Differentiating two partial nitrification mechanisms: Inhibiting nitrite oxidizing bacteria activity or promoting ammonium oxidizing bacteria activity

•Partial nitrification was closely associated to the specific biomass NH4+ load.•A low SRT was needed to maintain high specific biomass NH4+ load.•FA inhibition of NOB was insignificant for partial nitrification.•Increased AOB activity was the main reason for partial nitrification.

Partial nitrification is usually more easily achieved for high NH4+ concentration wastewater. In this study, a two-compartment aerobic nitrification reactor was used to raise the NH4+ concentration in the first aerobic compartment and to enhance partial nitrification for low NH4+ concentration wastewater. In addition to the regular sludge wastage to maintain sludge retention time (SRT), extra sludge wastage was used to increase the specific biomass NH4+ load. The experimental results indicated that partial nitrification was closely associated to the specific biomass NH4+ load. The modelling results indicated that a low SRT was needed to maintain the low biomass concentration and high specific biomass NH4+ load. The increased AOB (ammonium oxidizing bacteria) activity, instead of the free ammonia (FA) inhibition of NOB (nitrite oxidizing bacteria) was identified to be the main mechanism for partial nitrification in low NH4+ concentration wastewater treatment with high specific biomass NH4+ load.