Overflow performance in Scotland – A different approach

The regulation and implementation of overflow assessment, monitoring and regulation varies between Scotland, England and Wales but the challenges of pollution are still the same across the board. From a Scottish perspective, how will these challenges be addressed?

28 Nov 2021

Scottish Water provides and maintains the sewer network across the whole of Scotland.  This is a diverse area with a large amount of natural land, a heavily populated Central Belt and numerous clustered island communities.  Historically, Scotland has had a focus on outdoor activities with a large industry promoting them. During Covid-19 lockdowns and the subsequent international travel restrictions, the use of waterways for recreational activities and exercise has further increased.

Unlike in England and Wales, where water and sewerage was privatised in the late 1980’s, Scottish Water is a statutory corporation entirely owned by the Scottish Government and regulated by the Scottish Environmental Protection Agency (SEPA). The agency itself is a non-departmental public body of the Scottish Government.

river outflow

Combined Sewer Overflows (CSOs) and Event Duration Monitors (EDMs)

There are approximately 3,600 Combined Sewer Overflows (CSOs) in Scotland, of which around 630 are currently classed as discharging unsatisfactorily. These overflows are licensed by SEPA to discharge to the water environment, for example rivers, under the Water Environment (Controlled Activities Scotland) Regulations.  Licences contain conditions set by SEPA to protect the environment and include overflow settings, storage, screening, event recording and reporting requirements. RPS has been undertaking overflow performance analysis as part of strategic catchment projects and modelling studies, advising Scottish Water and identifying assets where further investigation and intervention is required.

Whilst significant pressure has been placed on water companies in England and Wales to install Event Duration Monitors (EDMs), a similar level of pressure has not been exerted on Scottish Water. SEPA’s Regulatory Method (RM-07) identifies that the majority of licences for overflows do not require permanent flow monitors or spill event monitors to be installed. However, a major focus for monitoring has been conducted at sites that are within 1km of a Bathing Water (BW) with proposed expansion of the programme to within 5km, effectively focussing on high amenity coastal discharges. There are long-term plans to use a 'Scotland Significance Matrix/Event Monitoring' which will outline the level of monitoring provision required, based on a variety of factors including:

  • waterbody amenity classification
  • proximity to Designated Bathing or Shellfish Waters
  • number of spills per annum
  • existing Screening type
  • Network PE & Q95 Dilution

Again, these are a separate set of criteria to those required by the Environment Agency in England and Natural Resources Wales. RPS has however, supported Water companies in England with overflow classification as part of EDM requirements and will be in a position to support Scottish Water in determining asset score in the significance matrix.

This increased focus on network performance stems from customer and environmental impact from CSOs and a perceived increase in Urban Wastewater Treatment Directive (UWwTD) compliance risk as well as the Water Framework Directive. This shows consistency with the public pressure on the water companies in England and Wales, which will only heighten with the potential for a reach of the River Almond targeted for future inland bathing water (IBW) designation. RPS is currently gaining insights working with Severn Trent Water to enhance understanding of the impact on sewerage networks and overflows of an IBW designation. We will therefore be well-placed to support Scottish Water in transitioning catchment performance to meet these more stringent conditions.



So whilst the regulation, requirements and implementation of overflow assessment, monitoring and regulation varies between Scotland, England and Wales, the challenges of pollution are still the same. Subsequently, the requirements for enhanced water quality continue to grow within the public arena and RPS will continue to support Scottish water to meet their performance challenges


Regulatory Method (WAT-RM-07) Sewer Overflows 

Supporting Guidance (WAT-SG-13) Municipal Sewage Treatment Works (STW) Guidance

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If you would like to discuss this topic further and understand how RPS can support you, please contact me anytime.


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