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The widespread antibiotic resistance genes (ARGs) severely jeopardizing human health have been widely concerned. Wastewater treatment plants (WWTPs) are commonly considered the important sources and sinks of ARGs due to their roles in depositing ARGs and provoking ARGs' proliferation. Current WWTPs often use biological treatment to remove pollutants, whereas the purification efficacy of ARGs is unclear. Thus, this study investigated the seasonal changes of ARGs (tetA, tetC, tetM, tetO, tetQ, tetW, and tetX, as representative) and a class of integron (intI1) in the sludge of different treatment units of four WWTPs to understand the role of WWPTs in ARGs spread. It is found that ARGs abundance varied between summer and winter depending on the types of ARGs and treatment units. The cumulative abundance of ARGs in the same treatment unit showed no obvious difference between seasons. By comparison, ARGs abundance was generally lower in the anoxic unit than in the oxic unit. Moreover, the WWTPs with Cyclic Activated Sludge System (CASS) process reduced the amount of ARGs in sludge, while the WWTPs with Anaerobic-Anoxic-Oxic (A2O) increased ARGs abundance. In conclusion, this study demonstrated that the excess sludge still posed a risk for ARG spread after biological treatment.