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National Environmental Services Agency (NESA). Tom addresses the issues that bug you the most. And Tom knows!! With 35 years experience in providing environmental support services to public and private sector clients on a wide range of environmental issues. Tom has also co-authored and presented training courses on wastewater treatment systems.
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Everything You Wanted To Know
About Water Softening
Guest article by Gary Schreiber, The Purolite Company
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How Softening Works
 During
the softening process, water is passed through a column of ion
exchange resin. The calcium and magnesium ions present in the water
are exchanged on the resin beads for an equivalent amount of sodium
ion. The softened water exiting the water softener is significantly
higher in sodium than the raw water. This basically is the “Ion
Exchange Process”.
The exchange of hardness for sodium
is not perfect or complete and a small amount of “hardness” usually
passes through the softener in the treated water. However, testing
of the softened water discharging from a properly operating softener
unit for hardness at this stage will usually not detect the trace
amount of hardness. Eventually, more and more hardness will escape
in the water and it can be detected with the normal water hardness
test kit. At some stage, it will be necessary to rejuvenate or
regenerate the resin so that the quality of softened water can be
maintained at the required standard.
Regeneration of the resin is achieved
by passing a solution of salt, (brine or sodium chloride), through
the resin to displace the calcium and magnesium ions that have been
taken up by the resin beads from the water. The sodium from the
brine replaces the calcium and magnesium ions on the resin and when
this process is complete the resin is ready to be used again for
softening water.
Generally, the more brine that is
used the more efficiently is the hardness displaced from the resin.
So for more demanding applications, such as softening of boiler
water, more brine is used to minimize the amount of hardness left on
the resin when the softener is brought back into service. If
insufficient brine is used “hardness leakage” from the softener will
be higher.
Water Softening Resin Selection
A variety of ion exchange resins can
be used for water softening. They are the heart of a Water Softener.
When hard water, which contains calcium and magnesium (the primary
hard water constituents), passes through a bed of resin calcium and
magnesium are removed from the water. Together these two impurities
are referred to as the total hardness (TH) of the water. The photo
below is the general appearance of standard water softening ion
exchange resin beads as they appear under a microscope.
Depending on the accuracy needed,
water hardness can be tested by titration or by the simpler
dropper-bottle test. The result for the total hardness test can be
expressed as either “parts per million” (or ppm) of hardness or
“grains per gallon” (or gpg), or even mg/l, and usually expressed as
Calcium Carbonate (or CaCO3). (Grains per US Gallon x 17.118 = parts
per million).
Household water containing less than
about 1 grain per gallon, (or 17.1 ppm Total Hardness as CaCo3), is
generally considered to be “soft water”. However, water for
industrial or commercial use, (e.g. boiler feed water or other more
demanding application), may require the water hardness to be reduced
to less than 1 ppm total hardness.
However, in both cases similar
softening processes can be used, but to achieve the lower level of
total hardness required for the industrial application, the design
for the industrial and commercial units is likely to be more
stringent.
In addition to removing hardness from
water ion exchange resins will also remove soluble Iron from the
water. It is therefore important to test water for the presence and
quantity of soluble Iron. Standard resins are generally limited to a
maximum of 3 ppm of soluble Iron but can be applied to remove higher
Iron levels provided steps are taken to prevent Iron fouling of the
resin by either removing the Iron before feeding the water through
the resin or by applying a resin cleaning chemical during the
regeneration process.
Softening Basics
Standard ion exchange resins for
softening water are manufactured using an inert co-polymer of
polystyrene and DVB, which gives the resin good mechanical strength.
Functional groups, such as sodium, are added to the resin during the
manufacturing process to give the resins their ability to soften the
water.
Manufacturers typically produce a
variety of grades of ion exchange resin. Resins to be used in
potable water treatment are put through an additional manufacturing
step to ensure compliance with FDA guidelines.
Polystyrenic gel resins, having an 8%
DVB crosslinking, are the primary standard softening resins. These
are produced as spherical beads varying in diameter from 0.3
millimeter to 1.2 millimeter. This is referred to as the standard
Gaussian bead distribution, or “16 to 50 US mesh” beads. These beads
provide the maximum surface area for the ion exchange process, while
not being so small in diameter as to inhibit water flow through the
resin. These resins are used for softening water in most residential
and commercial/industrial applications.
Other Water Softening Resins Include
a) Fine Mesh Resin:
Resins that have a range of bead sizes that are generally smaller in
diameter (40 to 70 mesh) than standard resin. Fine Mesh resins offer
the advantage of lower salt usage, less rinse water requirement and
faster kinetics (the rate at which calcium and magnesium hardness is
exchanged onto the resin in place of sodium). Fine Mesh resins are
generally not used in commercial or industrial applications, but are
typically used only in residential applications.
b) UPS Resin: A resin with
Uniform Particle Size, (bead size generally 30 to 40 mesh), designed
to provide a higher operating capacity. They can be used in any
softening application but are primarily designed for
commercial/industrial applications. They offer higher operating
capacity than standard resins, require lower salt doses, have lower
hardness leakage and lower service pressure loss. The exact
difference in capacity will depend on the actual water quality and
operating conditions.
c) High-Velocity or High-Flow Resin:
This is a coarser grade of resin beads (16 to 35 mesh) that are
designed to allow higher flow rates without the disadvantage of the
higher pressure drop that would occur with the standard resin. This
allows for smaller vessels. The kinetics of these bead sizes are not
as good as Standard, Fine Mesh or UPS resins. Therefore they are
usually limited to low hardness waters with no soluble Iron content.
d) 10% DVB Crosslinked Resin:
Referred to as a 10% crosslinked product, emphasizing the higher 10%
DVB content (as compared to 8% DVB in standard softening resins).
The higher DVB content makes the product more resistant to chlorine
or similar oxidizing chemicals (such as Hypochlorite, ozone,
hydrogen peroxide or permanganate). So in situations where chlorine
concentrations are difficult to control, 10% DVB crosslinked resin
may be a better option, although it too will eventually be attacked
by the chlorine.
e) Macroporous Resin:
Manufactured differently from the standard softening resins that are
referred to as gel resins. Gel resins have no porosity. Macro resins
are manufactured to have large pores and higher DVB concentrations.
These 2 features make them more resistant to high temperature and to
DVB de-crosslinking. When chlorine or other oxidizing chemicals are
present in significant concentrations in the water to be treated,
these oxidizing agents dissolve the DVB from the resin. Macro resins
having more DVB will have longer life than standard resins,
particularly in situations where the oxidizing agents in the water
to be softened are in excess of 1 ppm.
A new softening resin technology,
representing a revolutionary way of manufacturing resin, with a
shell and inner core sections of the beads is now available. The
process is known as Shallow Shell Technology or Salt Saving
Technology. The core of the resin beads is inert and does not take
part in the ion exchange process. All softening occurs in the outer
shell area that is closer to the surface of the beads. It is ideally
suited for co-flow softening designs (the most popular in North
America) in which the water to be treated and the brine used
afterward for regeneration enters and leaves the ion exchange resin
bed in the same direction (usually from top to bottom).
During regeneration, it is easier and
faster for the brine to reach all of the hardness inside the shell
of the bead. Because of this, considerably less salt is required
compared to standard softener resins, and the resin will still
achieve the same or lower hardness leakage during the next service
cycle. Less rinse water is needed for the same reason. From an
environmental standpoint, the reduced salt usage and the lower rinse
water requirement makes it quite attractive compared to standard
resin. The extremely low hardness leakage allows it to excel in
industrial applications, such as softening feed water for use in low
and medium pressure boilers, feed water for use in Reverse Osmosis
systems, and softening of oil field type (high total dissolved
solids) water.
Even though Shallow Shell Technology
resin sells at a premium over standard softening resins, it is
usually possible in most cases to recover this premium in just a few
months from the savings in both salt and water used for
regeneration. The lower hardness leakage translates to higher
efficiency and greater reliability of the equipment being served.
Your resin manufacturer of choice can
help you to select the right grade of ion exchange resins for water
softening.
Summary
Water softening using ion exchange is
very flexible, cost effective and can be applied anywhere there is
the need to solve a water hardness related problem.
About our author:
Mr. Gary Schreiber, CWS VI
The Purolite Company
150 Monument Road
Bala Cynwyd, PA 19004
Telephone: 800-834-8784
Fax: 507-448-3508
Email:
gschreiber@puroliteusa.com
Web site:
http://www.puroliteusa.com/
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