Partial nitrification in an airlift activated sludge reactor with experimental and theoretical assessments of the ph gradient inside the sponge support medium

An airlift-type activated sludge reactor using a sponge support medium was fed with wastewater containing 500 mg-N/L of NH ₄ ⁺-N. In order to establish partial nitrification, the reactor was kept at 35°C. Moreover, the reactor was started up with the NaHCO ₃ solution as an alkali agent to supply inorganic carbon for enhancement of partial nitrification. The partial nitrification was established during the period when NaHCO ₃ was used as an alkali agent. However, soon after changing the alkali agent to a mixture of Na ₂HPO ₄ and NaOH, the production of NO ₂ ^--N started to decrease. The change in pH inside the sponge medium revealed by the model simulation and microelectrode study showed relatively high free ammonium concentration inside the medium. Thus, partial nitrification was proved to be affected by inorganic carbon in the alkali agent. Furthermore, it was suggested that the establishment of partial nitrification in this study was due to the combination of high operating temperatures, addition of inorganic carbon, and the inhibition of NOB by the free ammonium concentration.