Effects of porous media type and nozzle design on the backwashing regime of pressurised porous media filters

Abstract

Pressurised sand filters used in drip irrigation need periodic backwashing to flush the contaminant particles out of the porous media. This process consumes high amounts of energy and water. The selection of more efficient backwashing operational conditions requires accurate information of the pressure drop and the bed expansion, the latter being not measured in commercial filters. An experimental study with a scaled filter that used a window to observe the bed expansion was conducted with three porous media types (glass microspheres and two silica sands), two packed media bed heights (200 mm and 300 mm) and four nozzles (one commercial and three prototypes). The 24 combinations of the filter experimental configuration were investigated for different superficial velocities. Both data and video recordings for all the 705 tests conducted were carefully analysed to obtain mean values and standard deviations of the height of the expanded bed. The behaviour of the fluidised bed dynamics was characterised. Results indicated that the nozzle design had a strong influence on the pressure drop, and, in consequence, on the power required for backwashing. It also had an observable impact on the fluidised bed dynamics although its effect on determining the overall height of the expanded bed was limited, this being more dependent on the type of the porous media. The most effective combination in terms of energy efficiency and porosity of the expanded bed was obtained with microspheres, though its retention efficiency might be questionable from the literature review, and the frustoconical nozzle geometry.