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Upgrade to improve the River Ribble estuary and the coastal environment
The majority of United Utilities sewerage systems are made up of combined sewers. Combined sewers transport wastewater from homes and industry as well as carry surface water run off from gutters, drains and some highways. Newer separate sewers transport only sewage, with surface water run off being directed to its own dedicated drainage system.
Heavy or prolonged rainfall can rapidly increase the flow in a combined sewer until the volume becomes too much for the sewer to carry and excess storm water is then discharged to a water course, such as streams, rivers or seas, via Combined Sewer Overflows (CSO).
How a CSO works…
Dry weather flow
Wet weather flow
A well designed CSO acts as an essential relief valve, preventing overloading to the sewer which could otherwise lead to flooding of properties and sewage treatment works. However, older CSOs, some of which were built in Victorian days, were designed to a much lower standard that is now considered unacceptable and does not comply with the current European Legislation. These are classed by the Environment Agency (EA) as Unsatisfactory Intermittent Discharges (UID).
Some common faults are:
- The downstream sewer pipe diameter is too small causing the CSO to operate too frequently
- The discharge is unscreened
- Some are simply holes knocked into manhole walls to alleviate flooding.
As part of our current five year investment programme we aim to replace and improve 235 UIDs acoss our region with modern, well designed and well functioning ones that comply to today's European legislation.
There are currently 39 CSOs in Preston that do not comply with this legislation. These overflows spill into watercourses during rainstorms and cause water quality issues in the River Ribble and the coastal region stretching from Southport to Blackpool.
These 39 CSOs are split into two major schemes of work. Seven of the UIDs form part of our Preston Tunnels project which includes the construction of 3.5km of tunnels underneath Preston and Penwortham as a pumping station and 5km of risng mains. The other 32 UIDs are split into 5 catchment areas across Preston and Penwortham.