Geoscience Reference
In-Depth Information
23 Water Treatment Processes
Pure water is the world's best and foremost medicine.
—Slovakian Proverb
23.1 INTRODUCTION
Because of huge volume and flow conditions, the quality of natural water cannot be modified sig-
nificantly within the body of water (Gupta, 1997). Consequently, the quality control approach is
directed to the water withdrawn from a source for a specific use. The drawn water is treated prior to
its use. Typically, the overall treatment of water (for potable use) consists of physical and chemical
methods of treatment—unlike wastewater treatment where physical, chemical, and/or biological
unit processes are used, depending on the desired quality of the effluent and operational limitations.
The physical unit operations used in water treatment include
Screening —This process removes large floating and suspended debris.
Mixing —Chemicals known as coagulants (e.g., alum) are mixed to make tiny particles
stick together.
Flocculation —Water mixed with coagulants is given a low motion to allow particles to
meet and floc together.
Sedimentation (settling) —Water is detained for a sufficient time so flocculated particles
settle by gravity.
Filtration —Fine particles still remaining in water after settling and some microorganisms
present are filtered through a bed of sand and coal.
Chemical unit processes used in treating raw water, depending on regulatory requirements and the
need for additional chemical treatment, include disinfection, precipitation, adsorption, ion exchange,
and gas transfer. A flow diagram of a conventional water treatment system is shown in Figure 23.1.
23.2 WATER SOURCE AND STORAGE CALCULATIONS
Approximately 40 million cubic miles of water cover or reside within the Earth. The oceans contain
about 97% of all water on Earth. The other 3% is freshwater: (1) snow and ice on the surface of
the Earth contain about 2.25% of the water, (2) usable ground water is approximately 0.3%, and (3)
surface freshwater is less than 0.5%. In the United States, for example, average rainfall is approxi-
mately 2.6 ft (a volume of 5900 km 3 ). Of this amount, approximately 71% evaporates (about 4200
km 3 ), and 29% goes to stream flow (about 1700 km 3 ). Beneficial freshwater uses include manufac-
turing, food production, domestic and public needs, recreation, hydroelectric power production,
and flood control. Stream flow withdrawn annually is about 7.5% (440 km 3 ). Irrigation and industry
use almost half of this amount (3.4%, or 200 km 3 /yr). Municipalities use only about 0.6% (35 km 3 /
yr) of this amount. Historically, in the United States, water usage has been increasing (as might be
expected); for example, in 1900, 40 billion gallons of freshwater were used. In 1975, usage increased
to 455 billion gallons. Estimated use in 2000 was about 720 billion gallons.
605
 
Search WWH ::




Custom Search