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
Annika Anders, Harun Cakir, Frank Ohnemüller, Harald Platen, Ulrich Kornhaas, Harald Weigand
Abstract
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.