Observations of Particle Detachment during Filter Backwashing

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Abstract
The behaviour of filter grains and detaching deposits during backwashing was investigated at University College London using a High Speed Video and endoscope technique. A detailed understanding of the backwashing process was required after decades of backwashing on a trial and error basis. This investigation provided detailed visual information on detaching deposits and bed behaviour. Experiments were performed using standard filter sand and synthetic suspensions. The endoscope viewed inside the sand bed and a high speed video camera recorded what happened, during the backwashing process, on a pore/grain scale with a video screen magnification of about 100X. The video material was analysed and grain velocities obtained. Results showed that combined air and water wash at collapse-pulsing gives best cleaning, due to the high degree of bed agitation resulting in high fluid shear stresses and maximising grain collisions. It has been observed that fluid shear forces dominate detachment, whether the bed is cleaned by water fluidisation or collapse-pulsing. During collapse-pulsing grains exhibit velocities as high as those measured for a fully fluidised bed. Since this degree of agitation is obtained for flow rates of around 40% of the water fluidising rate this represents a saving in the use of clean water for backwashing purposes.

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