What Is The Link Between Storm Overflows And Sewage Water Pollution?

Storm sewage overflows are hydraulic relief points within combined sewerage systems. In these networks, foul wastewater and surface water can enter the same sewer. During rainfall, therefore, runoff from roofs, roads, gardens and impermeable areas adds volume to the foul flow already moving towards the pumping stations, storage and wastewater treatment works.

Unfortunately, when the combined flow exceeds the capacity available downstream, an overflow can discharge to a river, estuary or coastal water to reduce the risk of sewer flooding, network surcharge and backing-up into properties. That hydraulic function explains why storm overflows exist. The sewage pollution water issue begins when the relief route carries diluted wastewater into the receiving environment instead of forward for treatment. This is the primary link between storm overflows and sewage water flows, but can anything be done about it?

How Combined Sewer Capacity Creates The Overflow Pathway

A storm sewage overflow event is not generated by rainfall alone. Rainfall supplies the trigger, certainly, but the overflow response itself depends on the catchment and asset conditions. These can extend to the connected impermeable area, groundwater infiltration levels, sewer capacity, pump station performance, storage volume, downstream treatment acceptance and restrictions caused by silt, fats, wipes, roots or other blockages.

If the network can pass the incoming flow forward, the overflow does not need to operate. It is only if the hydraulic load exceeds the available pass-forward capacity that flow reaches the overflow level and the relief route becomes active. ‘Pass-forward capacity’ is the volume the system can continue conveying towards treatment through its pumps, pipes, controls and downstream works. When that capacity is constrained, the spill risk increases even if rainfall has not changed significantly – which is why sewage pollution water instances can occur even during periods of average or below average rainfall.

Why The Discharge Is Not Only Rainwater

The discharge from a combined sewer overflow is usually diluted by stormwater, but dilution does not remove the wastewater component. Depending on the catchment, flow conditions and timing of the event, the discharge can contain fine and gross solids, organic matter, nutrients, ammonia, fats, oils, grease, litter and microbiological contamination.

Screens are intended to retain larger solids and sewage litter before the liquid fraction passes through the overflow route. However, their performance depends on the screen design, maintenance conditions and the nature of the incoming material. If a screen ‘blinds’ or becomes obstructed, hydraulic performance and solids retention can deteriorate together. That is why an overflow assessment should include the mechanical condition of the chamber and screen, as well as the spill record.

Why Duration, Load And Location Change The Risk Profile

Event Duration Monitoring records when a storm overflow operates and for how long. This is valuable operating evidence, but duration is not the same as volume, pollutant concentration or ecological effect. Two spills of the same duration can present pollution different risks if one carries a higher wastewater fraction, occurs after a long dry period, or discharges into a low-flow or ecologically sensitive reach.

Your storm overflow data should therefore be read alongside rainfall records, flow data, upstream asset condition and receiving-water context. A short event into a larger, fast-moving watercourse is not the same operational or environmental problem as a prolonged discharge into a smaller channel with limited dilution and existing water-quality pressure.

How Targeted Intervention Reduces Pollution Pressure

Reducing sewage pollution water courses from storm overflows depends on identifying the local limiting conditions. In one catchment, for instance, the most effective intervention may be removing surface water from the combined system. In another, infiltration sealing, sewer cleaning, pump reliability, added storage, real-time control or increased treatment inlet capacity may produce the required reduction. For your network, the most practical route is to connect the spill pattern with the hydraulic constraints and asset conditions that create it, then direct intervention towards the part of the system controlling overflow operation.

Speak To Samatrix

If storm overflow performance is affecting your wastewater network or receiving-water risk, please contact Samatrix to discuss our pumping, hydraulic and sewage infrastructure support services.