Calcium phosphate precipitation in biological phosphorus removal systems

Abstract

Since the early days of the use of the excess biological phosphorus removal (EBPR) process, it has been discussed whether and to what extent, chemical precipitation contributes to the phosphorus removal that is seen in the process. In this study, calcium phosphate precipitation in EBPR systems was investigated. Four laboratory-scale fill and draw activated sludge systems operating in parallel, fed with natural Ca-rich wastewater, with additions of acetate and phosphate were studied. Two of the systems were operated as EBPR systems and the remaining two as aerobic systems. The results showed that calcium phosphate precipitation occurred in all the systems at sufficiently high concentrations of calcium and phosphate. Based on differences in the effluent water, the Ca/P molar ratio of the precipitate was found to be around 1–1.3. Redissolution occurred when the concentrations of calcium and phosphate had fallen below the saturation curve for Ca 3(PO 4) 2 at neutral pH, and below the saturation curve for CaHPO 4 at pH ≈ 8.5. The results also showed that the concentration of phosphate in the effluent from the EBPR system could be substantially lower than from the aerobic system, for the same effluent concentration of calcium. This indicates that there is an important time factor involved, which implies that simple equilibrium equations cannot be used alone to judge whether or to what extent, precipitation will take place. The results suggest that at neutral pH the phosphate concentration must be at least 50 mg P l −1 at a calcium concentration of 100 mg Ca l −1, and in soft water much higher, before precipitation starts. This, together with the fact that redissolution starts at phosphate concentrations far above 1 mg P l −1 indicates that in most cases calcium phosphate precipitation in EBPR processes with municipal wastewater does not constitute a significant phosphorus removal mechanism.