Page 146 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.14 CHAPTER THREE

called high rate or rapid sand filters, different designs are available that offer flow rates
higher than those available for gravity designs. Three different kinds of granular filters are
used: single medium filters with sand media; dual media filters, consisting of a top layer
of anthracite and a bottom layer of sand; and multimedia filters, consisting of a three-layer
bed of anthracite on top, sand in the center, and garnet on the bottom.

Granular Filter Flow Rates
2
A single medium filter is usually operated at a flow rate of 3 to 4 gpm/ft of cross-sectional
2
area. A dual media filter is operated at a flow rate of about 6 gpm/ft . A multimedia filter
2
has a typical flow rate of 6 to 15 gpm/ft . Another consideration is the face velocity of
the feedwater, which is the velocity of the feedwater through the surface layer area of the
vessel. The manufacturer establishes a face velocity, based on actual tests, that cannot be
exceeded.
Backwashing
Eventually, the suspended particles removed from the water will accumulate in the top
layer of the filter medium and obstruct the flow of water through the filter. These solids
are removed by backwashing, in which clean water flows backward through the filter bed
and is discharged to drain. The backwash flow and volume should be capable of expanding
the filter media by 50 percent to permit complete dislodging of the trapped particles. The
optimum rate is determined by the manufacturer of the equipment and is usually in the
2
range of 10 to 15 gpm/ft of filter area for multimedia filters. For single and dual medium
filters the rate is lower than that of multimedia filters. A backwash pump provides the pres-
sure needed if the water pressure is not enough, and the typical length of backwash time
is between 5 and 15 mm. Where surface water is being filtered and to speed the cleaning
process, compressed air mixed with the backwash water may be used to effectively remove
algae and other organisms. This is called air scouring. The need to backwash is indicated
when, generally the pressure drop falls to between 7 and 11 psi (49 to 77 kPa) above the
clean pressure value.
During the backwash cycle, the action of the water in the single medium type filter
distributes the coarsest sand at the bottom and the finest at the top. This distribution will
result in the accumulation of a majority of particulates removed from the raw water in the
fine sand layer on the top where the raw water enters the filter. This quickly reduces its
effectiveness, thereby requiring frequent backwash.

Granular Filter Selection Criteria
The advantage of the granular filter is its low initial cost. The advantage of the multimedia
filter is the consistent distribution of the various sized media after backwashing, thereby
increasing the time between backwash cycles. Tests have shown that the multimedia filter
is more effective in removing particulates than the single medium sand filter. It is often
cost effective to prefilter a liquid containing large amounts of particulates with a high solid
capacity sand filter before putting it through a cartridge filter.
Sand filters can be obtained with either vertical or horizontal tanks, depending on the
headroom available. The horizontal tank has a larger filter bed area, which requires a higher
backflow rate. Compartmentalization of the vessel may offset this requirement somewhat.
The key to operation for this type of filter is the arrangement of the filter media, best
arranged from coarse to fine in the direction of water flow. In addition, activated carbon
added to the filter bed will remove excessive odor and bad taste, although this is usually
done in a separate filter.

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