Southern Water is one of ten regional water authorities in England and Wales, and as part of its river basin management role it is responsible for sewage disposal in the counties of Hampshire, Sussex, Kent and the Isle of Wight. The volume of sewage effluent which SWA has to deal with amounts to some 1100 megalitres per day (ml d−1 from a population of 3 7 million, and 70% is discharged directly into estuaries and coastal waters following treatment or partial treatment.

The standards to which each individual discharge must conform are set having regard to the nature of the receiving waters, whether they be rivers, estuaries or the sea Effluent discharges entering rivers and estuaries are generally controlled in terms of their biochemical oxygen demand (BOD), suspended solids, and ammonia loads Appropriate discharge consents for these are set having regard to river and estuary quality Rivers and estuaries are accorded a quality standard based on the 1979 National Water Council Classification (ref 1) Some 2200 km of rivers in Southern Water's area, represented by some 540 river stretches, come within this classification system, of which 1460 km meet the standards of Classes 1A and 1B (good quality) and 700 km meet Class 2 (fair quality) Likewise, 380 km of its estuaries lie within the NWC Estuary Classification (ref 21, of which 320 km are assigned to Class A (good quality) and 60 km to Class B (fair quality).

Unlike discharges to rivers and estuaries, those made to the sea seldom receive any treatment other than screening and maceration of solids The design and siting of an outfall must ensure that the EC directives on the quality of bathing waters (ref 3), the discharge of dangerous substances (ref 4) and the quality of shell fish waters (ref 5) are complied with

The effects of proposed discharges on non-tidal rivers are usually assessed on the basis of river flow, existing quality, and the proposed effluent loadings Stochastic techniques can be used to assess more precisely the effluent quality required to ensure 95% overall compliance However, assessing the effects of discharges into tidal waters is more complicated because of the tidal regimes and !he complex biochemical processes that occur Increasingly, ematical models are employed to help assess these effects, and the nature of the receiving waters determines the type of model which is needed.

This chapter describes the environmental effects of sewage discharges into estuarine and coastal waters Two case studies illustrate the mathematical modelling techniques currently being used within Southern Water to assist the design process The first case study describes a model of the River Stour Estuary, and its use in determining appropriate BOD and ammonia standards for a new sewage works serving Ramsgate The second case study describes a tidal model of the Solent, which has been used to simulate the dispersion of sewage from a new sea outfall at Cowes on the Isle of Wight

Fig. 1 The Stour Estuary, Kent (Available in full paper)

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