Improving urban waters

Our vision is for towns and cities to use nature-based, blue-green solutions to absorb and safely convey rainwater.

This will:

  • help strengthen their resilience to the intense downpours they face under climate change
  • minimise the risk of polluting sewage spills by keeping rainwater out of sewers
  • create fantastic places for people to live and work

Urban wastewaters

Over the past 20 years, SEPA has worked with Scottish Water to deliver significant improvements to the water environment through upgrades to the sewage collection and treatment systems. Between 2010 and 2021, SEPA required Scottish Water to improve 279 sewer overflows and 104 wastewater treatment works. These were prioritised in Scottish Water’s previous investment programmes in 2010-2015 and 2015-2021 because they were causing significant pollution.

As a result, water quality is now at its highest level to date, with 87% of Scotland’s waters having good or better water quality. However, sewer spills have become of increasing concern in recent years. The pandemic has seen a shift in public appreciation of their local water environment and there has been a significant increase in outdoor swimming. Climate change and customer disposal of inappropriately flushed items continues to put increased pressure on the sewer network.

The River Basin Management Plan 2021-27  recognises that there is more to be done to improve our urban waters. We need to move towards a circular economy for urban water management in Scotland and achieve SEPA’s sector plan vision.

Route map for improving urban waters

During 2021, SEPA wrote to Scottish Water setting out our expectations and timetable for a route map to improve urban waters as part of the actions required in the River Basin Management Plan 2021-2027. We highlighted the need for a step change in our efforts to tackle the most significant environmental impacts as soon as possible and to take a One Planet Prosperity approach to improving our urban waters for the long term.

We recognised that actions and resources are required not only by Scottish Water but by many other stakeholders, including SEPA, Scottish Government, Local Authorities and customers, if we are to deliver a circular economy approach for urban water management in Scotland. We highlighted that there are a number of issues relevant to sewage discharges which have emerged in recent years, such as microplastics, antimicrobial resistance and an increase in wild swimming. These are not considered in the current legislation nor in SEPA’s current regulatory policy, however we intend to engage with relevant stakeholders, including Scottish Water and NGOs, to discuss these issues and the long-term plan to progressively eliminate litter and substantially reduce spills.

In December 2021, Scottish Water has published a route map for improving urban waters highlighting the need for investment to deliver it. This sets out actions required by Scottish Water, as a public body and responsible authority for River Basin Management Planning, as well as significant work needed with other partners to deliver long term improvements.

The route map sets out how Scottish Water will work with partners to:

  • improve water quality to support Scotland’s RBMP objectives
  • increase monitoring and reporting
  • significantly reduce sewer related debris in the environment
  • reduce spills from the sewer network

SEPA has welcomed the route map, indicating that we will use regulatory tools if necessary to ensure that actions are progressed to address the most significant water quality and litter impacts in accordance with the agreed timetable. We also confirmed our intention to engage with relevant stakeholders on the long term ambitions for the urban water environment from 2022.

What is a Combined Sewer Overflow (CSO) and why do they spill?

There are over 3,600 Combined Sewer Overflows (CSOs) within the 50,000km of sewer network in Scotland (equivalent of one for every 15km). These are designed to spill during heavy rainfall to prevent sewer flooding of properties. However, an increasing number of CSOs spill with high frequency due to the following causes:

  • Hydraulic overloading from increased flows since the sewer was originally designed. This can be caused by:
    • the connection of additional impermeable surfaces to the sewerage network (housing/business growth and when permeable areas are paved over);
    • increases in overall rainfall and rainfall intensity due to climate change;
    • network issues (e.g. siltation and infiltration).
  • Blockages (over 35,000/year) caused by inappropriately flushed items that customers dispose to sewer (e.g. wet wipes or cooking oils) and from sewer collapses and deterioration of the sewer system.

What impacts do CSOs have?

  • Spills can cause acute environmental pollution incidents, usually due to blockages. There have however been very few major incidents in the last decade in Scotland. There are generally fewer than 10 significant incidents and around 200 minor incidents reported each year.
  • Screens are fitted to many CSOs where frequent spills occur. However, around 80% of CSOs in Scotland do not have a screen and therefore have the potential to spill during rainfall causing sewage litter in the watercourse. Even where screens are provided, not all sewage debris can be retained within the system since the best performing screen is likely to be only 90% efficient in retaining debris. Over 600 CSOs are currently confirmed as generating unacceptable litter. As spill frequencies increase, further improvements in screening will be required.
  • A small number of hydraulically overloaded CSOs result in known chronic pollution impacts across significant stretches of rivers (due to widespread increased ammonia and/or reduced dissolved oxygen). There are 24 combined sewer overflows which are identified in the River Basin Management Plan 2021-2027 as impacting on water quality classification.
  • Bacterial pollution from CSOs is investigated and action taken to meet bacterial standards at the 85 designated bathing water areas and shellfish water protected areas.
  • There are a number of environmental impacts which are not fully understood or assessed. With the exception of some pharmaceuticals, these are not specifically considered in the River Basin Management Plan. These include the effect of macro and micro plastics on the freshwater and marine environment, other persistent chemicals in raw sewage including pharmaceuticals and hormones, viral pollution and anti-microbial resistance.