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Anaerobic digestion of sludges favors the formation of struvite because ammonia, phosphate, and magnesium are solubilized by the digestion process. Struvite (MgNH4P04 · 6H20(s)) can cause problems through scale formation. This paper provides a rational method for predicting the optimum FeCl3 dose for preventing struvite formation during anaerobic digestion. Based on continuous flow anaerobic digestion experiments, the minimum dose to prevent struvite formation in the San Francisco Southeast Water Pollution Control Plant is 13.5 mM FeCl3/L or 100 kg FeCl3/ton total solids (TS). Approximately 68% of the total magnesium and total phosphorus present in the feed was available for chemical precipitation and the average ratio of soluble P04‐P removed to Fe added was 0.37. Additional iron demand was attributed to FeS(s) and FeC03(s) precipitation. Anaerobic sludge digestion batch studies showed similar soluble P04‐P removal efficiencies by FeCl3 and FeS04 dosing, indicating that Fe(III) is reduced readily to Fe(II) in anaerobic sludge digesters.

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Temperature and low mean cell residence time (MCRT) effects on enhanced biological phosphorus removal (EBPR) were investigated in continuous flow bench-scale activated sludge systems treating settled domestic wastewater supplemented with 50 mg/l acetate over ranges of MCRT and temperature of 2-4 days and 13.5-20° C respectively. Temperature effects (10-37°C) on anaerobic soluble COD (CODsol) uptake and soluble P release (Psol) and aerobic Psol uptake rates were studied in batch. For the temperature range studied, EBPR functioned efficiently at MCRT ≥ 2.9 day; below 2.9 day MCRT EBPR is lost at an MCRT value that depends on temperature. Temperature effects on MCRT for EBPR (13.5-20°C), anaerobic CODsol uptake and Psol release rates and aerobic Psol uptake rates (10-30°C) are described by similar Arrhenius plots indicating a similar temperature dependency for all biological processes involved in EBPR.