The backwashing process used La clean triple media filters has been fo
und to cause significant disturbance to the stratification of the sand
and fine garnet layers. A laboratory-scale study identified the media
size as the dominant factor influencing stratification, The disturban
ce dr mixing also increased with lower water temperatures and higher f
luidising flow rate. A review of stratification/mixing mechanisms indi
cated that mixing was initially mainly by fine garnet moving up into t
he sand layer, However, under ideal laminar flow conditions with no ra
ndom grain movement, stratification should occur, Therefore, a further
critical element to the extent of mixing was the degree of turbulence
, increasing the random grain movement, A pilot scale study using size
selected media for stratification supported this conclusion.