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