Polymers
in Water Purification
Advances
in the field of polymer technology are expected to significantly contribute
in solving problems of water purification.
Introduction
Water is an abundant and important resource on
our planet. Nature has designed many intriguing systems for its constant
reuse by all living organisms, and man has been learning from nature
for eons. Science using the new knowledge of building blocks of nature
molecules (i.e. polymer chemistry) continues this tradition. There
are 752 substances that are present in tap water as identified by
the WHO. Out of these only 66 are monitored by water authorities.
Role of Polymers in Water Purification Systems
Contaminants typically found in water and their
related filteration process are shown in Fig. 1. Speciality industrial
applications along with scarcity of required quality of water for
human use, is a constant impetus for improving water purification
systems. Modern polymers (polypropylene, polyethylene, nylon, PTFE,
UHMWPE) have a major role in purification systems such as filters,
diffusers, aerators and osmotic processes. Filters remove coarse particles
of sizes greater than 1 micrometer (µm). Filtration of smaller
sizes is possible with membranes of modified polymers of nylon, PTFE,
acrylic, polyamide, and cellulose. These are usually laboratory or
intensive industrial processes.Ceramic filters are usually gravity
filters. Most ceramic filters made in India are hydrophilic (wettable)
– i.e., they absorb water and the flow is not pressure related.
Pore size ranges from 50 – 500 µm.
UHMW HDPE filters have negligible water absorption and are non-toxic
(also used in joint replacement surgery). Sintered UHMW filters give
a good pressure related flow and can be used in online systems. They
can be made in various sizes and shapes. The sintered matrix produces
a random porous structure. Particles get trapped in the tortuous network
of the flow channels. The principle retention mechanisms are random
adsorption and mechanical entrapment throughout the depth of the matrix.
The material lends itself to machining and snap fitting. This makes
it versatile for various applications. With depth type cartridges,
filtration efficiency improves with use, as particles are trapped
within the matrix. Pore size range from 5 – 200 µm. PP
filters are available as non-woven fabric or friction spun yarn. Nonwoven
is a group of fabrics made of PP amongst other polymers which could
be needle punched, thermally bonded, chemically bonded or spun bonded.
The fabrics can be used in various ways, but are usually made into
pleated or bonded cartridges or bag filters. Another type of cartridge
commonly used in water filtration is made of friction spun yarn of
PP.
Fig. 1: Contaminents found in Water and the
associated Filteration Process.
....contd.
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