Environmental Engineering Reference
In-Depth Information
These filter types also have problems with suspended particles clogging the
surface, requiring the filter to be manually scraped clean. These units take
up a considerable amount of land area because slow filtration rates require
a greater filter surface area to produce the necessary filtered water qualities.
In modern water treatment plants, the rapid filter has largely replaced the
slow sand filter. The rapid filter consists of a layer of carefully sieved silica
sand ranging from 0.6 to 0.75 m in depth on top of a bed of graded gravels.
The pore openings between the grains of sand are often greater than the size
of the floc particles that are to be removed, so much of the filtration is accom-
plished by means other than simple straining.
Note:
The ideal filter medium is coarse enough for large pore openings to
retain large quantities of floc, yet sufficiently fine to prevent the passage
of suspended solids. It must have adequate depth to allow relatively long
filter runs and be graded to permit effective cleaning during backwash.
DID YoU KNoW?
In granular filtration, the higher the temperature the better the filtration.
Adsorption, continued flocculation, and sedimentation in the pore spaces
are also important removal mechanisms. When the filter becomes clogged
with particles (which occurs approximately once a day, depending on the
turbidity of the water), the filter is shut down for a short period of time and
cleaned by forcing water backward through the sand for 10 to 15 minutes.
After cleaning, the sand settles back in place and operation resumes.
Other Common Filter Types
Rapid flow filters are the most common type used for treating water sup-
plies, primarily because they are the most reliable, but other types of filters
are sometimes used to clarify water, including pressure filters and diato-
maceous earth filters. A pressure filter is similar to a rapid filter in that the
water flows through a granular filter bed; however, instead of being open to
the atmosphere and using the force of gravity, the pressure filter is enclosed
in a cylindrical steel tank and the water is pumped through the bed under
pressure. They are not as reliable as rapid filters, because pressure may force
solids through the bed in the effluent. Because of this problem, they are sel-
dom employed in municipal water treatment works but instead are used for
filtering water for industrial use or for swimming pools. Diatomaceous earth
filters contain a thin layer of a natural, powdery material formed from the
shells of diatoms; they are also used primarily for industrial or swimming
pool applications because they are not as reliable as rapid sand filters.
