Phosphorus recovery with electro-chemical process from sewage sludge ash

According to population projections, the world population is expected to exceed 9 billion by the middle of the 21st century, and accordingly, food production needs to be increased by approximately 30%. Phosphorus, one of the most essential elements for food production, is considered a strategic raw material in this context. However, it is estimated that, even under the most optimistic scenarios, global phosphorus reserves will only be sufficient to meet the world's needs for about 200 years. Therefore, the need to identify alternative phosphorus sources is gaining increasing importance. In this study, a two-stage method was applied to evaluate the recoverability of phosphorus from sewage sludge ash (SSA), which is generated by the incineration of wastewater treatment sludge. Characterization of the SSA revealed that the total phosphorus content was calculated as 7.66% in the form of P2O5, indicating a significant potential for phosphorus recovery. In the first stage, phosphorus was extracted from the ash using acid leaching, while in the second stage, the extracted phosphorus was recovered through an electrochemical precipitation method. To optimize both stages and evaluate the interactions between parameters, the Box-Behnken Design (BBD), within the framework of the Response Surface Methodology (RSM), was employed and statistical analyses were conducted. The statistical results confirmed that both models were highly explanatory and statistically significant. Phosphorus extraction achieved yields above 90%, while the electrochemical recovery stage reached up to 99%, demonstrating remarkably high efficiency. The energy consumption of the electrochemical precipitation process was found to be low (0.08–1.72 kWh/m3). Moreover, XRD analyses of the obtained precipitates revealed the presence of agriculturally valuable compounds such as K-struvite and potassium sulfate. It was also determined that the concentrations of heavy metals such as Pb, Cu, Ni, Zn, Cr, and Sn in the precipitates remained below the limit values specified in the relevant regulation. These findings indicate that the obtained precipitates are both safe and effective candidates for use as fertilizers.