**2. Effects of operating parameters on the inhibition of NOB in municipal wastewater treatment**

### **2.1 Dissolved oxygen (DO)**

Continuous aeration mode keeps DO at a low level in the CANON process reactor. However, Liu et al. found through experiments that under long-term low DO (0.16 ~ 0.37 mg/L) operation, the dissolved oxygen affinity coefficient Ko2, AOB will be higher than Ko2, NOB, that is, NOB is more competitive than AOB. As a result, part of NO2 − -N in the reactor is further converted to NO3 − -N, and the lack of stable nitrite supply greatly endangers the overall nitrogen removal performance of autotrophic nitrogen removal process [8]. In view of this phenomenon, Regmi et al. believed that Nitrobacter would increase significantly more than Nitrospira in a long-term state of low DO [9]. Nitrospira is the strategy-K (low specific proliferation rate, high matrix affinity), while Nitrobacter is the strategy-R (high specific proliferation rate, low matrix affinity). As a result, Nitrospira is more amenable to compete with DO. The previous thought that Ko2, NOB was higher than Ko2, AOB might be due to the dominance of Nitrobacter. Controlling DO concentrations of medium (<1.0 mg/L) and low (<0.5 mg/L) is indeed beneficial to Nitrosomonas, but it inhibits the Nitrobacter of strategy-R rather than the Nitrospira of strategy-K (which grows at a rate close to the maximum).

### **2.2 Transient hypoxia**

Transient hypoxia caused by on/off aeration has been proven to be an effective method to inhibit NOB. The lag time of NOB activity at the beginning of aeration section is due to the following two reasons: (1) Lack of one or two substrates (nitrite and oxygen) [10]. (2) In the aerobic environment after a brief period of hypoxia, compared with AOB, NOB faces metabolic mechanism inactivation and adaptive lag in the

recovery period [11]. The delay of nitro Spirillum activity after anoxic period (5–15 min) effectively inhibited NOB in PN/A process based on SBR [11]. Intermittent aeration is also effective in integrated fixed-film activated sludge (IFAS) process [12]. However, intermittent aeration is not conducive to the stability of effluent quality in continuous flow process and frequent opening and closing of blower will increase the failure rate of equipment. However, in a continuous system that consumes nitrous oxide in time, an aerobic/aerobic alternating strategy can effectively carry out partial nitrification [11, 13].
