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Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110   Fax: 020 7405 4967

Environmental Impacts of Combined Sewer Overflows


The purpose of this PPS is to identify the key issues related to management of the environmental impact of combined sewer overflows (CSOs).

CIWEM Considers:

1. CIWEM accepts that the capacity of the underground drainage infrastructure is finite and that therefore CSOs, which are the safety valves of the system, are inevitable. However, CIWEM believes that it is environmentally desirable that discharges from CSOs should be minimised where there is a negative environmental impact, wherever this is technically and economically feasible. CIWEM also recognises that it is not economically feasible to construct an infrastructure with a capacity to cope with the most extreme events. CIWEM acknowledges that changes in rainfall patterns associated with climate change affect the operation of CSOs, with the potential "peakiness" of discharges increasing, and therefore the difficulty of the task is increasing.

2. CIWEM considers it is essential that maintenance of the hydraulic capacity of the existing infrastructure, including proactive cleaning, is adequately funded in order to maximise conveyance and minimise the frequency of CSO operation.

3. Where CSOs cannot be eliminated, the frequencies, volumes and pollutant loads of CSO spills should be controlled so as not to compromise the desired beneficial uses of receiving waters. The performance of CSOs should be managed through the application of standards which have direct relevance to the quality of the environment and which have a demonstrable scientific justification. This means that, wherever possible, target standards should be expressed in terms of the pollutants that directly compromise the beneficial use in the receiving water, as opposed to surrogate measures which may be easier to assess, but which do not relate directly to the environmental impact of the discharge. Where environmental standards are not feasible, or are not available, for example as is the case with aesthetic pollution, an alternative approach based on emission standards is appropriate.

4. An holistic approach should be adopted in setting the required performance standards for CSOs, which takes account of the interaction between CSOs and other polluting discharges, such as sewage treatment works effluents, surface water outfalls and diffuse sources.

5. The UK water industry has invested heavily in research into the impact of CSO and other storm related discharges and in the development of the  UPM Procedure to provide environmental and cost effective management of them. CIWEM believes that maximum use should be made of the products of this investment, since it offers the opportunity to effectively manage the environmental impact of CSOs and to improve on historical practice that has been shown to be inadequate by virtue of the problems that have become apparent over recent years.

6. The efficient planning and implementation of holistic solutions takes time and ways should be found to accommodate this process within the current 5 year planning cycle that controls water industry investment in England and Wales. Within this rigid framework, the identification of optimal solutions is currently often suppressed by the need to achieve time and budget deadlines.

7. In principle, CIWEM endorses the use of Sustainable Drainage Systems (SUDS) techniques in preference to conventional piped sewer systems. SUDS techniques are likely to be particularly suitable for new development areas, but CIWEM recognises that there are current uncertainties regarding the long-term performance of some types of SUDS that could lead to problems over adoption and maintenance. Research is ongoing to address these uncertainties. However, it should be recognised that in certain circumstances conventional storage and treatment options might offer a more cost effective and sustainable approach to CSO control.

8. CIWEM advocates the practice of post-project monitoring and appraisal in the context of CSO improvement schemes. Post-project monitoring should be a feature of all such schemes, since this is the only way that future practice can be enhanced. The increasing trend for telemetry links to monitor CSO operation for regulatory purposes makes post project appraisal more readily viable. However, effective management and use of these data is a major issue that should addressed if the industry is gain the potential benefit of this practice.

The Chartered Institution of Water and Environmental Management (CIWEM) is the leading professional body for the people who plan, protect and care for the environment and its resources, providing educational opportunities, independent information to the public and advice to government. Members in 96 countries include scientists, engineers, ecologists and students.


The function of a sewerage network is to convey household and industrial wastewater and surface water runoff from impermeable surfaces, to an appropriate location for treatment and disposal. Sewer systems may carry combined flows of household and industrial wastewater together with surface water runoff in a single pipe system for treatment at the sewage treatment works. Alternatively, separate piped systems may be provided for each type of flow, with surface water runoff being discharged to the nearest receiving water and only household and industrial wastewaters being taken to the treatment works. Up to the middle of the 20thcentury, most sewers were constructed as, or evolved to become, combined systems. Since that time, separate sewer systems have become the norm for new development. Hence, the majority of conurbations in the UK are served by systems with a central core of combined sewers, with peripheral, more recently developed areas being drained by separate systems. In many cases, the local topography and distance from a suitable receiving water dictates that new separate pipe systems discharge flows into the older, downstream combined sewers. Surface water may also enter separate sewers because of infiltration and illegal connections.

The combined volume of household & industrial wastewater and surface water runoff generated from an urban area during a significant rainfall event is such that it is generally not economically feasible or environmentally cost effective either to transport the total flow for large distances via a combined sewer system or to treat it at the sewage treatment works when delivered. For these reasons, and also to minimise the risk of sewer flooding, it has been customary to provide CSOs that serve as "safety valves" for the pipe system by limiting the quantities of flow passed forward to treatment to a level that the downstream sewer and sewage treatment system can practically and economically accommodate. Historically, the quantity of flow passed forward at CSOs has been based on a multiple of the base (or dry weather) flow carried in the sewer. The rationale behind this approach was that the heavily polluted base flow would be sufficiently diluted by relatively clean surface water runoff. Hence, it would be environmentally acceptable to discharge excess flows into a local watercourse which, it was assumed, would also have increased in flow from the same rainfall event.

In 1989, at the time of privatisation of the water industry in England and Wales, there were approximately 20,000 CSOs in England and Wales (plus significant additional numbers in the rest of the UK). The environmental regulator (then the National Rivers Authority) estimated that some 30% of the total stock of CSOs was unsatisfactory, judged against a range of environmentally based criteria. Since that time, a major priority of successive Asset Management Plans of the Water Service Companies has been the improvement of these CSOs. The Urban Wastewater Treatment Directive is driving a similar programme of improvement in other parts of the UK and elsewhere in Europe. By the time of completion of Asset Management Plan 3 (AMP3) in England and Wales in 2005, the majority of unsatisfactory CSOs should have been improved to provide environmentally acceptable performance.

Key Issues

1. It is generally accepted that it is, and will remain for the foreseeable future, impractical to totally eliminate discharges from CSOs. The cost of upsizing combined sewer systems and treatment facilities to accommodate all flows is prohibitive, as is the cost of universal separation of all existing combined sewers systems. However, the practice of spilling all flows in combined sewer systems above an arbitrary multiple of dry weather flow is unacceptable in some situations, as witnessed by the large number of existing CSOs identified to be causing excessive environmental impact.

2. Despite the considerable dilution of household and industrial wastewater base flows by surface water runoff, storm sewage discharges from CSOs may contain significant loads of a wide variety of pollutants, including bacteria and viruses, oxygen demanding and toxic pollutants, as well as persistent materials such as heavy metals, Polycyclic Aromatic Hydrocarbons (PAHs), etc. The presence of gross solids of obvious sewage origin is also a frequent problem. Although only discharged over short periods of time on an infrequent basis, these pollutants can seriously compromise many beneficial uses of receiving waters such as fisheries, shellfisheries, bathing and recreational water use, as well as the perceived amenity value of the waters. In extreme cases, CSO discharges can result in fish mortalities, shellfish unfit for human consumption, public health hazards and visual and odour problems.

3. The problem of CSOs and the inadequacy of traditional design practice have been recognised for many years. A major programme of research and development to improve planning and design practice was carried out by the UK water industry in the period 1985 to 1994. The aim was to develop a practical planning framework, which allowed an holistic approach to sewerage upgrading and the achievement of environmental targets, whilst recognising the financial constraints within which these objectives have to be delivered. The principal product of this research was the Urban Pollution Management (UPM) Manual, which was first published in 1994 and, subsequently updated in 1998, in the light of implementation experience.

4. The essential requirement in dealing with unsatisfactory CSO discharges is to consider them in the context of the system of which they form a part. Whereas traditional practice has been to consider the sewer system, the treatment works and the receiving water as separate entities, it has been clearly demonstrated that there is significant benefit to be gained from recognising their interrelationship as elements of a single system. This means that the total system should be considered so that the required environmental performance, in terms of receiving water quality, can be achieved in the most efficient and cost-effective way.

5. UPM provides a planning framework which, when implemented through the provision of appropriate data and the use of modelling tools, allows the holistic performance of existing urban wastewater systems and proposed upgrading measures to be effectively tested against target environmental criteria.

6. Within the overall planning framework, designers may choose from a wide variety of potential upgrade techniques to achieve required improvements in environmental performance of sewer systems. Additional transport capacity may be provided through larger sized or supplementary pipe runs. Peak flows may be attenuated by detention tanks. Flow rates and volumes may be reduced by disconnecting and diverting runoff from contributing areas. Aesthetic pollution problems may be addressed through a combination of reduced CSO spill volumes and frequencies and good chamber design, coupled with screening to provide effective solids separation, where appropriate.

7. Traditional practice has been to remove all surface water runoff and wastewater flows from the urban area as quickly as possible via piped drainage systems. Urbanisation of an area increases both the volume and speed of storm runoff, as well as introducing pollutants into the flow. Hence, widespread urbanisation can and has resulted in river flooding problems as well as water quality issues. The tendency for home improvement in the form of hard surfacing (patios, drives, etc.) extensions and conservatories adds to the volume of surface water. Recognition of this has generated interest in alternative drainage management techniques, which seek to control flows at, or as near as possible, to their source with the objective of maintaining overall flow and quality of discharges close to the natural condition. In the UK, such techniques have become identified by the generic title of Sustainable Drainage Systems (SUDS). These include methods such as permeable surfaces, filter drains and strips, swales and infiltration devices as well as ponds, basins and artificial wetlands. Environmental regulators throughout the UK actively encourage the use of SUDS wherever it is feasible. The use of SUDS, where conditions allow, can potentially be an effective means of attenuating rainfall inputs into combined sewer systems and, hence, of managing CSO discharges.


FWR (1998) UPM Urban Pollution Management Manual. Second edition October 1998 FR/CL0009 published by FWR, Marlow, SL7 1FD

November 2004

Note: CIWEM Policy Position Statements (PPS) represent the Institution's views on issues at a particular point in time. It is accepted that situations change as research provides new evidence. It should be understood, therefore, that CIWEM PPS's are under constant review, that previously held views may alter and lead to revised PPS's.




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Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110   Fax: 020 7405 4967