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Storm Overflows

Storm Overflow Outlets

NI Water believes that sharing more information in regard to our wastewater system and the effect that it has on the water quality of our rivers, loughs and seas, will help improve accountability, grow understanding and inform decision making about the investments we need to make as a society to protect nature and enable economic development.

The map below provides detail on the outlet locations of both our storm overflows and wastewater treatment works. It allows you to zoom in and out using the icons in the top left of the map image. By zooming in you will see the locations of the outlets. By clicking your mouse arrow on one of the coloured location dots, a box will appear that will provide you with the name of both the asset and the receiving water body. It will also provide the location co-ordinates for the outlet. This information is being provided as part of an approach which is designed to explain more about our wastewater system.

In this regard we have also developed a short explanation of what a Storm Overflow is, how and when it works. This is located on the top left of this web page.

NI Water strongly advises that you do not visit Storm Overflow sites which are often hazardous. Please put the safety of yourself and others first by following this guidance.

NI Water’s wastewater treatment works are facilities that process wastewater so that it can be returned to the environment safely via an outlet to a water body.

Storm overflows are designed to release excess storm water when the capacity of the sewer network prior to the wastewater treatment works is overwhelmed. Without storm overflows many more properties would to be flooded with wastewater. Storm overflows are therefore essential to the sewage system.

To monitor and record spills from storm overflows, Event Duration Monitors (EDMs) are being installed across NI Water’s wastewater network in a multi-year investment programme. These measure the times when a spill is occurring and for how long.

NI Water is committed to ensuring that storm overflows are compliant with the standards that are set by the NI Environment Agency. This requires appropriate funding.

To help explain more fully how our wastewater system operates, the challenges it faces due to a legacy of underfunding and what is now being addressed following the decision by the Utility Regulator to approve investment plans for the current price control we have developed a document entitled “Northern Ireland’s wastewater system”. This, together with a link to information about the modelled performance of storm overflows in the wastewater system, is also located in the top left of this web page.

It is hoped this web page is helpful in answering questions that you may have.

How do I know if a Storm Overflow is operating?

Storm overflows are designed to release excess storm water when the capacity of the sewer system is overwhelmed. Signs of water flowing out of drainage pipes during or after heavy rainfall can indicate that a Storm Overflow is in operation.

What should I do if I see a storm overflow operating?

NI Water is providing the locations of its storm overflows as part of an approach that is designed to share more information about our wastewater system. These details are not provided to encourage people to visit these assets which are often hazardous because of their siting.

If you see water coming out of a storm overflow it is most likely that the storm overflow is operating as designed, particularly if during or after heavy rainfall. Remember the safety of yourself and others must always be the top priority. NI Water strongly cautions that you do not enter a water body during or after a period of heavy rain.

If you have concerns that an overflow is not operating correctly or need further advice, please contact NI Water by email to waterline[AT]niwater[DOT]com or by using the Web Chat icon on screen.

Does the spillage of wastewater from Storm Overflows impact my drinking water quality?

NI Water’s number one priority is the quality and safety of your drinking water. We can assure our customers that the water supplied from all our water treatment works is safe to drink and use as normal.

On a daily basis, we monitor our raw water intakes from all sources, at our treatment works and at customer taps to ensure that drinking water supplied meets strict quality standards.

NI Water has a robust testing and sampling system which sees over 120,000 samples lifted and analysed each year. Sampling and analysis are carried out 365 days per year. Samples are taken from customer homes, reservoirs and treatment plants.

Is it safe to swim in water after there has been heavy rain?

Poor water quality can result after periods of heavy rainfall, which is why the Department of Agriculture, Environment and Rural Affairs (DAERA) advise not to bathe during heavy rainfall or for up to 48 hours after heavy rainfall. There are two main sources contributing to poor water quality at these times, farm runoff and spills from stormwater overflows.

Further information can be found on DAERA’s website under Bathing Water FAQs.

Why do we have storm overflows?

Most modern property developments now have separate drainage systems where foul water from kitchens and bathrooms goes to the wastewater system (sewer) while rainwater from roofs and driveways goes to a separate storm drain and off into a nearby watercourse.

However, in common with many other places, most homes and businesses in Northern Ireland are served by what is called a combined sewer system. This means foul water (including sewage) from within homes and businesses is mixed with rainwater as it makes its way to our treatment works. During times of heavy rainfall, large volumes of this storm water will enter the system and cause it to become overwhelmed in places.

Storm overflows exist so wastewater can be released to designated waterways if the system is at risk of being overloaded during times of heavy rainfall. Without storm overflows many more properties would be flooded with wastewater.

When do they operate?

Storm overflows primarily operate as a form of pressure relief valve during times of heavy rainfall. Whilst storm overflows exist, some may not operate at all for many months whereas others might operate routinely when it rains.

Storm overflows can also spill during periods of sustained rainfall when the storage within the system is not sufficient to manage the volume of water arriving at the location.

Are storm overflows monitored?

NI Water carries out manual inspections of storm overflows as part of our maintenance and monitoring activities. These inspections are conducted to assess the condition and performance of the asset. This includes checking for any signs of blockage or malfunction.

To monitor and record spills, Event Duration Monitors (EDMs) are being installed across NI Water’s wastewater network over a multi-year investment programme. These measure the times when a spill is occurring and for how long.

The first phase of our deployment, as agreed with the NI Environment Agency (NIEA), is focused on discharges to bathing waters and shell fisheries. By June 2024 we will have many EDMs fully installed and confirmed to be sending reliable information back to our Alarm Management Centre.

By the end of this PC21 Price Control period, (ending spring 2027), we aim to have over 700 EDMs deployed for an investment of around £20m. In preparing to submit our business plan for the next PC27 Price Control covering the period 2027 to 2033, we will engage with NIEA, the Utility Regulator and the Department for Infrastructure (DfI) to make the funding case for EDM installation at all storm overflows.

How much raw sewage does NI Water put into our waterways each year?

The quantity of untreated sewage contained within wastewater that spills from a storm overflow during heavy rainfall depends on several factors, such as the volume and intensity of the rainfall, the capacity and design of the local sewer system, and the amount and type of pollutants in the sewage. It is therefore difficult to give a precise or universal answer to the question of how much raw sewage is contained in the wastewater that is discharged.

We know that in general during periods of heavy rainfall, the sewage discharged is heavily diluted and will be further diluted when it enters adjacent water bodies which are themselves also receiving large volumes of additional rainwater. We estimate that around 1-2% of what is spilt is raw sewage.

We are making the results of our drainage modelling available on our Storm Overflow web page Storm Overflows in the first half of 2024. This predicts the volumes of waste water spilled to help inform investment decisions.

Why can’t storm overflows be removed?

If storm overflows were simply removed, then homes and businesses would be flooded with wastewater at times of heavy rain.

The engineering works to create an entirely separate rainwater drainage system would cause a huge amount of disruption for many decades and require billions of pounds of investment to remove all storm overflows. This is deemed unaffordable and impractical for our society.

Storm overflows therefore remain a necessary part of our system. Nevertheless, NI Water is committed to ensuring that they are compliant with the standards that are set by the NI Environment Agency (NIEA) to ensure that nature is protected. This requires appropriate funding.

How are unsatisfactory storm overflows going to be addressed?

Primarily we need solutions in place to deal with the excess volume of wastewater during times of heavy rainfall. The most effective solution is to stop storm water getting into and overwhelming our combined sewer system by separating and diverting it elsewhere. The Living with Water Programme for Belfast includes examples of the use natural drainage to relieve pressure on the sewer network. The installation of temporary storage for storm water and screens to remove visible material also help to meet the standard.

People can also assist by avoiding putting items down the drain that cause the sewer to block or restrict the flow. Wet wipes as well as fat and oil are particularly problematic.

How much will it cost to turn all unsatisfactory storm overflows back to satisfactory status?

Our assessment is on-going but NI Water’s very early estimate is that this could cost between £3 - £4billion at today’s prices. It is estimated that a further £3 billion could be required to bring these up to the new standards that are now being adopted in England. This quantum of investment raises major questions of affordability. These will be considered by the Utility Regulator in assessing NI Water’s business plan for the next Price Control (2027-2033).

Why do storm overflows affect the ability to connect customers to the wastewater system?

Connecting new customers will increase sewage in the system. If a storm overflow is already failing the consent level set by the NI Environment Agency (NIEA), then this additional connection brings the risk of causing increased harm to the receiving water. Careful assessment is therefore needed for each connection on a case-by-case basis.

How accurate are the outlet location co-ordinates provided by NI Water?

Gathering and retaining 100% accurate information on the many thousands of assets that NI Water owns is challenging, particularly when they belong to a category that is not routinely visited for maintenance. NI Water believes that sharing the location data held on our systems is an important step in providing more information about our wastewater system even though there will be some inaccuracy in the location co-ordinates provided. NI Water welcomes engagement with any stakeholder who identifies where there is opportunity to improve the location co-ordinates for any outlet.

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