Learn how to build a business case for a successful CBM strategy to ensure a good return on investment.

How can water companies reduce storm overflow discharges? (Part 1)

Table of contents

Reducing storm overflow discharges is a strong focus for water companies in order to improve service and protect the environment. Achieving zero harmful storm overflow discharges will undoubtedly involve a multi-pronged solution to optimize cost effectiveness and maintain service as sustainably as possible. Taking the UK wastewater sector as a prime example, we will look at a recently published government report which outlines 5-yearly discharge reduction targets1. This breaks down the costs of different solutions to achieve zero storm overflow discharges by 2050. As this report only presents capital and operating costs (CAPEX/OPEX) for high-CAPEX options in isolation, we will highlight the benefits of utilizing a joint strategy. This will be based on using up-front, quick-to-implement, low-CAPEX solutions in the short-, medium- and long-term together with one or more high-CAPEX options. Such a joint strategy would maximize the overall benefits by reducing environmental impact, lead times and public disruption.

Storm overflow pipes and system

What are combined storm overflows?

Combined storm overflows (CSOs) are a common element of many wastewater networks. They are intended to act as a safety valve for releasing untreated wastewater into the natural environment during periods of heavy rainfall, when pressures on the system become too great. Due to environmental impact they tend to be stringently monitored and regulated. Water companies in some countries can even face significant fines for overusing storm overflows and creating pollution events outside of storm conditions. Nowhere is this more poignant than in the UK. Only recently the UK Environment Secretary proposed to raise the ceiling on fines for water companies by 1000% up to £250 million (€290 million) per pollution incident1. This comes at a time when regulatory authorities have also set strict targets for water companies in eliminating the harmful impact of storm overflow discharges up to 2050.

Current water pollution landscape: pressures, regulations and targets

There are around 15,000 storm overflows in England alone. However, these safety valves are being used more and more often outside of heavy rainfall periods. In 2021, 90% of storm overflows discharged at least once, with 5% discharging more than 100 times, including in high-priority nature sites such as sites of special scientific interest and designated bathing areas. Like many networks globally, the wastewater networks across the UK have come under pressure from many external factors. This includes changing weather patterns due to climate change, a growing population, urban development and bad consumer habits causing pump blockages in the sewage network.

Five-yearly targets on discharge reduction in England have recently been published by regulatory authorities to cover 75% of storm overflows at high-priority sites by 2035 and all storm overflows by 2050 (Figure 1)2. In addition to increased fines, regulators plan increased monitoring to cover 100% of storm overflows by the end of 2023 and water companies have been strapped with compulsory investment of £56 billion (€65 billion) to improve associated infrastructure.

CSO blog1 table1 updated
Figure 1. Indicative spill reduction targets that should be achieved based on 2020 baseline figures (2).

Strategies for reducing storm overflow discharges

To meet the five-year targets, the main focus for water companies will fall on upgrading aging network architecture, employing green infrastructure and improving operational efficiency. This will have to account for factors such as climate change causing dryer summers and wetter winters which increases pressures on the sewage system.

The high-CAPEX route to eliminate storm overflow discharges

Separation of the combined sewer network and construction of additional storage are two principal options highlighted in the associated government report. These are the only options that could eliminate storm overflow use independently as a solitary solution. Separating the combined sewer network would involve construction of new sewers for wastewater running parallel to the network across the country. The combined sewer system would then only be used for rainwater. To eliminate discharges using the increased storage option would require increased capacity equivalent to 40,000 Olympic-sized swimming pools to accommodate all rainwater.

Apart from these two principal options, a third possibility is the use of Sustainable Drainage Systems (SuDS). These systems divert rainwater away from the sewer network by slowly filtering rainwater back into the natural environment. This option includes various measures such as trees in pits, street side rain gardens and property level measures. However, it would need to be combined with other strategies to achieve zero storm overflow discharges.

CSO blog1 table2 updated
Figure 2. Total CAPEX and yearly OPEX costs for different strategies for storm overflow discharge reduction rounded to nearest £10 billion (for CAPEX) and £10 million (for OPEX).

Limitations of high-CAPEX strategies

All of these three options are calculated to incur high capital expenditure levels (Figure 2), long lead times, high levels of embedded carbon (total CO2 emissions associated with the construction of the project)3 as well as a negative impact on consumer bills (Figure 3). The principal options also have a high risk of environmental harm and would cause years of disruption to public life. All three options carry significant uncertainties.

For the complete separation option, no calculations were made of a benefit-cost ratio, embedded carbon or operating costs, yet this option has the highest level of capital expenditure. The benefit-cost ratio for increased storage is unfortunately low, but inherent CAPEX costs might be inflated by 30%. Furthermore, achieving zero discharges using only increased storage would cost over £100bn (€115bn) more than achieving an average of only 10 spills a year using the same option (€23bn–38bn). This is a massive difference which potentially provides great scope for a joint strategy. For the SuDS option where benefit-cost analysis provides a more positive picture, there are also uncertainties. For example, cost estimates and the significantly higher operating costs associated with them might also have hidden costs related to emissions. That said, SuDS do hold great environmental benefits with regard to natural hydrology, bringing improvements to the environment and ecology, and having positive societal impact.

CSO blog1 table3
Figure 3. Central estimate of yearly increases in consumer bills for different high-CAPEX strategies for storm overflow reduction (rounded to nearest £10).

Using low-CAPEX options up front to address underlying issues and lower capital expenditure

The final option proposed in the government report is termed “alternative measures to reduce sewage discharges and remove inefficiencies in the network.” It is within this option where low-CAPEX/OPEX solutions can be found which are quick to implement and don’t have the significant carbon footprint of the high-CAPEX options. These solutions range from public awareness campaigns aimed at changing consumer behavior to using more data-driven analytical methods. For example, a data-driven approach could be used by maintenance teams to resolve clogging issues, which, if left unresolved, lead to water supply disruptions and pollution incidents. This provides a positive case for selecting a joint low-CAPEX/high-CAPEX strategy as the most beneficial way to eliminate storm overflow discharges. The implications here are multifold:

Reducing the pressure on meeting reduction targets early on

First, the high capital expenditure options have long lead times and therefore take longer for reduction targets to be achieved. Implementing successful low-CAPEX solutions up front would help in reducing discharges more quickly, allowing companies to reach targets faster and more efficiently.

Reducing environmental impact

There are low-CAPEX options which can be implemented with comparatively minimal embedded carbon levels when contrasted to all the high capital expenditure options3. The complete separation and increased storage options would cause significant environmental harm. Reducing the requirements of any high-CAPEX option will additionally have a positive effect in reducing the embedded carbon of the high-CAPEX elements of a joint strategy. Furthermore, low-CAPEX options exist which could actually reduce emissions associated with industrial operations by improving the efficiency of industrial equipment. This is a big plus for water companies in moving toward a more sustainable future.

Dealing with issues not addressed by high-CAPEX options

One problem with relying on the high-CAPEX options is that they may not actually deal with inherent problems in the system that could be the cause of pollution events such as pump clogging. For example, according to analysis of UK Environment Agency data4 between 2015–2020, blockages and asset failures were responsible for 80% of sewer overflow pollution events. In 2020 alone, this was 87%. Any joint strategies chosen should cover all principal causes of pollution events and storm overflow use to effectively eliminate CSOs.

Reducing disruption to public life

Additionally, low-CAPEX options once implemented would involve minimal or even no disruption to public life. This reduces any negative impact that extensive engineering projects could have on corporate image. By reducing requirements on high-CAPEX elements of a joint strategy, this would also have a positive knock-on effect in reducing disruptions.

Reducing the scope and costs of high-CAPEX options

Lastly, the extent of the capital expenditure costs for the high-CAPEX options is related to the scope of the requirements. If a low-CAPEX option was implemented early, even on a smaller scale, its impact on reducing discharges could quickly be measured and extrapolated to estimate its impact covering the whole sewer network. A reduction in yearly discharges by low-CAPEX methods implies a reduction in overall requirements for the high-CAPEX options. This reduction in requirements would most likely lead to reduced capital investment levels for the high-CAPEX elements of a joint strategy.

As a prime example, reducing the requirements of the increased storage option could reduce overall costs significantly. This is based on the fact that the increased CAPEX for going from an average of 10 spills per year to zero spills using only increased storage costs a factor of 3-4 times more. Combining increased storage with an up-front low-CAPEX option could mitigate the need for such extensive additional capital expenditure. With the SuDS option this would by definition need to be combined with other options.


Deciding on the optimal solution to reduce CSO discharges is a difficult task with many uncertainties. A joint strategy proposed in this article would prove to be the most cost-effective for water companies, where low-CAPEX methods are implemented up front and their effectiveness determined at an early stage. This would be followed by a recalculation of reduced requirements for the high-CAPEX options and selection of the optimal high-CAPEX solution as part of a joint strategy. As a result, this would reduce environmental impact and disruption to society while improving service and operations. It would also generate tangible results quicker and deal with some causes of pollution events not covered by the high-CAPEX options (e.g. clogging).

In article 2 we discuss the effectiveness of low-CAPEX options in more detail, focusing on improving wastewater network efficiency. This could help water companies in the UK and globally in developing cost-effective strategies in reducing storm overflow use and pollution events.


  1. Fines and targets listed in this report only apply to water companies operating in England. Other UK countries have different regulatory bodies covering the water sector.
  2. Information and calculations in this article have principally been taken from Storm Overflows Discharge Reduction Plan. UK Dept. for Environment, Food and Rural Affairs. (August 2022).
  3. No calculations for embedded carbon were performed for the complete separation option but a high level of embedded carbon would be expected.
  4. Understanding storm overflows: Exploratory analysis of Environment Agency data. UK National Audit Office (2021)

30.000+ professionals at industrial companies get our insights and best practices delivered monthly to quarterly.

Please select listing to show.