Page 76 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Water Contaminants 31

2.5.1 CONTAMINANTS AND WATER USES
As stated, contaminants that may be found in water number in
the thousands. The utility of water covers a wide variety of uses
such as drinking water, industrial water, cooling water, esthetic
appreciation, recreation contact, ecological habitats, irrigated
agriculture, etc. Each of these categories of water use may be
divided further. The objective of water quality criteria is to know
(a) (b)
the limits of each contaminant level that will not impair the uses
intended. The scope of the undertaking may be depicted in a
matrix of m uses in columns (perhaps several hundred columns
would adequately depict the uses) and of n contaminants in rows
(which may number several thousand). The number of possible
interactions, i.e., criteria, are several thousand, with size, m n.
Since not all contaminants have a bearing on each use, the
problem may be reduced considerably, but still there are hun-
dreds or thousands of criteria that must be delineated. Examples
(c) (d)
of such criteria include (1) the electronics industry, where par-
FIGURE 2.1 Examples of microorganisms found in water (the ticle-free water is required; (2) boiler feed water for electric
Giardia trophozoite is not found in water but occurs in the gut of a energy-generating plants, which must be ‘‘ion-free’’ water; (3)
warm-blooded animal after excystation). (a) Giardia trophozoite, growing of citrus crops, where boron levels must be less than 0.5
10 mm width. (Courtesy of Dr. Judith L. Isaac-Renton, Public Health mg=L,etc.Thesethreeexamplesindicatethenatureofthecriteria
Laboratory Director and Professor, Medical Microbiology, The Uni-
determination issue, e.g., that in-depth knowledge about the
versity of British Columbia, Vancouver, BC.) (b) Giardia cyst,
contaminant–use interaction must be generated. Most often,
10 mm length, (c) Cryptosporidium oocyst, 5 mm width, (d) Oocystis
criteria are more complex than just a simple number, e.g., tem-
parva, 400 . (Courtesy of Dr. Greg Sturbaum, CH Diagnostics and
perature affects how fish will survive under low dissolved oxy-
Consulting Services, Loveland, CO.)
gen conditions, the effect of TDS in irrigated agriculture depends
on the plant species and the amount of water applied, etc.
2.5 UTILITY OF WATER QUALITY DATA 2.6 COMBINATIONS OF QUALITY OF SOURCE
WATERS AND PRODUCT WATERS
In water treatment, uses of water quality data include
Source water quality and the required product water quality
1. Preliminary selection of unit processes: Source dictate the design and operation of a treatment train. Each
water quality must be characterized over the time source water is different, and standards for product water vary
period in which cyclical variation occurs (usually with the kind of use. Examples of uses include drinking water,
an annual cycle), and product water quality must be discharge of municipal wastewaters to a receiving body, pro-
specified. duction of particle-free and molecule-free process water for
2. Process control: Requires water quality monitoring microchip production, etc. The number of combinations is as
at selected node points in the treatment train in large as the number of possible water sources times the
order to verify proper operation and to diagnose number of possible water quality standards.
problems. To the extent possible in water treatment where ambient
3. Regulatory compliance: Requires sampling of the water is used, the water of highest quality is sought as a
effluent to document compliance with standards or source. Since high-quality sources have been mostly appro-
criteria. priated or are not available in some regions of countries,
4. Monitoring of receiving waters: Confirms whether a secondary sources have been committed to use to an increas-
stream standard has been violated. ingly greater extent. Thus, wastewaters are considered as
5. Laboratory analyses: Data from the laboratories must sources of water, assuming the associated water rights to
be organized, understood, and utilized. their use can be established. Also, as unit process technologies
6. Data archiving: Data must be archived in a format develop and expand, the number of treatment options
suitable for long-term storage and retrieval. increase, which may facilitate the use of lower-quality waters.
7. Supervisory control and data acquisition (SCADA) The use of membranes, for example, has become increasingly
systems. more economical since about the mid-1990s, which has made
feasible the use of even seawater as a source.
Monitoring sensors report to computers for display and use in Table 2.6 indicates the variety of source waters and product
plant control. Pumps, valves, and motors are controlled. water requirements. Other columns indicate examples of