The sewage system explained - part two - combined sewage system

​Here is part two of our blog trilogy from Rob Hellawell, aka #urbanpollutionhunter. In part one he talked about the two different sewage systems; combined sewage and separate sewage. In part two he will go into more detail about the problems with combined sewage systems, problems that have regularly made the news in recent months.

A handful of sewage litter.

As discussed previously, we have two types of sewer systems in the UK, combined sewer systems and separate sewer systems. Here we look at how sewage can get into our rivers from combined sewer systems. ​

Combined Sewer Systems are designed to discharge sewage via Combined Sewer Overflows or CSOs as they are often referred to. They act as a safety valve, releasing storm water containing sewage into our rivers in times of rain. They are only allowed to discharge in this way when the volume of flow in the combined sewer reaches an agreed rate. This is usually during, or following, heavy rainfall. The precise details are given in the Discharge Consent Permit issued by the Environment Agency (EA) for each CSO. All Discharge Consent Permits are held by the EA and can be obtained free of charge by the public. ​

Permission to discharge effluent to a watercourse is needed by businesses and private individuals not just water companies, such as Yorkshire Water. If the discharge occurs within the limits set out in the discharge consent permit, then the discharge is said to be a “permitted discharge”. This is not considered to be pollution by the EA.  ​

Pollution is said to occur when a CSO discharges outside the agreed limits of its permit. In this case, it will discharge well before the permitted, sewer flow rates in the discharge consent permit. So, if it is not raining now, and it has also been dry for a few hours, then there should be no sewage escaping from any CSO. Sewage escapes from CSOs in dry weather mean that there is a problem in the sewer system. Discharges from CSOs during dry weather conditions is considered to be pollution by the EA. ​

CSO discharge in dry weather – River Aire Baildon.

The sewage discharges which have made the news recently are largely from CSOs that serve as overflows to Wastewater Treatment Works (WWTW). Here the concentration and sheer volume of sewage discharged can have a detrimental impact on the health of our rivers. An unsightly aspect of CSOs is the substantial amounts of sewage litter which they discharge into our rivers. Much of this is plastic based such as wet wipes, nappies and other sanitary products, adding to our growing problem of plastic pollution in our seas. It is a sobering thought that anything which we flush down the toilet has a chance of ending up in your nearest beck or river. Sewage litter caught along the banks of our rivers is evidence of CSO discharges nearby. ​

The sewage litter along Bradford Beck, Shipley.

Only the “Three Ps” should be flushed down your toilet! These are pee, poo and paper. Sadly, not everyone follows this guidance, and everyday all kinds of debris are flushed down our toilets that can lead to blockages in sewers. Pollution of our rivers can then follow as the CSO upstream of the blockage does its job and discharges sewage into the river instead of that sewage backing up and spilling out in our homes and streets.  ​

Hard times for Peppa Pig! – Caught in a wet wipe blockage in foul sewer at Shipley.

You can find out if you have a CSO, or other permitted discharge, near you by looking on the Rivers Trust sewage map which can be found on the Rivers Trust website. 
https://www.theriverstrust.org/key-issues/sewage-in-rivers
In the next instalment we will look at pollution to our rivers from separate sewer systems and finally learn what a misconnection is. 
This blog originally appeared on the River Worth Friends website.