Multi media filtration
Multi-media filters represent a significant improvement over single-media filters. This is 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.
The multi-media filter is radically different. The multi-media filter bed, in comparison to the sand filter bed, is upside down. Coarse, but lighter, particles backwash to the top, whereas finer, but heavier, particles remain at the bottom of the bed. The innovation lies in the selection of suitable media. This configuration has many advantages. The entire bed acts as a filter, rather than only the top few inches. Turbidity is trapped throughout the bed, enabling the filter to hold far more solids filtered from the water before backwashing is necessary.
Typically, the filter bed is made up of three layers of filter media. The total bed depth is about 660 to 1000 millimeters. In a three layer filter the top layer is made up of large, lighter weight particles of anthracite coal and is from 380 to 450 millimeters in depth (particle size 1.0 to 1.5 millimeters, density 1.35 to 1.75). The middle layer contains from 200 to 380 millimeters of heavier and smaller particles of calcined aluminum silicate or sand (particle size 0.5 to 0.6 millimeters, density2.65). The bottom layer contains from 75 to 150 millimeters of heavier garnet (particle size 0.2 to 0.3 millimeters, density 4.0to 4.2). This semiprecious red silicate mineral is 50 to 60% heavier than sand.
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.
In a four-media filter a fourth or top layer contains from 75 to 150 millimeters of lighter and larger plastic pillows (particle size2.0 to 4.0 millimeters, density 1.1 to 1.2). Their density is slightly above the density of water which is 1.0
- The multi-media filter can 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.
- 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.
More rapid filtration flow rates in multi-media filtration allow the use of smaller diameter tanks with equal or better results.
- A very high degree of clarity is achieved in the filtered water because of the fact that the finer particles of garnet at the bottom trap finer turbidity particles.
- Another important advantage is that the multi-media filter can clarify water at a much higher flow rate than a single-media sand filter (21 to 30 litres per minute, as compared to 6 to 10 litres per minute in a 12 inch diameter tank). This is 54 to 58 lpm per square foot of bed area, as compared to 8 lpm per square foot of bed area. This is a very important difference in the production of filtered water.