Page 344 - Water and wastewater engineering

ION EXCHANGE 8-13

Column 1 Column 2 Column 3
FIGURE 8-4
Two columns in series with one column as standby. After exhaustion of column 1, it will be taken out of service and regener-
ated. Column 2 will become “lead” and column 3 will follow in series. When column 2 is exhausted, it is taken out of service
and column 3 becomes “lead.” Its effluent is passed through the regenerated column 1. This system has been called a “merry-
go-round” system.

Multiple Columns
With the exception of home water softeners that may be shut down for a short period at night or that
may be replaced by a service technician, the exhausted column that is taken out of service must be
replaced by bringing another column on line. Although there are many alternate arrangements, three
schemes are more common than others. They are (1) a standby column, (2) columns in series (known
in the trade as the “merry-go-round” system), and (3) columns in parallel (the “carousel” system).
In the standby system there is a minimum of two columns. One is in service while the other
is being regenerated and placed in standby. The operating time of each column must be long
enough to allow for regeneration of the out-of-service column. This system does not provide any
redundancy if only two columns are provided. A three column arrangement provides one extra
column in the rotation and allows for backup during maintenance.
As show in Figure 8-4 , the first column in the merry-go-round system serves as a roughing
column and a second column serves as a polishing step.
In the carousel system, three columns are run in parallel while one is out of service. The three
columns are in various stages of exhaustion: up to and including breakthrough, less than break-
through, and substantially less than breakthrough. The water from the three columns is blended
to achieve a consistent product water. This system is more likely to be used to meet an MCL
requirement for a toxic constituent than for softening.

8-4 ION EXCHANGE PRACTICE

Typical design criteria for cation and anion exchange systems are summarized in Table 8-2 . The
following paragraphs elaborate on the design parameters.

Resin Selection
There are several hundred different resins available from United States and European manu-
facturers. Of these, the resins based on the polystyrene-DVB matrix are most widely used. The
operating capacity (meq/mL as CaCO 3 ) serves as the primary selection criterion. This differs