What are Unsatisfactory Combined Sewer Overflows? 

There are a number of possible reasons that a combined sewer overflow (CSO) can be considered Unsatisfactory.
 
Discharges via CSOs can be unsatisfactory due to:

  • Operation causing water quality impacts
  • Operation during dry weather
  • Operation causing aesthetic (sewage related debris) impact
  • Failure to meet licence conditions

Prioritisation of Unsatisfactory CSOs 

Studies have been undertaken to prioritise unsatisfactory sewer overflows for phased improvement as follows and summarised in the table below.

High Priority 
An Unsatisfactory CSO 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 (>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 here

Urban Waters Routemap

Download the Scottish Water Urban Waters Route Map below.

Download here

CSO FAQs

Frequently asked questions about combined sewer overflows.

View here

Medium Priority 

An Unsatisfactory CSOs is Medium Priority if some sewage litter is observed during ground surveys (<1000 items) and multiple, verified environmental complaints/incidents have been recorded.  
 
Low Priority
Some sewage litter is observed during ground surveys (<1000 items) but no environmental complaints/incidents have been verified.

The majority of studies have been completed and any unsatisfactory sewer overflows 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 to 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