Nitrite Looping: Plausible Phenomenon or Mathematical Nuance?

Two biochemical pathways were used to model a counter-diffusion biofilm that performed simultaneous nitrification-denitrification (SND): one characterized by parallel denitrification and the other by sequential denitrification. Nitrite looping, a phenomenon in which denitrification-produced- NO2− is oxidized to NO3− rather than following the traditional denitrification pathway to N2, was shown to occur in sequential denitrification models. In the mathematical simulations conducted, nitrite looping triggered suboptimal conditions including (i) inefficient COD utilization, (ii) inefficient O2 utilization, and (iii) NO2− accumulation. Simulations suggested that thick biofilms (700μm vs 300μm) would minimize the magnitude of nitrite looping. From this theoretical study, it was proposed that nitrite looping within SND biofilms is a plausible occurrence. The theory of nitrite looping simply requires that denitrification-produced NO2− interacts with NOOs prior to being reduced to N2, which could occur due to close spatial proximity of bacteria or mechanisms that transport denitrification–produced NO2− from anoxic to aerobic regions.