Advances in Water Treatment and Enviromental Management

RIBBLE ESTUARY WATER QUALITY IMPROVEMENTS 247In these, quantities of dye (rhodamine) and bacterial spores (Bacillus globigii) were added tothe effluent from Southport Sewage Treatment Works (STW), Preston STW and the tidal sewagestorage tanks at Fairhaven. In all, tracer tests were carried on 6 occasions in 1988 and 1989,involving 2 releases from Southport and Preston and 4 from Fairhaven. The main findings ofthese tracer releases were:discharges from Preston STW could be detected in the outer estuary in the watersoff Lytham, but not off Southport;discharges from Southport STW were well dispersed in the outer estuary and werenot detectable in the waters off either Southport or Lytham;discharges from Fairhaven Tanks were detectable in the waters off Lytham but notSouthport, and during a period of strong south westerly winds could also be detectedin water further north along the Fylde Coast.From the determinations of the numbers of B. globigii found in the samples taken during thecourse of the tracer releases, it is probable that bacteria contained in the discharges fromPreston STW and Fairhaven Tanks would lead to concentrations in the waters off Lytham inexcess of those allowable by the Directive. The numbers of spores recovered from the watersoff Southport and the Fylde Coast indicate that any effect from the above sources is small andunlikely to result in failure to comply with the Directive.MODELLING OF THE SEWERAGE SYSTEMBackground and VerificationThe hydraulic performance of each of the sewerage systems draining to the existing outfallswas examined in conjunction with the Water Research Centre (WRc) using the WallingfordStorm Sewer Package (WASSP) (DOE/NWC, 1983). With this package it is possible to simulatethe effects of surcharging and flooding and to incorporate a wide range sewer ancillaries,including overflows and storage tanks. Information on the pipe networks and contributingareas was obtained from the local authorities. Surveys of flow and impermeable area werecoordinated by WRc, and soil moisture deficit values for calculating the urban catchmentwetness index were supplied by the Meteorological Office. The validity of the flows predictedby the model was checked by means of an extensive series of flow measurements carried outat key points on the sewer network. In all, data from some 50 flow monitors and 20 raingauges were used in the validation process. The hydrographs produced by the WASSP modellingwere used to calculate the inputs to the dispersion model for the various combinations oftreatments investigated.Storm Water ManagementSince it is necessary to ensure that any scheme to achieve bacteriological compliance of thebathing waters during the bathing season must be capable of dealing with all but the severeststorms {this is taken by the UK regulatory authority to include storm events of up to a 1 in 5year return period ) a considerable effort was directed towards modelling the effects of stormson the sewerage systems. This work provided estimates of the volumes of storm water liableto overflow from the existing sewerage systems under a wide range of rainfall conditions.Following on from the work previously carried out for the Fylde Coast studies (Crawshaw &Head, 1989; Head et a/., in press) hydrographs for use with the storm discharge dispersionmodel were obtained by running the WASSP models for Fairhaven and Preston with a sampleof 44 summer storms from a time series rainfall suite representing the storms of a typicalyear, and 177 summer storms derived from rainfall data at Blackpool Airport. In the case ofSouthport, the most severe summer storms from the annual time series were used inconjunction with 25 summer storms derived from a local 15 year rainfall record.DISPERSION MODELLINGBackgroundThe model used to simulate the dispersion of effluent from outfalls to the estuary and coastalwaters was developed by WRc. The dispersion processes are represented by a random walkprocedure superimposed on the water movements derived from a hydrodynamic model coveringthe majority of the Irish Sea. The hydrodynamic model was validated by comparison with