THE SCREEN EFFICIENCY OF A NOVEL SELF-CLEANSING CSO

The paper presents the results of testing of a novel self-cleaning CSO device at the UK National CSO Test Facility at Hoscar Wastewater Treatment Works, Wigan, when fitted with mesh screens of aperture sizes of 6 mm, 4 mm, 2 mm and 1 mm. The current standard in the UK for Combined Sewer Overflows (CSOs) and other intermittent discharges is based around the removal of aesthetic pollutants and the reduction of spill frequency. The most stringent consent standards (relating to discharges to high amenity waters) require a 'significant removal' of all gross solids, these being defined as particulate material, 6 mm in 2 directions or larger. The results demonstrate that a significant performance gain can be achieved using a 4 mm aperture screen in place of a 6 mm screen but that the 2 mm screen does not offer a similar gain over the 4 mm screen. Initial testing with a 1mm screen indicates another significant gain in performance but with the penalty of a reduced hydraulic loading rate. It is concluded that CSO screening systems offering a 4 mm two directional screening standard can be significantly more efficient than 6 mm screens and may not entail any additional cost. The performance of a 4 mm screening system currently represents BATNEEC (Best Available Technology Not Entailing Excessive Cost) for a CSO screening device.