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.

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.

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.

Poor bathing water quality can result after periods of heavy rainfall, which is why the Department of Agriculture, Environment and Rural Affairs (DAERA) advises 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 run-off and spills from stormwater overflows.

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

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 older 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.

Storm overflows primarily operate as a form of pressure relief valve during times of heavy rainfall to protect homes and businesses from flooding caused by wastewater backing up in pipes. 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.

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 locations near to shellfish breeding grounds.

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. This will give us around 30% coverage of all storm overflows. 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.

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.

You can view the results of our drainage modelling spills here. This predicts the volumes of waste water spilled to help inform investment decisions.

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.

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 of 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 toilet or drain that cause the sewer to block or restrict the flow. Wet wipes, as well as fat and oil, are particularly problematic.

Our assessment is ongoing but NI Water’s very early estimate is that this could cost between £3 - £4 billion 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 period (2027-2033).

Connecting new properties will increase sewage in the system. If a storm overflow is already failing the consent level set by the NI Environment Agency (NIEA), then these additional connections bring the risk of causing increased harm to the receiving waters. Careful assessment is therefore needed for each connection on a case-by-case basis. As such, NI Water strongly urges developers to engage with us early in their design via the Pre-Development Enquiry process.

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[AG1] . 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.

EDMs, which are being deployed by UK water utilities, do not provide volumes spilled; they provide evidence of spill frequency and duration. The data from EDMs is crucial in identifying potential sources of contamination and the measures to prevent future incidents.

NI Water endorses the assessment made by Water UK that the cost to install EDMs across the entire network is already significant and the investment required to upgrade this technology to monitor spillage volumes would be more effectively spent fixing the problems, rather than improving their measurement. There may be exceptions to this rule, depending on particular circumstances, and NI Water will remain open to these and alert to the application of any lower cost new technologies that may help.

NI Water supports the view that our collective environmental focus should be on better understanding the quality of our receiving water bodies and determining appropriate action plans to encourage nature to thrive. In so doing, we believe that it is important that we must own our part and manage our assets so that they are demonstrably compliant with NIEA’s standards.

Our drainage modelling does forecast the volume of wastewater that spills in an average rainfall year up to 18 million m³ (cubic metres), yet we believe it is more realistic to consider this to be a range of 16-20 million m³ to reflect limitations in the simulation. We estimate that around 80-90% of all wastewater that comes into our system is processed through our treatment works and not spilled.

When deciding where to invest, the volume of spillage is important but is not the only factor that needs to be considered; for example, the volume of water in the receiving water body and its status also are relevant.

The Urban Waste Water Treatment Regulations (Northern Ireland) 2007 (UWWTR) set out the minimum EU standards for both wastewater networks and treatment plants. At the time this was enacted, it was recognised that a legacy of underfunding in Northern Ireland’s wastewater system meant that it was not possible to fully comply with the legislation. Guidance was therefore provided to NI Water that recognised that the investment backlog would have to be addressed before the standards could be met and stipulated that no detriment to our aquatic environment should be incurred in the interim.