Page 346 - Water and wastewater engineering

ION EXCHANGE 8-15

The larger the laboratory or pilot scale column, the better will be the results from scale-up.
Although 60 cm long, 1 to 5 cm diameter columns are adequate for laboratory studies, larger
diameter columns (for example, 10 cm) that have resin bed depths greater than 1 m are recom-
mended (Reynolds and Richards, 1996; MWH, 2005).
Breakthrough curves are obtained from laboratory scale or pilot scale data such as that shown
in Figure 8-5 . The design breakthrough concentration, shown as b in Figure 8-5 , may be used to
V
estimate the capacity of the resin by calculating the area between the influent concentration ( C 0 )
and the effluent concentration and dividing by the mass of resin in the column.

Flow Rates
The flow rate through the column affects the kinetics of the absorption bed. The longer the water
is in contact with the resin, the greater is the opportunity for the mechanisms of the exchange
process to come into play. Thus, the longer the contact time, the longer the time to reach break-
through. There are two parameters used to control the contact time: (1) empty-bed contact time
(EBCT) and (2) service flow rate (SFR) or exhaustion rate. The EBCT is calculated as the vol-
ume occupied by the resin ( R ) divided by the flow rate:
V

V R
EBCT (8-19)
Q

The service flow rate is

Q
SFR (8-20)
V R

The EBCT and SFR are used for ease of calculation. An actual detention time in the bed
would have to account for the porosity. Typical EBCTs range from 1.5 to 7.5 min and SFRs
3
3
3
range from 200 to 1,000 m of water per day for each cubic meter of resin (m /d · m ). The SFR
C
7 0
6 5
C, meq/L as CaCO 3 4 3 Area 6.572 meq/L 2,350 L

15,444 meq

1 2
0
0 1 2 V b 3 4 5 6
Volume, m 3
FIGURE 8-5
Ion exchange softening breakthrough curve.