Multimedia Filtration

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Multimedia Filtration is designed so that the larger grain particles are at the top of the filter and the smaller to the bottom of the filter. This has several advantages over a single media filter.

Increased filtration rates
Longer filter runtimes
Reduced backwash water consumption

A bed of anthracite placed on top of the sand layer provides the filter with:-

Larger grain particles on the surface of the filter which delays binding and pressure drop across the filter.
A larger reservoir in which the floc may be held before backwashing.
A prefilter for the filtration sand below.

The grades of media used in a multimedia filter are chosen to have similar backwash expansion characteristics.

Single Media Filter
Media	Grade	Depth	Bulk Density
Sand	0.60 - 1.18 mm	850 mm	1.56 tonne/m3
Gravel	6.7 - 13.2 mm	150 mm	1.60 tonne/m3

Duel Media Filter
Media	Grade	Depth	Bulk Density
Anthracite	1.18 - 2.50 mm	600 mm	0.74 tonne/m3
Sand	0.60 - 1.18 mm	600 mm	1.56 tonne/m3
Gravel	6.7 - 13.2 mm	150 mm	1.60 tonne/m3

Triple Media Filter
Media	Grade	Depth	Bulk Density
Anthracite	1.18 - 2.50 mm	400 mm	0.74 tonne/m3
Sand	0.60 - 1.18 mm	400 mm	1.56 tonne/m3
Garnet	0.30 - 0.60 mm	150 mm	2.10 tonne/m3
Garnet	1.40 - 2.36 mm	75 mm	2.10 tonne/m3
Gravel	6.7 - 13.2 mm	150 mm	1.60 tonne/m3