Prioritisation of Overflows in Scotland
Scottish Water has more than 4080 overflows on its network and around 3200 of these have been assessed as satisfactory.
Around 900 overflows have been assessed as unsatisfactory and these are the overflows which are considered against our prioritisation for investment criteria as outlined below.
What are Unsatisfactory Overflows?
There are a number of possible reasons that an overflow can be considered Unsatisfactory.
These might include
- Operation causing water quality impacts
- Operation of a sewer overflow during dry weather
- Operation causing aesthetic (sewage related debris) impact
- Failure to meet licence conditions
Prioritisation of Unsatisfactory Overflows
Studies have been undertaken to prioritise unsatisfactory overflows for phased improvement as follows and summarised in the table below.
High Priority
An unsatisfactory overflow is High Priority if it:
- spills in dry weather; and/or
- causes or significantly contributes to failures in water quality standards, as identified in the River Basin Management Plan; and/or
- has very significant quantities of sewage litter observed during ground surveys (more than 1000 items); and/or
- a number of environmental complaints/incidents have been recorded (3 or more) and the presence of sewage litter has been confirmed.
Improving Urban Waters High Priority Assets
Download this document to view 108 high priority assets that have been identified through study and subsequently prioritised for improvement
Download hereMedium Priority
An unsatisfactory overflow is Medium Priority if some sewage litter is observed during ground surveys (less than 1000 items) and multiple, verified environmental complaints/incidents have been recorded.
Low Priority
Some sewage litter is observed during ground surveys (less than 1000 items), but no environmental complaints/incidents have been verified.
The majority of studies have been completed, and any unsatisfactory overflows have been prioritised. It is expected that there will always be a small amount of study work ongoing.
Satisfactory Design
Note that sewer overflows can be considered to be ‘satisfactory’ in their design but can still occasionally cause pollution due to blockages elsewhere in the sewer network. These blockages can be caused by inappropriate customer products being disposed of in the sewer, sewer blockage, collapse, or sediment build-up. For information about what not to flush down the toilet, check out our Nature Calls campaign here.
Table 1 – Summary of prioritisation criteria
|
Criteria |
Ground survey (items of litter observed) |
Verified complaints or incidents |
Indicative Timescale |
|||
|
|
>1000 |
1-1000 |
1-2 |
>3 |
Identify solutions |
Deliver solutions |
|
Causing water quality impacts |
High |
High |
High |
High |
2024 |
2027* |
|
Spills in dry weather |
High |
High |
High |
High |
2024 |
2031* |
|
Sewage-related debris |
High |
|
|
High |
2024 |
2031* |
|
|
|
Medium (both criteria met) |
|
2027* |
2031* |
|
*Timescales are subject to Scottish Water investment prioritisation and planning
March 2026 Update.
Since publishing the Improving Urban Waters Routemap in December 2021, we have been working hard to develop the solutions required to address 108 High Priority UID needs with an aim to deliver solutions in line with the indicative dates in Table 1 (summary of prioritisation criteria). We have also begun looking at a wider list of Medium Priority UIDs and some others where there are hydraulic links between overflows.
For some High Priority UIDs, particularly in heavily connected and highly urbanised areas, we have found that more extensive work is required to both understand the problems we see and how to solve these, and, therefore, solutions will take longer to develop and deliver.
We have been continuing work with SEPA to identify and reprioritise UIDs and are using new information to ensure that we can maintain good progress on delivering improvements in line with our initial expectations. The Improving Urban Waters High Priority Assets shows the intended delivery schedule for the 145 UIDs currently in development, with 97 solutions predicted to be completed before December 2027.
