Process conditions for the backwashing of filters with simultaneous air and water

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