10 Years Experience of CSO Management in the United Kingdom

The United Kingdom’s ageing sewerage infrastructure meant that by the late 20th century the country was facing a major environmental problem associated with storm sewage discharges from more than 8,000 unsatisfactory combined sewer overflows (CSOs). Addressing this problem presented a major challenge to the UK water industry, but it is a challenge that has been met by a combination of farsighted measures which have included both reorganisation of the institutional arrangements of the industry and the development of sophisticated and integrated planning and management tools. As a consequence, the UK is now nearing the end of a multi-billion dollar investment programme which will see the end of the CSO problem in terms of both visual and water quality issues.

The paper reviews the key steps that have brought this about over the last decade and highlights the changes in approach that the industry and water professionals have had to make to allow it to happen. Finally, the paper considers the extent to which the lessons learnt could be applied beyond the UK and, in particular, in the U.S.

The urban wastewater infrastructure of the United Kingdom has evolved since the dawn of the industrial revolution in the first half of the 19th century up to the present day. The open gutters and natural watercourses that drained early industrial conurbations evolved into the culverted watercourses and combined sewers which still constitute the core of the sewer systems that serve almost all of the UK’s major cities. Given the very different duties to which these conduits are now subjected, it is hardly surprising that in more recent years there have been numerous environmental problems associated with combined sewer overflow (CSO) and other wet weather discharges from urban wastewater systems. At the beginning of the 1990’s it was estimated that there were around 25,000 CSOs in England and Wales and that some 8,000 were causing pollution problems. The cost of improving these CSOs was forecast to be in the order of $4.5 billions.

The Urban Wastewater Treatment Directive (CEC, 1991) requires European Union member states to take action to limit pollution from storm overflows and to improve unsatisfactory intermittent wet weather discharges from combined Sewer Overflows (CSOs) and storm tanks at Wastewater Treatment Plants (WTPs). In addition, there now is the wide reaching Water Framework Directive (CEC, 2000), which requires that all receiving waters achieve “good status” in terms of environmental and ecological targets by 2015. While there are no specific requirements, such wet weather intermittent discharges can influence the achievement of these targets and also contribute to failures to meet standards set by other Directives, such as the Freshwater Fish, Bathing Water and Shellfish Waters Directives. As a member of the European Union, the UK is obliged to meet the requirements of these, and all other Directives, emanating from the European Commission.

Historically, CSOs in the UK, if they were designed on any rational basis at all, were constructed to discharge at fixed multiples of the dry weather flow (dwf) in the sewer, usually 6 times dwf. Flows in excess of 6 times dwf would be spilled and the remainder carried forward to the treatment plant. Problems arising from this pragmatic approach were recognised as early as the 1960s when a more rational approach, known as ‘Formula A’ (HMSO, 1970), was proposed to calculate an acceptable pass forward flow from CSOs. This formula is still in use today as a minimum CSO spill setting criteria. Also, in the 1960s, new engineering designs for overflows structures incorporating the use of storage, and stilling pond or high side weir overflows were adopted to provide for retention of pollution in the sewer system. An early method for estimating storage requirements was developed in the late 1970s. Referred to as the Scottish Development Department method (SDD, 1977), this is based on a Formula A spill threshold, plus storage within the CSO related to the available dilution in the river and the upstream sewer population equivalent. Neither the Formula A nor the SDD approach can be used directly to ensure compliance with environmental quality standards in the receiving water.

Unsatisfactory performance of CSOs is currently defined in the UK on the basis of the following criteria (DETR, 1997):

  • causes significant visual or aesthetic impact due to solids or fungus in the receiving water or there is a history of justified public complaint; or,
  • makes a significant contribution to a deterioration in river quality; or,
  • makes a significant contribution to a failure to comply with the quality standards set by the EC Bathing Waters Directive and Shellfish Waters Directive Quality Standards for identified bathing and shellfish waters, respectively; or,
  • discharges in dry weather conditions; or,
  • causes a breach of water quality standards.

This paper identifies the advances that have occurred in the UK in recent years to meet the challenges presented by the new legislation and raised public expectations for improved pollution control and environmental standards. The paper highlights the enhanced regulatory, financial and technological capabilities that have been made to allow progress to be made in the control of CSO discharges. In particular, it describes the Urban Pollution Management (UPM) Procedure that has been the principal vehicle by which these advances have been achieved in the UK. The paper also examines objectively the experience which has been gained, particularly in the last 5 years, in the practical application of this technology. In the latter part of the paper consideration is given to whether these developments could have application in places other than the UK, and in particular in North America.

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