Pilot scale biofilm reactor for heterotrophic denitrification of groundwater: the Milan city case study

A pilot scale biofilm reactor has been applied to study the efficacy of an heterotrophic denitrification process for the removal of 40 mgNO3/L from Milan city groundwater, characterized by contextual pollution of VOCs and pesticides. The biological submerged filter (Biofor, Degrémont) with expanded clay as biomass support (Biolite, Degrémont) was fed with a constant water inflow of 0.6 m3/h, and the effluent was subjected to an aeration step (3 min HRT) and a GAC filtration (8 min EBCT). Tracer tests using LiCl were performed before and after bacterial colonization, revealing a mean residence time of 22 min and suggesting a PFR with axial dispersion (increasing with the colonizing degree), as the best interpretative model. A continuous dosage of sodium acetate (CH3COONa) and potassium phosphate (KH2PO4) was assured, with a variable organic carbon dosage, obtained fixing the C/DO ratio (0.94 gC/gDO for a concentration of 6.2 ± 0.55 mgDO/L) and varying the C/N ratio. Data observed during 70 days of stationary phase have shown that an applied C/N ratio equal to the stoichiometric value of 1.46 gC/gN led to a stable removal efficiency higher than 88.5%, while lower applied C/N values presented a significant occurrence of nitrite in the effluent. Stoichiometry was used to verify the aerobic and anoxic organic carbon consumption along the biofilter, since a concomitant removal of nitrogen and dissolved oxygen was observed, and suggested that no biomass stratification was present. The observed removal efficiencies did not highlight any inhibition mechanism associated to influent micro-pollutants, and any co-metabolic removal of VOCs or other organic pollutant could be inferred.