Phosphate recovery from aerobic wastewater Bio-P Sludge through precipitation process: understanding, experimentation and optimization.

Struvite precipitation process is one the most effective approaches for recovery of phosphorus in the form of a valuable product, which could be used as a fertilizer. Despite struvite added value as a product, its application at full scale is not very common worldwide due to its economic concern of requiring the addition of Mg and NaOH to the system. The process of struvite precipitation is generally applied to the dewatered sludge after anaerobic digestion where the concentration of phosphorus is sufficiently high. Although it could be possible to use the aerobic sludge taken from the activated sludge process in which phosphorus release is happening in the anaerobic stage and a luxury uptake occurs in the aerobic zone. Lab-scale experimentation was carried out in this thesis on the process of struvite precipitation using the aerobic sludge taken from Nosedo WRRF, Milan, Italy. The impact of different reaction parameters has been studied on the process of struvite precipitation. Particular attention was paid to the effect of pH, Mg:NH4:P molar ratio and presence of calcium. The results showed that abundant amount of Mg and NH4 are required for significant struvite precipitation. The analysis of precipitates using FTIR, XRD, TGA-MS, ICP-AES and Elemental analysis showed the presence of struvite in notable amounts for the experiments with 5:5:1 molar ratio at pH=8.5 and 9.0. in addition, the use of Ca(OH)2 as an economic pH adjustment reagent showed a decent potential for obtaining an amorphous calcium phosphate (ACP) precipitates alternatively to the pure struvite. Chemical equilibrium modeling and statistical optimization of the process of struvite precipitation were conducted using PHREEQC software and Response Surface Methodology (RSM) respectively with Saturation Index and P removal % as response variables. The results showed that struvite, ACP and calcite are three main solid phases that could precipitate in operating conditions of the experiments. It is also noted that the only parameter affecting significantly on the P removal was the pH while Ca:Mg and NH4:P were also significantly important for the struvite SI. It can be concluded that as the pH increases the P removal also increases but the possibility of struvite precipitation diminishes since the struvite SI is decreasing. Pilot-scale study of the phosphate precipitation process was implemented using synthetic solution with the characteristics of aerobic sludge. The effect of pH and retention time of the reaction were studied. The pH of the reactor was controlled at two points using PID-Time Proportional control system. The result of the pH control was very satisfactory and kept the pH level sufficiently stable during the entire experimentation. The results demonstrated that P removal is not much dependent on the retention time and is decreasing rapidly with the start of the reaction and then reaches at the stable level where it does not improve anymore. It was also observed that RT of 60 minutes is enough for reaction completion but not for substantial crystal growth. The FTIR results showed the possibility of having a ACP-rich product. In addition, using sodium alginate demonstrated a potential for increasing struvite precipitation possibility by reducing calcium concentration by 20% and improving the Mg:Ca ratio.