Opoka Rock as a Sustainable Material for Phosphorus Removal from Domestic Wastewater: Effects of Calcination Temperature and Hydraulic Retention Time

The aim of this study was to investigate the effectiveness of calcined opoka rock as a sustainable phosphorus sorbent and to assess the effect of calcination temperature and hydraulic retention time on phosphorus removal efficiency. Opoka was included in this study due to its local availability and low acquisition cost. Filters filled with opoka rock (2–6 mm) calcined at temperatures ranging from 400 to 900 °C were tested under laboratory conditions at flow rates of 0.96 and 1.92 L d−1, corresponding to hydraulic retention times of 2 and 1 h, respectively. While previous studies have predominantly focused on opoka rock calcined at temperatures of 900 °C and above, this study systematically evaluated a broader calcination range (400–900 °C) to identify the optimum temperature for phosphorus removal and to improve the energy efficiency of sorbent preparation. A statistically significant non-linear relationship was found between the calcination temperature of opoka rock and total phosphorus removal efficiency. The highest phosphorus removal efficiencies were achieved in filters containing opoka rock calcined at 700 and 800 °C (72–78%), whereas the lowest efficiencies were observed for opoka rock calcined at 400 and 500 °C (3–11%). Significantly higher phosphorus removal efficiency was obtained at the lower flow rate (0.96 L d−1) and the longer hydraulic retention time (2 h). The calcination temperature had a strong and significant effect on the phosphorus removal efficiency, with a more pronounced effect observed after 2 h than after 1 h of retention time (partial correlation coefficients of 0.71 and 0.60, respectively). Furthermore, a global multiple polynomial regression model was successfully developed to predict phosphorus removal efficiency based on operational and influent parameters, including calcination temperature, retention time, influent pH, and influent phosphorus concentration. The model identified the optimal calcination temperature as 715.15 °C, closely confirming the experimental results. Importantly, the identified optimum temperature is lower than the temperatures typically reported for the preparation of calcined opoka sorbents. This finding suggests the possibility of reducing energy demand and the environmental footprint associated with sorbent production while maintaining high phosphorus removal efficiency. Filters filled with opoka rock calcined at 700 and 800 °C and operated at a hydraulic retention time of 2 h exhibited the longest lifespan and the greatest operational stability. Therefore, filters filled with opoka rock calcined at 700 and 800 °C and operated at a hydraulic retention time of 2 h can be recommended as an effective, low-cost, and sustainable solution for phosphorus removal in small-scale wastewater treatment plants.