DUGi

Alternative underdrain designs for reducing pressure drop across sand media filters

http://dugi.udg.edu/item/http:@@@@hdl.handle.net@@10256@@28674

Reduction of pressure drop across sand media filters – which are broadly used for preventing emitter clogging – should help reducing energy consumption in microrrigation systems. Previous research has shown that most of the pressure drop is located at sand filter underdrain. In this work, the effect of the nozzle geometry on the pressure drop of a sand filter was experimentally studied. Four nozzles were analysed: one commercially produced with a conical shape and three alternative cylindrical underdrains that were built for the experiment and that differed in the location and the number of slots. Experiments in both filtration mode and backwashing conditions for a wide range of superficial velocities were carried out at the laboratory. The results reported that a reduction of the filter energy consumption greater than 20% could be achieved by simply modifying the position of the slots above the surface of the underdrain element. The effects of the nozzle were further investigated by means of an analytical model that predicted the pressure drop of the water flow through the filter. The model confirmed that the distribution of the slots in the underdrain was a critical factor for determining the length of the region with a nonuniform flow within the sand. When using the commercial nozzle at flow rates >0.85 l/s, this region produced the major contribution to energy losses in the filter due to increases in the tortuosity of the water path within the porous medium. From these results, it is suggested that an affordable way to increase the energy efficiency of already existing installations would be to replace the current underdrain elements with new improved designs that have 40% more slots in its cylindrical surface

The authors would like to express their gratitude to the Spanish Ministry of Economy, Industry and Competitiveness for their financial support for this experiment though grant AGL2015-63750-R. P. Sala, S. Saus and J. Vicens provided very helpful technical assistance

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Wageningen University and Research