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. More recently, we have extended our coverage of EDMs to storm overflows that spill into waterbodies which support shellfish growth and production.
We have 153 storm overflows which spill within 2km of designated bathing waters and 111 storm overflows which spill within 2km of shellfish areas. Under DAERA policy these must operate to different standards than other storm overflows.
As of October 2024, 132 EDMs across bathing waters and shellfish areas are operational and returning information on frequency and duration of spill. We are committed to improving our EDM coverage; 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 | 132 |
| Total number of wastewater spills recorded | 3,751 |
| Total duration of wastewater spills recorded (hrs) | 20,636 |
| Average number of spills per overflow | 28 |
| Storm overflows spilling fewer than 10 times per annum | 60 |
| Storm overflows that did not spill at all in the year | 11 |
The storm overflows with the greatest number of spills are listed in the table below.
| Asset Name | Sensitive Waterbody Type | Sensitive Waterbody Name | 2023 spill count |
|---|---|---|---|
| Mill Road Newtownabbey CSO | Shellfish Water | Belfast Lough | 254 |
| Brook Street Holywood CSO | Shellfish Water | Belfast Lough | 148 |
| Tullybrannigan Road CSO | Bathing Water | Newcastle | 146 |
| Stewart Place CSO | Shellfish Water | Belfast Lough | 142 |
| Glenabbey Crescent CSO | Shellfish Water | Belfast Lough | 127 |
| Curran Road Loughview CSO | Bathing Water | Browns Bay | 126 |
| The Diamond Ballycastle CSO | Bathing Water | Ballycastle | 120 |
| Articlave CSO | Bathing Water | Portstewart | 115 |
| Newpark Shore WwPS | Shellfish Water | Larne Lough | 102 |
| Net Walk CSO | Shellfish Water | Skate Rock | 101 |
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 2023.
| Asset Name | Sensitive Waterbody Type | Sensitive Waterbody Name | 2023 spill count | 2023 spill count (hrs) |
|---|---|---|---|---|
| Mill Road Newtownabbey CSO | Shellfish Water | Belfast Lough | 254 | 2954 |
| Articlave CSO | Bathing Water | Portstewart | 115 | 1711 |
| Tullybrannigan Road CSO | Bathing Water | Newcastle | 146 | 1532 |
| Brook Street Holywood CSO | Shellfish Water | Belfast Lough | 148 | 1193 |
| Red Bay WwPS | Bathing Water | Waterfoot | 69 | 887 |
| Net Walk CSO | Shellfish Water | Skate Rock | 101 | 818 |
| Curran Road Loughview CSO | Bathing Water | Browns Bay | 126 | 774 |
| Stewart Place CSO | Shellfish Water | Belfast Lough | 142 | 659 |
| Ballyhaskin WwPS | Bathing Water | Millisle | 61* | 560 |
| Bushfoot WwPS | Bathing Water | Portballintrae Salmon Rock | 82 | 545 |
* additional information has become available for Ballyhaskin WwPS
The number of spills and duration 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.
Are EDMs in operation at all times?
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. EDMs at the following storm overflows have been identified as having interruptions to data availability in the 2023 reporting period:
- Antrim Street Carrick CSO
- Hibernia Street CSO
- Pinewood WwPS
- Woodburn Road Killaloe CSO
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).)