Many of the sewer pipes in Wales are ‘combined sewers’ This means that they usually combine both wastewater from our homes and businesses (toilets, sinks, showers, baths etc.) and clean rainwater from roads, hardstanding areas and roofs.

During heavy rainfall the capacity of these pipes can be exceeded, which means possible flooding of sewage works affecting the treatment process, and the potential to back up and flood peoples’ homes, businesses, roads and open spaces, unless it is allowed to spill elsewhere.

This is why storm overflows (often referred to as Combined Sewer Overflows) were developed to act as overflow valves to reduce the risk of sewage backing up during heavy rainfall.

Combined Storm Overflows (CSO’s) are used during periods of heavy rain fall to help protect properties from flooding and prevent sewage from overflowing into our streets and homes.

As discharges of storm sewage only occur during wet weather, the sewage in the sewers is diluted and the rivers have high volumes of water in them. This ensures that the impact of storm sewage entering the watercourse are minimised.

Our role as a regulator

We issue permits for storm overflows which require water companies to ensure they only discharge during rainfall, when the capacity of the sewer has been exceeded due to rainfall and/or snow melt.

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

We have required the water companies in Wales to install monitors on storm overflows to record storm spills, these are referred to as Event and Duration Monitors (EDM). The work to install the monitors completed in March 2020. These monitors mean we now have data to show how often the discharges happen, and how long for.

When we receive a report of a pollution incident from a storm overflow, we carry out an investigation and take action if an offence or pollution to the environment occurs.

We set targets for DCWW and Hafren Dyfrdwy to reduce the number of pollution incidents they cause, and every year we publish their performance

Read our Annual Environmental Performance Assessment Reports.

Storm Overflow discharges during dry weather

Unfortunately, discharges do sometimes occur in dry weather because of sewer blockages, where the pipe is blocked and causes sewage to back up and spill from a storm overflow. As the discharge would not be diluted with rainwater it can cause an environmental impact.

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.

Where and when a pollution occurs NRW will assess the event and uses its enforcement powers appropriately.

How often do storm overflows occur in Wales?

This table shows the number of spills recorded per year and the number of permitted overflows in Wales with EDM installed.

  2020 2019 2018 2017 2016
No. of spills recorded by EDM 105,751 73,517 48,499 29,878 14,485
Number of storm overflows with EDM installed 2041 1665 1359 983 545

NOTE Re: Table:

  • The increase in spills year on year is the result of an increased number of event duration monitors (EDM) being installed, and subsequent available data reported.
  • We are aware that DCWW and HD are working to improve the reliability of data reported via EDM. At the end of March 2020, EDM installation was completed on all storm overflows.
  • Both water companies are required to provide a summary of spill data from their monitors to NRW on an annual basis. The information is also reported on their websites.

What are we doing to reduce the impacts of storm overflows?

We are reviewing our approach to storm overflows in Wales, in terms of permitting and regulation of existing discharges, as well as our ambition for the future.

EDM data is being used to inform development of a DCWW programme of targeted investment of over £20m aimed at reducing environmental impacts from overflows identified as ‘high’ spillers (more than 40 annual spills).

NRW is working closely with DCWW on this programme to ensure investment is targeted appropriately. We are also requiring water companies in Wales to target investment at increasing capacity and storage of wastewater in the sewerage network, and to carry out additional monitoring so that we can better understand issues that arise and wherever necessary carry out appropriate regulation

Currently, there is a presumption against permitting new storm overflows, unless as part of a wider scheme delivering a net improvement in water quality.

What are we doing about 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. It is vital that nothing inappropriate is put down the sewer as this often causes blockages leading to discharges from storm overflows on dry days. Wet wipes often contribute to blockages in sewers and 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 leading to blockages.

Climate change, together with a growing population and the paving over of green spaces is resulting in additional pressure on the sewerage network, and we are working to ensure that this does not result in increased spills to the environment.

We are working with Welsh Government, OFWAT, the Consumer Council for Water as well as the two water companies that operate in Wales to improve how we address the environmental impacts from storm overflows in Wales.

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