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.