**2.4 Aerobic sludge residence time**

In the side-flow anaerobic process, the growth rate of AOB is higher than that of NOB, so short-time aerobic SRT is used to inhibit and flush NOB [17]. In the activated sludge process with temperatures between 28 and 30°C, aerobic SRT of 2.5 d is one of the main factors to ensure stable nitrogen removal in the Singapore Changi Regenerating water plant [18]. The control aerobic SRT of large-scale activated sludge denitrification process in the Southwest Wastewater Treatment Plant in St. Petersburg was 3.5 d, which is another reference case of this operation mode [19]. In the mainstream process, Blackburne and Oleszkiewicz found that shortening SRT was beneficial for AOB growth [19, 20]. Stinson et al. used short-term aerobic SRT as an intervention to achieve inhibition of NOBs at moderate and low temperatures [21].
