Recovery of phosphorus from sewage sludge ash: Influence of incineration temperature on ash mineralogy and related phosphorus and heavy metal extraction

Phosphorus (P) can be effectively recovered from sewage sludge ash (SSA) by applying wet chemical extraction techniques. However, considerable amounts of heavy metals can be co-extracted depending on the type of extraction liquid used. In this study, the influence of the incineration temperature on the ash mineralogy and related P and heavy metal extraction using sulfuric acid, hydrochloric acid, oxalic acid and sodium hydroxide was studied. Results showed that the P extraction efficiency with sodium hydroxide decreased with increasing incineration temperature, related to a decrease in Al/Fe-phosphates in the SSA. Furthermore, P extraction efficiency for the acidic extraction liquids reached a maximum for SSA obtained around 850 degrees C (> 86% P extracted). Heavy metal co-extraction generally decreased with increasing incineration temperature, which could be linked to the immobilization of the heavy metals in the SSA matrix. For all extraction liquids considered, the lowest heavy metal co-extraction was found in SSA samples obtained at 1000 degrees C (< 21% total heavy metals extracted). However, the incorporation of P into silicate melt agglomerates at that incineration temperature reduced the P extraction. As a result, an incineration temperature in the range 800 - 850 degrees C offered a good trade-off between high P extraction and low heavy metal co-extraction.