Microbial Population Dynamics of the Low-strength Partial Nitritation System and Its Correlation of Nitrifiers to the Model-predicted Biomass

Using a lab-scale activated sludge system equipped with an external selector for suppressing nitrite oxidising organism (NOO), the microbial population dynamics were investigated during about 400 days of the continuous operation. In the varied selector operations and excess sludge withdrawal, the nitrification, partial nitritation, and nitrification loss followed by the recovery of partial nitritation consecutively took place. The DNA analysis with the high-throughput sequencing revealed that only Nitrosomonas spp. retained during the nitritation–nitrification and only Nitrospira spp. could grow in nitrification, respectively. Proteobacteria was the most predominant heterotroph in the main aeration tank, whilst Bacteroidota was abundant in the external selector in association with the reduced microbial diversities. In the external selector, due to the high nitrite and ammonium, the ammonium oxidising organism’s (AOO) inherent enzyme genes (Amo and Hao) per total microbial gene copies were as low as about 50% of that in the main aeration tank. The dynamically calculated AOO and NOO concentrations using the IWA-ASM1-based model could be linearly correlated to both relative abundances in the activated sludge. The correlations suggested that the DNA analysis could be potentially utilised as an alternative tool to estimate the nitrifier biomass of activated sludge instead of performing kinetic modelling.