Page 335 - Water and wastewater engineering

8-4 WATER AND WASTEWATER ENGINEERING

sites, (4) exchange of the ions by reaction, (5) diffusion of the exchanged ions outward through
the pores to the solid surface, (6) diffusion of the exchanged ions through the boundary layer, and
(7) movement of the exchanged ions into the bulk solution.
For a column of resin, the exchange reactions begin to saturate the upper levels before the
lower levels. The progress of this saturation through the column results in a “wave” of satu-
ration as shown in Figure 8-2a . If samples are taken at the bottom of the column, a curve of
increasing concentration is detected as shown in Figure 8-2b . This curve is called the break-
through curve. At some point in time the effluent concentration exceeds the design criteria,
for example at concentration C c in Figure 8-2b . The column is then taken out of service and
regenerated.

) ( ) ( ) ( )
( 1 2 3 4
Inlet
concentration C 0 C 0 C 0 C 0
Exchange Exchange
zone zone

Effluent
concentration C a C b C c C d
(a)

C 0 Breakthrough C d
Concentration of gas in effluent
curve

C a C b C c Breakpoint
0
( ) ( ) ( ) ( )
1 2 3 4
Volume of effluent
(b)
FIGURE 8-2
Ion exchange wave ( a ) and breakthrough ( b ). ( Source: Treybal, 1968.)