This paper reports the results of a study to investigate the performance of fixed-film two-stage reversible bio-denitrification reactors operated at very short detention times for the removal of nitrates from contaminated groundwater. The results demonstrate that these systems imparted lower concentrations of organics and suspended solids into the treated effluent than traditional single-stage systems while maintaining higher levels of nitrate removal rates at HRT values as low as 30 minutes. The lower detention times translate into substantial reductions in capital cost on scaled-up systems. The results also demonstrate the ability of two-stage reversible flow systems to withstand the stresses associated with low HRT and flow cycle reversals.

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