Advances in Water Treatment and Enviromental Management

RIBBLE ESTUARY WATER QUALITY IMPROVEMENTS 251works at a rate of 6DWF. Discharge flows in excess of those for the 1 in 5 year stormto sea via a short outfall (about 1 km) near the bathing waters.For all these options it was necessary to determine the amount of storage that would berequired to limit the number of discharges from any combined sewer overflows, to inland ormarine waters, to a frequency that would be acceptable in terms of general environmentalimpact, and in the case of those to sea to that which would not prejudice bathing waterquality.Options for Treating Storm FlowsNorth bankFor Fairhaven, investigations concentrated on determining which storms would lead to spillsfrom the tanks outside the 3 hour period following high water (the tidal window), whendischarges receive maximum dilution and tend to move offshore away from the bathing area.This involved modelling the effect, in terms of probable spill volumes, of all the time seriessummer storms, starting at hourly intervals through the tidal cycle. After combining some ofthe output to take account of similar spill volumes from different storms starting at differenttidal states, an analysis of rainfall data from nearby Blackpool airport (Head et al., in press)was used to determine the number of spills to be expected during an average summer. FromFigure 5 it can be seen that increasing the existing storage from 10 000 m 3 to around 25 000m 3 would dramatically reduce the number of spills liable to occur. Above this figure thebenefit in terms of a reduction in the numbers of spills diminishes. Increasing the tidal windowfrom 3 to 4 hours has a relatively small effect on spill frequency, whereas the removal of3DWF for treatment elsewhere would greatly reduce the number of spills.A much more limited examination of the storm water management with regard to receivingwater quality was required in the case of Preston, as the initial investigations showed that thebacterial loads from the combined sewer overflows on the sewerage system only affected theupper and middle estuary, and that significant numbers of bacteria did not reach the bathingareas.Figure 5 Relationships between storm water storage requirements and number ofdischarges during an average summer for Fairhaven Tanks