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 H 2 SO 4 (0.5 N, 10 ml/g, 120 min) and oxalic acid (0.5 N, 12.8 ml/g, 120 min). H 2 SO 4 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.

The overall aim of this research was to evaluate the valorization potential of the poultry litter ash produced in the biomass power plant of BMC Moerdijk (the Netherlands), since the ash contains phosphorus (P) and potassium (K), which are both essential nutrients. As a first step, the ash was characterized by means of chemical analysis. Ash collected in the superheater section of the boiler had the highest P concentration (10.6%). Furthermore, the P concentration in the ash decreased as it was collected further downstream in the boiler and flue gas cleaning part of the installation. K showed an opposite concentration trend, that is, its concentration was the lowest in the superheater ash (9.4%) and increased to 15.5% in the electrostatic precipitator ash. Based on the results of the chemical analysis, different valorization options could be considered. Although poultry litter ash has the same heavy metal/P ratio as poultry litter and is free of pathogens and toxic organic substances, its recycling as a P/K fertilizer is hindered by legal constraints. Furthermore, the use of the ash in/as animal feed is not straightforward because of its origin (animal feces) and waste status. Besides P and K, other ash elements such as calcium, silicon, magnesium, iron and aluminum can also be valorized, for example by using the ash as building material or in cement production. However, in these applications the high P and K concentration of the ash can be a technical obstacle rather than a benefit. In this regard, it can be interesting to separate the fertilizer elements, that is, P and K, from the rest of the ash by means of for example a wet chemical extraction after which the remaining solid residue better meets the composition requirements for building material or raw material for cement production.

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Phosphorus (P) rich ash from biomass incineration is a potential promising alternative for non-renewable phosphate rock. This study considered the P recovery potential of poultry manure ash, sewage sludge ash and meat and bone meal ash through wet chemical extraction. XRD analysis showed that these three ash types had a distinct P mineralogy. If inorganic acids were used for the extraction, the P extraction efficiency was not or only slightly affected by the P mineralogy. Contrarily, for the organic acids, alkaline extraction liquid and chelating agents considered, the P extraction efficiency was highly affected by the P mineralogy, and was also affected by the elemental composition of the ash and/or the chemical characteristics of the extraction liquids. Alkaline extraction liquids showed in general low heavy metal co-extraction, in contrast to the inorganic acids. From an economic point of view, of all extraction liquids considered, sulfuric acid was the most interesting to extract P from all three ash types. Oxalic acid could be a more sustainable option for P extraction from sewage sludge ash. In addition, extraction of poultry manure ash with ethylenediaminetetraacetic acid showed a relatively high P extraction efficiency combined with relatively low heavy metal co-extraction.