Advances in Water Treatment and Enviromental Management

Chapter 24RIBBLE ESTUARY WATER QUALITYIMPROVEMENTS: AN INTEGRATEDWASTE WATER MANAGEMENT PLANTO ACHIEVE BATHING WATERCOMPLIANCEP C Head and D H Crawshaw (North West Water Ltd, Warrington, UK),S K Rasaratnam and J R Klunder (North West Water Ltd, Preston, UK)As part of the United Kingdom’s programme to bring its identified bathing waters up to thestandard required by the EC Bathing Water Directive, North West Water has developed anintegrated waste water management plan to improve the water quality of the bathing beachessituated at the mouth of the Ribble Estuary. This plan is based on an extensive investigationto determine the most appropriate way of dealing with both base flows (flows during dryweather or moderate rain), and storm flows (overflows from combined sewerage systems duringwet weather). Mathematical models were used to examine the hydraulic behaviour of thesewerage systems and the subsequent dispersion of effluent into the receiving waters.To determine the discharge characteristics of the existing sewerage systems discharging tothe estuary, extensive hydraulic models were set up and validated by means of flowmeasurements gathered from critical points of the sewer networks. These models were thenused, in conjunction with a time series rainfall record for the area, to investigate the effect ofthe intensity and duration of storm events on the volumes of sewage to be discharged. Estimatesof the base and storm flows in the systems, derived from the hydraulic models, were used todetermine the inputs to the dispersion model.Dispersion modelling of various possible waste water management schemes, involving differentdegrees of sewage treatment and long sea outfalls, individually and in combination, inconjunction with tracer releases demonstrated that the use of a long sea outfall for all or partof the base flow was unlikely to lead to consistent compliance at the bathing waters situatedat the mouth of the estuary. A suitable degree of compliance could be achieved by means ofa sewage treatment scheme designed to remove at least 99 per cent of the bacterial load fromthe largest input.INTRODUCTIONThe well known seaside resorts of Southport and Lytham St Anne’s, which are situated nearthe mouth of the Ribble estuary (see Figure 1) were not included in the original list of 27bathing waters submitted by the UK Government to the European Commission in 1979 fordesignation under the Bathing Water Directive (Council of European Communities, 1976).This caused considerable local concern which led, in 1986, to a formal complaint being madeto the Commission and a national review of how the Directive should be applied. As a resultof this review, which identified more than 350 additional beaches, an extensive samplingprogramme was instigated to determine the extent to which these bathing waters compliedwith the microbiological standards contained in the Directive. This confirmed earlier studies©1991 Elsevier Science Publishers Ltd, EnglandWater Treatment—Proceedings of the 1st International Conference, pp. 245–254245