Recovery of Potassium and Phosphorus from Digested Sludge Ash via Selective Multi-Step Leaching

Digested sludge represents a waste biomass derived from anaerobic digestion processes that require proper management due to its harmful pathogen content. The most common management route is incineration, which allows a reduction in volume. Within a circular economy framework, the valorization of these sludges is essential, considering that they are a suitable source of potassium (K) and phosphorus (P), essential macronutrients for agriculture. In this study, a process for P and K recovery was developed based on a multi-step leaching approach applied to ash produced from co-combustion with NaCl. The ash was characterized to assess compositional variations induced by NaCl addition. The first selective leaching step was performed to extract K, followed by a second leaching step targeting P recovery. Different leaching systems based on H2SO4/citric acid mixtures were investigated. After optimization of the leaching conditions, an EDTA-assisted precipitation step was applied for P recovery. NaCl addition promoted heavy metal volatilization and enhanced P extraction. Water-based leaching was selective for K, whereas the use of H2SO4 combined with citric acid was detrimental due to the co-extraction of heavy metals. The EDTA-assisted precipitation step limited heavy metal co-precipitation, yielding a P-based product compliant with the EU Regulation 2019/1009.