OPTIMUM BACKWASHING OF FILTERS WITH AIR SCOUR - A REVIEW

The problems occurring in the filtration facilities of water treatment plants, such as mudballs and poor initial filtrate quality are associ ated with the effectiveness of backwashing. Backwashing with water alo ne to fluidize the media is an inherently weak cleaning process. The e ffectiveness of backwashing is significantly improved by the use of ai r scour simultaneously with subfluidization water wash. When air and s ubfluidization water flowed concurrently through porous media, a condi tion termed ''collapse- pulsing'' occurred in which air pockets formed and collapsed within the bed. A theory has been developed that predic ted the combinations of air and water flows that caused collapse-pulsi ng to occur. Model filter systems have shown that the fluid mechanical condition termed ''collapse-pulsing'' is also the condition of optimu m removal of particles from the media grains during backwashing. The t heoretical aspects of collapse-pulsing have been confirmed with an exp erimental filter system having an endoscope, high speed video system w ith freeze frame capability for visual observations and an XY Coordina tor with computer hardware and software which enabled the determinatio n 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 an d are valid at the plant scale too.