Overflow performance in Scotland – A different approach

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.

Summary

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

References

Regulatory Method (WAT-RM-07) Sewer Overflows 

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