Page 145 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

SOLID-LIQUID SEPARATION AND INTERCEPTORS 3.13

FIGURE 3.5 Typical pressure sand filter.

of sand, and a bottom layer of garnet. An additional layer of gravel may sometimes be used
as an underlayer on the bottom of the unit to support the media bed. Anthracite usually
has 1.1-mm grain size with a specific gravity of not less than 1.4. Typical depth is usually
between 8 and 12 in. Silica sand has an effective size of between 0.35 and 0.50 mm with
a specific gravity of 2.6. The sand layer is usually between 8 and 12 in in depth. Garnet
or ilmenite typically has 0.2-mm grain size and 4.2 specific gravity. Ilmenite is commonly
substituted for garnet of the same grain size, has a specific gravity of 4.5, and has a typical
depth of about 4 in. The gravel depth can range from 6 to 24 in, but usually falls between
10 and 18 in.

Gravity Filters
Roughing-type filters of the gravity type are used for initial treatment (pretreatment) of
surface water, and are made to handle large flows economically and remove large particles.
These types of filters are most often rectangular, concrete tanks. Conventional gravity filters
usually operate on pretreated water and are housed in steel tanks. Gravity filters are usu-
2
ally rated at 2 to 4 gallons per minute per square foot (gpm/ft ) of cross-sectional bed area.
Aluminum or ferric sulfate (which is composed of multivalent ions) could be added as a
coagulant to the feedwater in order to neutralize the surface charge of the colloids, thus
making their removal easier. The resulting flocculated aggregates removed by the filter are
discharged to drain. This type of filter is used to treat large volumes of water for process
use and is outside the scope of this handbook.

Pressure Filters

Pressure filters are the most common types used for general utility and service systems.
They usually operate under normal water pressure, but may require pumps to overcome
excessive friction loss through the unit if available water pressure is too low. Commonly

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