Page 144 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.12 CHAPTER THREE

process is called ultrafiltration. Ultrafilters retain material ranging in size from 1,000,000 to
1000 daltons while allowing water to pass through (1 dalton = 1/12 mass of carbon atom).
Filters retaining material below 1000 daltons are often called nanofilters. Often, different
manufacturers’ nomenclature of filter categories becomes blurred. Ultrafiltration and nano-
filtration are considered purification methods and are discussed in Chap. 4.
In general, ultrafilters and nanofilters are used to concentrate and purify fluids and to
remove particulate contaminants, and microfilters are used to clarify a solution for applica-
tions where quantitative retention is not required. In addition, filters are used to sterilize
various solutions that cannot use heat due to the loss of biological activity after exposure
to elevated temperatures.

TYPES OF FILTERS

Filters are divided into two general categories, depending on their filter media, granular
and preformed. Granular filters are depth-type filters using individual grains such as sand
and charcoal. Preformed filters can be either screen-, surface-, or depth-type, ranging in
thickness from a single thin membrane element to a thick filter mat. Often, filter elements
are contained within a housing called a cartridge.
Granular filters are larger units generally used to remove suspended particles larger
than 10 μm. Examples of granular filter media are single or multimedia sand and activated
charcoal contained in a vessel, tank, or column and septum filters.
Cartridge filters are relatively small, and generally used to clarify a previously filtered
stream of water containing suspended particles smaller than 10 μm. When the media is
plugged, the cartridge is replaced. Another type of cartridge filter is the capsule filter, in
which the filter media is contained in a sealed housing. When plugged, the entire capsule is
discarded. Commonly used materials for cartridge filters are paper, cloth, polymetric fibers,
and various combinations of these.

DEEP BED GRANULAR (SAND) FILTRATION

Deep bed sand filters consist of a tank containing either silica or garnet sand of constant
size (grade), or layers of a multimedia type consisting of a variety of graded material such
as anthracite, silica sand, garnet sand, and quartz. This type of filter is most often used as a
prefilter to remove larger-sized suspended solids in order to extend the duty cycle of finer
filters downstream. It has a relatively large retention capacity of solids and removes par-
ticles 10 μm and larger. During normal operation, the raw water to be treated enters at one
end (or the top) of the unit, the suspended solids adhere to the media, and the clear water
collects at the other end or on the bottom.
Sand filters are either gravity or pressure type. If the tank is atmospheric and water flows
through the unit with no assistance from pumps, it is a gravity filter. If the filter is in line and
uses the pressure of the water supply to force its way through the filter, it is a pressure type.
The pressurized filter is the most commonly used because of its smaller size and higher
flow rate. A typical pressure sand filter is illustrated in Fig. 3.5.

Types of Granular Filter Media

The filter media type, particle size, and specific gravity are primarily selected for particle
retention capability and ease of restratification of the media in the filter tank after back-
wash. A typical multimedia filter arrangement has a top bed of anthracite, a middle layer

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