Sewers, drainage and stormwater

What is the difference between a combined and a separate sewer system?

• Combined: one pipe. Foul and rainfall travel together to treatment.
• Separate: two pipes. Foul goes to treatment, surface water goes to a watercourse.
• Storm overflows exist because of combined systems. Rainfall can exceed what the pipe can carry.
• Misconnections break the separate system. A foul appliance plumbed into the surface pipe pollutes directly.

What each one does when it rains

On a dry day the two systems behave almost identically. The difference appears entirely in wet weather, and it is the whole reason storm overflows exist.

In a combined system, rainfall falling on roofs and paved areas enters the same pipe as foul sewage. Flow can rise by a large multiple within minutes. The pipe and the treatment works downstream cannot be sized for that peak economically, so a relief point is built in. When capacity is exceeded, the storm overflow discharges the diluted mixture to a watercourse rather than allowing it to back up into streets and buildings.

In a separate system, rainfall never meets sewage. Surface water goes down its own pipe straight to a watercourse, and the foul pipe carries only foul flow, so it stays close to its dry weather rate throughout the storm. There is nothing to overflow, because the foul system does not receive the rainfall in the first place.

Why the UK has both, and cannot simply convert

Combined systems are the older design. Much of the Victorian and early twentieth century urban network was built that way, at a time when the priority was removing waste and rainwater from streets rather than protecting rivers. Separate systems became standard for newer development.

The result is a patchwork. Older urban cores are largely combined, newer estates are separate, and many catchments are partially converted, with separate systems draining into combined trunk sewers further downstream.

Converting a combined catchment to a separate one means laying a second pipe network and reconnecting every property to it, under existing streets and buildings. The cost and disruption are why the practical strategy is usually to reduce how much surface water enters the combined system rather than to separate it entirely, through sustainable drainage, disconnection of roofs and yards, and attenuation upstream.

The failure mode unique to separate systems

A separate system removes the storm overflow problem and introduces a different one, which is easy to miss because it produces no dramatic event.

Because the surface water pipe discharges directly to a watercourse with no treatment, anything wrongly connected to it goes straight into the river. A washing machine, sink or toilet plumbed into the surface water pipe pollutes continuously, every time it is used, in all weathers. There is no overflow event to record and no obvious trigger to investigate.

These misconnections are found by looking for foul indicators in a pipe that should carry only rainfall. Continuous water quality monitoring at a surface water outfall is well suited to it, because a misconnection produces a signature that persists in dry weather, when a surface water pipe should be close to inactive.