Substrate inhibition and pH effect on denitritation with granular biomass

Undissociated HNO₂ (up to 2 mg dm⁻³) was confirmed as substrate inhibitor for granular biomass from a denitritation upflow sludge bed reactor used for biological removal of nitrite. On the contrary, total nitrite nitrogen (N-NO₂ up to 500 mg dm⁻³) and methanol (COD up to 2000 mg dm⁻³) were not proven to be inhibitors. pH also affected the denitritation efficiency (optimal pH was 5.9). Reduction of HNO₂ concentration in the reactor by effluent recycling is recommended.