Page 48 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

1 Water Treatment

The topics covered in this chapter include a review of unit The process principles must be adhered to but they must be
processes, the genesis of water treatment as a technology, a provided the means for being implemented, i.e., in terms of a
discussion of units, and an overview of how the book is plant. The latter is not excluded in this book, but is not the main
organized. focus. The two areas are complementary. Process design can be
taught in school. But in putting it all together, experience is
most important.
1.1 WATER TREATMENT IN-A-NUTSHELL
Any water treatment is done in the context of a treatment 1.1.2 RESIDUALS
train, a collection of unit processes. Such unit processes may
As an axiom of water treatment, residuals are always a
include screening, sedimentation, flotation, coagulation, filtra-
by-product. They are unwanted, but must be dealt with. For
tion, adsorption, ion exchange, gas transfer, oxidation, bio-
example, in membrane treatment, the ‘‘concentrate’’ water
logical reactions, and disinfection. The aggregation selected,
flow is often limiting with respect to the feasibility of an
i.e., the treatment train, if applied to full scale with all of the
installation. The residuals stream in water treatment includes
needed appurtenances and engineering to make it function on
the sludge from settling basins and the backwash water of
a continuous basis, is a water treatment plant (WTP). The
filtration. Settling ponds must be provided to decant the
objective of water treatment, by this treatment train, is to
water and to dispose of the resulting sludge. In wastewater
effect a required change in water quality.
treatment, a variety of treatment trains may be employed to
The water being treated may be any water, e.g., ambient
process the solid streams, with some kind of recycle or
water used as a source for drinking water, municipal or
disposal being the end result.
industrial wastewater, contaminated groundwater, brackish
Some of these required tasks can be taught and other aspects
water, seawater, or the product water from any treatment
must be learned by experience. Some introduction is helpful,
plant. The treatment train employed in a WTP depends, first,
but experience and visits to installations is essential. Many
upon the source water and the objectives for the effluent water
questions can be answered readily and the problems may be
and, second, on other factors, such as capital and operating
put into perspective by visits to plants complemented by dis-
costs, reliability, ease of operation, traditions, current prac-
cussions with operators and experienced engineers.
tices, etc. The relative importance of these other factors
depends on the project at hand.
1.2 ORGANIZATION OF WATER TREATMENT
1.1.1 WATER TREATMENT PLANTS KNOWLEDGE
To bring into existence an operating WTP requires more than a Common themes in organizing water knowledge include
consideration of unit processes. A well-conceived treatment
train and the proper design of its unit processes is the heart of 1. Treatment for a particular purpose, such as for drink-
any design. But much more is required to support the process ing water, wastewater, industrial wastewater, and
design. All plants must have, e.g., stated not in a particular contaminated groundwater
order, various kinds of sampling, metering and monitoring, 2. Treatment of particular contaminants
control of flows with pumps and valves of various types, 3. Unit processes
facilities for receiving and storing chemicals along with their
subsequent metering and injection, safety measures regarding From about 1880 to 1960, knowledge developed along the
chemicals and many other aspects of plant operation, labora- lines of the first approach, i.e., drinking water treatment,
tory support, utility tunnels, structural design of various tanks, wastewater treatment, and industrial wastes treatment. Then
hydraulic design for various purposes including setting the treatment of hazardous wastes emerged about 1980, employ-
hydraulic grade line of a plant, etc. In a plant with granular ing some of the same unit processes. Desalting of brackish
activated carbon (GAC), transport of the treated carbon to and waters and seawater became issues beginning about 1960.
from the reactors must be provided along with provision Thus, we have had books and persons who think along the
for regeneration either on-site or off-site. In addition, the stor- lines of drinking water treatment and others who think of
age tanks, usually steel, must be protected from corrosion. municipal wastewater treatment, with commensurate books
The distinction between the design of water treatment and journals, and yet another group who specializes in treat-
processes and a WTP is between principle and implementation. ment of industrial wastes, etc.

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