Hydraulic performance analysis of new versus commercial nozzle design for pressurised porous media filters

Abstract

Rapid water filtration with pressurised porous media filters is extensively applied in drip irrigation systems. In double-chamber filters, the underdrains are fixed to the base of the inner plate to sustain the media above while draining water. Here, a new underdrain design intended to reduce the filter energy consumption is presented. The main difference with commercial underdrain units corresponds to the distribution of the slots, being in a horizontal plate to uniformise the flow trajectories inside the porous media. Both commercial and new underdrain designs have been tested in laboratory in both filtration and backwashing modes with three media types, two media heights, and superficial velocities ranging from 20 to 120 m h−1. In filtration mode, results indicate that the new design reduces the filter pressure drop by 31% at 60 m h−1 in comparison with the commercial one. The exploration with an analytical model that correctly reproduces the filter pressure drop, reveals that its value at 60 m h−1 is only 15% higher than the ideal scenario (uniform flow throughout the porous media bed). In backwashing mode, the pressure drop in comparison with the commercial design is reduced by 65% at 80 m h−1, while having the same trend for the bed expansion, which is also predicted by a simple analytical expression. Thus, the new underdrain design produces a more homogeneous fluidised regime than the commercial one at low-moderate superficial velocities.