09 Water Purification: Physical Treatment: Slow Sand Filter
Description
This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
09 Water Purification: Physical Treatment: Slow Sand Filter
Slow sand filters pass water slowly through a bed of sand. Pathogens
and turbidity are removed by natural die-off, biological action, and
filtering. Typically the filter will consist of 24 inches of sand,
then a gravel layer in which the drain pipe is embedded. The gravel
doesn't touch the walls of the filter so that water can't run quickly
down the wall of the filter and into the gravel. building the walls
with a rough surface also helps. A typical loading rate for the
filter is 0.2 meters/hour day (the same as .2 m^3/m^2 of surface
area). The filter can be cleaned several times before the sand has to
be replaced.
Slow sand filter construction information: Slow sand filters should
only be used for continuous water treatment. If a continuous supply
of raw water can't be insured (say using a holding tank), then another
method should be chosen. It is also important for
the water to have as low turbidity (suspended solids) as possible.
Turbidity can be reduced by changing the method of collection (for
example, building an infiltration gallery, rather than taking water
directly form a creek), allowing time for the material to
settle out (using a raw water tank) prefiltering or flocculation
(adding a chemical such as alum to cause the suspended material to
floc together.)
The SSF filter itself is a large box, at least 1.5 meters high. The
walls should be as rough as possible to reduce the tendency for water
to run down the walls of the filter, bypassing the sand. The bottom
layer of the filter is a gravel bed in which a slotted pipe is placed
to drain off the filtered water. The slots or the gravel should be no
closer than 20 cm to the walls. again to prevent the water from
bypassing the sand.
The sand for a SSF needs to be clean and uniform, and of the correct
size. The sand can be cleaned in clean running water, even if it is
in a creek. The ideal specs on sand are effective size (sieve size
through which 10% of the sand passes) between .15 and .35 mm,
uniformity coefficient (ratio of sieve sizes through which 60% pass
and through which 10% pass) of less than 3, Maximum size of 3 mm, and
minimum size of 0.1 mm.
The sand is added to a SSF to a minimum depth of 0.6 meters.
Additional thickness will allow more cleanings before the sand must be
replaced. .3 to .5 meters of extra sand will allow the filter to work
for 3-4 years An improved design uses a geotextile layer on top of the
sand to reduce the frequency of cleaning. The outlet of a SSF must be
above the sand level, and below the water level. The water must be
maintained at a constant level to insure an even flow rate throughout
the filter. The flow rate can be increased by lowering the outlet
pipe, or increasing the water level.. One common idea for maintaining
the water level is to use a elevated raw water tank or pump, and a
ball valve from a toilet .
While the SSF will begin to work at once, optimum treatment for
pathogens will take a week or. During this time the water should be
chlorinated if at all possible (iodine can be substituted). After the
filter has stabilized, the water should be safe to drink, but
chlorinating of the output is still a good idea, particularly to
prevent recontamination.
As the flow rate slows down the filter will have to be cleaned by
draining and removing the top few inches of sand. If a geotextile
filter is used, only the top 1/2" may have to be removed. As the
filter is refilled, it will take a few days for the biological
processes to reestablish themselves.
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