Impact of Rainfall Events on the Performance of WWTPs

Background
Rainfall events alter influent properties by introducing shock loads of organic matter, suspended solids, and pathogens into the system, overwhelming the plant's capacity. The flooding caused by these events also damages key infrastructure, including pumps, clarifiers, and disinfection systems, further reducing treatment efficiency. Hydraulic overloading impacts the settling efficiency in clarifiers and reduces the effectiveness of biological and chemical treatment processes. The increased turbidity in stormwater also interferes with disinfection systems, resulting in bacterial exceedances in the effluent. This study evaluates the impact of rainfall events, on effluent quality dynamics at a full-scale activated sludge wastewater treatment plant (WWTP).

Study area
Houston, one of the fastest-growing cities in the USA, is confronted with significant challenges in managing the increasing demands on its wastewater infrastructure. With a population of approximately 2.5 million and a daily wastewater generation of around 250 million gallons, Houston relies on 38 wastewater treatment plants owned by the City to handle and treat this large volume. WWTPs must adhere to specific effluent permit limits to protect public health and the environment.

Objective and Methodology
The primary objective is to analyze how rainfall events contribute to WWTP effluent violations for parameter including CBOD, NH3-N, TSS, and E. coli, and to identify strategies for mitigating these rain-induced disruptions. A comprehensive analysis was conducted using operational and effluent quality data from a WWTP operating under the City of Houston. Comparisons of wet and dry weather conditions were performed to assess the correlation between rainfall events and effluent violations.

Results
Key findings reveal that rainfall events introduce shock loads of organic matter, suspended solids, and pathogens into the treatment system, overwhelming plant capacity and disrupting processes. Hydraulic overloading reduced clarifier settling efficiency and impaired both biological and chemical treatment. Increased turbidity in stormwater interfered with disinfection systems, leading to bacterial exceedances in effluent. Additionally, flooding caused significant damage to critical infrastructure, including pumps, clarifiers, and disinfection systems, further reducing the plant's ability to maintain compliance. Violations persisted for 2—3 days after rainfall, emphasizing the lasting effects of rain-induced disruptions.

Several solutions are proposed to mitigate the impact of rainfall events on WWTP performance. Installing equalization tanks to buffer peak flows and installing screens and grit removal systems upstream of the primary tank help reduce the concentration of high solids entering the system. Adding polymer dosing helps to improve settling in clarifiers during high TSS periods. Installing pre-filtration to reduce turbidity entering the UV system and maintaining dynamic chlorine dosing adjustments is recommended to prevent bacterial violations.

Conclusions
This study underscores the importance of understanding wet weather flow dynamics and their impact on WWTP effluent quality. By identifying the causes of rainfall-induced violations and proposing targeted solutions, this study provides a framework for enhancing WWTP performance and ensuring compliance with regulatory standards under extreme weather conditions. The findings contribute to developing adaptive and robust wastewater management practices, which are increasingly critical in regions experiencing frequent rainfall and flooding.