DETERMINATION OF FERRIC-CHLORIDE DOSE TO CONTROL STRUVITE PRECIPITATION IN ANAEROBIC SLUDGE DIGESTERS

Anaerobic digestion of sludges favors the formation of struvite because ammonia, phosphate, and magnesium are solubilized by the digestion process. Struvite (MgNH4PO4.6H2O((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 PO4-P removed to Fe added was 0.37. Additional iron demand was attributed to FeS((s)) and FeCO3(s) precipitation. Anaerobic sludge digestion batch studies showed similar soluble PO4-P removal efficiencies by FeCl3 and FeSO4 dosing, indicating that Fe(III) is reduced readily to Fe(II) in anaerobic sludge digesters.