THE PERFORMANCE OF A NOVEL COMBINED SEWER OVERFLOW WITH PERFORATED CONICAL SCREEN

Increasing concern over the discharge of sewer solids from combined se wer overflows (CSOs) has encouraged the introduction of installations which incorporate either mesh or bar screen arrangements, or both. Gro ss solids create visual and aesthetic pollution if they reach watercou rses. An entirely new arrangement for a CSO with a perforated screen o f 6 mm diameter apertures is described in the paper. The arrangement i s circular, with tangential flow across the screen to facilitate its c leaning. High velocities across the screen holes ensure that solids wh ich have been prevented from discharging are swept clear from the scre en. As a result, a sufficient screen area remains free from trapped so lids to permit discharge of flow. The device has been designed to meet all anticipated performance criteria for the removal of solids, and t o require a minimum of maintenance. A thorough testing programme in di fferent conditions is described. Laboratory testing utilised a wide ra nge of gross solids and flows to determine efficiencies of solids remo val 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 180l/s using both raw and treated sewage to simulate storm sewage of different strengths. Both the device and its testing a re described, together with the results of the testing programme. (C) 1998 IAWQ. Published by Elsevier Science Ltd.