Performance and characteristics of a nitritation air-lift reactor under long-term HRT shortening

• The HRT was shortened to 2.1 h which elevated the NLR to 4.07 kg m−3d−1.
• The combined effect of DO, pH and FA contributed to rapid nitrite accumulation.
• Organic functional groups present on biomass surface helped for metals absorption.
• The attachment of iron precipitates on biomass increased the sludge settleability.
• High-throughput pyrosequencing results revealed microbial diversity in the reactor.

The start-up and subsequent operation of nitritation process was carried out in an airlift reactor (ALR) over 253 days under the HRT shortening condition. The nitrogen loading rate (NLR) of the ALR was finally raised to 4.07 kg m−3d−1 by shortening the HRT from 22.7 to 2.1 h under 350 mg l−1 of influent ammonium. The improvement of removal efficiency and nitrite accumulation to 98.7% and 92.4% elevated the ammonium removal rate (ARR) and nitrite production rate (NPR) to 3.89 and 2.81 kg m−3d−1, respectively. The settling property of the nitritation biomass was improved substantially under the high hydraulic loading condition. Scanning electronic microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were used to evaluate the physicochemical characteristics of the biomass. The attachment of chemical precipitates on biomass surface was an important reason contributing to the enhancement of nitritation performance through increasing the sludge settleability. The taxonomic complexities of microbial communities from phylum to species level were dealt with the high-throughput pyrosequencing of 16S rRNA gene sequence. It was revealed that Lactobacillus, Thermomonas, Ignavibacterium and Aquimonas were the most abundant genera in the biomass. The low abundance of Nitrospira and Candidatus Brocadia confirmed the possible occurrence of nitrite oxidation as well as anammox in the ALR.