Advances in Water Treatment and Enviromental Management

250 WATER TREATMENTFigure 4. Effects combined discharges from Preston and Fairhaven following variouslevels of bacterial removal. Dispersion model simulations, E. coli concentrationsgreater than 2 000 per 100 ml: a 95 per cent removal, b 99 per cent removal.South bankThe existing drainage system for Southport is designed to convey flows of combined foul andstorm sewage for primary treatment at a works discharging via Crossens Pool to the Ribbleestuary. Although the effluent from this works comprises a large proportion of the flow inCrossens Pool, leading to local oxygen depletion problems, particularly in the summer, itseffects on bathing water quality are small. Inadequate capacity of the existing sewerage systemleads to premature operation of existing combined sewer overflows and the flooding ofproperties. In planning improvements to the system to alleviate these problems, it wasnecessary to determine how best to deal with the base flow so that the effects of discharges ofstorm water on bathing water quality could be reduced to a level compatible with therequirements of the Bathing Water Directive. Since only part of the system involves combinedoverflows discharging to areas near the bathing waters, it was necessary to consider conveyingdifferent multiples of the base flow to the site of the existing works, depending on whether theoverflows were to inland water courses or to the sea. Additionally the possibility ofconstructing a long sea outfall for some or all of the base or storm flows needed to beconsidered. After taking into account the various constraints associated with existing landuse and obtaining permissions for new constructions it was possible to narrow the options tobe considered to:convey all flows up to a 1 in 5 year storm, from areas where overflows would need tobe to the sea, to the existing sewage works site. Allow volumes in excess of these tobe discharged via a relatively short outfall (about 1 km) near the bathing waters. Forareas with overflows to inland water courses, transfer approximately 6DWF to theexisting sewage works site. Discharge flows in excess of 6DWF arriving at the worksvia an overflow to Crossens Pool;convey all flows from areas where overflows would need to be to the sea to a coastalsite for pretreatment and possibly storage before discharging to the sea. Dischargevolumes up to a 1 in 5 year storm via a long outfall (about 4.5 km) and those inexcess of this via a short outfall (about 1 km) near the bathing waters. Transfer flowsof up to 6DWF from inland areas to the existing sewage works site;convey flows up to 6DWF from all areas to the existing sewage works site. Dischargeflows of between 6DWF and a 1 in 5 year storm via a long sea outfall (about 4.5 km),with or without storage to limit the time of the discharge; or store and transfer to the