Recovery of phosphorus from sewage sludge ash: Influence of chemical addition prior to incineration on ash mineralogy and related phosphorus and heavy metal extraction

Sewage sludge ash (SSA) rich in phosphorus (P) is a promising alternative for non-renewable phosphate rock. However, wet chemical P extraction from SSA can co-extract considerable amounts of heavy metals depending on the type of extraction liquid used. In this study, addition of chemicals to sewage sludge (SS) prior to incineration was investigated for its influence on the ash mineralogy and related P and heavy metal extraction using sulfuric acid, hydrochloric acid and oxalic acid. The additives selected consisted of Al, Ca, Fe, Mg and Na chlorides, sulfates and carbonates. Results showed that the addition of chemicals to SS prior to incineration changed the P mineralogy in the SSA, but this had generally almost no effect on the P extraction efficiency. The only exception was the addition of calcium chloride, sodium chloride or carbonates, which decreased the P extraction efficiency. Conversely, addition of chemicals to SS prior to incineration significantly affected the extraction of heavy metals from the SSA. Overall, the addition of aluminum chloride, iron(III) chloride, magnesium chloride or aluminum sulfate to SS prior to incineration offered the best trade-off between high P extraction and low heavy metal co-extraction.