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In most cases filters
containing specially graded and sized gravel and sand
are effective in screening out turbid particles. With
such units a periodic backwashing to remove the filtered
material is all the maintenance necessary.
Municipal and industrial
systems frequently make use of the coagulation process
to aid in the removal of turbidity.
In cases where turbid
particles are very small, a coagulating agent, such as
aluminum sulfate, is fed into the water. After rapid
mixing, the coagulating agent forms a "floc".
As the floc forms, it
tends to collect or entrap the turbid particles and form
them into larger particles which can then be removed by
the filter.
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