Development of a new underdrain for improving the efficiency of microirrigation sand media filters

Microirrigation is an irrigation technology with high potential water use efficiency. For preventing emitter clogging, which reduces the uniformity and can lead to inoperability of microirrigation systems, filtration is mandatory. Sand media filters are broadly used to protect microirrigation systems, but their manufacturer's pressure requirements for backwashing are high and contribute to energy consumption. Previous studies have shown that pressure losses in sand media filters can be more important in the underdrain elements than in the filter bed. The main objective was to design a new underdrain that could reduce sand media pressure drop. The new underdrain has wider passages to the water chamber located at underdrain outlet, is surrounded with a granular medium with higher hydraulic conductivity and it modifies the flow curvature above the underdrain. The new underdrain was built and tested under filtration and backwashing conditions and with and without filter bed using a laboratory filter which was scaled from a commercial filter. Results show that the new underdrain reduced pressure loss by 50% compared with a scaled commercial filter taken as reference, especially under backwashing conditions. A Computational Fluid Dynamics (CFD) model was developed to further study those filter areas where the underdrain achieved a pressure loss reduction. The new underdrain also affected pressure profile across filter bed, which was closer to the theoretically predicted by the Ergun equation. The new underdrain improves hydraulic performance of sand media filters for microirrigation system, thereby increasing both water and energy use efficiency.