Civil Engineering Reference
In-Depth Information
regulations concerning the treatment of water supplies to eliminate any possibility of
the type of outbreak that occurred in Milwaukee.
TRENDS IN RAW-WATER QUALITY
Increased Water Withdrawals
Surface water sources are subject to ever-increasing withdrawals to supply a growing
population, industry, and agriculture. Heavy withdrawals of water for consumptive use,
such as irrigation, decrease stream flows available for downstream dilution of waste-
waters. In the case of underground water supply sources, excessive withdrawals may
have an adverse effect on the chemical quality of the supply, such as increasing the
content of iron or manganese or of total dissolved solids.
Waste Discharges
Ever-increasing quantities of domestic, industrial, and agricultural wastewaters are be-
ing discharged directly or indirectly to water supply sources. Domestic sewage con-
tinues to add increasing quantities of bacteria, viruses, algal nutrients, substances with
a high oxygen demand, suspended and dissolved solids, and taste- and odor-producing
substances to water supply sources. Return irrigation waters in some locations are
producing substantial increases in the total dissolved solids content of receiving
streams and aquifers, and many of these substances, such as sodium sulfate, chloride,
and others, are not removed by conventional treatment methods or other economically
feasible means. Serious changes are also produced by industries that are discharging
a wide variety of complex chemical pollutants to the waters of the nation.
Objectives in Water Treatment
The primary objective of water treatment for public supply is to take water from the
best available source and to subject it to processing that will assure that it is always
safe for human consumption and is aesthetically acceptable to the consumer. For water
to be safe for human consumption, it must be free of pathogenic organisms or other
biological forms that may be harmful to health, and it should not contain concentra-
tions of chemicals that may be physiologically harmful. To provide safe water, the
treatment plant must be properly designed and skillfully operated.
The general requirements of an aesthetically acceptable water are that it be cool,
clear, colorless, odorless, and pleasant to the taste; also, it should not stain, form scale,
or be corrosive. Treatment plants must be designed to produce water of uniformly
good quality despite variations in raw-water quality and plant throughput. Because the
consumer is interested in the quality of water at the tap rather than at the treatment
plant, precautions must be taken to preserve water quality in the distribution system
and to control water quality from tests of tap water samples as well as plant samples.
Many of the advantages of a high-quality water supply are difficult to express in terms
of exact economic return, but they are, without question, quite substantial. An excellent
supply of high-quality water assures good public relations and favors industrial and
community growth.
