Combined Sewer Overflows (CSOs) explained
Three initial definitions
Sewage:
All types of liquids and particles that pass through the sewerage (i.e. the sewer pipe drainage system).
Storm water sewage:
Rainwater (or storm water as it is commonly described) refers to the untreated water that falls as rain and is collected from roofs, driveways and roads. This water is usually safe to return to the environment – including brooks such as the Yazor - assuming it doesn’t fall on potentially contaminated areas, such as fuel-station forecourts, where oil and fuel spills may be present. However, after a long period without rain, all roads, (particularly busy roads) will have accumulated oils, tyre rubber particles and litter, which make the first flush of stormwater into the drains quite contaminated.
Foulwater sewage:
Foulwater sewage, on the other hand, refers to the water that drains from toilets, sinks, baths, showers, washing machines and dishwashers in homes and all other types of premises, that needs treatment before being discharged into water courses or the sea. It also includes permissible discharges from factory processes. Foulwater can be further broken down into foul water from toilets etc .and factory processes, and greywater from sinks etc. that could (but rarely does because of the high cost) receive basic treatment and be reused for certain purposes. The aim is for both types of foulwater to be transported by the drainage system to local waste water treatment works.
Combined Sewers:
Properties built after 1970 began being designed with separate drainage systems for storm water and foulwater so that the stormwater could be discharged into a watercourse instead of unnecessarily going through a sewage treatment works. However, if a post 1970 property has been renovated/extended, or had a new appliance installed by an unqualified plumber or inexperienced DIY enthusiast, there is a chance that the wrong connections were made…and a direct discharge to a water course could be causing a water pollution problem. HYBRP and Welsh Water refer to these wrong connections as “misconnects” and acknowledge that there are many of them along the course of the Yazor/Widemarsh/Eign system. However once the stormwater sewer reaches the boundary of the new development site, if there is not a suitable water course or storm drain very close by to discharge into, it is common for the stormwater sewer to be connected to a sewer carrying foulwater to the sewage treatment works. Such sewers are then known as combined sewers.
Combined Sewer Overflows (CSOs)
Combined sewer pipes obviously have a limited capacity and when the flow within them exceeds the maximum the pipes can carry, a choice must be made. If nothing is done to cope with that excess flow then properties would not be able to discharge any fluid (foul water and rain water) into the sewer. Those properties would then flood internally with this fluid and also with back flowing sewage from the sewer. Hence special manhole chambers are constructed on the sewer with an overflow branch pipe higher than the sewer to release the excess flow safely; these are the Combined Sewer Overflows (CSOs - sketch diagrams will be posted soon). There are very many thousands of CSOs across the UK which are water company assets licenced by the Environment Agency.
Combined Sewer Overflows (CSOs) have been a standard part of a sewerage system (i.e. the sewage pipes) ever since rainwater pipes were connected to the foul sewer pipes, probably since the late 19th century, or quite possibly much earlier. A new development will have sewers carrying foul sewage to the existing combined sewers while storm water, instead of going into a combined sewer, will hopefully be carried by a rainwater drain to a Sustainable Urban Drainage System (SUDS) [See description (1) of SUDS below] or directly to a local stream. For example, the discharge of rainwater from the new Redrow Homes built on the site of Whitecross School goes into a SUDS with an overflow and probably a very restricted discharge to slowly drain the SUDS, to the so-called City Leat that runs alongside the development.
Importantly, and for very many years - probably from pre-war - the sewage undertaker has been required to size sewers so that they can carry 6 times the maximum foul sewage flow. This is still the standard required by the Government and OFWAT who until now have considered and responded to the public preferences for the customers’ water rates to be spent dealing with other problems (e.g. leaking water pipes, drinking water quality, water supply reliability, improved quality of treated sewage discharges to water courses and the sea etc). Until 1974 all town/city councils were the sewage treatment providers who frequently spent an absolute minimum on the sewage system in order to minimise the council rates - since the majority of the public had little interest in what happened to their sewage and demanded that council rates be spent on other services. Countless frighteningly dilapidated sewage assets were handed over to the Water Authorities when they were created to take over and operate the water and sewage assets in 1974. The 1989 Water Act privatised the Water Authorities in England and Wales, with the Government selling the businesses to the public and others for £7.6 billion. The Government did this to avoid having to borrow the mind-boggling sums of money needed to replace, maintain or improve the dilapidated assets that the Water Authorities had identified during their first 15 years.
What is the trigger for discharges through CSOs?
The trigger is a sewer which normally flows with no pressure, reaching its capacity, which causes pressure in the pipe. This results in some of the sewage flow rising up inside a special manhole to an overflow pipe which is higher than the sewer, which then discharges the overflow to a water course. Hence there is no human or mechanical activation of CSOs.
Why do CSOs sometimes discharge when there is not excessively high flow in the sewer due to heavy rainfall (so-called “dry spills”)?
One cause of “dry spills” is identified by Southern Water head of wastewater John Penicud, in The Institution of Civil Engineers’ News, 7.9.23: “High groundwater conditions can lead to rising water finding the path of least resistance into a network of sewer pipes and manholes [ i.e. through no longer watertight pipe and brickwork joints], and a discharge made up of groundwater can happen in dry weather. The problem is especially challenging in areas prone to flooding, as mitigation measures such as sewer relining and manhole sealing redirect flows. Groundwater can then cause flooding”.
Less frequently the cause can be due to a partial blockage (or blockages), in the sewer which prevent 6 times the waste water flow passing through it. These partial blockages are caused by people using toilets as waste bins for wet wipes, nappies, fat, etc. etc., which creates the well-publicised "fat-bergs". Also, occasionally unscrupulous people lift manhole covers and tip rubbish into the sewer instead of taking it to a proper place. Another problem is tree roots which force their way through leaking sewer joints or leaking cracks in a sewer and cause partial blockages as debris in the sewage gets caught by the roots. Only when a less than severe rainstorm occurs and the CSO operates (and is hopefully detected) does the problem reveal itself, so a repair can be organised.
CSOs on the City brooks
Research into open source data by HWTCB Chair, Anne Cottringer has found that there are at least four CSOs along the course of the City Brooks: Grandstand Road CSO (Moor Park, just downstream of Trinity School) discharged 28 times for a total of 75.5 hours in 2022, 3 Elms Road (actually just upstream of Yazor Road) 14 times in 2022 for a total of 4.25 hours, “Miss Chave’s” CSO (on Widemarsh Way) spilled 2 times in 2022 for a total of 0.5 hours and Eign Road CSO (just upstream of the confluence with the Wye) 38 times for a total of 29.25 hours. HYBRP knows that a “dry spill” seems to have occurred from “Miss Chave’s” CSO in late 2021; investigations are ongoing into the nature and cause of this discharge.
The future
CSOs look set to continue to pollute watercourses and the sea with sewage until huge investment is made into the UK’s network of drains and sewers. Where that investment comes from is a matter for urgent, informed debate and decision-making. It’s also important to note that every home and business should adopt a thoroughly responsible approach to what is flushed into the network, no matter how much is invested in the latter.
1. Sustainable Urban Drainage Systems (SUDS) are a recent innovation that is increasingly required by planners for development approval in order to reduce the rate that stormwater will go into rivers, and thereby reduce flooding from rivers. SUDS provide a reservoir for rainwater that will drain the water very slowly into the ground beneath them and possibly a watercourse nearby. Only when a SUDS reservoir is full will overflow take place on to adjacent ground, into watercourses or possibly into a combined sewer, SUDS thus lower flood risk. Sadly, it is difficult to get anyone take responsibility for maintaining SUDS beyond their initial construction and this has delayed their introduction for many years.
HYBRP encourages construction and retrofitting of SUDS wherever and whenever opportunities exist; our tree planting programme has received Environment Agency funding as part of natural flood management, which complements SUDS and other strategies to reduce flooding by catching the rainfall and lowering the water table. HYBRP volunteers observed SUDS being built on the Redrow Homes development and their overflow outfalls are clearly visible into the City Leat.
Sources:
JDP (pipe supplier) website
Keith Pratley, retired senior Water Company water engineer
Institution of Civil Engineers’ News
Editing and additional text by Nic Howes
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