Increasing concern over the discharge of sewer solids from combined sewer overflows (CSOs) has encouraged the introduction of installations which incorporate either mesh or bar screen arrangements, or both. Gross solids create visual and aesthetic pollution if they reach watercourses. An entirely new arrangement for a CSO with a perforated screen of 6 mm diameter apertures is described in the paper. The arrangement is circular, with tangential flow across the screen to facilitate its cleaning. High velocities across the screen holes ensure that solids which have been prevented from discharging are swept clear from the screen. As a result, a sufficient screen area remains free from trapped solids to permit discharge of flow. The device has been designed to meet all anticipated performance criteria for the removal of solids, and to require a minimum of maintenance.

A thorough testing programme in different conditions is described. Laboratory testing utilised a wide range of gross solids and flows to determine efficiencies of solids removal under all anticipated operating conditions. The performance of the CSO in a sewer environment was evaluated at a sewage treatment works with flows up to 1801/s using both raw and treated sewage to simulate storm sewage of different strengths. Both the device and its testing are described, together with the results of the testing programme.

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