Policy Position Statement
Environmental impacts of combined sewer
overflows (CSOs)
Purpose
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.
CIWEM, 106 to 109 Saffron Hill, London, EC1N 8QS. Charity Registration No. 1043409 (England & Wales) SC038212 (Scotland)
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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 UPM1 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.
CIWEM is the leading independent Chartered professional body for water and
environmental professionals, promoting excellence within the sector.
Context
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 20th
century, most sewers were constructed as, or evolved to become, combined systems. Since
FWR (1998) UPM Urban Pollution Management Manual. Second edition October 1998 FR/CL0009 published by FWR,
Marlow, SL7 1FD www.fwr.org
1
CIWEM, 106 to 109 Saffron Hill, London, EC1N 8QS. Charity Registration No. 1043409 (England & Wales) SC038212 (Scotland)
[email protected] | 020 7831 3110 | www.ciwem.org
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
9.
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.
10.
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
CIWEM, 106 to 109 Saffron Hill, London, EC1N 8QS. Charity Registration No. 1043409 (England & Wales) SC038212 (Scotland)
[email protected] | 020 7831 3110 | www.ciwem.org
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.
11.
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.
12.
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.
13.
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.
14.
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.
15.
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
CIWEM, 106 to 109 Saffron Hill, London, EC1N 8QS. Charity Registration No. 1043409 (England & Wales) SC038212 (Scotland)
[email protected] | 020 7831 3110 | www.ciwem.org
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.
November 2004
Note: CIWEM Policy Position Statements (PPS) represents 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 and that previously held views may
alter and lead to revised PPS’s. PPSs are produced as a consensus report and do not represent the view of
individual members of CIWEM.
CIWEM, 106 to 109 Saffron Hill, London, EC1N 8QS. Charity Registration No. 1043409 (England & Wales) SC038212 (Scotland)
[email protected] | 020 7831 3110 | www.ciwem.org