Combined Sewers Overflows (CSOs) Explained:

One of the most important things we do is protect public health by taking away waste water from homes, businesses and communities to be treated and returned safely to the environment. Waste water can include waste from toilets, sinks and showers and rainwater from roofs and gardens.
Most of our sewer pipes are ‘combined sewers’. This is usually one single pipe that combines both waste water from our homes and businesses (toilets, sinks, showers, baths etc) and surface water from roofs and gutters. 


When our sewer system is operating normally waste water gets taken to our treatment works, where it is cleaned, treated and returned safely to watercourses and the sea. 

During heavy rainfall storms, more water is getting into these pipes than they can cope with, so they have been designed with a relief mechanism to safely relieve the pressure on the network. These relief mechanisms are known as Combined Sewer Overflows, or CSOs. Storm water is released by an overflow pipe, which spills at designated points, into watercourses or the sea.


During heavy rainfall, we have to protect our customers and their homes and businesses from sewer flooding so this CSO ‘release’ system is vital. Without it the sewer network would back up, flooding homes, businesses, roads and open spaces with diluted sewage and rainwater, from manholes, drains and toilets.

Usually, this only happens when there’s been a heavy rain downpour or prolonged wet weather, so any water that is released into rivers or the sea is unlikely to cause environmental damage.

We have permission for sewers to overflow during heavy rainfall via environmental licences from our environmental regulators, Scottish Environment Protection Agency (SEPA).  

All discharges into water courses must comply with quality standards set in these licences. The levels of untreated sewage in storm water that is discharged is very dilute so is unlikely to cause harm to the environment.  SEPA monitor and take samples from the watercourses to understand if there is any impact to the environment.

What can impact the environment from CSOs discharging, is when there are items which have wrongly been flushed down the toilet mixed in with the storm water. Items commonly flushed down the toilet are wet wipes, cotton buds and female sanitary products. Items commonly put down drains are fats, oils and grease. These items can make their way into rivers, burns and coastal waters from CSOs, however this is easily preventable by only ever flushing the 3Ps (pee, poo and toilet paper). More advice here: Scottish Water Cycle