ObjectiveFilamentous bulking is one of the most severe operational issues in wastewater treatment plants (WWTPs), which typically causes reduction of treatment capacity and excessive effluent suspended solids. Particularly for sequencing batch processes, the normal operation would be destructed by filamentous bulking due to failed sludge-water separation in biological systems. Therefore, controlling the excessive growth of filamentous bacteria is significantly important for stable operation of WWTPs.MethodsThis paper introduced a case study of cyclic activated sludge system (CASS) process WWTP in a certain county town with a design capacity of 20 000 m³/d, that experienced severe sludge bulking due to the combined effects of influent load shock and low influent temperature in winter. The effluent COD, ammonia nitrogen and TN parameter were exceeded. In order to restore normal operation of CASS process, a two-phase system adjustment was implemented. In phase I, the measures such as polyacrylamide (PAM) dosing, extended aeration duration, and dissolved oxygen (DO) control were employed, but without achieving significant improvement in operation effects. In phase Ⅱ, a comprehensive measures were implemented, including accelerated sludge wasting, manual floating sludge removal, operation sequence optimization, influent loading rate stabilization, aeration and DO strictly controlling. Fortunately, the sludge bulking was effectively controlled.ResultsThe settling velocity (SV₃₀) was reduced from 98% to below 70%. And the major effluent parameters decreasing from averages (COD: 37.14 mg/L, ammonia nitrogen: 3.60 mg/L, TP: 0.33 mg/L, TN: 16.12 mg/L) to compliant levels. System recovery was achieved within 40 days, and the treatment capacity was restored from 14 000 m³/d during the commissioning period to designed 20 000 m³/d.ConclusionLow temperature constitutes the primary factor for filamentous bulking. And influent loading rate shock directly causes effluent quality non-compliance. Meanwhile, high sludge loading rate combined with sustained low DO levels promotes filamentous bacteria proliferation as indirect contributing factors. It is recommended that WWTPs should promptly discharge and replace sludge and adjust their operating conditions before entering winter to inhibit the growth of filamentous bacteria. In daily operation, it is necessary to arrange reasonable operating procedure and strict control of parameters such as loading rate, DO and sludge retention time (SRT) to keep stable operation in WWTPs.