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14 Chapter 1 Introduction to Water Systems
1.7 PURIFICATION WORKS

The quality of some waters from surface or ground sources is naturally satisfactory for all
common uses. Disinfection may be the only required safeguard. Other waters contain ob-
jectionable substances that must be removed, reduced to tolerable limits, destroyed, or oth-
erwise altered in character before the water is sent to the consumer. Impurities are acquired
in the passage of water through the atmosphere, over the earth’s surface, or through the
pores of the earth. Their pollution is associated with man’s activities; in particular, with his
own use of water in household and industry and the return of spent water to natural water
courses. Some of the heavy metals (lead, copper, zinc, and iron) come from the corrosion
of metallic water pipes. Contamination of distribution systems through cross-connections
with impure water supplies and through backflow in plumbing systems is another hazard.
(Backflow permits water drawn into a fixture, tank, or similar device to flow back into the
supply line by gravity or by siphonage.)
How to treat a given supply depends on its inherent traits and on accepted water qual-
ity standards. Municipal works must deliver water that is (a) hygienically safe, (b) aesthet-
ically attractive and palatable, and (c) economically satisfactory for its intended uses. The
most common classes of municipal water purification works and their principal functions
are as follows:
1. Filtration plants remove objectionable color, turbidity, and bacteria as well as other
potentially harmful organisms by filtration through sand or other granular sub-
stances after necessary preparation of the water by coagulation and sedimentation
(Fig. 1.4a).
2. Iron and manganese treatment plants remove excessive amounts of iron and man-
ganese by oxidizing the dissolved metals and converting them into insoluble flocs
removable by sedimentation and filtration.
3. Softening plants remove excessive amounts of scale-forming, soap-consuming
ingredients, chiefly calcium and magnesium ions (a) by the addition of lime and
soda ash, which precipitate calcium as a carbonate and magnesium as a hydrate
(Fig. 1.4b), or (b) by passage of the water through cation exchange media that
substitute sodium for calcium and magnesium ions and are themselves regener-
ated by brine.
Today most water supplies are either chlorinated or ozonated to ensure their disinfec-
tion. Lime or other chemicals are often added to reduce the corrosiveness of water to iron
and other metals and so to preserve water quality during distribution and ensure a longer
life for metallic pipes in particular. Odor- or taste-producing substances are adsorbed onto
activated carbon, or destroyed by high doses of chlorine, chlorine dioxide, or other oxi-
dants. Numerous other treatment methods serve special needs. The perspective of a water
treatment plant in northern Portugal is shown in Fig. 1.5.
Water purification plants must take into consideration these design functions:
1. Process design: An understanding of unit operations that bring about the removal
or modification of objectionable substances.
2. Hydraulic design: A knowledge of how water flows through the structures com-
posing water purification plants: channels, pipes including perforated pipes,
gates, measuring devices, basins, beds of sand and other granular materials, and
pumps.
3. Structural design: A comprehension of the behavior of needed structures under load.
4. Economic design: An appreciation of treatment costs and associated benefits.