Page 102 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

4 Unit Process Principles

This chapter outlines the spectrum of unit processes and organics, Giardia lamblia cysts, and various disinfection by-
associated technologies that are available for water treatment. products. Not only were new contaminants included, but the
The themes of ‘‘transport’’ and ‘‘sinks’’ are described, as is the traditional contaminants had lower limits. All of this meant
notion of a ‘‘reactor.’’ The materials balance principle is the that new treatment technologies had to be explored and the
basis for modeling a reactor. The materials balance principle traditional ones had to perform to achieve lower effluent
is the foundation for much of what follows in later chapters, concentrations and perform more reliably. For example, to
for example, for those that deal with fluidized-bed and handle disinfection by-products, the idea of ‘‘enhanced coagu-
packed-bed reactors. lation’’ was proposed. While this is merely using higher
dosages of alum or ferric coagulant to react with the natural
4.1 UNIT PROCESSES organic matter (NOM), it might be considered as a variation in
the established technology. Other technologies that have been
The unit processes for water treatment number only 10–15, explored include granular activated carbon (GAC), biofilters,
depending upon how they are categorized and counted (see, and nanofiltration membranes. Thus, there is nothing ‘‘set in
e.g., Section 1.1, Table 1.1, Table 3.1). Under each of the unit concrete’’ about which unit processes are most appropriate.
processes there are several categories and under the categories In some cases there may be two or three alternatives in the
there may be numerous technologies. As with contaminants, selection of a unit processes for a treatment train. In the
there are different ways to categorize. The categorization used treatment of municipal drinking water, for example, rapid
for this chapter is based upon unit processes. filtration has been traditional in the United States, since
about 1910. Other processes that may work include slow
4.1.1 SPECTRUM OF UNIT PROCESSES AND TECHNOLOGIES sand filtration, diatomaceous earth filtration, and microfiltra-
tion. The selection depends upon the ‘‘context’’ (see Box 1.1).
Table 4.1 (an expansion of Table 3.1) lists unit processes in
In addition to technical considerations such as water quality,
the left-hand column. As noted, there are only about 15. The
effluent or drinking water standards, flow, etc., the context
respective principles operative for each unit process are indi-
includes such factors as population served, location (e.g.,
cated in the second column. The third column lists the several
urban or rural), proximity to supplies, operating and mainten-
categories for a given unit process. The fourth column lists
ance costs, financing capacity of the community, political
technologies that implement the unit processes; the list is not
factors, operator capabilities, etc. The ‘‘form,’’ that is, the
inclusive. A fifth column, if added, could list the variations for
treatment train selected, should ‘‘fit’’ the context.
each technology. For example, under settling tanks, there are
rectangular tanks, upflow tanks, center feed circular tanks,
4.2 PRINCIPLES
peripheral feed circular tanks, etc. Also, manufacturers have
developed their own variations for implementing each tech- Two themes are common to most unit processes: (1) a ‘‘sink’’
nology. of some sort, and (2) transport to the sink. This section
summarizes these ideas. They are amplified further in the
unit process chapters.
4.1.2 MATCHING UNIT PROCESS WITH CONTAMINANT
Some words should be said about the match between contam-
4.2.1 SINKS
inants to be treated and the unit processes that could do the
job. A tabular array could be developed with contaminants as A ‘‘sink’’ is a site within a unit process where something
rows and processes and technologies as columns. The cells of ‘‘happens,’’ for example, where a contaminant is removed. To
the matrix would indicate the extent to which a given tech- illustrate, a sink may be a surface where a particle settles, an
nology could treat the contaminants listed in the rows. Usu- adsorption site within a particle of activated carbon, a surface of
ally, there is more than one unit process that may treat, granular media to which particles bond, or the multitude of
economically, a given contaminant. In some cases, laboratory microlocales within a reactor where two reactants collide by
testing followed by a pilot plant study is required. turbulence or molecular diffusion and a reaction occurs.
The kinetics of the reaction, that is, its rate, depends upon
4.1.2.1 Contextual Changes and New Treatment the rate of transport to the sink or the rate of the reaction. One
Demands or the other is rate limiting. Generally, the rate limiting mech-
Since the 1974 Safe Drinking Water Act, the number of anism is the rate of diffusion, which is a transport process,
regulated contaminants has expanded to include synthetic rather than the rate of reaction.

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