Sand filters are used to remove suspended solids from water and wastewater. Because the sand filter only has one layer of filter media, it removes only relatively large solids (normally in the range of 40-100 microns). The effective size of each granule of sand media typically varies from 0.35mm to 1.2mm. Larger granules are for coarser filtration. Finer granules are for finer filtration.
Sand filters are typically used for:
1. Irrigation water
2. Storm water
3. Cooling water
4. Swimming pool water
5. Water in fountains and other water features
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, the sand filter can remove suspended solids as small as 5 micron.
Sand filter units are skid-mounted, factory assembled and wired, hydro-tested, and delivered ready for installation. Piping, valves, pressure gauges and accessories are included with filter systems. Solar power, media air scour, and rinse cycle are available as options. Filter systems are engineered to require little maintenance.
Dual and multi-media filters are typically used for:
1. Industrial process water
2. Waste water
3. Potable (drinking) water
The size of each filter, the number of filters required, and the type of filter media used depend upon:
1. how small a solid needs to be removed,
2. how dirty the water is
3. how the water will be used once it is filtered
4. the source of backwash water needed to clean the filters.
Size of solids to be removed: 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.
Concentration of suspended solids: The higher the concentration of suspended solids, the larger the diameter of the filter media needs to be.
Source of backwash water: Backwash water is used to clean the filters and flush out the suspended solids that have been trapped in the media bed. Water for backwashing can come either from the filters themselves or from an external source of clean water (such as a domestic water line or a storage tank).
If backwash water comes from the filter system, there must be more than one filter in the system. Filters are backwashed one-at-a-time. During the backwash cycle, a portion of filtered water exiting the other filter(s) in the system is used to clean the dirty filter. During these periods (which typically last for several minutes per filter), the amount of filtered effluent flowing downstream of the filter is reduced by the amount of water required for backwashing.
In the cases where this temporary reduction in flow is acceptable, there will be two or more filters in the sand filter system. If no decrease in the flow of filtered water is allowed, then water for backwashing should be supplied from an external source or the pump supplying water to the filters must increase its output.