The effect of influent organic compounds on the performance of a biological nutrient removal system was investigated using a pilot plant system operated as a UCT (University of Cape Town) process. The system was fed domestic sewage and operated at a sludge age of 13 days. The effects of separate addition of formic, acetic, propionic, butyric, isobutyric, valeric, and isovaleric acid on phosphorus release under anaerobic conditions, and phosphorus uptake under aerobic conditions, were studied. The effects of the organic acid additions on the removal of nitrogen and COD, and changes in SOUR and MLSS, were also studied. All added substrates, except formic acid, caused significant increases in phosphorus release in the anaerobic stage, and subsequent phosphorus uptake in the aerobic stage with an increase in phosphorus removal efficiency. It was also found that the branched organic acids, isobutyric and isovaleric, caused more phosphorus release in the anaerobic stage and better phosphorus removal efficiencies in the system, compared with the nonbranching forms of the same organic acids. The most recent biochemical model, proposed by Comeau et al. (1986) and Wentzel et al. (1986) was also tested using the data collected in this investigation. Both models, in most cases, overestimated the ratios of phosphorus release to volatile fatty acid utilized. All added substrates caused no change in either COD or TKN removals. For engineering applications, it is suggested by this research, that at least 20 mg COD equivalent of acetic acid is needed for the removal of 1 mg phosphorus.

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