Event Duration Monitors (EDMs) are being installed across our wastewater network to monitor and record discharges (commonly known as spills) from storm overflows. These measure the times when a spill is occurring (count) and for how long (duration).
As well as enabling our teams to respond to alerts, EDMs also increase our understanding of wastewater spills and enable the better targeting of investment to improve the performance of our wastewater network.
Around 700 EDMs are planned to be operational by 2027 giving circa 30% coverage of all storm overflows.
The EDMs send data back to our alarm centre. The alerts are assessed and responded to by field crews as necessary. An alert may, for example, suggest a blockage or collapse has occurred and needs to be cleared or repaired. Or it may be the network is operating normally with the spill being caused by heavy rain.
Progress
We are taking a phased approach to EDM installation. Our initial focus has been to install EDMs on storm overflows which spill into Northern Ireland’s 26 designated bathing waters. To date, we have EDMs located in 22 of the 26 designated bathing waters.
We have 153 storm overflows which spill within 2km of these designated bathing waters. Under DAERA policy these must operate to different standards than other storm overflows.
As of the end of May 2024, 82 EDMs are operational and returning information on frequency of spill. We are committed to improving our EDM coverage of bathing waters; by summer 2025 we anticipate EDMs at most designated bathing and shellfish waters.
Results
The storm overflow map shows where operational EDMs are monitoring the outlets of storm overflows. By clicking on each pin, you can see a summary of data for the 12-month period ending 31 December 2023.
The table below outlines some of the key facts relating to our operational EDMs.
| 2023 | |
|---|---|
| Number of EDMs operating | 82 |
| Total number of wastewater spills recorded | 1,785 |
| Average number of spills per overflow | 22 |
| Storm overflows spilling fewer than 10 times per annum | 36 |
| Storm overflows that did not spill at all in the year | 7 |
Although we have not yet shared the spill duration data referred to in the table above, we will do so before the end of the summer.
The storm overflows with the greatest number of spills into bathing water beaches are listed in the table below.
| Asset Name | Bathing Beach | 2023 spill count |
|---|---|---|
| Tullybrannigan Road CSO | Newcastle | 146 |
| Curran Road Loughview CSO | Browns Bay | 126 |
| The Diamond Ballycastle CSO | Ballycastle | 120 |
| Articlave CSO | Portstewart | 115 |
| Sandy Bay WwPS | Browns Bay | 86 |
| Bushfoot WwPS | Portballintrae Salmon Rock | 82 |
| Bonamargy WwPS | Ballycastle | 70 |
| Red Bay WwPS | Waterfoot | 69 |
| Orlock Sandeel WwPS | Groomsport | 68 |
| Ballyhaskin WwPS | Millisle | 55 |
The above data highlights frequent spillers without consideration being given to the duration of these events. Once the duration data becomes available this information will be used to populate a further table which will provide greater insight into the performance of Storm Overflows.
The number of spills recorded will vary from year to year. 2023 was the third wettest on record in Northern Ireland, with 1,399mm of rain reported by the BBC using Met Office data. Record rainfall means our storm overflows have been spilling more frequently than we would anticipate.
Frequently asked questions and answers
What is an Event Duration Monitor (EDM)?
To build upon the manual inspections that NI Water carries out on storm overflows as part of our maintenance activities, including checks for any signs of blockage or malfunction, installing Event Duration Monitors (EDMs) will enable us to monitor and record spills in real time. 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 it lasts. Having EDMs gives us telemetry data that can be used to supplement the results of our models. They can also be used to trigger operational response if abnormal spills are occurring such as a blockage or equipment failure.
What method is used to count the number of spills?
The annual spill frequency reported from the models is counted using the industry standard 12/24 counting method which works as follows:
- Start counting when the first discharge occurs.
- Any discharge (or discharges) in the first 12-hour block is counted as one spill.
- Any discharge (or discharges) in the next and subsequent 24-hour blocks are each counted as one additional spill per block.
- Continue counting until there is a 24-hour block with no discharge.
- For the next discharge after the 24-hour block with no discharge, begin again with the 12-hour and 24-hour block spill counting sequence.
When will information be available on duration of spills for the operational EDMs?
We anticipate information will be available on duration of spill at operational EDMs by the end of the summer.
Can NI Water quantify how EDMs help to reduce storm overflow spills?
It is too early for us to measure this, but other water utilities are reporting that improved operational response guided by EDMs has in some cases reduced spills in the range of 10 to 15% depending on specific localised circumstances.
Is NI Water deploying flow meter technology to record the volumes being spilled?
NI Water is following 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 spill volumes would be more effectively spent fixing the problems rather than improving their measurement.
Currently, the volume of spill is assessed using the results of Drainage Area Models.
Who is responsible for measuring and assessing water quality of receiving waterbodies?
This is the responsibility of NIEA and is set out in the River Basin Management Plans (River basin management | Department of Agriculture, Environment and Rural Affairs (daera-ni.gov.uk).)