Page 341 - Water and wastewater engineering

8-10 WATER AND WASTEWATER ENGINEERING

potential of causing physical damage to the resin beads, the hydraulic requirements of the resin
rather than the kinetics for ion exchange govern the selection of the resin particle size.
Ion exchange resin beads are spherical. They are produced in particle diameters ranging from
0.04 to 1.0 mm. In the United States, the particle sizes are sold by standard sieve screen or “mesh”
sizes. A table of U.S. Standard Screen sizes and their equivalent diameters is given in Appendix B.
The common sieve size ranges used are 16 to 50 and 50 to 100. The smaller number is the largest
diameter sieve, and the larger number is the smallest diameter sieve. The manufacturer’s specifi-
cation is generally given the notation 16 50 or 50 100. Thus, for a 16 50 resin, all of the
resin beads will pass the number 16 sieve, and none will pass the number 50 sieve.
Other data provided by the manufacturer includes the effective size ( d 10 ) and the uniformity
coefficient. The effective size is the mesh size that passes 10 percent of a sieved sample. The
uniformity coefficient is the ratio of the d 60 to the d 10 resin sizes. These data are provided to
facilitate hydraulic design.

Structural Stability and Service Life. As noted above, high pressure drops through the bed
have the potential to cause resin bead compression. This, in turn, can cause inadequate liquid
distribution and reduced flow. In addition the resin beads are also susceptible to swelling, shrink-
ing, and abrasion from excessive backwashing. These effects reduce the structural integrity of the
resin and shorten its operating life.
Oxidation of the resin beads, especially strong acid sulfonated polystyrene-DVB resins, from
chlorination prior to ion exchange will significantly reduce service life. If prechlorination is essential,
resins with high cross-linking are recommended (MWH, 2005).
Excessive concentrations of iron and manganese, if oxidized, will form precipitates that will
foul the resin. GLUMRB (2003) specifies that iron, manganese, or a combination of the two
should not exceed 0.3 mg/L in the water applied to the resin. Organic compounds may foul the
resin by irreversibly binding to strong base anion exchange resins.
Turbidity should not exceed 5 NTU in water applied to cation exchange softeners (GLUMRB,
2003).
Some of these issues are remedied with the selection of an appropriate resin and proper
arrangement of the sequence of pretreatment processes.

8-3 PROCESS OPERATION
To contact the water with the ion exchange resin, it is passed through a columnar pressure vessel
as shown in Figure 8-3 . The water is passed through the column until the effluent no longer
meets the treatment objective. The column is then regenerated. The two common methods for
regeneration (cocurrent and countercurrent) are used to identify the operating schemes.

Cocurrent Operation
In this scheme the regeneration step is conducted in the same flow direction as the treatment
flow. The direction of both flows is usually downward. For softening operations where some
leakage of hardness in the effluent can be tolerated, this operational scheme is frequently chosen.
It is usually the lowest cost design and the simplest to operate. The domestic water softener is a
familiar example of this type of design (Brown, 1998).