Influent-driven antibiotic resistance dynamics are constrained by low temperatures in conventional activated sludge systems

Wastewater microbial communities contain high abundance of microorganisms carrying antibiotic resistance genes (ARGs), many of which persist through conventional activated sludge (CAS) treatment. This study aimed to evaluate the effect of temperature on microbial community composition and ARG abundance in CAS systems, framed by treatment performance. Two laboratory CAS systems were operated for 147 days at an urban wastewater treatment plant (UWWTP) under identical conditions, except the test variable—temperature. The Test system was sequentially operated at 10, 5, 15, and 28 °C, whereas the Control remained at ~ 20 °C. Feed (primary UWWTP effluent), effluent, and sludge samples were analysed for physicochemical parameters, faecal indicators, ARGs, and bacterial community composition. Although the effluent ARG patterns were influenced by the feed, temperature exerted additional influence during treatment. Operation at 5 °C resulted in significantly (p ≤ 0.05) lower abundance of intI1 and clinically relevant ARGs, including blaIMP and qnrS, compared with the Control (~ 20 °C). These differences were associated with shifts in microbial communities, particularly the decrease of genera such as Acinetobacter and Escherichia-Shigella, potential ARG hosts. No significant differences were observed in sludge ARG abundance, indicating that temperature mainly influenced ARG persistence in the liquid phase. Faecal indicator removal was also temperature-dependent, with significantly (p ≤ 0.05) greater reductions of Escherichia coli at 5 °C. However, low temperature impaired nutrient removal, although overall effluent quality remained within discharge limits. Overall, low-temperature operation reduced ARG abundance in CAS effluents, likely due to reduced fitness of key bacterial hosts, highlighting temperature as an important operational driver of antibiotic resistance dissemination in wastewater treatment systems. Key points • ARG prevalence is driven by influent composition and modulated by temperature. • Operation at 5 C significantly reduces ARG abundance in CAS effluents. • ARG variation during treatment can be associated with microbial community structure.