Hydraulic characterisation of the backwash process in sand filters used in micro irrigation

The objective was to evaluate effects of structural design, particle size and filter media height on pressure loss and surface velocity in the expanding filter layer during the backwashing process. The hydraulic behaviour of three commercial sand filter designs were compared. Experiments were conducted in an experimental closed circuit module. The treatments were based on the evaluation of three sand particle sizes combined with three filter bed heights for each filter design. The results showed that for the evaluated sand filters, with their different drainage structures in terms of area and arrangements, had different hydraulic behaviours during the backwashing. Coarser sand particles and greater filter layer heights produced larger slopes in the surface velocity curves as a function of percentage of filter bed expansion. No difference was observed in the minimum fluidisation velocity with changing the filter media heights, but an increase in the minimum fluidisation velocity was observed with increasing sand particle size. Filter beds composed of coarse sand particles and low filter bed heights produced lower increases in the pressure loss with increasing backwash surface velocity. Determining the minimum fluidisation velocity using the graphical analysis method to determine the pressure loss across the filter bed proved to be a reliable method for evaluating the equipment used. •We evaluated different commercial sand filter models.•Different sand filter drainage led to heterogeneous hydraulic behaviours.•The use of coarser particles and lower layer heights led to lower pressure loss.