Sand filters are used to remove suspended solids from water. Because they have one layer of filter media, sand filters remove only relatively large solids (normally in the range of 40 micron and larger). The effective size of each granule of sand typically varies from 0.35mm – 1.2mm. Larger granules yield coarser filtration. Finer granules yield finer filtration. Removing suspended solids smaller than 40 microns requires filters with either two or three layers of filter media. These are referred to as dual-media filters or multi-media filters. When sized properly and with the correct configuration of filter media, these filters can remove suspended solids as small as 5 micron.
The smaller the solid to be removed, the slower the water must pass through the media and the finer the granule of media must be. Very often the end user doesn’t know how small a solid must be removed. In those situations, how the filtered water will be used will determine how small a solid should be removed. The higher the concentration of suspended solids, the larger the diameter of the filter needs to be. All filters have a maximum concentration of suspended solids that can be managed without compromising filter performance.
The size of each filter, the number of filters required, and the type of filter media used depend upon:
- how small a solid needs to be removed,
- how dirty the water is,
- how the water will be used once it is filtered,
- and the source of backwash water needed to clean the filters.
Dual and Multi-media filters are typically used for:
- Industrial process water.
- Waste water.
- Potable (drinking) water.