The DynaSand filter is based on thecounterflow principle. The water tobe treated is admitted through theinlet distributor (1) in the lowersection of the unit and is cleaned as itflows upwards through the sand bed,prior to discharge through the filtrateoutlet () at the top. The sandcontaining the entrapped solids isconveyed from the tapered bottomsection of the unit (3), by means of anairlift pump (4), to the sand washer(5) at the top. Cleaning of the sandcommences in the pump itself, inwhich impurities are separated fromthe sand grains by the turbulentmixing action. The contaminatedsand spills from the pump outlet intothe washer labyrinth (6), in which it iswashed by a small countercurrentflow of clean water. The separatedsolids are discharged through thewash water outlet (7), while thegrains of clean sand (which are heavier)are returned to the sand bed (8).As a result, the bed is in slow,constant downward motion throughthe unit. Compressed air for the sandpump is provided via the controlpanel (9).Thus, water purification and sandwashing both take place continuously,enabling the filter to remain inservice without interruption.
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