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News
CROWN
Solutions, Inc. Technical Resource
Water Softener Elution Studies
By James McDonald, PE, CWT
Ion
exchange is a physical process involving velocity, concentration,
and time. Regeneration is a combination of the same
three ingredients, and if one or two of the variables
are askew, proper regeneration will not occur. Then
each subsequent regeneration will result in steadily
reduced throughput.
During
softener regeneration, calcium and magnesium ions are
“eluted” from the resin beads with a highly concentrated
salt (NaCl) solution. This is one of the most critical
steps in the entire ion exchange process and one of
the first places for problems to pop up. Monitoring
the parameters surrounding this “elution” step, or performing
an “elution study,” is a very effective and easy investigative
tool for the water treatment engineer.
Tools
Tools required to perform an elution study include:
- Salometer:
A specialized hydrometer that measures the percent
saturation of a brine solution (0-100%).
- 250-mL
Graduated Cylinder: This is used as a hydrometer jar
for taking the Salometer readings.
- Timer:
Stopwatch, wristwatch, etc.
- Graph
Paper: To record Salometer readings versus time.
Sampling
Points
There are two sampling points for an elution study:
- Brine
tank
- Regenerant
drain line going to the sewer.
Procedure
1. Measure the concentration of the brine in the brine
tank using the Salometer. The brine tank should be 90%
to 100% saturated (90 to 100 Salometer degrees) at room
temperature.
2.
When the softener is ready for a regeneration and has
completed its backwash in the normal manner, make note
of the time the brine cycle started.
3.
Take a sample from the drain line as soon as the softener
switches to the brine cycle. Note: Be sure the sample
is taken at a point before water from a given softener
unit has mixed with any other water source.
4.
Transfer the sample to the 250-mL graduated cylinder,
insert the Salometer, and take a reading.
5.
Take samples every 2 to 3 minutes until the Salometer
reading has dropped below 5 Salometer degrees.
6.
Make note of the time the brine cycle was complete.
7.
Make note of the time when the softener switches from
slow rinse to fast rinse.
8.
Plot the Salometer readings on graph paper with time
(minutes) on the X horizontal axis and Salometer readings
on the Y vertical axis.
9.
Connect the data points to make a curve.
10.
Draw a horizontal line at 30 Salometer degrees.
Interpreting
the Results
Ideally, there should be 25-30 minutes where the curve
is above the 30 Salometer degree reading. This is known
as the “30-30 Rule” where a solution of 30 Salometer
degrees is in contact with the resin for 30 minutes.
There
are a number of operational factors that may affect
the appearance of the elution curve. Figure 1 illustrates
various possibilities and states possible causes for
the shape of each curve. Factors that can adversely
affect the elution curve include:
- Not
enough brine drawdown.
- Channeling
of the resin bed.
- Restricted
brine line or eductor resulting in a slow brine draw.
- Restricted
distribution system.
- Improper
adjustment of brine rinse flowrate.
- Unsaturated
brine in brine tank.
Brine
Usage
The brine usage can be calculated by measuring the diameter
of the brine tank and the depth of the brine before
and after the brine draw. You will need the following
information:
- Volume
=
where 
= 3.14, r = radius, & h = height of brine used.
- Assume
a void space of 40% between the solid salt pellets.
- At
100% saturation, 1 gallon of brine contains 2.647
pounds of salt.
Conclusions
An unconventional elution curve may not be able to pinpoint
the cause of a problem, but it can confirm that a problem
exists. After a review of the operating procedure and
perhaps an evaluation of a sample of resin has been
completed, corrective action can be taken, and a second
elution study can confirm that the efficiency of operation
has been improved.
Figure
1: Interpretations of Elution Curves
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