25 May 2005
Sand Filter Description. A sand filter is a bed of sand (or anthracite coal)
located below a set of distribution headers and resting on a support layer of coarse
rock. The collection header, through which the clarified water is drawn, is situated below
the sand filter. Water flows downward through the filter bed, either due to gravity or by
Method of Action. The sand or anthracite acts as a support bed for a
layer of SS laid down on top of the bed as a result of the filtering process. This layer of
deposited solids, formerly SS, does most of the actual removal of solids from the water
and is known as the filter cake.
Filter Cycle. As the thickness of the filter cake builds up, the water flow
decreases and the backpressure increases. When the flow rate becomes too low or the
back-pressure too great, the water flow can be reversed, with the filter cake then being
backwashed with the wash water to a waste collection point. The filter is then returned
to service and the cycle is repeated.
Sand Filtration Rates. Sand filtration rates are typically 2 liters per
second per square meter (3 gallons per minute per square foot). Backwash rates are
8.1 to 10.2 liters per second per square meter (12 to 15 gallons per minute per square
foot) for sand filters and 5.4 to 8.1 liters per second per square meter (8 to 12 gallons
per minute per square foot) for anthracite filters. A variety of filter types are available.
Cartridge and Bag Filters. Cartridge and bag filters are available in
various mesh and pore sizes, which determine the size of the particles removed.
. Centrifuge separators represent still another
to remove suspended materials by passing water through a centrifuge chamber where
particles are removed due to density instead of size. Using separators is limited to
removing very small particulates. An advantage of separators is that they do not require
backflushing or change-outs of filter cartridges or bags.
Lime-Soda Softening. The lime-soda process is often used for treating
large volumes of water (i.e., 37,850 liters per day [10,000 gallons per day] or higher) for
potable and industrial uses. The process is used primarily to reduce the levels of
hardness and alkalinity, but also to reduce the quantity of silica and SS. The process
could be applicable to an entire base or facility; usually this process is not practical for
individual small site locations. The process is labor intensive and can produce large
amounts of sludge from the precipitated materials.
Method of Action. The process involves adding hydrated lime (calcium
hydroxide) and soda ash (sodium carbonate) to the water in an open reaction tank. The
calcium and magnesium concentration is reduced by the resulting precipitation of solids.
Bicarbonate and carbonate alkalinity is also reduced, as may be some silica. The sludge