Effects of hydroxylamine and ammonia concentration on the stability of CANON process

This study investigated the effects of hydroxylamine (NH2OH) dosage and duration on the restoration of the deteriorated completely autotrophic nitrogen removal over nitrite (CANON) process due to the overgrowth of nitrite oxidizing bacteria (NOB) under conditions of low and high ammonia concentration. The results showed that the nitrogen removal efficiency of CANON process could be restored to more than 80% in 5 days after 5 mg/L NH2OH addition. However, once the NH2OH was inexistence in the reactor under the low ammonia concentration, the NOB was restored and thus producing nitrate. Upon shock by 133 mg/L nitrite and low temperature, the activity of the functional consortium was inhibited and then recovers under the assist of NH2OH. Compared to low ammonia concentration, the NO3−-Nproduced/NH4+-Nconsumed ratio was more stable and closer to 0.11 under high ammonia concentration after stopping NH2OH addition due to NOB was also inhibited by free ammonia (FA). The analysis for microbial community showed that the relative abundance of anaerobic ammonia oxidation bacteria (AAOB) increased from 6.11% to 12.7% with the enhancement of NH2OH dosage, while barely affected by ammonia concentration. However, the relative abundance of ammonium oxidizing bacteria (AOB) increased slightly from 2.96% to 3.62%, and 3.62% to 4.25%, respectively, under the improvement of NH2OH dosage and ammonia concentration. The relative abundance of NOB was less affected by NH2OH dosage, while it decreased from 1.45% to 0.78% when influent NH4+-N increased from 50 mg/L to 200 mg/L.