~ Pergamon PII: S0043-1354(96)00282-5 Wat. Res. Vol. 31, No. 5, pp. 1047-1055, 1997 © 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0043-1354/97 $17.00 + 0.00 CALCIUM PHOSPHATE PRECIPITATION IN BIOLOGICAL PHOSPHORUS REMOVAL SYSTEMS HANS CARLSSON t, HENRIK ASPEGREN 2, NATUSCKA LEE~@ and ANDERS HILMERt~ ~Departmem of Water and Environmental Engineering, PO Box 118, S-221 00 Lund, Sweden, 2Malm6 Water and Sewage Works, S-205 80 Maim6, Sweden and 3Department of Biotechnology, Chemical Center, PO Box 124, S-221 00 Lund, Sweden (First received December 1995; accepted in revised form July 1996) 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 l-l.3. Redissolution occurred when the concentrations of calcium and phosphate had fallen below the saturation curve for Caa(PO4)2 at neutral pH, and below the saturation curve for CaHPO4 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 - ' at a calcium concentration of 100 mg Ca I ~, 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- ' indicates that in most cases calcium phosphate precipitation in EBPR processes with municipal wastewater does not constitute a significant phosphorus removal mechanism. © 1997 Elsevier Science Ltd Key words---biological phosphorus removal, chemical precipitation, calcium phosphate INTRODUCTION Since the early day,; 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 (Marais et al., 1983; Arvin, 1983). This question has not been fully resolved, and the role of the cations in the influent wastewater, especially calcium, is still under debate. The chemical precipitation of phosphate in activated sludge systems in which inorganic and organic material and biological activity are present simultaneously is a very complicated process. Chemical equilibrium models describing the precipi- tation process cannot be used without further consideration (Morgan and Fruh, 1972). There may also be interactions between the chemical and biological systems with respect to phosphorus removal in the EBPR system, as elevated concen- trations of phosphorus and certain cations can be expected in the bulk solution under anaerobic conditions. It has I:,=en shown in numerous studies that potassium and magnesium are released and taken up together with phosphate under anaerobic and aerobic conditions, respectively (Wentzel et al., 1991), These cations and possibly also small amounts of calcium, function as counterions in the intracellu- lar polyphosphate granules. Calcium phosphate precipitation in the EBPR process has been discussed by Arvin (1983). Possible interactions between dissolved and precipitated phosphate during an EBPR batch experiment are illustrated in Fig. 1. During the anaerobic phase, phosphate, together with potassium and magnesium, is released from the biomass to the bulk liquid. If the phosphate concentration rises above a certain level (the precipitation limit), depending on the concen- tration of calcium, precipitation will occur. As the dissolved phosphate concentration decreases during aeration, due to EBPR activity, it may fall below the precipitation limit. If the phosphate concentration becomes sufficiently low, redissolution may occur, which makes further phosphate uptake by the micro-organisms possible. The overall effect of calcium phosphate precipitation in the EBPR process 1047