Untreated sewage can enter a river by two main means: through a combined sewer overflow (CSO) or through a misconnection.
Combined Sewer Overflows
Sewerage systems built before the 1920s carry rainwater, wastewater and sewage to the sewage treatment works in a single pipe. In heavy rain, the pipe may not be able cope with the amount of water and to prevent it backing up into houses, the water is sent to a nearby river via a combined sewer overflow which discharges the sewage (diluted by the rain and wastewater, into the river). This is called a combined sewer system. This older type of system is now struggling to cope with the volumes of water being sent to it for a number of reasons:
- An increased population – using more water and generating more wastewater.
- Changes in lifestyles and demand for water – more showers, washing machines, car washes etc.
- Increased surface water run off due to an increase in the impervious surfaces e.g. paving, concrete etc. resulting from urbanisation.
- Heavier rainfall events.
Discharges of untreated sewage from CSOs has increased over the years and a 2023 Guardian article suggested that many are even discharging in light rainfall events.
You can use this interactive map from the Rivers Trust to find the location of CSO’s near you and the frequency of their discharge.
More modern houses/urban areas operate a separate sewer system, where rainwater falling on houses and roads is sent, via a surface water drain, to a river/stream or ditch. Waste water and sewage are sent in a separate foul water sewer to a sewage treatment works. This system has a greater capacity for dealing with heavy rain. About half the houses in the UK work on a separate sewer system.
Misconnections
According to a 2019 Defra report about 150,000 to 500,000 homes in the UK have drain misconnections. A household sewage or wastewater pipe connected to a surface (clean) water pipe will result in polluted water being sent to a nearby river. Clean water being misconnected to a foul water sewer won’t necessarily cause pollution but will increase the likelihood of flooding due to the increased load of water. The most common misconnections are washing machines (35%), sinks (10-15%) and toilets (5%).
Other resources:
How to report sewage pollution
Spotting signs of pollution in waterways
Water Care Partnership 