multimedia filtration
Multimedia Filtration has advantages over a single media
system.
The Ideal grain distribution in a filter bed is for the largest grains
to be at the top with progressively smaller grains below. This can
be achieved by placing a lower density but larger grained media
on top of the filter. The most common combination is to place
anthracite on top of sand.
To ensure that the bed will backwash correctly the media types
must be matched to have the same backwash characteristics. These
may be obtained from the backwash curves.
| Backwash Rate 35 m/hour | |||
| Media | Grade | S.G. | Expansion |
| Anthracite | 1.40 - 2.50 mm | 1400 kg/m3 | 8% |
| 1.18 - 2.50 mm | 1400 kg/m3 | 10% | |
| Sand | 0.60 - 1.18 mm | 2650 kg/m3 | 11% |
| 0.50 - 1.00 mm | 2650 kg/m3 | 13% | |
| Garnet | 0.30 - 0.60 mm | 4100 kg/m3 | 12% |
The above medias all have approximately the same backwash
expansion rates and would be suitable in a multimedia filter.
Filter Media Examples
| Examples of filter media installations | ||||
| . | Bed Depth mm | |||
| Filter Media | Grade mm | Single | Duel | Triple |
| Anthracite | 1.18 - 2.50 | . | 400 | 300 |
| Sand | 0.60 - 1.20 | 900 | 500 | 400 |
| Filter Garnet | 0.30 - 0.60 | . | . | 200 |
| Support Garnet | 1.40 - 2.50 | . | . | 50 |
| Gravel | 6.0 - 12.0 | 150 | 150 | 100 |