Phosphorus activation in sewage sludge ash: Enhancing bioavailability through chemical modification

Due to the depletion of high-quality phosphate rock reserves, alternative sources such as sewage sludge ash (SSA) – a by-product of municipal wastewater treatment, are gaining attention for phosphorus recovery. However, SSA typically contains phosphorus in poorly soluble forms, limiting its direct use as a fertilizer. This study evaluates the effectiveness of chemical acidification using hydrochloric, nitric and sulfuric acids to enhance the bioavailability of phosphorus in SSA. Different acid concentrations (10-40%) and solid to liquid (S/L) ratios (1:0.5-1:1.25) were tested to identify optimal conditions for phosphorus activation. Sequential phosphorus fractionation and X-ray diffraction analysis revealed that acid activation significantly increased the proportion of readily available phosphorus in SSA (from 0.1% P₂O₅ up to 52.7% P₂O₅). A clear trend was observed, indicating that higher acid concentrations and increased solid-to-liquid ratios were associated with improved phosphorus availability. Pot experiments with radish ( Raphanus sativus L. ) confirmed increased phosphorus uptake and plant biomass at lower application rates of activated SSA, though higher doses led to potential phytotoxic effects. Heavy metal speciation analysis indicated moderate increases in cadmium and lead mobility after acid treatment, which was confirmed in pot trials. Despite economic and environmental challenges, acid-activated SSA shows potential as a sustainable phosphorus source for fertilizer production.