Moving Bed Sand Filter

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In a moving bed sand filter developed by ASTRASAND, (Figure 10), wastewater is dosed ‘in-line’ with a polyelectrolyte before being fed into the filter at (1) and then into the filter bed at (4) through the supply manifold, (2) and distributors, (3). The water is filtered as it passes upwards through the sand bed with the treated ‘filtrate’ discharged in the upper part of the unit, at (5). The sand bed is continuously moving downwards as the water rises. The ‘dirty’ sand (6) is abstracted from the bed and raised to the upper part of the filter, after which it is released back on top of the sand bed (7). The sand circulation is based on the airlift principle forcing a mixture of dirty sand and water upwards through a central pipeline (8). The intensive scouring separates the impurities from the sand particles. At the top of the pipeline the air is released, the dirty (wash) water discharged (9) and the sand settles in the washer (10). As the sand passes through the washer labyrinth the particles are finally rinsed by a small amount of clean filtrate flowing through the washer counter-currently. The filtrate flow is generated by a difference in discharge level between the filtrate (11) and the washwater (10). The filter performance can be optimised by varying the airlift to control the sand circulation speed.

Figure 10. A schematic presentation of a moving bed sand filter, (Adopted by Kramer, J.P., Wouters, J.W., Noordink, M., Moving bed biofiltration for dynamic denitrification of 3,600 m3/h sewage effluent published at the World Filtration Congress, Brighton, UK, 1999)

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