Comparing the leaching behavior of phosphorus, aluminum and iron from post-precipitated tertiary sludge and anaerobically digested sewage sludge aiming at phosphorus recovery

Phosphorus (P) is a key nutrient for agriculture and global food security, but the remaining phosphate rock reserves are finite and contain increasing toxic impurities. Thus, phosphorus recovery from secondary waste sources, such as sewage sludge, became a strategic goal for the European Union to reduce its import dependency and stimulate circular economy. Previous studies focused on phosphorus recovery predominantly from primary, activated and anaerobically digested sludge but not from post-precipitated (tertiary) chemical sludge. In this research, leaching tests were performed with aluminum and iron containing tertiary sludge from two different wastewater treatment plants. The feasibility to re-dissolve the phosphorus bound into the sludges was investigated systematically by applying acidic or alkaline treatment. The shift in pH causes the dissolution of phosphorus which makes it readily available for further recovery as a high-value fertilizer product. For the aluminum containing sludge, the phosphorus re-dissolution efficiency after acidic leaching was similar to 95% at pH 2.0. For the iron containing sludge, similar to 80% efficiency was achieved only after significant acidification to pH 1.5. Alkaline leaching at pH 13 showed considerably lower potential with similar to 70% phosphorus re-dissolution for the aluminum containing and similar to 40% for the iron containing tertiary sludge. Additionally, the co-dissolution of the associated aluminum and iron metals was also analyzed and all experimental results were compared against data obtained from the leaching of anaerobically digested sewage sludge and synthetically precipitated sludge. In conclusion, the optimal pH for phosphorus dissolution depended strongly on the type of metal-phosphate compounds in the sludge. (C) 2019 Elsevier Ltd. All rights reserved.