Low Energy Filtration Using Buoyant Media

Abstract

The use of a spherical buoyant media in filtration has many potential advantages. The media stratify during backwashing such that the largest spheres are at the top of the bed and the smallest are at the bottom. During upflow filtration solids may be removed by both straining on the underside of the filter and by depth filtration. The accumulated solids may be readiiy dislodged by a brief downflow backwash and gravity assists i n this cleaning process. Basic studies on the removal of coagulated clay suspensions indicated that the clay was removed throughout the depth (D.3m) of the filter for media sizes between 0.6 and 1.0 mm. Straining was not observed to be significant but it would become more important for smaller media sizes or more flocculant solids. Headloss development was monitored and followed the expected behavior for depth filtration. Low headlosses could be maintained by using shallow bed of media and backwashing frequently. These preliminary studies indicate that if the buoyant media are to be used in an effective low energy filter, the system design wi11 be very important. A straining mechanism may be preferred since the solids are retained on the underside of the filter and gravity wi1'l assist in cleaning the bed during backwash. However, to avoid rapid headloss development depth filtration must be used also. Additional studies are needed to characterize the backwashing behavior of the media and to identify the best operating strategies for long-term use.