Combined sewer overflows: growing awareness, growing problem

This article first appeared on The Water Report

The UK water industry is facing the greatest challenge in the way it operates its wastewater networks in more than a generation. The Environment Bill has placed a legal requirement on companies to progressively reduce the adverse impacts of storm discharges within their wastewater networks, including those from combined sewer overflows (CSOs), amidst growing public concern and growing national environmental ambitions.

Companies need to act if they are to avoid heavy fines, criticism from their customers and damaging media headlines. In assessing the right course of action, companies will need to consider how significant the issue is and what solutions are available to tackle it. The answer for each area will be different given the varying asset bases and physical and human geographies.

CSOs – growing problem or growing awareness?

Most of the wastewater network comprises combined sewers dating back to the Victorian era, which take combinations of foul sewage and rainwater. CSOs were installed as a pragmatic way of managing storm flows or asset failures by spilling excess flow to a neighbouring watercourse, to avoid the need to construct excessive network capacity or intrusive and expensive storm storage.

Since CSOs are primarily used for stormwater discharges, the spills are normally heavily diluted. Further, many CSO discharges are protected through the use of course screens and dip tubes intended to remove gross solids before they reach the water course.

Notwithstanding this, their usage has been a subject of considerable recent public attention and media interest. Concerns over the potential public health and environmental implications of discharges, combined with an increase in demand for safer bathing waters, culminated in the measures in the Environment Bill.

Companies need to separate the problem from the awareness to tackle this challenge in the optimum way, first examining the causes and then the actuality of CSO discharges. Three primary causes are considered: climate change, urbanisation and sewer blockages.

Causes of CSO use

There is clear evidence of climate change driving the level of discharges. The Met Office’s latest State of the UK Climate report highlights there were 4% more extremely wet days compared to the 1961-90 average, however importantly, there was 17% more rain on those days. This creates more surge flows, exceeding the networks’ capacity more often and more rapidly.

In addition, the increase in population and urbanisation means fewer areas of natural environmental mitigations for surface water runoff and increased loading on existing sewers. In England for example, urban population has increased by 2,716,844 from 2011-2020.

On sewer blockages, the evidence of change is less clear. Blockages cause flow restrictions, resulting in build ups within the network and unintended spills from CSOs. Between 2015-2020, blockages were attributed as the cause of just under 80% of all pollution events at storm overflows. There is however little evidence that the frequency of their occurrence has changed over the last decade.  

Extent of CSOs

A key issue in understanding and addressing CSO discharges, is the paucity of data to confirm when, and where overflows are used. As recently as 2016, only 6% of all storm overflows were monitored. However, this is changing, and recently, significant progress has been made, with many companies now reporting greater than 90% coverage. The EA expects that all overflows will be monitored by EDM’s by 2023. 

Installing EDM’s has come with its own challenges; it required companies to trace locations for CSOs that may not have been accurately mapped and, once located, carry out extensive survey work to understand how the overflow is connected and how it would behave during high flow periods. The initial data from EDM monitors sent to EA has required some water companies to revisit EDM installations to understand and address spurious data. This has conversely provided benefits in a far greater understanding of CSOs.

While it will take time for a reliable picture to be established, EDM data provides a far improved baseline for water companies to understand and analyse CSO performance. For example, figure 3 indicates that it is possible to identify the spillage rates for many CSOs.

What are the possible solutions?

The industry is gaining momentum on this issue and corralling understanding and ideas, and companies have committed £3.1bn. This amount is welcome, but tiny compared to the Storm Overflow Taskforce’s estimate of c. £280bn to retain overflows and put mitigations in place and c. £600bn to completely separate out surface water. The industry will need to address the challenge with the right mix of alternative, innovative, cost-efficient approaches. We consider some approaches below. 

Reducing demand on the network

Finding ways to reduce surface water entering the network would naturally reduce the risk of discharge. This could be achieved through:

Nature based solutions.These use natural ‘green’ infrastructure to mitigate the impact of peak surface water flows or provide temporary storage. For example, ‘Sustainable Urban Drainage Systems’ or ‘rain gardens’ provide a natural buffer between surface waters and the sewerage network. These are low-cost approaches with additional socioeconomic and biodiversity benefits. However, they require overground land, potentially extensive landscape re-engineering and a risk of consumer rejection.  

Constructed wetlands are another approach, treating CSO existing discharges to remove harmful pollutants before discharging, hence retaining the CSO but reducing its potential for harm. The land requirement is considerable and mostly limits their use to rural environments.

Continuing to reduce infiltration. Poor condition sewers below the water table can attract high inflow, especially in sandy or silty soils. Flow monitoring can identify these areas allowing proactive intervention, such as sewer lining. However, tracing infiltration is notoriously difficult and expensive as it varies depending on ground water table levels and other soil conditions. It also requires extensive CCTV survey work and root and debris removal.

Continue to tackle blockages.This requires a combination of physical intervention and customer behavioural change. Blockages can be caused by collapsing sewers due to poor condition, tree roots, misalignment in pipe joints and a build-up of silt during low flows. As with infiltration, resolving these issues is often expensive and time consuming. On the behavioural side, sewer misuse remains the primary cause for most blockages. Companies have worked hard to engage customers and some progress has been made in this area.

Optimise existing capacity

Smart Networks will be a key, cost-effective alternative to deploying additional infrastructure to manage storm flows and can be achieved in the (relatively) short term; the industry has made, and continues to make, significant advances towards smart waste and water networks by improving monitoring and network control.

Network monitoring– the first step in enabling truly ‘smart’ networks is to ensure there is enough monitoring and data to inform actions to optimise the network. This can:

• Identify early-stage network blockages or infiltration occurrences through, for example, flow mapping, which can be used to prioritise maintenance activities, and proactive intervention to these areas.
• Develop confidence-based weather and flow forecasting to give granular, catchment area level data on potential storm weather events and their implications on the network. Helping inform the control response.

Advanced Network Control– a greater understanding of network performance, combined with a large asset suite to control, can be used as an effective tool to optimise the capacity within the existing network. For example, using predictive analytics to take advantage of existing storage in the network in anticipation of severe weather events and hence evacuating pumping station wet wells and storm storage towards areas where there is anticipated to be spare capacity.

Smart networks can drive significant progress in CSO use, complementing the development of infrastructure measures. The prerequisites are that enough assets are available to control, enough reliable data and monitoring to understand flow, and the capability to analyse and utilise this data. Notwithstanding this, the effectiveness still relies on having sufficient, vacant, network storage to utilise and a risk appetite to manage flows without other infrastructure fail safes.

Developing an approach for sustainable progress

If elimination of CSOs is the industry’s aim, then it potentially poses the biggest change to the wastewater network in generations. To address this challenge, the industry must:

• Focus on data: CSO reduction needs a deliberate approach to create an affordable, acceptable solution for each location. However, there is a paucity of quality data to determine which CSOs to address and how. The industry has made good progress and a knowledge base is emerging.
• Prioritise outcomes: The choice of which solution to pursue for each part of the network will have operational, financial, and societal implications. Re-engineering CSOs should be an opportunity to ensure that solutions that add wider social and environmental value are distributed throughout the customer base.
• Be risk focused: Data will provide the necessary information to inform a risk-based approach, minimising impact to the customer, while demonstrating the industry is making progress. This approach should also target the use of low cost, low carbon solutions.
• Smart networks: Smarter networks and ‘digital twins’ – mirroring the physical network in an IT solution – have been progressing in the industry and can be used to target solutions.
• Education: The negative media attention is understandable and justified but does risk creating a misunderstanding about CSOs. Customer education could help the water companies’ position and reinforce how the consumer themselves can continue to help by monitoring their own waste disposal.

The demand from customers, regulators and policymakers to reduce CSO use has never been greater. There is no one-size-fits-all solution. Resolving this issue requires systemic industry change and a need for innovation and investment prioritisation, to drive improvements in CSO performance in a sustainable yet equitable way, whilst minimising the cost to the consumer for current and future generations.