Nitrification at low levels of dissolved oxygen with and without organic loading in a suspended-growth reactor

Abstract

Laboratory-scale mixed flow reactors with DO (dissolved oxygen) control were operated at 25°C using substrate containing 80 mg l −1 of NH 3-N. DO of 0.5 mg l −1 produced no effect on ammonia oxidation of the reactor. Determination of the biomass index based on the substrate utilization rate revealed that low levels of DO elevated the growth yield of ammonia oxidizers by double. This high growth yield compensated the reduced specific substrate utilization rate at low levels of DO. Kinetic analysis showed that the μ m value was not significantly changed by low DO because of the elevated growth yield. Growth yield of nitrite oxidizers was not increased by low DO. Nitrite oxidation was strongly inhibited by 0.5 mg l −1 of DO, and accumulated nitrite reached almost 60 mg l −1. The saturation constant in terms of DO ( K o ), estimated based on specific substrate utilization rate, was 0.32 mg l −1 for ammonia oxidation, while this value for nitrite oxidation could not be obtained. The interacting effect of low DO and organic loading was examined by adding glucose to the substrate. The inhibitory effect on ammonia oxidation by organic loading through heterotrophic activity was enhanced by low DO. The results suggest that the K o value in a system with organic loading is larger than the above value which was estimated in a system without organic loading. Simultaneous nitrification and denitrification took place to some extent when glucose was fed at low DO.