Effect of inorganic carbon on nitrite accumulation in an aerobic granule reactor

Pilot scale experiments were performed to evaluate the potential of nitrite type nitrification process with an airlift reactor and granular biomass. Initially, oxygen limitation was used as the main control parameter for accumulating nitrite in the effluent. After 30 d operation, the maximum nitrite conversion rate reached 2.5 kgNO₂‐N m⁻³ d⁻¹, average diameter of the granule was 0.7 mm. Nitrite type reaction continued over 100 d, but nitrate formation increased after 150 d of operation. Once nitrate formation increased, oxygen limitation could not eliminate nitrite oxidising bacteria from granule. To overcome nitrate formation, laboratory scale batch experiments were conducted and it revealed a high concentration of inorganic carbon which had a significant effect on nitrite accumulation. Following this new concept, inorganic carbon was fed to the pilot scale reactor by changing pH adjustment reagent from NaOH to Na₂CO₃ and nitrite accumulation was recovered successfully without changing DO concentration. These results show that a high concentration of inorganic carbon is one of the control parameters for accumulating nitrite in biofilm nitrification system.