Many of the sewer pipes in Wales are combined sewers. This means that they combine both wastewater from our homes and businesses (toilets, sinks, showers, baths, washing machines etc.) with rainwater from roads, gardens and roofs.

Although sewers are no longer built in this way, it’s estimated that about 60% of housing in Wales uses combined sewers.

During heavy or prolonged rainfall the capacity of combined sewers can become overwhelmed. This can cause flooding and has the potential to back up into homes, businesses, roads and streets.  

Storm overflows were developed to act as overflow valves to prevent this from happening. They were designed to operate automatically during periods of heavy rainfall discharging the extra rainwater and wastewater into rivers or seas.

As the operation of storm overflows should only occur during wet weather, when river volumes are high, the wastewater from the ‘combined sewer’ is diluted.

Our role as a regulator

As Wales’s environmental regulator, we recognise the concern about the number of spills and their impact they may be having on our rivers.

We regulate the use of storm overflows and protect the environment by granting permits for them in certain circumstances.  

In Wales there are over 2000 permitted storm overflows on the sewer network.

The permits for storm overflows require wastewater companies to ensure they only discharge during heavy rainfall, when the capacity of the combined sewer has been exceeded due to rainfall and/or snow melt.

In some instances, permits also require that the wastewater is screened prior to discharge to prevent sewage litter such as wet wipes, cotton buds and sanitary wastes entering the environment.

We were the first regulator to require the wastewater companies operating in Wales to install monitors on storm overflows to record storm spills. These are known as Event and Duration Monitors (EDM). The work to install the monitors was completed in March 2020 and we now have data to show how often the discharges happen, and how long for.

We set targets for the two water companies operating in Wales, Dŵr Cymru Welsh Water and Hafren Dyfrdwy to reduce the number of pollution incidents they cause, and every year we publish a report on their performance.

Read our Annual Environmental Performance Assessment Reports

Discharges during dry weather

Unfortunately, discharges do sometimes occur in dry weather. This can be because of sewer blockages and it can cause wastewater to back up and spill from a storm overflow.  

Blockages often occur when household items are flushed down the toilet or poured down the sink, such as wet wipes, sanitary products and cooking fats and oils.

If we are made aware of discharges occurring contrary to permits issued, we can take enforcement action. This can take several forms including a warning, the issue of an enforcement notice or a prosecution. Water companies have previously been prosecuted by NRW and the Environment Agency for allowing unauthorised spills.

Reducing the impact of storm overflows

In July 2022, Minister for Climate Change Julie James convened the Wales Better River Quality Taskforce, to tackle the pollution caused by storm overflows. It brings together Welsh Government, NRW, Dŵr Cymru Welsh Water, Hafren Dyfrdwy and Ofwat, with independent advice from Afonydd Cymru and Consumer Council for Water.

The Taskforce has collaboratively developed action plans to gather greater evidence on the impact of storm overflows on our rivers, to reduce the impacts they cause, to improve regulation and to educate the public on sewer misuse.

View the actions plans on the Welsh Government website.

Through the Taskforce we have refined our regulatory guidance to clearly define the conditions under which an overflow should operate. This puts us in a stronger position to take action when we have evidence of a poorly performing overflow. 

As well as requiring enhanced screening to be installed on overflows, we are also overseeing a programme of investment of £20m by Dwr Cymru Welsh Water and Hafren Dyfrdwy to further reduce the impact of storm overflows, prioritising high spillers.

You can read more about our aims and ambitions to reduce the impact of storm overflows in our annual storm overflow spill data report.

Unpermitted storm overflows

We have put a programme in place to ensure that all unpermitted storm overflows that have been identified in Wales are brought within our regulatory framework.

In this way we will assess whether there is an environmental impact from the discharges and require the appropriate investment to deliver improvements to the wastewater infrastructure.

Everyone can play their part

Everyone connected to a sewer can play their part in reducing spills from storm overflows.

Flushing wet wipes, sanitary products and cotton buds can contribute to blockages in sewers, causing unnecessary spills. They should be disposed of in a bin, not down the toilet. Fats, oils and grease should not be washed down sinks either as they stick to the inside of sewer pipes causing ‘fat bergs’ and leading to blockages.

You can also check to ensure that the pipes from kitchens and bathrooms are properly connected to the foul sewer not the surface water drain. Some areas have separate drains, and properties which are incorrectly plumbed are known as ‘misconnections’. This means that all the dirty water from toilets, sinks and washing machines goes directly into a river or stream instead of to a wastewater treatment works. 

If you spot a storm overflow operating in dry weather you can report it to us on our incident hotline on 0300 065 3000 24 hours a day. You can also report it online.

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