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Re-inventing CSOs and implementing SUDs are key to meeting WFD challenges. Professor David Balmforth, technical director of MWH, explains why

Combined 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 screens. This has controlled the frequency and volume of spill and prevented visible pollutants entering receiving waters. This upgrading programme has made a major contribution to

the clean-up of UK rivers and coastal areas. However, the industry now faces new challenges.

WFD and the carbon reduction challenge

The UK Government has set a carbon reduction target of 80% to be achieved by 2050. The EU has introduced its Water Framework Directive (WFD) that requires inland water bodies to achieve good quality status, population is growing and climate change means that storm flows are likely to increase significantly. A major reconstruction of our sewerage infrastructure would run counter to a carbon reduction agenda, as well as being highly disruptive and probably unaffordable. But without this CSOs are set to discharge more frequently, causing increased pollution, which runs counter to the requirements of the WFD. And blocking up CSOs is not the answer as major flooding would occur.

So what is the answer? For a number of years, other countries have been reinventing their drainage systems, using techniques that mimic natural systems.

They keep surface run-off separate from wastewater and where practical manage these separate surface water flows above ground. Both flow and water quality is

managed locally, using surface features such as storage ponds and wetlands.

Such sustainable drainage systems (SUDs) are now being featured in new development, but legislative change is needed if they are to become widely adopted. The Government’s new Floods and Water Bill is set to deliver this change. However the real challenge is not to get SUDs into new development but to retrofit them into existing urban areas so that the flows in our combined sewerage systems can be reduced to offset the effects of climate change and urban growth. This requires the various bodies responsible for urban drainage to work together more effectively, and the Defra Integrated Urban Drainage Pilots have demonstrated what can be achieved when this happens.

Imaginative applications of retrofit approach

Our experience working in Europe and Holland in particular has shown that the best way to separate surface water and implement flow reduction strategies using SUDs is to integrate them with general urban regeneration work. This allows the whole urban design to benefit from improved drainage management and the amenity value of surface water to be fully exploited. Often the new urban layout is very different from what preceded. For example there will be many more surface water features such as small ponds and wetlands. Another key learning point is that those who ultimately will live and work in the ‘retrofit’ area need to be engaged early in the planning process to ensure the final design is compatible with their culture and life style. Public engagement becomes a vital part of the design process.

MWH has been pioneering this retrofit approach in Holland and plan to transfer these techniques into the UK. We believe that flow reduction strategies using SUDs provides a sustainable means of addressing the future challenges facing urban drainage. A number of water companies are now actively pursuing pilots and with imagination, these new approaches will not only solve our drainage issues, but will substantially enhance the amenity value of our urban areas.

David Balmforth is a technical director with leading environment and water engineering specialists, MWH, and a visiting professor at Imperial College. For more information please email [email protected] or visit

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