitate is valuable in terms of soil pH regulation, while providing Ca, Mg, and P as plant nutrients. However, the poor overall recovery of 1.9 % of total sludge P clearly highlights the challenges of re-dissolution and phase separation at the pilot-scale. These arise from the combined biological and chemical P elimination strategy used at the particular WWTP, variations in acetate-induced re-dissolution kinetics, and inefficient sedimentation for solid/liquid separation. Perspectives for process optimization and improved overall recovery are discussed.