Results of a biological denitrification demonstration plant for drinking water, operated for about two years under different conditions, are here summarized. The fixed film anoxic reactor worked downflow at flowrates up to 20 m3/h, using a groundwater with quite variable NO3 concentration (32-52.5 mg NO3/L); thus, nitrate volumetric load Cv range was 0.40-1.45 kg NO3-N/m3 of active bed per day (1.8-6.5 kg NO3/m3·day). These figures were obtained with varied flow (and related hydraulic contact time HCT), partial recirculation of treated water and NO3 addition (fixed HCT); for a short period Cv reached 1.95 kg NO3-N/m3·d, for both NO3 extradosing and flowrate increase. Nitrogen removal rates, using sugar or glucose syrup as organic carbon sources for heterotrophic denitrification, were usually above 80% for the whole biological section (taking into account NO2 oxidation), but NO3 removal rate in the anoxic reactor often reached 95%. For different operating conditions, both substrates gave comparable results, requiring a quite low degree of post-treatments. Heterotrophic denitrification using sugars seems to be a practicable way to solve the problem of increasing NO3 concentrations in drinking water.

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