OBSERVATIONS OF PARTICLE DETACHMENT DURING FILTER BACKWASHING

The behaviour of filter grains and detaching deposits during backwashi ng was investigated at University College London using a High Speed Vi deo and endoscope technique. A detailed understanding of the backwashi ng process was required after decades of backwashing on a trial and er ror 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 happen ed, during the backwashing process, on a pore/grain scale with a video screen magnification of about 100X. The video material was analysed a nd grain velocities obtained. Results showed that combined air and wat er wash at collapse-pulsing gives best cleaning, due to the high degre e of bed agitation resulting in high fluid shear stresses and maximisi ng grain collision. It has been observed that fluid shear forces domin ate detachment, whether the bed is cleaned by water fluidisation or co llapse-pulsing. During collapse-pulsing grains exhibit velocities as h igh as those measured for a fully fluidised bed. Since this degree of agitation is obtained for flow rates of around 40% of the water fluidi sing rate this represents a saving in the use of clean water for backw ashing purposes.