Page 350 - Water and wastewater engineering

ION EXCHANGE 8-19

volume should be estimated in the test column after the test column has been backwashed
and settled over several cycles.
2. The assumption of zero leakage is not realistic. As noted above, it will usually be some
concentration less than 5 mg/L as CaCO 3 .
3. The estimate of the hardness removed by the resin is determined by computing the area
under the breakthrough curve at the “design breakthrough.” For a two column system
the design breakthrough is some hardness concentration above the leakage. For three
columns in series, the design breakthrough may be as high as complete bed exhaus-
tion. Likewise, in the parallel system with four columns, exhaustion of the bed may be
selected as the design breakthrough.

Regeneration
Resins operated on the sodium cycle are usually regenerated with a 5 to 10% brine solution. The
3
3
mass loading ranges from 80 to 320 kg NaCl/m of resin with 80 to 160 kg NaCl/m of resin
2
3
being typical. The liquid flow rate is 60 to 120 m /d · m of surface area or in terms of bed vol-
umes, about 2–5 BV/h (Reynolds and Richards, 1996; MWH, 2005).
Slow Rinse
The water rinse to push the regenerate through the bed is at the same flow rate as the regeneration.

Cycle Time
A minimum of two columns is recommended for redundancy: one in service and one in regenera-
tion or standby. One column in service with storage is an alternative, but it provides no redun-
dancy for mechanical or resin rehabilitation. Even with two columns, the out-of-service time must
be less than the operating time for the in-service column to reach breakthrough. The following
may be used to estimate the out-of-service time (Clifford, 1999):

tos tbw tsr tfr (8-23)
tr

where t os out-of-service time
t bw time for backwashing, 5 to 15 min
t r time for regeneration, 30 to 60 min
t sr time for slow rinse, 10 to 30 min
t fr time for fast rinse, 5 to 15 min

Using the maximum estimate for each of these steps, the total out-of-service time is about
two hours.

Example 8-3. An alternative three column design for Hard Times ( Example 8-2 ) is to be evalu-
ated. In this alternative, the columns will be in series and exhaustion of the resin is the “design
breakthrough.” The total out-of-service time is estimated to be two hours.