How a combined sewer overflow works

In dry weather everything goes to treatment. In heavy rain, flow exceeds what the sewer can pass forward, and the excess spills over a weir to a watercourse.

Design intent: The spill is meant to be heavily diluted storm water containing some sewage, discharged rarely. Frequent spills, or spills in dry weather, indicate a capacity or asset fault.
Why counts rose: Monitoring coverage in England went from a small minority of overflows to effectively all of them within about seven years. Part of the recorded increase is increased measurement.
The fix: Separating surface water from foul sewers removes the cause, but means rebuilding urban drainage. Storage tanks and sustainable drainage reduce frequency at lower cost.

Text description

In dry weather, foul sewage and a small amount of runoff flow along the combined sewer to the treatment works. In wet weather, rainfall runoff greatly increases the flow. The sewer can only pass a limited flow forward to treatment, so once that limit is exceeded, the excess passes over an overflow weir and discharges, diluted, to a river. An event duration monitor records when and for how long this happens.