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 1000 EDMs are planned to be installed by 2028 giving circa 40% coverage of all storm overflows.
The EDMs send data back to our telemetry system and alarm centre. The alarms are assessed and responded to by field crews as necessary. An alarm 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 33 designated bathing waters and 10 designated shellfish water protected areas. To date, we have EDMs located in 31 of the 33 designated bathing waters. More recently, we have extended our coverage of EDMs to inland waterbodies and rivers, including Lough Neagh and Lough Erne.
We have 209 storm overflows which spill within 2km of designated bathing waters and 109 storm overflows which spill within 2km of shellfish areas. Under DAERA policy these must operate to different standards than other storm overflows.
As of August 2026, 500 EDMs, including EDMs across bathing waters and shellfish areas are operational and returning information on frequency and duration of spill.
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 2025.
The table below outlines some of the key facts relating to our operational EDMs.
|
2025 |
|
|
Number of EDMs operating and reporting |
500 |
|
Total number of wastewater spills recorded |
12938 |
|
Total duration of wastewater spills recorded (hrs) |
48688.36 |
|
Average number of spills per overflow |
27 |
|
Storm overflows spilling fewer than 10 times per annum |
173 |
|
Storm overflows that did not spill at all in the year |
43 |
The storm overflows with the greatest number of spills are listed in the table below.
|
Asset Name |
Bathing or Shellfish Waterbody |
Bathing or Shellfish Waterbody Name |
2025 spill count |
|
Inisharoan WwPS |
Shellfish Water |
Paddys Point & Reagh Bay |
223 |
|
Mayobridge WwPS |
Not Applicable |
Not Applicable |
198 |
|
Lisburn Road Cranmore CSO |
Not Applicable |
Not Applicable |
168 |
|
Alexander Loop CSO |
Not Applicable |
Not Applicable |
167 |
|
Alfred Street Florence CSO |
Not Applicable |
Not Applicable |
156 |
|
Drumsesk Header Tank Rostrevor |
Shellfish Water |
Carlingford Lough |
146 |
|
Killysuggan CSO |
Not Applicable |
Not Applicable |
141 |
|
Victoria Road CSO |
Not Applicable |
Not Applicable |
136 |
|
Albert Place CSO |
Not Applicable |
Not Applicable |
129 |
|
Fortwilliam Park Dunlambert CSO |
Not Applicable |
Not Applicable |
129 |
The above data highlights frequent spillers without consideration being given to the duration of these events.
The table below displays the storm overflows which are measured to spill for the longest duration (hours) to the environment in 2025.
|
Asset Name |
Bathing or Shellfish Waterbody |
Bathing or Shellfish Waterbody Name |
2025 spill count |
2025 spill duration (hrs) |
|
Mayobridge WwPS |
Not Applicable |
Not Applicable |
198 |
3607 |
|
Inisharoan WwPS |
Shellfish Water |
Paddys Point & Reagh Bay |
223 |
3293 |
|
Alexander Loop CSO |
Not Applicable |
Not Applicable |
167 |
2430 |
|
Drumsesk Header Tank Rostrevor |
Shellfish Water |
Carlingford Lough |
146 |
1564 |
|
Lisburn Road Cranmore CSO |
Not Applicable |
Not Applicable |
168 |
1158 |
|
Drumhaw WwPS |
Not Applicable |
Not Applicable |
87 |
1151 |
|
Millbay WwPS |
Shellfish Water |
Larne Lough |
63 |
839 |
|
Castlewellan 1 WwPS |
Not Applicable |
Not Applicable |
67 |
718 |
|
Killysuggan CSO |
Not Applicable |
Not Applicable |
141 |
684 |
|
Bleach Green WwPS |
Not Applicable |
Not Applicable |
86 |
652 |
The number of spills and duration recorded will vary from year to year, depending on rainfall.
Frequently asked questions and answers
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.
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.
There can be a number of reasons why an EDM may not have been operational for 100% of the reporting period, for example capital/maintenance works affecting EDM operation, communication failures, power failures or sensor failures.
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.
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.
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).)