Biological denitrification of a textile effluent in a dynamic sand filter

Post-denitrification of a pre-treated textile effluent was tested in a pilot-scale dynamic up-flow sand filter, which has been used as a biofilm reactor, together with filtration of suspended solids (SS) and decolorization. The potential application of the reactor as a three-in-one unit (decolorization, filtration and denitrification) has been successfully tested. Biomass growth and the sloughing of biological film did not prevent the removal of high concentrations of influent SS. Both pilot- and bench-scale tests confirmed that the intrinsic denitrification kinetics was zero-order, corresponding to a half-order removal rate if nitrate concentration is lower than 10 mgN l⁻¹. Zero-order and half-order kinetic constants have also been calculated. At low nitrate loading rates (up to 2 kgN m⁻³ d⁻¹) the filter followed the ideal plug-flow hydrodynamic model. In the lower part of the filter, zero-order kinetics fitted denitrification removal rates, while in the upper part of the filter denitrification followed half-order kinetics. At nitrate loading rates higher than 2.5 kgN m⁻³ d⁻¹, nitrogen bubbles developed and partially mixed the reactor. Consequently, flatter concentration profiles were detected in the reactor and denitrification followed half-order kinetics along the entire filter.