Grain Displacement during Backwash of Drinking Water Filters

Backwashing rapid sand filters causes inadvertent displacement of filter media grains from theirprevious depths. This displacement can affect the hydraulic function of filters by mixing or segregatingmedia grains, and the function of biofilters through displacement of active biomass and coatings fromproper depths. This study quantifies grain displacement in a pilot-scale filter using tracer grains ofcolored sand, glass beads, anthracite and garnet to determine the effect of grain size, density andshape on grain displacement. Statistical moments are used to describe the depth distributionsresulting from displacement during backwashing. Results show that significant grain displacementoccurs during backwash consisting of air scour, air-and-water wash and sub-fluidization water-onlywash. Here, displacement is largely independent of grain size, density and shape. When fluidizationbackwash is used, greater displacement and more dependence on grain characteristics is seen.A variety of grain movement phenomena can be observed during the backwashing steps, indicatingthat grain movement and therefore the resulting displacement is highly inhomogeneous in fourdimensions. These results have direct practical implications for the design of rapid sand filters and theoptimization of backwashing procedures, while suggesting that the current widespread backwashingpractice used in the case study country (Denmark) should be abandoned