THE THAMES TIDEWAY STRATEGIC STUDY

There has been an ongoing national improvement programme of unsatisfactory storm discharges since privatisation of the UK water industry in 1989. Consequently, most major conurbations have seen substantial improvement but London, with its Victorian sewerage legacy, remained the exception and it was agreed that a review should be carried out of the storm sewage discharges to the tidal River Thames. The Thames Tideway Strategic Study (TTSS) was set up as a 3-year project to assess the environmental impact of intermittent discharges of storm sewage on the Tideway, identify objectives for improvement and propose potential solutions, having regard to cost benefit. The key objectives of the TTSS are reduction of sewage-derived litter, preservation and improvement of water quality for bathing and recreational activities and maintenance of dissolved oxygen concentrations to support a sustainable fish population. Four broad strategies were considered for reducing the impact of the combined storm overflow (CSO) discharges;Before the sewerage system,Within the sewerage system,At the CSO/river interface,Within the river.The only viable strategy was at the interface between CSOs and the river. The solution options within this strategy comprise some combination of flow interception, storage and/or transfer of flow, and treatment. Eight different options were identified and a shortlist of five was tested for compliance with the objectives. A Cost Benefit study was also implemented to identify and evaluate the costs/benefits of the various proposed solutions. Only one CSO solution satisfied all hydraulic and operational requirements, showed modelled compliance with the objectives and gave a favourable cost benefit analysis. This involves the construction of a large diameter tunnel to provide storage and attenuation prior to pumping out for screening and treatment and discharge to the river.