Page 211 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.47

calcium and magnesium hardness and alkalinity (bicarbonates). Source water from agricul-
tural areas often has high levels of nitrates, phosphates, and organic pesticides. Water from
public utilities has residual chlorine, fluorides, and chlorimines as well as iron oxides and
other pipe-related impurities. It is important to establish or obtain historical water analysis
data from different times and seasons of the year in order to properly design a system with
enough flexibility to obtain the required purity under worst case conditions.
It is an FDA requirement that the feedwater for USP Purified Water and WFI systems
meet the EPA guidelines for potable water. Of particular concern is the microbial level.
The water should be virtually free of coliform, which is a pathogenic marker organism.
The feedwater must also meet the 400 cfu/mL, the maximum value specified in USP XXII.
Another requirement is that it shall contain no added substances. This is an issue that
appears to be interpreted differently by individuals and organizations involved in the design
of PW systems. There are systems presently in use that appear from an overall perspective
to violate this position. Items such as the chlorination of raw water supplies, acid addition
to adjust pH, and the use of ozone to control microbial growth are subject to interpretation.
All of the concerns should be resolved during the application phase for FDA approval and
validation of the system.

PURIFICATION SYSTEM DESIGN

General
Specific methods of purification are capable of removing various types of impurities better
than others. None can be depended on to remove all the impurities necessary to achieve the
purity level required for USP purified water. It is accepted practice to use a combination of
technologies, each of which is designed to remove a specific type of impurity.
The methods used to produce this water are dependent on the feedwater supplying the
facility. In most instances, that feedwater has a high level of some classes of contaminants.
It is much more economical to pretreat that water to remove the bulk of large quantities
and/or the more concentrated of those impurities, and then use the purification equipment
to “polish” the water to the desired purity level. In some cases, the removal of individual
impurities is necessary to allow the use of specific types of purification equipment.

Pretreatment

General. Pretreatment is considered for two reasons: to prevent potential damage to the
membrane selected and to increase membrane filtration quality. The need for pretreatment
is determined by an analysis of the raw water supply. The decision is generally governed
by the cost efficiency of the pretreatment method, specifically whether the cost of purchas-
ing and installing the pretreatment equipment will reduce the initial cost of the main treat-
ment equipment and lower the operating cost of the system as a whole enough to justify
its installation.

Water Temperature. Membrane productivity (flux) is usually rated with feedwater at
77°F, and is inversely proportional to the feedwater viscosity. When the water temperature
is lower, additional membrane area is required. The flux increases with increased feedwater
temperature. Heating the feedwater lowers the viscosity. The water can be heated with a
separate water heater or with a blending valve using domestic hot water mixed with feed-
water to provide the necessary temperature.

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