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.

Three main areas will be covered regarding the design and operation of a pure oxygen activated sludge plant. Air entrainment occurred in RAS and influent lines and gas entrainment occurred at the sixth stage effluent weirs in an enclosed pure oxygen aeration basin. This caused the oxygen feed to be shut off due to overpressurization and resulted in a combination of high apparent O2 usage and high vent gas purities. The use of an anaerobic selector for Nocardia control is discussed. Anaerobic digester operation has been adversely affected by Nocardia-dominated waste activated sludge. The effect of thick anaerobic digester foams on the ability to pump digested sludge is discussed.

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