Deciphering the role of hydraulic retention time (HRT) in integrated fixed-film activated sludge (IFAS) system under aniline stress: Effects on microbial succession and communication

Integrated fixed-film activated sludge (IFAS) system offered an energy-efficient option for nitrogen removal from aniline wastewater. The regulation of hydraulic retention time (HRT) has been reported to affect the fate of functional microbial communities with different generation cycles, but the role of HRT in IFAS under aniline stress has not been sufficiently revealed. Herein, it was observed that shortening the HRT altered the mechanisms underlying the adaptation of a continuous flow micro-aerated IFAS system to aniline stress, thereby negligibly impacting the removal of aniline. The total nitrogen removal rate in Y2 (HRT = 0.5 day) was 40.28 % higher than Y1 (1 day), and 23.7 % higher than Y3 (0.25 day). This could be attributed to Y2 having an ample supply of electron donors for denitrification compared to Y1, as well as showing weaker stress to inhibit nitrification compared to Y3. Furthermore, with the progressive intensification of stress induced by HRT reduction, microorganisms gradually enhanced their adaptation to aniline. This led to an increased migration of microorganisms towards the inner layers of the biofilm, resulting in enhanced functional complementarity between the sludge and biofilm in Y2. Also, there was a distinct development of autotrophic denitrification, with arenimonas predominant in Y1, and Thauera predominant in Y2 and Y3. Moreover, Y2 possessed a high abundance of functional genes related to the production and reception of quorum sensing signal molecules, which may enhance the electron transport-dependent nitrogen metabolism reaction.