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. 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

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).)