Performance and mechanism of triclosan removal in simultaneous nitrification and denitrification (SND) process under low-oxygen condition

Wastewater treatment under low dissolved oxygen (DO) conditions is promising for its low energy consumption. However, the removal process of some organic micropollutants, such as triclosan (TCS), could be inhibited under anaerobic conditions. So, it is worth investigating the TCS removal performance at low-oxygen condition. In this study, simultaneous nitrification and denitrification (SND) process, with DO ranging from 0.30 to 0.80 mg L−1, was chosen to investigate. Results showed that the water quality of the effluent was deteriorated after TCS addition at the beginning, with removal efficiency of NH4 +-N dropped from almost 100 ± 0.70 to 88.30 ± 0.98% and COD decreased from 95.15 ± 1.55 to 65.81 ± 2.42 %. However, the performance recovered from the 3rd day and almost stabilized on the 14th day with the removal efficiencies of NH4 +-N were over 98.00 ± 0.60 %, and COD was above 94.00 ± 1.70 % in effluent. Besides, TCS removal efficiencies were more than 93.00 %, and the contributions for TCS removal by the water effluent, sludge sorption, and other effects including biodegradation were 6.46 ± 2.25, 16.27 ± 3.30, and 77.27 ± 4.45 %, respectively. Although the results of absolute abundances of related genes showed no difference (P > 0.05), Illumina MiSeq sequencing analysis presented the variation of microbial community after TCS addition, in which T-45 had the highest Shannon and Simpson diversity index, followed by T-0 and T-2. Relative abundances of alpha and beta-Proteobacteria, which were related to TCS biodegradation, were increased. Compared with Bacteroidetes in T-0, the abundance of Bacteroidetes took up more than 15.6 % in T-45, which should play a more important role under low-oxygen conditions with TCS addition.