Page 147 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

SOLID-LIQUID SEPARATION AND INTERCEPTORS 3.15

Problems have resulted from failure of the interior lining of the filter due to abrasion of
the lining by the sand during backwash. This abrasion process creates iron corrosion products
that can pass through the filter and interfere with downstream purification processes.
It is important to operate these units at the face velocities recommended by the manufac-
turer to prevent the forcing of the impurities through the filter. This phenomenon is called
breakthrough.

Precoat (Septum) Filters

Precoat filters, often called diatomaceous earth filters, are depth-type filters used to clarify
water when the solids concentration is low. One of the most frequent uses is to clarify
water for swimming pools. A septum is a thin, porous membrane with relatively large pores
that has little resistance to the flow of water. The filter itself consists of a tank contain-
ing one or more vertical filter leaves or plates (septa) onto which a thin coating of filter
aid is evenly deposited to form a filter surface, or precoat. The precoat could be pearlite
or diatomaceous earth. The most commonly used material is diatomaceous earth. As the
feedwater flows through the filter, it passes through the filter precoat first, which traps the
particulates. Although pressurized filters are available, most precoat filters use a pump to
create a vacuum to pull the water through the filter, with the inlet water level controlled by
a float valve. When the filter coat becomes plugged, it must be replaced. This is determined
by excessive pressure loss through the filter. The filter is cleaned by backwashing, and the
filter aid along with trapped solids are discharged to sewer. Care must be used in rede-
positing the new filter aid to ensure uniformity. Generally accepted filter aid coverage
2
is 0.1 lb/ft of filter septum surface. The diatomaceous earth media usually have a dry weight
3
of 8 to 10 lb/ft and range from 5 to 64 μm in size.
ACTIVATED CARBON FILTERS

Activated carbon is a depth-type granular filter that depends on adsorption to separate
contaminants from the feedwater. It is discussed in this chapter because it falls in the granu-
lar filter category and conforms to the definition for filtration. It is not specifically used
for particulate removal but rather for reduction of free chlorine, removal of TOCs, soluble
organics, and trihalomethanes. Chlorine removal is necessary to protect some RO membranes
and ion-exchange resins from attack.
Activated carbon media are carbon granules that have been processed from raw high-
carbon materials, such as lignite and bituminous coal, wood, peat, and coconut husks. It is
available in powdered and granular forms. Another rarely used method is to have powdered
carbon added to a process stream and filtered out downstream. This method will not be
discussed.
The most common type of activated carbon for water treatment is the granular form,
often called activated charcoal, which will be the only one discussed here. Activated carbon is
manufactured by grinding the raw material into uniform sizes, adding a binder if necessary, and
reducing (burning) the mixture in the presence of steam. This creates skeletal granules with
a very large network of micro- and macropores, thereby becoming activated. Bituminous-
based coal has the highest bed density, and therefore a larger number of pores available for
attraction of contaminants. In addition, the bituminous product is more amenable to reacti-
vation. Because of these attributes, this is the material most often used. Granular carbon is
usually 8 to 30 mesh, with an effective size of 0.9 mm and a uniformity coefficient of 1.8.
The granules are placed within a housing and the feedwater to be treated is passed
through the granules. The organic removal capacity of the media depends on the diffusion
rate of the organic molecules through the pores of the media, the surface area of the media,

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