11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Combined Sewer Overflows - Do they have a Future?
P.E. Myerscough*(1) and C.J. Digman(2)
1
Yorkshire Water, Western House, Halifax Road, Bradford, BD6 2LZ, UK,
[email protected]
2
MWH, 1 Red Hall Avenue, Paragon Business Village, Wakefield, WF1 2UL, UK,
[email protected]
ABSTRACT
Combined sewer overflows (CSOs) have formed part of the United Kingdoms infrastructure
since the 19th Century helping to reduce the likelihood of flooding from the combined sewer
system. In more recent years, CSO structures have developed substantially to include screens
as our understanding on their performance and the chambers themselves under different flow
conditions increases. The standard WaPUG chamber is now well established as ‘best
practice’ however there is often the need to use a ‘non standard’ design. To enable such
structures to be used with confidence, physical modelling and CFD play an important role in
understanding how such CSOs will perform, and enable improvements to be made in design
and avoid costly mistakes. Further major refinements to CSOs are unlikely however in the
future, renewable energy may be an alternative source to power screens located in the
chamber. CSOs are likely to be required for many years, and although there is likely to be a
pressure to reduce CSO spills, the construction of large storage tanks is unsustainable.
Reducing the inflow into the combined sewerage system is likely to be key to returning the
CSO to its original concept of peak lopping in times of heavy rainfall.
KEYWORDS
Combined Sewer Overflow, CSO, Innovation, Modelling, Water Framework Directive,
Flooding
THE COMBINED SEWER OVERFLOW
In the 1830s and 1840s in England, outbreaks of Cholera were common place as a result of
raw sewerage dumped on the streets and stored in cess pits mixing with water supplies used
for drinking. During the rapid expansion of towns and cities during the industrial revolution,
the need manage foul sewage and storm water became apparent, and is still a challenge today
as we face the need to manage the risk of flooding and pollution. The construction of sewers
in England initially focused on removing the foul and surface water and disposing of it
directly into rivers. However, this quickly created pollution problems. For example in 1869
in Leeds, an injunction was obtained that prevented the discharge of any more sewage (as a
result of extending the existing sewer system) until it had been “sufficiently purified and
deodorized” (Sellers, 1997). This led to the construction of trunk sewers that intercepted the
early outfalls and conveyed sewage to a safe point for disposal.
However during the boom of constructing sewers in the 1850s and onwards, it was recognized
that the sewers did not and could not have the capacity to convey all storm flows to a point of
safe disposal. In London, original discharge points in existence prior to the construction of
intercepting sewers were retained as relief to the system, becoming the first CSOs. In Leeds
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
in 1883, it was recommended that a system of storm overflows be built on the interceptor
sewers and these overflows would only operate when six times the dry weather flow had been
reached. This provided benefit in reducing the flows to the treatment works and preventing
flooding in the drainage area. However, during storms, untreated sewage was discharged to
receiving waters leaving a legacy of pollution.
The challenge of managing the flow in combined sewers has remained ever since, and CSOs
are common place in the UK. These provide a balance between managing the risk of flooding
from the combined sewer system and polluting the receiving watercourse.
This particular challenge has been the focus of investment by the UK Water Industry in recent
years. Pollutants are typically in the form of aesthetic pollutants commonly known as gross
solids (greater than 6mm in 2D and of sewage origin) and dissolved or finely suspended
pollutants. To prevent the discharge of gross solids for all but the more extreme storm events,
many CSOs have been screened. In addition, to control the discharge of dissolved and finely
suspended pollutants, storage tanks have been constructed to enable the more polluted flow
flushed through in the early part of the storm to be retained before discharge takes place.
CSO Standards
The improvement of CSO discharges in the UK stemmed from the requirement to meet the
EU Urban Wastewater Treatment Directive (EU Directive No. 91/271/EEC 1991). This led
to the major upgrade programme from 1995 to date (March 2008) where over 6000 CSOs
nationally, and 950 in the Yorkshire have been addressed
In the UK, a national standard has been developed to design CSOs, for aesthetically deficient
and “water quality” overflows (water quality CSOs normally require additional storage to
retain dissolved and finely suspended pollutants). Aesthetically deficient CSOs are generally
designed (Figure 1) to the WaPUG Code of Practice (WaPUG, 2006) where screens are used
to retain the solids in the system, generally up to and including a 1 in 5 year storm event
Water Quality CSOs have additional requirements to limit the discharge of all pollutants,
usually by the provision of additional storage. The standards for pollutant discharge are
derived from the EU Urban Waste Water Treatment Directive and associated UK legislation,
and are summarized in the Urban Pollution Manual (FWR, 1998)
Incorporating screens into CSOs
The UK predominantly uses “6 mm” screens to prevent solids being discharged to receiving
waters. These either have a 6 mm diameter perforated plate or a 6 mm by 4 mm mesh and are
either mechanically brushed (powered or manually cleaned (static) screens. Such 6 mm
screens were tested in a number of national trials using real sewage eg: Wigan (Saul, 2000).
Yorkshire Water undertook their own trials at Knostrop to determine screen loading rates,
screen selection criteria, chamber design and influence their screening framework. The
Yorkshire Water research identified that initial loading on static screens should not be
beyond 50 l/s/m2 and powered screens not beyond 350 l/s/m2. It identified cleaning
requirements for static screens and developed a formulaic approach based on catchment
characteristics. This approach also restricted the number of spills per year at an overflow to
12 if a static screen was selected and only where the receiving water amenity value was
classified as low (National Rivers Authority, 1993).
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Combined Sewer Overflows - Do they have a Future?
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Din≥Dmi
inlet
length
(0.4D)
Weir /
screen
length
outlet
length
(1.5D)
Figure 1 Plan of side weir CSO chamber after WaPUG Guide (2006)
The WaPUG CSO Design guide incorporates such screens and uses a smaller footprint than
earlier designs, offering substantial efficiencies for new chambers and a greater potential for
retrofitting screens in existing chambers (Marples et al, 2007). This chamber configuration
was developed using Computational Fluid Dynamics (CFD), large scale physical testing and
post project appraisal. This work demonstrated that in many cases existing chambers could
be modified and screens retrofitted.
POST PROJECT APPRAISAL
To achieve an improvement in CSO design with a view to producing greater efficiencies,
Yorkshire Water undertook a number of post project appraisals to assess the performance of
recently constructed CSOs. This included undertaking an independent check on associated
sewer network, modelling, detailed design, site investigations and flow and event monitoring.
The results from a 2004 study (Hanson and Cutting, 2004) indicated that headloss and screen
loading rates determined from Yorkshire Water’s testing at Knostrop were appropriate.
However, it identified problems in cleaning static screens and further development of screen
cleaning equipment proved to be necessary. A further study in 2006 demonstrated the storage
volumes built to retain dissolved pollutants were appropriate in meeting receiving water
quality standards. However, where all pollutants to a receiving watercourse are considered
together, as in an integrated study for example, then savings in storage volume have proved
possible.
INNOVATIVE CSO SOLUTIONS
CSOs are sometimes located in awkward or constrained sites, where the construction of a
standard chamber would be particularly difficult and expensive. Within inner city areas, this
frequently occurs. Two particular examples are of Kirkgate in Wakefield and Thwaitegate in
Leeds.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Kirkgate screening chamber.
Kirkgate is located just to the east of Wakefield City Centre. The overflow discharges direct
to the River Calder (Figure 2) via four siphons. The design inflow to the CSO was 9m3/s with
a spill flow of 7.5m3/s. The CSO in the past was screened using 12mm aperture vertical raked
bar screens, which at times failed because of poor flow patterns in the CSO chamber and
uneven loading to the screens. As the spill flow was greater than 2m3/s Yorkshire Water’s
policy was to undertake physical or CFD modeling in such cases to gain a better
understanding of the flow characteristics. Physical modeling was selected over CFD due to
the component interactions with the CSO and the flows passing through the siphons were
selected to undertake the physical modeling. Scaling in physical modeling is always critical,
Hydrotec Consultants Ltd appointed to undertake the modeling selected a scale of 1 to 8 on
this occasion as the most appropriate to meet the varying critical conditions in the chamber
(entry, velocities etc). On this occasion, a Froude scale was considered critical to define the
hydraulic characteristics with negligible scale errors as long as the flow regime was fully
turbulent within the model (Hydrotec, 2005)
Figure 2 (left) Location of Kirkgate CSO housed within a large above ground building as
indicated by arrow and (right) solution being tested in the laboratory using 6 mecmex screens
Due to the site constraints (including the building being surrounded by major services) it was
important to keep the new CSO within the same footprint. The outline solution was
developed initially by Yorkshire Water, MWH and the screen suppliers. The result was a
bank of 6 mechanically brushed screens, as shown in Figure 2, however the physical
modeling indicated potential problems with the priming of the four siphons, and this led to a
rearrangement of the weir shape and position relative to the siphons. This allowed the
installation of new weirs and screens within the existing chamber, and the retention of the
existing siphons. The savings on cost, compared with a new build solution, were in excess of
£0.5M.
Thwaitegate Screening Chamber
The second example resulted in the simultaneous upgrading of 9 CSOs at Thwaitegate in
Leeds, by screening the common outfall. Individual solutions in the catchment would have
resulted in a costly and disruptive scheme that would have failed to meet the contractual and
regulatory output date. Only one location was available where all the flows joined up
resulting in 9.5m3/s inflow in a 1 in 5 year storm event. Due to the high flows, it was
necessary to undertake more detailed modeling. CFD was used to identify flow patterns and
flow split for the solution (Figure 3). CFD has commonly been used before to model such
chambers with side weirs (Burt et al, 2002) using the Eularian multiphase model to replicate
solid deposition most appropriately. The benefit of CFD in this case over physical modeling
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Combined Sewer Overflows - Do they have a Future?
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
was that it could be undertaken in a shorter time scale. The solution utilized an escalator
screen for low flows, and four mechanically brushed mesh screens to screen the larger storm
flows. The storm screens passed forward flow and solids to a pump which returned them to a
parallel combined sewer system for disposal.
The CFD importantly enabled the shape of the chamber to be optimized, especially the
position of a “splitting” wall and benching to enable an equal distribution of flow. Potential
drop out zones were also identified where sediments could be expected enabling benching to
be strategically positioned and hoppers designed to help reduce deposition. The final solution
resulted in a cost saving of approximately £0.8M.
Figure 3 Thwaitegate Screening chamber. (left) CFD modelling of the chamber with flows
from left to right and (right) screening chamber under construction, four Huber ROK2 screens
in the centre and a Longwood Escalator screen bottom centre
Utilizing CFD and physical modelling for the more complex schemes has reaped substantial
benefits. Although design costs are significantly higher, a more robust product has resulted
and greater efficiencies achieved.
FUTURE DEVELOPMENTS IN CSO DESIGN
Major improvements in CSO design is now unlikely to occur. The WaPUG CSO chamber is
at its most optimum and smallest size possible, and screening technology is unlikely to make
major further developments, only improving on quality.
A key aspect though is likely to be providing power efficiencies, hence reducing operating
costs, at a time when energy prices are increasing substantially. To this end, more renewable
sources of energy to drive screen motors are likely to come to the fore. A recent scheme
completed in Yorkshire saw the innovative use of a wind turbine and solar cells (Figure 4) to
charge batteries which subsequently supplied the Hydrok Mecmex screen. This was driven
because of the remote nature of the site, and the cost (financially and environmentally) of
bringing a power supply to its location. However, the potential benefits of such an approach
are far greater, and the introduction of such alternative power supplies will become a more
realistic solution, particularly as renewable technology moves further on. A far greater
challenge may be to construct equipment robust enough to survive the challenges of potential
mindless vandalism.
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Figure 4 Wind turbine and solar cells to charge batteries in the kiosk for the Hydrok
Mecmex screen in a remote area
CONSEQUENTIAL IMPACTS OF RETAINING POLLUTANTS
The introduction of screens into CSO chambers has resulted in substantial improvements to
the receiving water. This though creates a new challenge now being faced further
downstream in the network. The solids retained in the system predominantly include faeces
and toilet tissue, sanitary protection items and baby wipes. These solids are now prevented
from leaving the system and so are presented to CSOs further downstream in the catchment
and in particular to inlet screens (figure 5). The increase in solids loading now seen at
wastewater treatment works is considerable, particularly during the first foul flush, as
modeled using GROSSIM (Digman et al, 2002). This places substantial pressure on inlet
screens and solids handling equipment. In some, additional screening equipment and/or
additional maintenance have been required.
WHERE NEXT FOR CSOS – FUTURE CHALLENGES
From the various post project appraisals (Hanson and Cutting, 2004, Marples et al, 2007) and
general feed back from operations teams, it is clear that the new CSO structures constructed
over the last 5 years have proved very reliable. They have substantially contributed to an
overall improvement in urban river water quality in the UK. For the future, it is unlikely that
significant further achievements can be achieved using the same approach. It may be possible
to retain more solids by reducing screen mesh size, but current meshes are retaining large
volumes of visible solids. In addition, this would result in the need for larger chambers and
screens to minimize the extra headloss generated from the smaller mesh size. For the finer
pollutants additional storage is likely to result in diminishing returns on investment.
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Combined Sewer Overflows - Do they have a Future?
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
8000
7000
TT post AMP3
Solids Loading (g/s)
6000
TT pre AMP3
5000
4000
3000
2000
1000
0
08:00 08:20 08:40 09:00 09:20 09:40 10:00 10:20 10:40 11:00 11:20 11:40 12:00
Time
Figure 5 Example of solids loading at a wastewater treatment works pre and post CSO
upgrading during a 1 in 10 year return period event with 10 days antecedent dry weather
period
As further improvements to receiving water quality are needed, in response to the EU Water
Framework Directive (EU Directive No. 2000/60/EC 2000), other means of reducing CSO
discharges will be required. The construction of large storage tanks to reduce discharges and
then be treated downstream is likely to be unsustainable from a construction and operational
view point.
A major alternative for traditional below ground construction would be to consider major
separation of the sewer system. The UK is predominantly served by a combined sewer, and
currently separation is not common place. The actual cost benefit requires detailed analysis
however a recent costing estimate for separating the sewer system in London by Ashley et al
(2007) estimated the cost at £12-20 Billion. In addition to the cost, constructing a new sewer
creates severe disruption in the local area and the identification of correct connections can be
particularly challenging, time consuming and expensive. However the resulting flow
reduction would provide a substantial savings through lower energy use at WwTW as well as
a reduction in storm tank discharges.
However, a particular challenge is the pollution impact of surface water discharges to a
receiving watercourse. The discharged flow in terms of quantity and quality from traditional
surface water sewers can lead to deterioration in the receiving water by increasing diffuse
pollution and physically altering the watercourse. A German study identified that the
pollution load from surface water sewer discharges can be significant due to heavy metals and
COD (Brombach et al 2004). In the future the pollutant load from surface water discharges
may require to be treated (eg through end of pipe solution (Ellis et al, 2003 and Heal et al,
2005) to meet standards set in the Water Framework Directive. Therefore care will be
required if sewers are separated and end of pipe solutions may need to be sought.
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The potential of flow reduction methods by retrofitting sustainable drainage (Figure 6) and
using source control have and continue to be investigated (Atkins, 2004, DTI, 2006, City of
Bradford Metropolitan District Council, 2007, Ashley et al, 2007). However, numerous
systems have successfully been retrofitted into urbanized areas, in particular in Europe,
producing greater benefits for the communities. Such measures will remove a proportion of
surface water run-off from combined systems and may in some cases lead to a complete
separation of sewerage. The economics of various strategies have not yet been fully worked
through, but it is clear that where new sewerage is being provided as part of urban
regeneration, flow reduction and surface water separation can be cost effective. In addition,
through the retrofitting of Best Management Practices, pollution within surface run-off can
also be tackled.
Flow reduction also has the added advantage that CSOs will only operate very infrequently,
“peak lopping” the flow hydrograph for the more extreme events. Thus CSOs will revert to
their original concept of providing essential flood relief for the more extreme storm events
whilst preventing pollution of receiving waters through infrequent operation.
The CSO is therefore likely to remain as long as combined sewerage systems remain, though
their role will change to delivering strategic flood relief, rather than an outlet for frequently
overloaded systems
Figure 6 Example of sustainable drainage systems retorifitted into the urban environment in
Holland
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CONCLUSIONS
•
•
•
•
The development of the CSO has progressed substantially over the years, although its
function has primarily remained the same. The introduction of screens and storage has
substantially reduced the discharge of solids from the combined system. However, as
these solids are retained in the system the ‘solids problem’ has been passed
downstream, and is requiring further investment.
Physical modeling and CFD offer substantial opportunities to test out designs which
are non-standard. The use of such techniques not only improves the designs, it can
create substantial cost savings and further enhance knowledge.
The use of renewable energy sources to power CSOs should be considered more
frequently in the future.
CSOs are likely to remain, however with the need to meet the Water Framework
Directive in future years, there is likely to be a drive to reduce inflows to the
combined system. The construction of large storage volumes to reduce discharges and
then be treated downstream is an unsustainable process. The reduction of inflows
offers a more holistic solution and therefore will reduce the frequency CSOs operate.
DISCLAIMER
The views in this paper are those of the authors and do not necessarily represent the views of
Yorkshire Water or MWH.
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