Case Study

Environment Agency definition of 12/24 spill counting

Improving knowledge on sewer spills

Water company reports to Regulators now include data on sewer spill frequency and duration. The monitoring system, Event Duration Monitoring (EDM), was extended significantly during the recent business planning period. It was found that configuration and data handling has been inconsistent. As a result there have been difficulties in comparing spill performance both between locations and against current regulations.

To address this, UKWIR commissioned the project Sewer Analytics - Building on Event Duration Monitoring Expansion. The project has been undertaken by Servelec Technologies as the main contractor with support on modelling aspects from Margaret Williams of RPS.

At the project dissemination workshop, UKWIR Programme Lead, Alex Mortlock of Severn Trent Water, said that "we need to build confidence that EDM data reported by the industry to Regulators can be compared on an ‘apples with apples’ basis".


This means consistency in comparison between combined sewer overflows (CSOs), and between companies. He added that ‘this is not an exercise in dumbing down or fudging data. Higher frequency of reporting is likely to capture more spill events. This is fundamentally a good thing and results in better operational decisions’.

Servelec's George Heywood explained to the audience the concept of '12/24 spill counting', which was developed by the Environment Agency and is fundamental to spillage reporting. The process, illustrated in opposite, is as follows:

• spill counting starts when first discharge occurs

• any discharge(s) in the first 12 hour block is counted as 1 spill

• any discharge(s) in the next and subsequent 24 hour blocks are each counted as 1 additional spill per block

• this counting continues until there is a 24 hour block with no discharge.

• for the next discharge after 24 hour block with no discharge, the sequence begins again.

The Environment Agency has a matrix (see image below) setting out where monitoring is required, depending on the amenity class of the receiving water and the frequency of spills.

Matrix for determining monitoring requirements


A literature survey of configuration of collection and analysis parameters from outside the UK unearthed very little information on methods of counting, suggesting that the UK was in the forefront of work in this area.

All the water and sewerage companies in the UK replied to a questionnaire which found that there are wide variations in the numbers of storm overflows and variation between companies in monitoring coverage.

Margaret Williams took the audience through how to produce statistical estimates of spill frequency for unmodelled sewerage catchments.

There followed a series of detailed statistical analyses which provided a number of important recommendations including a proposed addition to the12/24 spill counting definition.

This project complements the work of the 21st Century Drainage Programme, a partnership of more than 40 organisations led by Water UK, which aims to identify the major risks to drainage in the future and provide options for how these risks could be addressed.

The 21st Century Drainage Programme has developed a series of tools and approaches to enable companies to target investments more effectively and provide customers and stakeholders with better information about the UK’s drainage and wastewater services.

One of these tools, which this UKWIR project complements, is a framework for valuing the benefits of further improvements to storm overflows, as part of a new ‘Storm Overflow Assessment Framework’.