Multi-media water filter systems represent a significant improvement over single-media filters. This is primarily due primarily to improved filter bed action based on the innovative use and selection of filter media. Multi-media filtration permits delivery of high quality filtered water at much faster flow rates, as compared to a conventional sand filter.
In a conventional sand filter, lighter and finer sand particles are found at the top of the filter bed, and coarser, heavier sand particles remain at the bottom after backwashing. Filtration takes place in the top few inches of the filter bed.
A multi-media filter is backwashed in the same manner as a sand filter, using reverse or upward flow of water through the filter bed. The various layers of media retain their stratification because each material has a different density.
Advantages
1. The multi-media water filter systems operate for much longer periods of time (five or more times as long at the same filtration rate), before backwashing is necessary because the bed can hold more turbidity. Turbidity is trapped and held throughout the entire bed depth, rather than the top one or two inches.
2. Multi-media filtration is much better suited for use in a closed pressure tank since cracking of the bed, and subsequent breakthrough of turbidity is virtually eliminated and the need for visual inspection is unnecessary.
The use of pressure tanks, rather than open basins or filters, is an obvious advantage for point-of-use filtration and could also be of real importance in the filtration of small community water supplies.
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