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Abstract--COD and nitrogen balances were performed on four different types of laboratory-scale activated sludge system: aerobic, anoxic, anoxic-aerobic and anaerobic-anoxic-aerobic (biological excess phosphorus removal systems). The systems included a variety of configurations, with differing wastewater characteristics and operating parameters. The results suggest that good COD balances are to be expected in aerobic and anoxic-aerobic systems. Systems incorporating anaerobic zones exhibit low COD balances (less than 80%). Fermentation in the anaerobic zone apparently is implicated in this "loss" of COD. The consequences of the COD "loss" include both a significant decrease in oxygen requirements and in sludge production compared to aerobic or anoxic-aerobic systems. Possible mechanisms for the loss of COD and areas which require further study are discussed.
