PROCESS CONDITIONS FOR THE BACKWASHING OF FILTERS WITH SIMULTANEOUS AIR AND WATER

Experimental data published by Amirtharajah et al. on the conditions f or washing filter media by simultaneous air and water have been re-exa mined. employing the classical relationships for pressure loss in gran ular media developed by Kozeny and Carman, making the assumption that the air and water move through the interstices at about the same veloc ity. It has been found that for sand Amirtharajah's experimental data, as correlated by his empirical equation, can be predicted fairly accu rately direct from the hydraulic size and voidage of the media. For lo w density materials where the buoyancy of the air is much greater than the submerged density of the bed it is suggested, with some support f rom direct observation, that bubbles are able to disrupt the structure and in some cases churn the bed at zero water flow.