STRATIFICATION IN TRIPLE MEDIA FILTERS

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.