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1.8: Ion Exchange

  • Page ID
    5730
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    Learning Objectives

    • Describe ion exchange chemistry
    • Define matter, ions, and compounds
    • Describe the aeration process and applications

    Ion Exchange Processes

    Most ion exchange units in use today use sulfonated polystyrene resins as the exchange media. Ion exchange can be defined as exchanging ions found in the source water for sodium ions or chloride ions that are attached to the ion exchange resins.

    Ions are atoms or molecules that have a non-zero net electrical charge. Its total number of electrons is not equal to its total number of protons. Matter occupies space and possesses rest mass, especially as distinct from energy. It comes in gas, liquid, or solid form. Ions are a form of matter. Compounds are made up of two or more atoms bonded together, and one form of bond is an ionic bond which is formed by the bonding of oppositely charged ions.

    A cation is a positively charged ion, while an anion is negatively charged. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds such as salts.

    Ions can be created by chemical means, such as the dissolution of a salt into water. Ions consisting of only a single atom are atomic ions. If they consist of two or more atoms, then they are called molecular ions.

    The treatment plant operator should be aware of the three types of ion exchange units:

    • An up-flow unit in which the water enters from the bottom and flows up through the ion exchange bed and out the top
    • A unit that is constructed and operated like a gravity rapid sand filter. The water enters the top, flows down through the ion exchange bed, and out the bottom
    • The pressure downflow ion exchange unit is the most common. These units may be horizontal or vertical. Vertical units are preferred because of less chances of short-circuiting

    The water enters the unit through an inlet distributor located in the top. It is forced (pumped) down through a bed of some type of media into an underdrain structure. From the underdrain structure, the treated water flows out of the unit and into storage or into the distribution system.

    The flow pattern through a filter and ion exchange unit are similar, the difference is the action that takes place in the media or bed of each process. The filter bed may be considered an adsorption and mechanical straining device used to remove suspended solids from the water. The bed usually consists of sand, anthracite, or a combination. Once the bed becomes saturated, with the insoluble material, the filter is taken out of service, backwashed, and returned to service. This pressure filter will continue to operate until the condition reoccurs and the procedure is repeated.

    The bed, media, or resin in an ion exchange unit is more complex. The resin serves as a medium in which an ion exchange takes place. As water is passed through the resin, the sodium ions on the resin are exchanged for cations in the water. The primary ions that are exchanged for sodium are calcium and magnesium. The sodium ions are released from the exchange resin and remain in the water, which flows out of the unit. The exchanged ions are retained by the resin, and is free of the exchanged ions.

    Once a unit has exchanged all the sodium ions and the resin is saturated, it will no longer remove the targeted ions. The unit must be taken out of service and the targeted ions removed from the resin by exchange them with sodium ions. This process is referred to as a regeneration cycle.

    In a regeneration cycle, the ions that have been retained by the resin must be removed and the sodium ions restored. For the exchange to take place, the resin must hold all ions loosely. Salt, in the form of a concentrated brine solution, is used to regenerate (recharge) the ion exchange resin. When salt is added to water it changes into or ionizes to form sodium cation and chloride anions. When the brine solution is fed into the resin, the sodium cations are exchanged. As the brine solution travels down through the resin, the sodium cations are attached to the resin. After regeneration has taken place, the bed is ready to be placed in service.

    Operation

    Several factors influence the procedures used to operate an ion exchange unit and the efficiency of the process. These factors include:

    • Characteristics of the ion exchange resin
    • Quality of the source water
    • Rate of flow applied to the unit
    • Salt dosage during regeneration
    • Brine concentration
    • Brine contact time

    Each ion exchange unit will have at least four common stages of operation. These stages are:

    • Service
    • Backwash
    • Brine
    • Rinse

    Service

    The service stage of each unit is where the actual ion exchange occurs. Water is forced into the top of the unit and allowed to flow down through the exchange resin. As this process takes place, the targeted ions are exchanged with sodium on the resin. The sodium ions are released into the water and the exchange capacity of the unit is slowly exhausted.

    The length of each service stage is dependent on source water quality. The effluent from the unit has a reduction is TDS. If the source water has high TDS levels, then some leakage may occur. If the high TDS water has high sodium levels, then the process may be hindered because a local exchange on the media for sodium may occur. The amount of leakage depends on the TDS and the salt dosage used for regeneration.

    Other factors involved ae the size of the softener and the exchange capacity of the resin. The unit should produce enough water without the targeted ions so that blending of the source water and effluent from the exchange unit will produce a treated water with the desired ion content.

    The exchange resin also varies in its removal capacity. The removal ability of the resin is usually expressed in grains of material removed per cubic foot of resin.

    The source water characteristics, the size of the unit, and the removal capacity of the resin will determine the amount of water that can be treated before regeneration. With a few calculations, an operator can determine the capacity of the units.

    Backwash

    The second stage of the ion exchange process is the backwash. In this stage, the unit is taken out of service and the flow pattern through the unit is reversed. The purpose of this activity is to expand and clean the resin particles and to free any material such as iron, manganese, and particulates that might have been removed during the service stage. The backwash water entering the unit at the beginning of this stage should be applied at a slow, steady rate. If the water enters the unit too quickly, it could create a surge in the resin and wash it out of the unit with the water going to waste.

    Ideal bed expansion during the backwash process should be 75 to 100 percent. When the unit is backwashed, the resin should expand to occupy a volume from 75 to 100 percent greater than when in normal service. As the bed expands, a shearing action due to the backwash water and some scrubbing action will free material that might have formed on the resin particles during the service stage.

    During backwash, a small amount of resin could be lost. The amount should be minimal and the operator should check the backwash effluent at different intervals to ensure that the resin is not being lost. A glass beaker can be used to catch a sample of the effluent while the unit is backwashing. A trace amount of resin should cause no alarm; however, a steady loss of resin could indicate a problem in the unit and the cause should be located and corrected. Too much loss of resin may be caused by an improper freeboard on the tank or wash troughs. The backwash duration and flow rate will vary depending on the manufacturer, the type and size of resin used, and the water temperature.

    Brine

    The third stage is most often called the regeneration or brine stage. At this point, the sodium ion concentration of the resin is recharge by pumping a concentrated brine solution onto the resin. The solution is allowed to circulate through the unit and displace all water from the resin in order to provide full contact between the brine solution and the resin.

    Most treatment plants use a brine solution for regeneration. The optimum brine concentration coming in contact with the ion exchange resin is around 10 to 14 percent sodium chloride solution. Concentrated brine is only used when the water within the unit serves as the dilution water. A 26 percent brine solution (saturated) cause too great of an osmotic shock on the ion exchange resin and can cause it to break up. The salt dosage used to prepare the brine solution is one of the most important factors affecting the ion exchange capacity, and it ranges from 5 to 15 pounds of salt per cubic foot of resin. Brine concentrations less than saturated require longer contact time and more solution must be applied to the resin to achieve regeneration.

    The regeneration stage is very important and the operator should be certain it is properly carried out. In the regeneration stage, the sodium ions present in the brine solution are exchanged. The ions on the resin were exchanged during the service stage. The regeneration rate is usually 1 to 2 GPM per cubic foot of resin for the first 55 minutes and then 3 to 5 GPM per cubic foot for the last 5 minutes. If the regeneration process is performed correctly, then the result is a bed that is completely recharged and ready for service.

    Rinse

    The last stage is the rinse stage. After adequate contact time has been allowed between the brine solution and the resin, a clear rinse is applied from the top of the unit to remove the waste products and excess brine solution. The flow pattern is very similar to the service stage except that the effluent goes to waste. The waste discharge contains high concentrations of chloride. Most rinse stages will last between 20 to 40 minute, depending on the size of the unit and manufacturer.

    The operator should pay close attention to the unit while it rinses. The rinse must be long enough to remove the heavy concentration of waste from the unit. If the rinse is not of sufficient time and the unit returns to service, a salty taste will be noticeable in the effluent. Taste the waste effluent near the end of the rinse stage to determine if the majority of chloride ions have been removed. The chloride ion concentration may also be measured by titration or the conductivity can be measured. If the water has a strong salty taste or excessive chloride ions are present, check the rinse rate and timer settings. The unit may need adjustment to increase the duration of the rinse stage.

    Typical ion exchange configuration.
    Figure \(\PageIndex{1}\): Ion Exchange – Image by the EPA is in the public domain

    Treatment for Iron and Manganese

    If the water contains manganese up to 0.3 mg/L and less than 0.1 mg/L of iron, an inexpensive and reasonably effective control can be achieved by feeding the water with one of the three polyphosphates. Chlorine usually must be fed along with the polyphosphate to prevent the growth of iron bacteria. The effect of the polyphosphate is to delay the precipitation of oxidized manganese for a few days so that the scale that builds up on the pipe walls is reduced.

    Polyphosphate Treatment

    The chlorine dose for phosphate treatment should be sufficient to produce a free chlorine residual of 0.25 mg/L after a five-minute contact time. Any of the polyphosphates can be used; however, sodium metaphosphate is effective in lower concentrations than the other polyphosphates. The proper phosphate dose is determined by laboratory bench-scale tests.

    Polyphosphate treatment to control iron and manganese is most effective when the polyphosphate is added upstream from the chlorine. The chlorine should never be fed ahead of the polyphosphate because the chlorine will oxidize the iron and manganese to insoluble precipitates.

    Ion Exchange

    Iron and manganese ion exchange units are similar to down-flow pressure filters. The water to be treated enters the unit through an inlet distributor located on the top. The water is forced down through the ion exchange resin into an underdrain structure. From the underdrain structure, the treated water flows out of the unit to the next treatment process.

    The location of the ion exchange resins with respect to other water treatment processes will depend on the raw water quality and the design engineer. If the water contains no oxygen, iron and manganese may be removed by ion exchange using the same resins that are used for water softening. If the water being treated contains any dissolved oxygen the resin becomes fouled with iron rust or insoluble manganese dioxide. The resin can be cleaned; however, this process is expensive.

    The primary advantage of ion exchange for iron and manganese removal is that the plant requires little attention. The disadvantages are the danger of fouling the ion exchange resin with oxide and high initial cost.

    To operate an ion exchange unit, operate as close as possible to design flows. Monitor the treated water for iron and manganese daily. When iron and manganese start to appear in the treated water, the unit must be regenerated. Regenerate with a brine solution that is treated with 0.01 pounds of sodium bisulfite per gallon (1.2 g/L) of brine to remove oxygen present. After regeneration is complete, dispose of the brine in an approved manner.

    Oxidation by Aeration

    Iron can be oxidized by aerating the water to form insoluble ferric hydroxide. This reaction is accelerated by an increase in pH. If the water contains organic substances, the rates will be significantly lower. A reduced temperature will also lower the rates. The oxidation of manganese by aeration is so slow that this process is not used on water with high manganese concentrations.

    Surface Aerator
    Figure \(\PageIndex{2}\): Aeration – Image by Trlabarge is licensed under CC BY-SA 3.0

    Since pH is increased by the removal of carbon dioxide, it is important that the aeration be as efficient as possible. Lime is sometimes added to the water to increase the pH along with the removal of carbon dioxide. The higher the pH the shorter the time required for aeration.

    The operation of the aeration process to remove iron and manganese requires careful control of the flow through the process. If the flow becomes too great, not enough time will be available for the reactions to occur. Flows are controlled by the use of variable speed pumps or the selection of the proper number or combinations of pumps. Carefully monitor the iron and manganese content of the treated water. If iron is detected, then flows may have to be reduced.

    Several methods are used for delivering aeration. The water being treated can be dispersed into the air or air can be bubbled through the water. Aeration may be achieved using compressed air that passes through diffusers in the water. These diffusers produce small bubbles that allow the transfer of oxygen in the air to dissolved oxygen in the water.

    Other aeration techniques include forced draft, multiple trays, cascades, and sprays. These methods may cause slime growths to develop on surfaces or coatings on media. Slime growths and coatings on media should be controlled to prevent the development of tastes and odors in the product water and the sloughing off of the slimes. Chlorination may be used to control slime growths and coatings. Regularly inspect aeration equipment for the development of anything unusual.

    A reaction (detention or collection) basin follows the aeration process. The purpose of the reaction basin is to allow time for the oxidation reactions to take place. The aeration process should produce sufficient dissolved oxygen for the iron to be oxidized to insoluble ferric hydroxide. A minimum recommended detention time is 20 minutes with desirable detention times ranging from 30 to 60 minutes. The pH of the water strongly influences the time for the reaction to take place. Sometimes chlorine is added before the reaction basin.

    The reaction basin is similar to a clarifier. Often the basin is baffled to prevent short-circuiting and the deposition of solids. Since no provisions for sludge removal are available, the basin must be drained and cleaned regularly. If the basins are not cleaned, slugs of deposits or sludge or mosquito and fly larvae could reach the filters in the next process and cause them to plug.

    Operators must be on the alert for potential sources of contamination. Basins should have covers and protective lids to keep out rain, stormwater runoff, rodents, and insects. All vents must be properly screened. The outlet to the drain must not be connected directly to a sewer or stormwater drain. An air gap or some other protective device to prevent contamination from back-flow must be present.

    After ferric hydroxide is formed in the aeration process, it is removed by sedimentation or by filtration. If only filtration is used, water from the reaction basin is usually pumped to pressure filters. The water may also be pumped or flow by gravity to rapid sand filters.

    The primary advantage of this method is that no chemicals are required; however, lime may be added to increase the pH. The major disadvantage is that small changes in raw surface water quality may affect the pH and soluble organic levels and slow the oxidation rates to a point where the capacity of the plant is reduced.

    Oxidation with Chlorine

    Chlorine will oxidize manganese to the insoluble manganese dioxide and will oxidize iron to insoluble ferric hydroxide, which can then be removed by filtration. The higher the chlorine residual, the faster this reaction occurs. Some compact plants have been constructed treating the water to a free chlorine residual of 5 to 10 mg/L, filtering, and dechlorinating to a residual suitable for domestic use. Do not use high doses of chlorine if the water contains a high level of organic color because excessive concentrations of total trihalomethanes could develop. The water is dechlorinated using reducing agents such as sulfur dioxide, sodium bisulfite, and sodium sulfite. Bisulfite is commonly used because it is cheaper and more stable than sulfite. When dechlorinating with reducing agents, be careful not to overdose because inadequate disinfection could result and if the dissolved oxygen level is in the water is depleted, fish kills could occur in home aquariums. Frequently, a reaction basin is installed between the chlorination and dechlorination processes to allow time for the reactions to occur.

    Oxidation with Permanganate

    Potassium permanganate oxidizes iron and manganese to insoluble oxides, and can be used to remove these elements in the same way that chlorine is used. The dose of potassium permanganate must be exact. Bench-scale tests are required to determine the proper dosage. Too small a dose will not oxidize all of the manganese in the water and too large a dose will allow permanganate to enter the system and produce a pink color in the water. Actual observations of the water being treated will tell if any adjustments of the chemical feeder are necessary. Most well water has relatively constant concentrations of iron and manganese. Therefore, once the chemical feeder is set, adjustments of the dosage usually are not necessary.

    Experience from many water treatment plants has demonstrated that a regular filter bed (rapid sand filter or a dual media filter bed) can remove manganese as long as iron and manganese concentrations are less than 1 mg/L. These plants use chlorine or permanganate to oxidize the iron and manganese before the water being treated flows through the filter bed.

    Potassium permanganate is often used with manganese zeolite or manganese greensand. Greensand is a granular material. After the greensand has been treated with potassium permanganate, it can oxidize iron and manganese to their insoluble oxides. The greensand also acts as a filter and must be backwashed to remove the insoluble oxides.

    Manganese greensand filters can be operated in three modes: continuous regeneration (CR), intermittent regeneration (IR), or catalytic regeneration. The method used will depend on the concentrations of iron and manganese in the water and the pH of the water.

    The manganese greensand continuous regeneration process can be used for water containing iron concentrations as high as 15 mg/L; however, with concentrations so high, frequent backwashing will be necessary. Generally, water having iron concentrations in the range of 0.5 mg/L to 3.0 mg/L can be treated with more acceptable run lengths of 18 to 36 hours before backwashing.

    In the continuous regeneration process, chlorine and potassium permanganate are added to the raw water ahead of the manganese greensand bed. Chlorine is added first to oxidize most of the iron and any sulfide. A slight excess of potassium permanganate is then added to oxidize the remaining iron and soluble manganese. This reaction produces insoluble oxides. When the raw water passes through the manganese greensand bed, two things occur:

    • The insoluble iron oxide particles are filtered out
    • Any remaining permanganate is reduced to manganese oxides by the greensand

    These manganese oxides attach to the grains of greensand; thereby, continuously regenerating the manganese greensand. As the run progresses, the filter bed becomes clogged with insoluble oxides and the differential pressure increases. When head loss reaches a predetermined point or a certain number of gallons of water have been treated, the filter must be backwashed to remove the filtered particulates.

    The intermittent regeneration process is suitable for raw water containing only manganese or mostly manganese with small amounts of iron. The raw water flows through a manganese greensand bed where oxidation of manganese occurs directly on the grains or greensand. Some iron will also be oxidized directly on the grains of greensand. If the iron concentrations are high, iron oxides will quickly coat or foul the media. To prevent fouling of the media, iron is sometimes converted to its insoluble form before the water enters the greensand bed by adding chlorine ahead of the filter or aerating the water before it enters the greensand bed. After treating a certain number of gallons of water or when head loss reaches a predetermined point, the capacity of the greensand to oxidize manganese and iron is used up and the media must be backwashed and regenerated. Regeneration consists of the down-flow passage of a dilute potassium permanganate solution through the bed using 1.5 ounces of potassium permanganate per cubic foot of media, followed by thorough rinsing of the media.

    When well water contains low concentrations of iron and manganese and the pH is greater than 7.0, the catalytic regeneration mode of operation may be a suitable method for removing iron and manganese.

    Operation of Filters

    When iron and manganese are oxidized to insoluble forms by aeration, chlorination, or permanganate, the oxidation processes are usually followed by filters to remove the insoluble material.

    Iron tests should be made monthly on the water entering a filter to be sure that iron is in the ferric state. Collect a sample of the water and pass the water through a filter paper. Run an iron test on the water that has passed through the filter. If the iron is still in the soluble ferrous state, iron is in the water. If aeration is being used to oxidize the iron from the soluble ferrous to the insoluble ferric state and iron is still present in the soluble state in the water entering the filter try adding chlorine or potassium permanganate. If chlorine or potassium permanganate is being used and soluble iron is in the water r entering the filter, try increasing the chemical dose. If potassium permanganate is being used, the sand may be replaced by greensand to improve the efficiency of the process.

    If oxidation is being accomplished by aeration or chlorination, a free chlorine residual must be maintained in the effluent of the filter to prevent the insoluble ferric iron from returning to the soluble ferrous form and passing through the filter.

    Most iron removal treatment plants are designed so that the filters are backwashed according to head loss. If iron break-through is a problem, filters should be backwashed when break-through occurs or just before a breakthrough is expected. Accurate records can reveal when a breakthrough occurs and when break thorough can be expected.

    Review Questions

    1. Describe ion exchange chemistry.
    2. Define matter, ions, and compounds.
    3. Describe the aeration process and applications.

    Test Questions

    1. ________ can be defined as exchanging ions found in the source water for sodium ions or chloride ions that are attached to the ion exchange resins.
      1. Softening
      2. Oxidation
      3. Reduction
      4. Ion exchange
    2. A ______ is a positively charged ion. An anion is negatively charged.
      1. Anion
      2. Cation
      3. Ion
      4. Electron
    3. An _____ is negatively charged.
      1. Anion
      2. Cation
      3. Ion
      4. Electron
    4. Once a unit has exchanged all of the sodium ions and the resin is saturated, it will no longer remove the targeted ions. The unit must be taken out of service and the targeted ions removed from the resin by exchanging them with sodium ions. This process is referred to as ________.
      1. Service cycle
      2. Backwash cycle
      3. Regeneration cycle
      4. Rinse cycle
    5. _______ is when the actual ion exchange occurs. Water is forced into the top of the unit and allowed to flow down through the exchange resin.
      1. Service cycle
      2. Backwash cycle
      3. Regeneration cycle
      4. Rinse cycle
    6. _____ is when the unit is taken out of service and the flow pattern through the unit is reversed. The purpose of this activity is to expand and clean the resin particles and to free any material such as iron, manganese, and particulates that might have been removed.
      1. Service cycle
      2. Backwash cycle
      3. Regeneration cycle
      4. Rinse cycle
    7. ______ is when the sodium ion concentration of the resin is recharge by pumping a concentrated brine solution onto the resin. The solution is allowed to circulate through the unit and displace all water from the resin in order to provide full contact between the brine solution and the resin.
      1. Service cycle
      2. Backwash cycle
      3. Regeneration cycle
      4. Rinse cycle
    8. After adequate contact time has been allowed during brine application, a ______ is applied to remove the waste products and excess brine solution. The flow pattern is similar to the flow through the unit except that the effluent goes to waste. The waste discharge contains high concentrations of chloride. This cycle will last between 20 to 40 minutes, depending on the size of the unit and manufacturer.
      1. Service cycle
      2. Backwash cycle
      3. Regeneration cycle
      4. Rinse cycle
    9. If the water contains manganese up to 0.3 mg/L and less than 0.1 mg/L of iron, an inexpensive and reasonably effective control can be achieved by feeding the water with _______. Chlorine usually must be fed along with the polyphosphate to prevent the growth of iron bacteria.
      1. Sodium chloride
      2. Polyphosphate
      3. Calcium carbonate
      4. Magnesium hydroxide
    10. Iron can be oxidized by aerating the water to form insoluble ferric hydroxide. This reaction is accelerated by __________.
      1. Increasing pH
      2. Increasing organic matter
      3. Decreasing temperature
      4. Decreasing pH
    11. After the greensand has been treated with _________, it can oxidize iron and manganese to their insoluble oxides.
      1. Sodium chloride
      2. Calcium carbonate
      3. Potassium permanganate
      4. Polyphosphate
    12. If oxidation of iron and manganese is being accomplished by aeration or chlorination, _______ must be maintained in the effluent of the filter to prevent the insoluble ferric iron from returning to the soluble ferrous form and passing through the filter.
      1. A pH of 8.2
      2. A permanganate residual of 0.3 mg/L
      3. A free chlorine residual
      4. None of these are required

    This page titled 1.8: Ion Exchange is shared under a CC BY license and was authored, remixed, and/or curated by John Rowe (ZTC Textbooks) .

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