Page 199 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.35

Filtration
Ultrafiltration and nanofiltration describe membranes that are categorized by their pore size.
Ultrafiltration membrane pore sizes range from 0.001 to 0.02 μm. Nanofiltration membranes
have pore sizes that will allow the passage of solids to 10,000 daltons. The 10,000-dalton
cutoff is recommended for the complete removal of pyrogens. Typical recovery rates for
ultrafilters range between 95 and 98 percent, with the remainder flushed to drain.
A membrane is manufactured by bonding the membrane onto a porous, supporting
substrate and then configuring it into elements. These filters are generally used for pretreat-
ment in the removal of colloids, bacteria, pyrogens, particulates, and high-molecular-weight
organics. Spiral wound and hollow fiber are the two configurations used most often.

MICROBIAL CONTROL

Chemicals
The most common disinfection method is the addition of oxidizing or nonoxidizing
chemicals. Chemicals could be either biocides, which are substances that kill microbes,
or biostats, which prevent the further growth of microbes. Commonly used chemicals are
chlorine, chlorine compounds, hydrogen peroxide, and acid compounds.
In order to be effective, the chemical must have a minimum contact time in the water. In
addition, a residual amount of the chemical must be present to maintain disinfection.
Chemicals add impurities to the water and are not generally suitable for a pure water
environment. They disinfect potable and process water and equipment by injection directly
into the fluid stream by means of a metering pump. They must be removed from feedwater
used for purification. Chlorine may produce trihalomethanes.

Ultraviolet Radiation
Ultraviolet (UV) radiation is an in-line process. UV light is generated using mercury vapor
lamps. The UV spectrum is divided into three wavelengths: UVA (315–400 nm), UVB
(280–315 nm), and UVC (less than 280 nm). Only UVB and UVC wavelengths can eco-
nomically produce the intensity and energy output necessary for the intended germicidal
treatment. Federal standard 209E and aseptic guidelines issued by the FDA provide some
guidance for the use and application of UV irradiation.
The 254-nm wavelength operates in the germicidal region, sterilizing by destroying
bacteria, mold, viruses, and other microorganisms. This wavelength is preferred for pure
water systems and will significantly reduce the multiplication of organisms.
The 185-nm wavelength operates in the ozone-forming region, where it has the ability
to break down organic molecules to carbon dioxide by the photooxidation process. It slowly
breaks the bonds in organic molecules by direct radiation, and also oxidizes organisms by
the formation of hydroxyl radicals. The UV spectrum is illustrated in Fig. 4.14.
A flow rate of approximately 2 ft/s is an industry standard for effective sanitization of
purified water. Flow rate through the UV device should be reduced compared to that of the
circulation loop to extend the necessary contact time. The recommended location is ahead
of deionization equipment.
Problems include generation of ions that lower the resistivity of water and the possible
leaching of silica from the quartz sleeve of the UV device. Glass, plastic, rubber, and similar
materials exposed to UV radiation will eventually crack, etch, discolor, and flake. Tests have
shown that only half the energy used by a new bulb is actually transmitted to the water and that
in time an additional 25 percent of the output will be lost compared to a new bulb.

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