Shock load effects on WWTP performance under low loads following capacity expansion for combined sewer overflow control

Expanding wastewater treatment plant (WWTP) capacity is an effective approach to control combined sewer overflow. However, this often leads to low-load operation in dry weather and sudden peak flow during rainfall, with limited research on their impacts on WWTP performance. In this study, the design capacity of pilot-scale anaerobic-anoxic-oxic reactors was same as the peak flow, while the actual inflow rate was 70 % and 50 % of the design capacity in Reactor 1 (R 1 ) and Reactor 2 (R 2 ), respectively. After a period of operation, the influent flow was increased to 100 % of design capacity following rainfall. Results showed R 1 maintained stable pollutant removal, sludge characteristics, and bacterial community structure, showing resilience to shock load. Nevertheless, R 2 experienced reduced COD, NH 4 + -N and TN removal efficiency, along with fluctuations in sludge concentration, activity, floc size, soluble microbial products (SMP), and extracellular polymeric substances (EPS). The bacterial communities associated with pollutant removal, filamentous bacteria, and EPS synthesis underwent significant changes, which might contribute to the performance difference. These findings highlight the importance of maintaining operational flows at no less than 70 % of peak flow to ensure stability under shock load. Operating at 50 % capacity may reduce resilience to the peak flow induced by rainfall. • Influent shock load by rainfall differently impacted WWTP performance under various low loads. • Stable pollutant removal achieved with operational flow at 70 % of peak flow. • Operation at 50 % of peak flow reduced resilience to shock load. • Shock load caused shifts in sludge characteristics and bacterial community.