Page 351 - Water and wastewater engineering

8-20 WATER AND WASTEWATER ENGINEERING

Solution:
a. The mass of resin is divided equally between the three columns.

b. Using the total meq from step h in Example 8-2 , the meq of hardness to be removed in
one column at bed exhaustion is

6

1 3710 meq 5
.
.
4 5710 meq

3
3
c. The time for one bed to become exhausted at a flow rate of 208.5 m /d (step f in
Example 8-2 ) is
5

.
4 5710 meq
0 33d or 8 h
.
3
3
,
.
.
0
( 6 572 meq/L)( 208 05 m /d)( 1000 L/m )
d. If breakthrough is 67% of the exchange capacity, the time to breakthrough is
5
.
.

(067 )(4 5710 meq )
.
.
3 3 022 d or 5 4 h
.
(6 572 meq/L )(208 05 m /d)(1 000, L/m )
.
/
e. The third column in the series can be regenerated in two hours. Therefore, this design
will work because the third column can be brought on-line before breakthrough occurs
in the second column.
Vessel Design
While pressurized tanks are used for small ion exchange columns, gravity flow is used in larger
applications. Fiberglass and steel are the usual tank materials. Fiberglass tanks are generally
limited to about 2.5 m in diameter, pressures under 1,000 kPa, and temperatures under 40 C.
Prefabricated steel tanks may be up to 3.6 m in diameter. Steel tanks must be lined and often are
more expensive than fiberglass.
Column heights are generally less than 4 m. Multiple columns in series are used if greater
height is required. The height of the bed to diameter ratio is usually in the range of 0.2:1 to 3:1
(Clifford, 1999; Reynolds and Richards, 1996). Resin bed depths of less than 0.9 m are not recom-
mended to avoid premature breakthrough due to nonideal resin bed behavior (GLUMRB, 2003).
Resin bed depths greater than 4 m are generally avoided because of pressure drop limitations of
the resin. The column height must be sufficient for expansion of the bed during backwashing.
The dimensional limits of prefabricated units are usually governed by roadway constraints for
underpasses and turning radii.
Table 8-3 summarizes the typical design criteria used in sizing the tank. Example 8-4 illus-
trates the design of the vessel.