Investigation of the performance of combined sewer overflow screens.

A major objective of the NRA guidance note for controlling combined sewer overflow discharges (NRA, December 1993) is to minimise the presence of objectionable solids and persistent material in watercourses. The guidance note states 'this can be achieved by a number of means, for example the design of the overflow structure or the provision of screens' (NRA, December 1993). In his review of the performance of storm sewage overflow structures with respect to aesthetic criteria, O'Sullivan (1990) found that there was a shortage of information about the quantities of gross solids discharged from combined sewer overflows (CSOs) and there was no consistent approach to the use of screens on CSOs. He recommended that further research work should be done to evaluate the effectiveness and viability of screens at CSOs.A field and laboratory study was carried out to investigate the performance and efficiency of different types of CSO screens, identify the sources and type of gross polluting solids and identify the factors which influence screen performance. Two stilling pond CSO bar screens and two sewage treatment works (STW) inlet bar screens were evaluated by collecting the gross solids retained by each screen together with any gross solids passing through the screen. Flow data were obtained using flow survey equipment. A series of tests were also carried out on five 6 mm screen meshes at two STW sites and in the laboratory. Additional tests were performed on the five screen meshes in the laboratory to determine head losses. Results have shown that screen retention efficiency is dependent on the aperture size of the screen face. The larger the aperture of the screen face the lower the retention efficiency. Mechanically raked bar screens with 6 mm spacings were found to have a maximum overall retention efficiency of 30%. For 6 mm mesh screens this figure was 60%. The main polluting gross solids were found to be of dry weather flow origin. Fine paper, leaves, sanitary towels and tampons formed the bulk of the gross solids samples with condoms and cotton bud sticks forming less than 0.1% of the overall sample mass. There appears to be different transport mechanisms for different types of gross solids where some are continually transported in the sewerage system, whilst others require a threshold velocity of flow before being transported. The bulk of the gross solids arriving at a CSO chamber during a storm event was found to arise from the dry weather flow prior to the storm event. The total mass of gross solids presented to a CSO screen was shown to be dependent on the mean overflow intensity of the storm event and this relationship was used to develop a predictive model, based on the upstream population and average usage figures of sanitary products and toilet tissue. The research has also shown that prediction of the screen retention efficiencies obtained in the field is possible with full scale laboratory tests providing care is taken in the laboratory when simulating gross solids.