Water treatment method

1. Southeast University
Department of Pharmacy
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2. Overview:
Sub-topics Page No.
1)Water Treatment Method 02
2)Introduction of Ion Exchange 03
3)Principle 03
4)Ion Exchange Resin 03
5)Ion Exchange Process 05
6)Instrumentation 05
7)Advantage 08
8)Disadvantage 08
9)Applicability 09
10)At a glance 09
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3. Water Treatment Method
Water treatment is necessary to remove the impurities that are contained in water
as found in nature. Control or elimination of these impurities is necessary to
combat corrosion, scale formation, and fouling of heat transfer surfaces throughout
the reactor facility and support systems.
There are three general reasons to treat water for its impurities:
1. To minimize corrosion, which is enhanced by impurities
2. To minimize ​radiation​ levels in the reactor facility
3. To minimize fouling of heat transfer surfaces
There are several processes used in reactor facilities to purify the water in the
systems and water used as makeup. Deaeration is used to strip dissolved gases,
filtration is effective in the removal of insoluble solid ​impurities​, and ion exchange
removes undesirable ions and replaces them with acceptable ions. Typical
ionized ​impurities​ found in water are shown in Table 1:
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4. Introduction of Ion Exchange
In 1850, Thomas and Way performed some of the first scientific research that
indicated the existence of an ion exchange process. In their experiment, a solution
of ammonium sulfate was passed through soil. The filtrate collected was
composed of calcium sulfate instead of ammonium sulfate. The importance of this
discovery (in ion exchange terms) was not fully understood until later in that
decade, when it was found that this reaction was reversible. Ion exchange was
then primary used to soften water.
The presence of calcium and/or magnesium in water results in water being
considered “hard”. Calcium and magnesium ions in water react with heat, metallic
plumbing and chemical agents such as detergents to decrease the effectiveness of
nearly any cleaning task. Hard water can be softened using an ion exchange
softening process.
Ion exchange processes can also remove various charged atoms or molecules (ions)
such as nitrates, fluoride, sulphates, perchlorate, iron and manganese ions as well
as toxic metals (radium, uranium, chromium, etc.) from water.
The most typical application of ion exchange is the preparation of high purity
water for industrial applications, water softening, recovery or removal of metals in
the chemical industry.
Principle
Ion exchange is a water treatment method where one or more undesirable ionic
contaminants are removed from water by exchange with another
non-objectionable, or less objectionable ionic substance. Both the contaminant and
the exchanged substance must be dissolved and have the same type of electrical
charge (positive or negative). A typical example of ion exchange is a process called
“water softening” aiming to reduce calcium and magnesium content. Nevertheless,
ion exchange is also efficient in removing toxic metals from water.
Ion Exchange resin
Synthetic and industrially produced ion exchange resins consist of small,
microporous beads that are insoluble in water and organic solvents. The most
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5. widely used base-materials are polystyrene and polyacrylate. The diameter of the
beads is in the range of 0.3 to 1.3 mm. The beads are composed of around 50%
water, which is dispersed in the gel-structured compartments of the material.
Figure: Ion exchange resins bed contains many fine pores that fill with water.
Figure: A fixed and a mobile ion are changing places in so-called ion exchange
reaction.
Since water is dispersed homogenously throughout the bead, water-soluble
materials can move freely in and out. To each of the monomer units of the
polymer, so called “functional groups” are attached. These functional groups can
interact with water-soluble species, especially with ions. Ions are either positively
charged (cations) or negatively charged (anions). Since the functional groups are
also charged, the interaction between ions and functional groups is exhibited via
electrostatic forces. Positively charged functional groups interact with anions and
negatively charged functional groups interact with cations.
The binding force between the functional group and the attached ion is relatively
weak. The exchange can be reversed by another ion passing across the functional
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6. group. This process can be repeated continually, with one exchange reaction
following another.
Ion Exchange process
Figure: The water softening and recharge process.
The main component of ion exchange equipment is a microporous exchange resin,
which is supersaturated with a loosely held solution. For water softening, this is
usually done with sulfonated polystyrene beds that are supersaturated with sodium
to cover the bed surface. As water passes through this resin bed, ions attach to the
resin beads releasing the loosely held solution into the water.
After a time, the beds become saturated and the exchange resin must be
regenerated or recharged. To regenerate, the ion exchange resin is flushed with a
salt brine solution. The sodium ions in the salt brine solution are exchanged with
the ions, which are flushed out with wastewater.
Instrumentation
The equipment used for sodium zeolite softening consists of a softener exchange
vessel, control valves and piping, and a system for brining, or regenerating, the
resin. Usually, the softener tank is a vertical steel pressure vessel with dished heads
as shown in Figure.
Major features of the softening vessel include an inlet distribution system,
free-board space, a regenerate distribution system, ion exchange resin, and a
resin-retaining underdrain collection system.
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7. Figure: Softener tank, Vertical steel pressure vessel & Dished heads
The inlet distribution system is usually located at the top of the tank. The inlet
system provides even distribution of influent water. This prevents the water from
hollowing out flow channels in the resin bed, which would reduce system capacity
and effluent quality. The inlet system also acts as a collector for backwash water.
The inlet distributor consists of a central header/hub with distributing
laterals/radials or simple baffle plates, which direct the flow of water evenly over
the resin bed. If water is not prevented from flowing directly onto the bed or tank
walls, channeling will result.
Water is softened by the bed of strong acid cation exchange resin in the sodium
form. The quantity of resin required depends on the water flow, total hardness, and
time desired between regeneration cycles. A minimum bed depth of 24 in. is
recommended for all systems.
The underdrain system, located at the bottom of the vessel, retains ion exchange
resin in the tank, evenly collects the service flow, and evenly distributes the
backwash flow. Uneven collection of water in service or uneven distribution of the
backwash water can result in channeling, resin fouling, or resin loss.
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8. Figure: ​Schematic Diagram​ of a Typical Ion Exchanger
Although several underdrain designs are used, there are two primary types–subfill
and resin-retaining. A subfill system consists of multiple layers of support media
(such as graded gravel or anthracite) which support the resin, and a collection
system incorporating drilled pipes or subfill strainers. As long as the support layers
remain intact, the resin will remain in place. If the supporting media becomes
disturbed, usually due to improper backwash, the resin can move through the
disrupted layers and exit the vessel. A resin-retaining collector, such as a screened
lateral or profile wire strainer, is more expensive than a subfill system but protects
against resin loss.
The main valve and piping system directs the flow of water and regenerant to the
proper locations. The valve system consists of a valve nest or a single multiport
valve. A valve nest includes six main valves: service inlet and outlet, backwash
inlet and outlet, regenerant inlet, and regenerant/rinse drain. The valves may be
operated manually, or automatically controlled by air, electrical impulse, or water
pressure. In some systems, a single multiport valve is used in place of the valve
nest. As the valve rotates through a series of fixed positions, ports in the valve
direct flow in the same manner as a valve nest. Multiport valves can eliminate
operational errors caused by opening of the incorrect valve but must be properly
maintained to avoid leaks through the port seals.
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9. For instance, each cubic foot of a mixed-bed resin is capable of exchanging
with 19.8 moles each of monovalent cations and anions. Mixed-bed resins are
available commercially and in ​practical applications​ several cubic feet are used in a
purification system.
The capacity of ion exchange resins to remove impurity ions is given in Table 2
along with other information on resins.
Advantage
1. One of the most appropriate technologies to removes dissolved inorganic ions
effectively
2. Possibility to regenerate resin
3. Relatively inexpensive initial capital investment
Disadvantage
1. Does not remove effectively bacteria
2. High operation costs over long-term
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10. 3. The process of regenerating the ion exchange beds dumps salt water into the
environment (regeneration)
Applicability
The most common applications of ion exchangers are water softening (remove
calcium and magnesium ions), water demineralisation (removal of all ions), and
de-alkalisation (removal of bicarbonates). Cation exchange resins can also remove
most positively charged ions in water such as iron, lead, radium, barium,
aluminium and copper among others. Anionic exchange units can remove nitrate,
sulfate, and other negatively charged atoms (called anions). Researchers are
developing resins to selectively remove nitrate more efficiently than can currently
be done. Ion exchangers are also used to remove or recover metal ions from
wastewater in the chemical industry. Some contaminants (such as arsenic, fluoride,
lithium ions) are difficult to remove with ion exchange due to a poor selectivity of
the resins.
Ion exchangers are also used to remove or recover metal ions from wastewater in
the chemical industry. Some contaminants (such as arsenic, fluoride, lithium ions)
are difficult to remove with ion exchange due to a poor selectivity of the resins.
At a glance
Working principle Undesirable ionic contaminants are removed from
water by exchange with another non-objectionable,
or less objectionable ionic substance.
Capacity/adequacy Relatively simple technology.
Performance Efficient technology to remove ionic substances
from water and to soften water.
Costs Relatively low costs.
Self-help
compatibility
Monitoring is necessary to manage the regeneration
process.
Operation &
M​anagement
Ion exchange resin must be regenerated regularly.
Reliability Reliable if ion exchange resin regenerated properly.
Main strength Efficient to remove dissolved inorganics.
Main weakness Do not remove particles or bacteria.
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