In this paper, a study of cyclically-operated fixed-film bio-denitrification to remove nitrates from drinking water is reported. The purpose of the study was to investigate the ability of these two-stage systems to remove nitrate and residual organics from treated water as compared to single-stage units. In the cyclically-operated system, the second stage (i.e. follow) reactor is operated in series with the first stage (i.e. lead) reactor. After a given period of operation, the flow regime is reversed so that the follow reactor becomes the lead one and vice versa. The active solids remaining in the follow reactor (previously the lead one) are capable of removing residual soluble organics and nitrates to levels below the concentrations provided by single-stage units. Cyclically operated fixed-film bio-denitrification reactors were found to reduce long term effluent residual organics concentrations without adverse effects on the nitrate concentrations. The system also was found to be more effective in reducing nitrates, nitrites and soluble organic concentrations in the effluent than a single-stage system operated at the same hydraulic retention times and organic loading rates.

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