Sand Filtration is a water treatment process that employs a pressure filter vessel utilizing a single
media. It is commonly used in industrial filtration applications and water treatment plants for removing
suspended solids using a single-size filtration media. The filtration process is determined by adjusting
the size of the filter sand. The ideal size of each grain usually varies from 0.35mm to 1.2mm.
In sand filtration, pressure vessels with sand media are used. During the cleaning cycle, called “backwash”, the bed is lifted (or “fluidized”) to loosen the filter media and release trapped dirt, which is removed in the backwash flow. After the backwash cycle, the bed is allowed to settle before the filter is returned to service (i.e., normal flow).
A sand filter typically uses one grade of sand alone as the filtration media. In a sand filter, during the
“settling” cycle, the finest or smallest media particles remain on top of the media bed while the larger and heavier particles stratify lower in the filter. This results in very limited use of the media since virtually
all filterable particles are trapped at the very top of the filter bed or within 1-2 inches of the top where the filter media particles have the least space between them.
Note that MMF can remove particles down to 10-15 microns (µm) in size compared to a simple sand filter that can eliminate particles down to 25-50 microns (µm) in size.
Sand for filters are rated by effective particle size and uniformity. The effective size is such that approximately 10% of the total grains by weight are smaller and 90% are larger. Therefore, the effective size is the minimum size of most of the particles. Uniformity is measured by comparison of effective size to the size at which 60% of the grains by weight are smaller and 40% are larger. This latter size, divided by
the effective size, is called the uniformity coefficient; the smaller the uniformity coefficient, the more uniform the media particle sizes.
Finer sands result in shallower zones for the retention of suspended matter. The most desirable media size depends on the suspended solids characteristics, effluent quality requirements and specific filter design. In general, sand filters use sand with an effective size of 0.35-0.60 mm (0.014-0.024 in.) and a maximum uniformity coefficient of 1.7. Coarse media, often 0.6-1.0 mm (0.024-0.04 in.), are used for closely controlled coagulation and sedimentation.
Sand filters can have graded support media layers or be directly installed over or on a distribution plate. The sand of different particles is stored in the filter vessel, and the bottom of the sand filter contains a porous collector. Sand filtration is often combined with other filtration techniques such as activated carbon.
