Clarifier Scum Removal Can Be Both Simple and Efficient

The presence of floating scum and trash has troubled many wastewater treatment facilities. These accumulations on circular primary clarifiers, secondary clarifiers, and sludge thickeners are an unsightly nuisance, reduce performance, and cause odors. The added Operation and Maintenance effort—sometimes unsuccessful—can cause a deterioration of the staff morale and adversely impact other plant operations. Skimmers have become more complex and costly, full radial and ducking types, in an attempt to solve these problems. However, a review of conventional scum skimmers revealed that they were not optimized to ensure that flotables could be rapidly moved to the scum ramp/trough. The common problems were low angles of attack of the skimmer, undersized ramps for scum removal, and adverse effects of wind. The recommended skimmer has a bend at a radial point which results in a 25–30° angle of attack and 200–300% higher transport capacity than the tangential skimmer. The paper provides a methodology to establish the quantity of flotables to be removed, design of the bent skimmer and sizing of the scum ramp/trough in a series of tables and graphs.