CSO / Stormwater

In a combined sewer system, sewer flows and surface water drainage are handled by a single network and passed to a local municipal treatment works. During heavy rainstorms, flow rate through the system can rise dramatically, causing a peak flow event at the works. Where storage for this peak flow is not available then, to avoid overloading of the works, the network’s relief structure can allow some of the combined stormwater and sewage to overflow the network and be discharged untreated into an adjacent waterway or body of water.

Emergency overflows or discharge points (outfalls) are incorporated along the length of a sewerage network to overcome excessive surging. These are called Combined Sewer Overflows, or CSOs, and are designed to allow any excess stormwater flow to spill into open/surface water courses, so discharging wastewater and storm water from a combined sewer system directly into a river, stream, lake or ocean. In order to prevent bulk solids causing unsightly carry over into these bodies of water, the overflows are screened.

A well-designed CSO acts, essentially, as a relief valve and prevents overloading of the sewer system, which could otherwise lead to flooding of properties and/or sewage treatment works. A modern CSO may be fitted with telemetry to advise an operator of when it is discharging.

Overflow frequency and duration will vary, both from system to system and from outfall to outfall, within a single combined sewer system. Some CSO outfalls discharge infrequently, while others activate every time it rains. During heavy rainfall, when stormwater exceeds the sanitation flow, the CSO sewage content will be diluted.

The stormwater component itself can also contribute a significant amount of pollutants to the CSO. Each storm is different in terms of the quantity and type of pollutants it contributes. For example, storms that occur in late summer, when it hasn’t rained for a sustained period, will contain the most pollutants due to accumulations in the network, or first flush as it is commonly referred to:

Common contaminants include:

• Sanitary products (and other products flushed down the toilet)
• Street rubbish 
• Grease
• Faecal coliform from pet and wildlife waste

Any and all of these can be flushed into a sewer system. Additionally, in cold weather areas, pollutants from cars, people and animals accumulate on hard surfaces and grassed areas during the winter, and are then flushed into the sewer system during heavy spring rains.