Compared with sidestream anammox, out-selection of NOB and heterotrophic denitrifiers is more challenging in the mainstream, because the ammonium concentration in mainline is not high enough to produce high free ammonium (FA) to suppress NOB growth [11]. Additionally, in the mainstream, high C/N ratios may be favorable to the growth of heterotrophic denitrifiers, whereas low temperature may decrease anammox activity [12]