The effect of media bed characteristics and internal auxiliary elements on sand filter head loss

Abstract

Sand filters are beds of granular material placed in cylindrical metal containers. They are used as one approach for irrigation water filtration, mainly to remove organic and algae contaminants. The field effectiveness of these devices depends on their proper operation. The objectives of this study were to evaluate three commercial sand filter models, to study the effects of the internal auxiliary elements (the diffuser plate and the underdrains) and its interaction to sand particle size and media bed depth on the head loss as a function of the water flow velocity at clean water conditions. The experiment was divided in two parts. First, to assess the effects of the internal elements, the tanks (empty sand filter containers) head losses were characterized for water flow directions coincident with the processes of filtering and backwash. Second, the interactions of the internal elements with three sand particle sizes and three media bed depths were evaluated in light of the filtering flow direction. The results for the tanks showed that the different internal auxiliary elements significantly affected head loss, with the backwash flow direction generating values higher than the filtering flow direction for all evaluated equipment types. Additionally, it was observed that the head loss values were significantly affected by the sand particle size, the bed depth, the filtration velocity, and the interactions among these variables. The different models of internal elements generate distinct hydraulic performances of the sand filters, showing that they may exhibit different removal efficiencies for identical operating conditions. The research identifies the need to develop new procedures for the design of the internal elements of sand filters and for field operation conditions in order to improve localized irrigation performance.