Detention storage volume for combined sewer overflow into a river

Area of Science:

• Environmental Engineering
• Water Quality Management
• Wastewater Treatment

Background:

• Current combined sewer system design criteria often neglect receiving water conditions, leading to inadequate performance.
• Combined sewer overflows (CSOs) significantly impact river water quality due to untreated wastewater discharge.

Purpose of the Study:

• To determine optimal storage volumes in combined sewer systems for preserving water quality.
• To evaluate the effectiveness of storage volume and wastewater treatment plant capacity on CSO impacts.
• To propose water quality criteria for assessing CSO effects on river ecosystems.

Main Methods:

• A simulation model was employed to assess a theoretical combined sewer system with a downstream storage tank.
• Eleven years of rainfall data from Santander, Spain, were used to simulate CSO events.
• Water quality parameters including fecal coliforms, Biochemical Oxygen Demand (BOD), and total nitrogen were analyzed.

Main Results:

• Bacteriological pollution (fecal coliforms) posed the most significant threat, requiring storage volumes of 45-180 m³ ha⁻¹ net.
• Biochemical Oxygen Demand (BOD) required 5-50 m³ ha⁻¹ net, while total nitrogen needed less than 4 m³ ha⁻¹ net.
• Increasing wastewater treatment plant capacity (2-3 times dry weather flow) reduced CSO frequency by up to 65%, proving more effective than solely increasing storage.

Conclusions:

• Bacteriological pollution is the primary driver for storage volume requirements in CSOs.
• Enhanced wastewater treatment plant capacity offers a more efficient strategy for mitigating CSO impacts than increased storage volume alone.
• The study provides crucial data for designing effective CSO management strategies to protect river water quality.