URBAN DRAINAGE
Back to the future
Re-inventing CSOs and implementing SUDs are key to meeting WFD challenges.
Professor David Balmforth, technical director of MWH, explains why
C
ombined Sewer Overflows (CSOs) have
been used on combined sewerage
systems throughout the world to manage
excess flow during rainstorm events, the excess
flow being discharged to a local watercourse or
directly to the sea. This is because in general it is
uneconomic to build sewerage systems
with the capacity to convey all the flow to
treatment works during storm events, and
in any case it prevents treatment works from
becoming overloaded.
However, as urban areas have grown, CSOs
have been spilling over more frequently causing
pollution. Addressing this pollution has
accounted for a significant portion of water
company investment in recent years. Indeed,
over the past eight years, MWH alone has been
involved in upgrading or redesigning more than
1,000 CSOs as part of its work for major UK
water company clients.
Most CSOs now achieve a minimum flow
prescribed by “Formula A” before they discharge
and are fitted with modern 6mm aperture Incorporating green roofs into developments helps cut down on excess run-off
screens. This has controlled the frequency and
volume of spill and prevented visible pollutants manage these separate surface water flows amenity value of surface water to be fully
entering receiving waters. This upgrading above ground. Both flow and water quality is exploited. Often the new urban layout is very
programme has made a major contribution to managed locally, using surface features such as different from what preceded. For example there
the clean-up of UK rivers and coastal storage ponds and wetlands. Such sustainable will be many more surface water features such
areas. However, the industry now faces drainage systems (SUDs) are now being as small ponds and wetlands. Another key
new challenges. featured in new development, but legislative learning point is that those who ultimately will
change is needed if they are to become widely live and work in the ‘retrofit’ area need to be
WFD and the carbon adopted. The Government’s new Floods and engaged early in the planning process to ensure
reduction challenge Water Bill is set to deliver this change. the final design is compatible with their culture
The UK Government has set a carbon reduction However the real challenge is not to get SUDs and life style. Public engagement becomes a vital
target of 80% to be achieved by 2050. The EU into new development but to retrofit them into part of the design process.
has introduced its Water Framework Directive existing urban areas so that the flows in our MWH has been pioneering this retrofit
(WFD) that requires inland water bodies to combined sewerage systems can be reduced to approach in Holland and plan to transfer these
achieve good quality status, population is offset the effects of climate change and urban techniques into the UK. We believe that flow
growing and climate change means that storm growth. This requires the various bodies reduction strategies using SUDs provides a
flows are likely to increase significantly. A major responsible for urban drainage to work together sustainable means of addressing the future
reconstruction of our sewerage infrastructure more effectively, and the Defra Integrated Urban challenges facing urban drainage. A number of
would run counter to a carbon reduction agenda, Drainage Pilots have demonstrated what can be water companies are now actively pursuing pilots
as well as being highly disruptive and probably achieved when this happens. and with imagination, these new approaches will
unaffordable. But without this CSOs are set to not only solve our drainage issues, but will
discharge more frequently, causing increased Imaginative applications of substantially enhance the amenity value of our
pollution, which runs counter to the retrofit approach urban areas. ■■■
requirements of the WFD. And blocking up CSOs Our experience working in Europe and Holland
is not the answer as major flooding would occur. in particular has shown that the best way to David Balmforth is a technical director with
So what is the answer? For a number of years, separate surface water and implement flow leading environment and water engineering
other countries have been reinventing their reduction strategies using SUDs is to integrate specialists, MWH, and a visiting professor at
drainage systems, using techniques that mimic them with general urban regeneration work. This Imperial College. For more information
natural systems. They keep surface run-off allows the whole urban design to benefit from please email mwhuk.info@mwhglobal.com
separate from wastewater and where practical improved drainage management and the or visit www.mwhglobal.com
September 2009 Water & Wastewater Treatment 33
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