**3.1 Carbon pretreatment process**

In view of the high carbon-nitrogen ratio of municipal sewage, carbon pretreatment is usually introduced. At present, there are three carbon pretreatment processes at home and abroad: (1) High-rate activated sludge (HRAS). For example, in the Strass sewage treatment plant, A stage of activated sludge process (SRT ≈ 0.5d,

HRT ≈ 0.5 h) can guarantee the removal rate of COD of 60% [23]. (2) Chemicalintensive pretreatment can remove about 80% ~ 90% TSS and 50% ~ 70% COD; (3) Methanogenic fermentation pretreatment, designed to maximize energy recovery in UASB reactors [24]. Generally, the three reasonably designed pretreatment processes can satisfy the PN/A influent COD/N ≤ 2 ~ 3.

### **3.2 One-stage and two-stage processes**

The one-stage PN/A process performs PN/A in one reactor, while the two-stage PN/A process separates the PN/A reaction in two reactors. The combination of PN and Anammox reactions in a reactor significantly reduces infrastructure and operating costs compared to a two-stage process [25]. One-stage reactors tend to operate under nitrite restriction and low DO concentrations.

One-stage PN/A process is also known as CANON (Completely autotrophic nitrogen removal over nitrite, CANON) granular sludge has a regular shape and compact, dense structure, high sludge concentration, good settling performance, etc. Winkler et al. believed that in typical CANON granular sludge, AOB bacteria were usually distributed in the outer layer of particles permeable by dissolved oxygen, while AnAOB bacteria were distributed in the inner anaerobic zone of particles [23]. However, with the in-depth study of CANON granular sludge, some scholars found that AOB and AnAOB bacteria co-existed in CANON granular sludge, without specific spatial distribution rules, the two bacteria interleave each other, and the nitrite matrix produced by AOB ammonia-nitrogen does not need to be transferred through a long chain, and is degraded as the substrate of AnAOB bacteria in a short time. The whole autotrophic nitrogen removal process can be efficiently completed [26, 27]. Chen's research shows that the combination of AnAOB and AOB forms a special olivar-shaped structure: (1) AnAOB mainly gathers inside the particle to form the kernel of the particle, AOB forms a thick wall in the outer layer of the particle, (2) AnAOB gathers into multiple clusters, and AOB is relatively evenly distributed in the whole particle without any clusters. (3) The many cracks clearly observed in the Anammox particles are likely conduits through which substrates and wastes flow. In this special structure, part of influent NH4 + -N is oxidized to NO2 − -N by AOB in the particle surface layer, and Anammox in the particle core uses residual NH4 + -N and generated NO2 − -N. In addition, the consumption of O2 by AOB covered by the outer layer of particles provides protection for AnAOB from inhibition of O2 and other environmental factors [27]. Statistics show that more than 50% of PN/A reactors operate in SBR mode, 88% of wastewater plants operate using single-stage systems, and 75% are used to treat sideflow municipal wastewater. Solid film substrate transport, aeration control, and nitrate generation are major operational difficulties [28]. Moreover, the single-stage CANON is mostly used in side-flow processes.
