Numerical study of the effects of pod, wand and spike type underdrain systems in pressurised sand filters

Abstract

Commercial sand media filters adopt different underdrain designs, with pod, wand and spike designs being the most common. Studies about the consequences of using these configurations are often not conclusive since auxiliary elements and dimensions vary between filters. Here computational fluid dynamics (CFD) simulations were carried out comparing underdrain designs in filters with equal sized diffuser plates and same inlet, inner and outlet diameters. Seven underdrain pod-type designs were analysed ranging from a market model to one with more than 50% of pods. Designs with equal number of pods but with different spatial distributions were investigated. Two wand- and two spike-type underdrain models were also evaluated. The main variables analysed were pressure and volumetric flowrate. Results confirmed that the flow uniformity through the filter was crucial to achieve low pressure drop values. Pressure losses through the sand were the most important contribution to the filter pressure drop for all cases. The water-only region at the inlet had low relevance in terms of pressure losses. All underdrain designs had similar pressure drops at the exit collector chamber (pod-type) or pipe (wand- and spike-type). Pod-type designs with the same slot open area as wand- and spike-type configurations clearly had a better efficiency since wand- and spike-type designs had faces opposing the incoming flow. Pod- and spike-type designs with similar horizontal projected upward slot areas behaved alike in both filtration and backwashing modes, with a better performance for the spike-type configuration. Recommended spike-type designs should cover most of the filter cross-sectional area