Nitrite accumulation in activated sludge and airlift reactors: Process performance comparison

Nitrogen removal via nitrite allows operational cost reduction in wastewater treatment of effluents with a low COD/N-NH₄⁺ ratio, due to lower oxygen requirements for partial nitrification and a further diminishing in requirements of the external carbon source for the denitrification step. Two nitrifying systems-a suspended activated sludge (AS) reactor, and an airlift biofilm (AB) reactor-were compared to determine the best alternative for obtaining a maximum nitrite buildup. Both systems were operated under similar operational conditions; synthetic feed of ammonia nitrogen was applied to both reactors, at an ammonia loading rate (ALR) of 3.3 kg N-NH ₄⁺/m³ and an hydraulic retention time of 4.3 h. Nitrite accumulation was studied by means of controlling dissolved oxygen (DO) concentration, keeping both pH and temperature constant. Similar results were obtained in both systems. In the activated sludge reactor maximum nitrite accumulation was achieved at 0.7 and at 3.5 mg DO/L in the airlift reactor. The obtained results showed that at a similar percentage of nitrite accumulation, the K_La value of the airlift reactor was always 3.6 times higher than the value obtained in the activated sludge reactor; however, the airflows required were similar in both reactors. The operation of nitrifying systems (biofilm and suspended biomass reactors) with low oxygen concentration to obtain nitrite accumulation make possible a decrease of required airflow, even at small nitrite accumulation. Airflow savings of 38 and 58% were obtained for the activated sludge reactor and the airlift reactor, respectively, at 15% of nitrite accumulation. When 40% of nitrite accumulation was detected in the activated sludge reactor and the airlift reactor, a savings of 80% and 65% were obtained, respectively.