Optimization of wet acid phosphorus extraction from sewage sludge ash: Assessment of main influencing factors using an empirical model

This research investigates the primary factor influencing the phosphorus extraction efficiency (PEE) from incinerated sewage sludge ash (ISSA) using hydrochloric acid (HCl) as the extracting agent. A comprehensive database, comprising both literature and new experimental data, was developed to support the creation of a robust regression model. Based on 152 data sets, this model showed a high coefficient of determination (R² = 0.86), indicating a solid fit. Key findings revealed that the acid concentration (C_HCl) had the most significant effect on PEE, followed by the liquid-to-solid ratio (L/S) and contact time (t). The model's predictive accuracy was validated with an R² of 0.75 and a Pearson correlation (r) of 0.96, confirming its reliability. The model effectively predicts PEE for ISSA with moderate calcium content (6–12 %) and offers valuable insights into the process, providing guidelines for optimizing the configuration of process parameters. Optimal conditions for achieving high PEE (> 80 %) with acceptable acid costs (0.30–0.35 €/kg P_extracted) are expected from an extraction variable (X) of 3.8 and at a moderate C_HCl (3–4 mol/L) & L/S ratio (2–3 L/kg). Still, the model's applicability is limited by the quality of the data, particularly for ashes with high calcium content. Further research is required to expand the database and enhance model accuracy, particularly for optimizing the phosphorus recovery process using wet acid extraction at an industrial scale.