FIELD EVALUATION OF NOVEL WET-WEATHER SCREENING SYSTEMS

The effects of the regulatory requirements for intermittent wet-weather discharges in Europe leading to the evolution of screening criteria, such as the Asset Management Plan (AMP2) requirements in England and Wales, are discussed. The resulting focus on aesthetic pollutants (including floatables) is set in context with regards to a holistic approach to resolution of environmental problems associated with CSOs and other intermittent wet-weather discharges. The most stringent consent standard (relating to discharges to ‘high amenity’ waters) calls for separation, from the effluent, of a significant quantity of persistent material and faecal/organic solids greater than 6 mm (1/4 inch) in any two dimensions.Recent developments in CSO technologies are outlined, particularly screening devices for the control of aesthetic pollutants. The characteristics of the ideal intermittent wet-weather screening system are described together with the development and evaluation of a novel self-cleansing CSO device - The Hydro-Jet Screen™.Results of independent field testing at the UK National CSO Test Facility at Hoscar Wastewater Treatment Works, Wigan, of the commonly used CSO chambers and the Hydro-Jet Screen™ are presented and compared. The results demonstrate that the commonly used chambers do not meet regulatory requirements and the use of a self-cleansing screening system such as the Hydro-Jet Screen™ provides significant performance enhancements. An assessment of mesh screens with differing aperture sizes - 1mm (1/24 inch), 2 mm (1/12 inch), 4 mm (1/6 inch) and 6 mm (1/4 inch) respectively - show the novel back-washing feature of the Hydro-Jet Screen™ to be effective over a range of mesh aperture sizes. The results suggest the optimum mesh configuration for CSO screening to meet the AMP2 requirements to be 4 mm (1/6 inch). The 4 mm (1/6 inch) mesh provided significant performance gains (in terms of gross solids removals) over the 6 mm (1/4 inch) mesh without incurring a penalty in hydraulic capacity.Recognition of the need for adopting sustainability principles in the coming millennium coupled with lower capital and operating costs associated with non-powered self-cleansing CSO screening systems, such as the Hydro-Jet Screen™, is used as a basis for arguing their superiority.