Closing the phosphorus cycle: Multi-criteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash.

The presence of heavy metals in concentrations above legal limit values is one of the main obstacles preventing closure of the phosphorus (P) cycle through directly applying wastewater treatment sludge ash as a fertilizer. Therefore, an alternative procedure is proposed to recover the valuable P from the sludge ash via wet chemical extraction. This comprehensive study uses several inorganic and organic acids, chelating agents and an alkaline solution to establish optimal and cost-effective conditions for wet P extraction from sludge ash. The optimization takes into account co-extraction of the following heavy metals: Cd, Cr, Cu, Ni, Pb and Zn. Design of experiments results show extraction liquid concentration, liquid/solid ratio and contact time all affect P and heavy metal extraction efficiency, both individually and through interaction. In addition, type of extraction liquid and pH at the end of the extraction procedure also affect P and heavy metal extraction efficiency. Combining results of XRD and SEM-EDX analysis with extraction data shows that at a pH <2, both Ca- and Al-phosphates in the ash dissolve easily. However, at slightly higher pH only Ca-phosphates dissolve well and at alkaline pH only Al-phosphates. The best trade-off between high P extraction, low heavy metal co-extraction and low operational costs is obtained with HSO (0.5 N, 10 ml/g, 120 min) and oxalic acid (0.5 N, 12.8 ml/g, 120 min). HSO outperforms the other extraction liquids in terms of extraction liquid costs per kg P extracted, whereas extraction with oxalic acid results in the lowest heavy metal co-extraction, thus reducing the downstream processing costs. None of the extraction liquids considered is appropriate for heavy metal removal prior to P extraction due to loss of P and insufficient heavy metal removal.