Page 503 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

458 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

Batch reactor:A defined-volume reactor, e.g., a tank, in will be placed in operation as operating column #3, which
which there is zero influent flow and zero outflow during a ensures that the breakthrough curve is perpetually within
period of reaction. column #2, or at least not extending to the end of column #3.
Continuous-flow reactor: A reactor in which the influent
flow >0 and the effluent flow >0. Usually, such reactors 15.1.1.4 Performance Measures
are either complete-mix or a column. The former are more Several measures of performance have been used. The
common for powdered activated carbon; the latter are ‘‘effectiveness’’ of the process is whether the effluent concen-
more common for granular activated carbon. tration is less than a target value, C b (the breakthrough value)
Column: A packed-bed reactor that is shaped as a column and i.e., C e C b . Another measure of effectiveness is the volume
of water treated when C e C b occurs, i.e., V b . A measure of
is a continuous-flow. The reactor column is nonhomogeneous
with respect to adsorbate concentration with depth, Z. efficiency is the carbon usage rate (CUR); CUR ¼ (mass of
carbon involved when C e C b occurs)=V b (Reed et al., 1996).
Breakthrough curve: A term given to the characteristic
A parameter used in research is ‘‘bed volumes’’ treated, in
S-shaped concentration versus time curve that is seen in the
which a bed volume is defined as the pore volume in the
effluent of a packed-bed column as the column nears exhaus-
GAC reactor bed, i.e., pore volume of reactor bed ¼ V(reactor
tion, i.e., as it approaches ‘‘saturation’’ near the bottom of the
bed) P (in which P ¼ porosity). The relation between ‘‘bed
.
volumes’’ treated and time of operation, t,is ‘‘bed volumes
column. The mathematical statement is C(t) Z¼Z max
Wave front: The concentration, C, versus distance, Z, curve at treated’’ ¼ Q t=(V(reactor bed) P).
a given time, i.e., C(Z) t within a column, or packed-bed
reactor. The emergence of the wave front (i.e., its translation)
at the bottom of the column defines the ‘‘breakthrough’’ 15.1.2 ADSORBENTS
curve. What is called here the ‘‘wave front’’ is also called,
The range of manufactured adsorbents is not large; the most
synonymously, the ‘‘mass-transfer zone’’ (see also Neethling
common is activated carbon. Therefore, it is described more
and Culp, 1990, p. 132, Snoeyink, 1990, p. 795). Johnson
extensively.
et al. (1964, p. 9) refer to the phenomenon of a wave front
(of dissolved organic matter) moving downward through the 15.1.2.1 Kinds of Adsorbents
column and that complete exhaustion is not accomplished
Almost any solid substance can serve as an adsorbent. In
until the total bed length has had this ‘‘front’’ pass through it.
nature, rocks in mountain streams may provide a surface for
biofilms; sand may serve the same function in the percolation
15.1.1.2 Process Description of water through soil to groundwater. Clay or silt may provide
The adsorption process is the assimilation of adsorbate mol- adsorption sites for bacteria (and probably viruses). In water
ecules or particles by the surface of a solid, i.e., the adsorbent. treatment, biological or chemical flocs may provide adsorp-
This process may be depicted by the reaction, tion sites for organic molecules.

X þ A ! A X (15:1) 15.1.2.1.1 Activated Carbon
The most common adsorbent used for engineered adsorp-
where tion systems is activated carbon, which can adsorb a wide
X is the concentration of adsorbent, X, in aqueous suspen- spectrum of organic compounds. Its large surface area, e.g.,
2
3
sion (kg=m ) 1000 m =g, provides the highest adsorption capacity of any
A is the concentration of adsorbate, A, in aqueous solution material. Its mean pore diameter is about 3–6 mm (McGuire
3
(kg=m ) and Suffet, 1978, p. 623).
A X is the concentration of A in adsorbed state on or
3
within X (kg A=kg X=m solution) 15.1.2.1.2 Synthetic Resins
Several commercial resins (i.e., ‘‘macroreticular’’ resins,
The adsorbate may be any dissolved molecular substance meaning that they resemble a net in internal appearance or
or particles in suspension. Usually, for a designed system, the structure) are available, e.g., XAD-1t, XAD-2t, XAD-4t,
adsorbent is activated carbon. XAD-7t, XAD-8t, etc. (Kunin, 1980) with surface areas
2
100, 300, 725, 450, and 160 m =g, respectively, with mean
15.1.1.3 Operation pore diameters 200, 100, 48, 85, and 150 Å, respectively. The
Regardless of the kind of system, an adsorbent, e.g., GAC, first three are nonpolar adsorbents and XAD-7t, XAD-8t are
will eventually become ‘‘saturated,’’ i.e., exhausted, by con- polar. The pore size distribution may be quite large with
tinuous exposure to a given adsorbate. Therefore, to utilize the XAD-7t having a range of 30–1800 Å. These resins are
GAC effectively, several columns, e.g., 2 or 3, may be polymeric, i.e., they are pure polymers without ion-active
arranged serially. An additional column may serve as standby. groups, and depend upon their large surface areas for their
The first column will become saturated first and then must be adsorption capacities. The XAD-8t resin (Rohm and Haas,
removed from operation. At same time, the standby column Philadelphia, Pennsylvania) has been used to concentrate