Optimum Backwashing of Filters with Air Scour: A Review

Abstract

The problems occurring in the filtration facilities of water treatment plants, such as mudballs and poor initial filtrate quality are associated with the effectiveness of backwashing. Backwashing with water alone to fluidize the media is an inherently weak cleaning process. The effectiveness of backwashing is significantly improved by the use of air scour simultaneously with subfluidization water wash. When air and subfluidization water flowed concurrently through porous media, a condition termed “collapse- pulsing” occurred in which air pockets formed and collapsed within the bed. A theory has been developed that predicted the combinations of air and water flows that caused collapse-pulsing to occur. Model filter systems have shown that the fluid mechanical condition termed “collapse-pulsing” is also the condition of optimum removal of particles from the media grains during backwashing. The theoretical aspects of collapse-pulsing have been confirmed with an experimental filter system having an endoscope, high speed video system with freeze frame capability for visual observations and an XY Coordinator with computer hardware and software which enabled the determination of velocities of filter grains during air scour. Additional studies on optimum removal of particles from media during air scour have also demonstrated that the results may be extended to dual media filters and are valid at the plant scale too.