|Abstract:||Concerns about micropollutants in wastewater treatment effluent is increasing during the past decade due to their potential adverse environmental effects. It has been hypothesized that ammonia-oxidizing bacteria (AOB) were responsible for micropollutants biotransformation, which contributes to the removal of micropollutants. Both inhibition studies and pure culture studies have been performed to test this hypothesis. By inhibiting ammonia oxidation in nitrifying activated sludge (NAS) with inhibitors including allythiourea and octyne, the biotransformation of a number of micropollutants was inhibited to different extents, which was restored when the ammonia oxidation activity resumed. The inhibition of the biotransformation of the micropollutants by the inhibitors of ammonia oxidation suggested the affinity between AOB and the biotransformation of these micropollutants. However, when using pure AOB cultures, such as Nitrosomonas europaea, only few of these micropollutants could be biotransformed. Such discrepancy might due to the facts that the tested pure cultures were disparate with the AOB in NAS, and the microorganisms dependently growing with AOB, such as nitrite-oxidizing bacteria (NOB) were not taken into consideration. Therefore, the objective of this study is to investigate the contribution of more environmental relevant nitrifiers to micropollutants biotransformation using a nitrifying community enriched from biofilm of a nitrification tower in a local wastewater treatment plant. We inoculated biofilm samples to two reactors using basal medium without adding organic carbon. Ammonium was supplied into one reactor upon depletion, and nitrite was supplied to the other reactor. In the first reactor, namely AOB/NOB enrichment reactor, both of AOB and NOB were expected to be dominant. Contrarily, NOB was expected to be exclusively enriched in the other reactor fed with nitrite, hence the reactor was named by NOB enrichment reactor. After about 8-months enrichment, we obtained a nitrifying enrichment culture with 75.9% AOB and 7.6% NOB in the community of AOB/NOB enrichment reactor, while the relative abundance of AOB and NOB in the NOB enrichment reactor were 0% and 25.6% based on quantitative PCR detection. We then investigated micropollutants biotransformation by this two nitrifying enrichment culture. We used fourteen micropollutants whose biotransformation were previously observed to be inhibited by inhibitor (i.e., allythiourea and octyne) treatment to NAS. Results showed that the AOB/NOB enriched cultures were able to significantly degrade five among fourteen micropollutants, including Fenhexamid (55%) and Rufinamide (30.14%), while all of the five compounds can also be similarly degraded by Nitrosomonas europaea pure culture. However, the biotransformation of Furosemide in NOB enriched cultures was significantly higher than Nitrobacter pure culture.