Control of aeration, aerobic SRT and COD input for mainstream nitritation/denitritation

This work describes the development of an intermittently aerated pilot-scale process (V = 0.34 m3) operated without oxidized nitrogen recycle and supplemental carbon addition optimized for nitrogen removal via nitritation/denitritation. The aeration pattern was controlled using a novel aeration strategy based on set-points for reactor ammonia, nitrite and nitrate concentrations with the aim of maintaining equal effluent ammonia and nitrate + nitrite (NOx) concentrations. Further, unique operational and process control strategies were developed to facilitate the out-selection of nitrite oxidizing bacteria (NOB) based on optimizing the chemical oxygen demand (COD) input, imposing transient anoxia, aggressive solids retention time (SRT) operation towards ammonia oxidizing bacteria (AOB) washout and high dissolved oxygen (DO) (>1.5 mg/L). Sustained nitrite accumulation (NO2-N/NOx-N = 0.36 ± 0.27) was observed while AOB activity was greater than NOB activity (AOB: 391 ± 124 mgN/L/d, NOB: 233 ± 151 mgN/L/d, p < 0.001) during the entire study. The reactor demonstrated total inorganic nitrogen (TIN) removal rate of 151 ± 74 mgN/L/d at an influent COD/NH4+-N ratio of 10.4 ± 1.9 at 25 °C. The TIN removal efficiency was 57  ±  25% within the hydraulic retention time (HRT) of 3 h and within an SRT of 4-8 days. Therefore, this pilot-scale study demonstrates that application of the proposed online aeration control is able to out-select NOB in mainstream conditions providing relatively high nitrogen removal without supplemental carbon and alkalinity at a low HRT.