Scottish Water has set out its plans to install sensors on a number of its 3,614 Combined Sewer Overflows (CSOs) within its 50,000km of sewer network in Scotland.

 Image: CSO on the River Tay in Scotland

The water company intends to install monitoring on approximately 1,000 network and treatment works CSOs discharging to the highest priority waters, including all designated shellfish and bathing waters.

According to Scottish Water, the improved monitoring activities, which are currently estimated to cost around £50 million to £70 million to deliver, will improve its understanding of how they are operating and will provide transparent information on their performance.

The utility will also examine the costs and benefits of extending monitor coverage to lower priority locations and to where spills are not predicted to be occurring. For around 2,600 CSOs where there is low predicted spill frequency, low amenity and an absence of evidenced environmental impacts, Scottish Water anticipates there will be little benefit from monitoring.

Spill trigger levels will be used to drive investigations at monitored network CSOs to determine cause, scale of need and scope for improvement, together with increased reporting of spill data from monitored CSOs.

By December 2022, Scottish Water will publish spill data annually for monitored CSOs where data are already currently reported to the Scottish Environmental Protection Agency (SEPA) which identify where possible the main reason for the spills (e.g. heavy rain, blockages or a flow issue at the treatment works).

Beyond that date, by December 2023 the company will publish spill data annually for all other monitored CSOs and by December 2024, publish near real-time spill data for all monitored CSOs.

Scottish Water also intends to develop a prioritisation framework to consider the need for increased capacity at treatment works where monitors identify that spills are occurring prior to meetinglicensed hydraulic capacity.

In addition, the utility is also planning to deliver intelligent wastewater networks in 3 catchments (East Calder, Erskine and Lossiemouth) to expand its intelligence on network behaviour during dry weather and during rainfall and develop a plan for further roll-out of this approach.

A network of sensors will be installed to provide data on sewer network performance within pilot catchments, transmitting data to Scottish Water’s Intelligent Control Centre. The water company said that data analytics will generate near real time insights that will allow staff to take a rapid, proactive response to avoid blockages. The insights will also contribute to an improved understanding of how the sewer network behaves under different rainfall events, including which conditions lead to spills.

Scottish Water also proposes to continue the prioritisation of around 630 CSOs currently causing sewer related debris (SRD problems) by reviewing available performance information and undertaking surveys at all CSOsmodelled to spill more than once in five years and/or, for monitored CSOs, thoseexceeding spill frequency triggers.

The utility will develop solutions for those CSOs at 85 locations that are already confirmed as being high priority having significant SRD impacts on rivers.

Subject to by the investment planning and prioritisation group (IPPG) that oversees the development of the rolling Scottish Water investment programme, Scottish Water will then agree delivery timetables for the high priority CSOs (currently estimated to cost around£100 million - £130 million) with the aim of delivering these CSO improvements by December 2027.

The plans are contained in Scottish Water’s Urban Waters Route Map – beyond 2027 the utility will develop solutions for all its medium priority CSOs at around 150 locations. Delivery timetables for these, currently estimated to cost around £150 million to £200 million, will be promoted as a priority investment - subject to approval, Scottish Water will plan to deliver improvements to these CSOs by December 2031.

While screens are fitted to many CSOs where frequent spills occur, around 80% of CSOs do not have a screen and have the potential to spill during rainfall causing sewage litter in the watercourse. According to the water company, further improvements in screening will be required as spill frequencies increase.

The actions outlined in the Route Map have significant investment demands and not all are currently set out in Scottish Water’s investment planning scenarios.

Click here to download Scottish Water’s Urban Waters Route Map – beyond 2027