Separating sewers

Until the 1950s, most sewers in the UK were constructed as, or evolved to become, combined systems – taking both sewage and storm water.

Photo by Tim Newbury

Since that time, separate sewer systems have become the norm for new development, meaning that the majority of towns and cities in the UK are served by systems with a central core of combined sewers, with peripheral, more recently developed, areas being drained by separate systems.

Changing rainfall patterns caused by climate change and urban development increasingly mean that local sewage systems are unable to cope with the combined flow from the sewage and storm water system. This is particularly problematic for areas with combined sewer and storm water systems, as during periods of high rainfall the overflow discharges storm water and sewage into local watercourses.

London’s system is a classic example. Even during periods of moderate rainfall, overflows of sewer and storm water discharge into the river Thames on average once a week. An average of 20 million cubic metres of untreated sewage is discharged into the Thames every year. A Thames Tideway Strategy Group was set up in 2000 to assess the environmental impact of combined sewer overflows (CSOs) and to prepare potential solutions and costs.

As part of accommodating growth in cities, the problem of combined sewer overflows during storm periods should be considered. City authorities and partners should consider opportunities to either increase the capacity or performance of existing systems or develop ways to separate the storm sewers and sanitary sewers, and these findings should inform the evidence base for local planning policy documents.

The Thames study concluded that the preferred option was to build a 35km long storage and transfer tunnel beneath the river to convey the discharge from 36 CSOs for collection and treatment at the Crossness sewage works in east London, where a plant to handle the storm flows could be built. The study estimated the cost of such a measure at £2 billion. However, alternative options including the more widespread use of sustainable drainage dystems (SUDS) were considered and it might be the case that these could be achieved at a lower cost when considered through whole-life costing.

As a principle, SUDS in new developments should link to storm sewers in systems where storm sewers are separated from sanitary sewers.

Priority: encourage sustainable water use
Tags: water, cities and towns