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Filtration - Classification - Sand Filters
Unit 12 - Management of ponds
Filtration
Classification according to form of materials - Mechanical filtersSand filters
Sand filter consists of a layer of sand or other particulate material through which water is forced to flow. Filtering is a mechanical process whereby particles too large to pass through the space between sand grains are trapped in the sand. The maximum particle size the filter will pass is determined by the sand grain size. Larger sand grains allow larger particles to pass through the filter. Sand particle sizes commonly range from 2.0 to 0.02mm. However replacing the sand with rocks, coal, gravel or other material extends the particle size to any desired larger size diatomaceous earth, clay or similar material may be used to reduce the lower particle size to the micron level. However flow rates are so slow though the finer particles that these are rarely used for sand filters.
Flow rate through a filter and rate of clogging are dependent on filter medium particle size and on the concentration and characteristics of particulate materials in the influent. Smaller filter particle sizes results in lower flow rates for similarly constructed filters operated at the same head loss. The more particulate material in the influent, the quicker the filter will plug or the more often it must be backwashed. Since backwash water must be discarded, greater quantities of makeup water must be supplied for frequent back washing.
Figure shows a gravity flow sand filter cross section. Water containing particulate material enters at the top and passes down through the filter bed. Perforated pipe or a simple perforated floor provides a collection system for the clean water, which is discharged at left side. Gravity provides the necessary energy to induce flow though the filter.
Flow rate through gravity filters is described by Darcy’s equation.
V = K h/d
Where,
V = Average velocity of flow through sand filter (m/sec)
K = Permeability constant
h = head (m)
d = depth of the filter (m)
Sand filter consists of a layer of sand or other particulate material through which water is forced to flow. Filtering is a mechanical process whereby particles too large to pass through the space between sand grains are trapped in the sand. The maximum particle size the filter will pass is determined by the sand grain size. Larger sand grains allow larger particles to pass through the filter. Sand particle sizes commonly range from 2.0 to 0.02mm. However replacing the sand with rocks, coal, gravel or other material extends the particle size to any desired larger size diatomaceous earth, clay or similar material may be used to reduce the lower particle size to the micron level. However flow rates are so slow though the finer particles that these are rarely used for sand filters.
Flow rate through a filter and rate of clogging are dependent on filter medium particle size and on the concentration and characteristics of particulate materials in the influent. Smaller filter particle sizes results in lower flow rates for similarly constructed filters operated at the same head loss. The more particulate material in the influent, the quicker the filter will plug or the more often it must be backwashed. Since backwash water must be discarded, greater quantities of makeup water must be supplied for frequent back washing.
Figure shows a gravity flow sand filter cross section. Water containing particulate material enters at the top and passes down through the filter bed. Perforated pipe or a simple perforated floor provides a collection system for the clean water, which is discharged at left side. Gravity provides the necessary energy to induce flow though the filter.
Flow rate through gravity filters is described by Darcy’s equation.
V = K h/d
Where,
V = Average velocity of flow through sand filter (m/sec)
K = Permeability constant
h = head (m)
d = depth of the filter (m)
Last modified: Thursday, 28 April 2011, 6:08 AM