Ion exchange process pros and cons in water treatment
INTRODUCTION
Ion exchange resins are used in ion exchange procedures. Ion exchange resins are synthetic copolymers of styrene and divinylbenzene/acrylic acid with functional groups attached that act as acids, alkalis, or salts. Ion exchange methods are used to remove hardness, alkalinity, and dissolved solids completely from water. Ion exchange is a reversible chemical reaction that removes dissolved ions from solution and replaces them with ions of the same or similar electrical charge.
APPLICATIONS OF ION EXCHANGE PROCESS IN WATER TREATMENT
Softening: A sodium-based strongly acidic cation exchange resin, for example, softens water by exchanging sodium ions in the functional group with calcium and magnesium ions in the water. When the ion exchange resin is depleted, it can be regenerated using common salt and used again.
Dealkalization: By exchanging hydrogen ions contained in the functional group with calcium and magnesium ions linked with alkalinity present in water, a weakly acidic cation exchange resin in hydrogen form removes carbonate (or temporary) hardness from water. When the ion exchange resin is depleted, it can be regenerated using mineral acid and utilized again. Permanent hardness is not removed by the resin.
De-silicisation: It is a method that removes silica from water by using a strong base anion exchange resin in hydroxide form. The procedure can be used on waters with low TDS but moderate to high quantities of silica.
Demineralization: A strongly acidic cation resin and a strongly basic anion resin are housed in two separate containers in series in this process.
ION EXCHANGE RESINS
For ion exchange resins, the matrix is the basic polymeric structure or solid support. The matrix is a three-dimensional copolymer that contains acidic or basic spots. Functional groups or functionality of the ion exchange resin refers to these acidic or basic locations. A functional polymer, often known as an ion exchanger or simply resin, is a matrix that contains functional groups.
Ion exchange resins are classified into the following four categories:
- Strong acid cation (SAC) exchange resin
- Weak acidic cation (WAC) exchange resin
- Strong base anion (SBA) exchange resin
- Weak base anion (WBA) exchange resin
Strong acid cation exchange resin
The chemical matrix of the major strong acid cation exchange resins used in industrial water treatment applications is made up of styrene and divinylbenzene. Sulfonic acid radicals are the functional groups. In most cases, a strong acid cation exchange resin with 6–8% divinylbenzene crosslinking is employed in water softening. This resin comes in two varieties: INDION 220 Na and INDION 225 Na.
The major resin used for demineralization is cation resin in hydrogen form, such as INDION 225 H.
Weak acidic cation exchange resin
Weak acid cation exchange resin is generally employed in industrial water treatment applications when there is a significant degree of hardness and alkalinity. This resin has a substantially higher capability than strong acid cation exchange resins for exchanging all cations associated with alkalinity. Polyacrylic acid divinylbenzene matrix with carboxylic functionality and gel structure is used to make a weak acid cation exchange resin. The primary benefit of this resin is that it can be regenerated with stoichiometric amounts of regenerant, making it much more efficient. INDION 236 is an example of this resin.
Strong base anion exchange resin
Type I and Type II anion exchange resins are the two types of strong base anion exchange resins. The Type I resin has the highest total basicity and hence produces the highest-quality effluent. The Type II resin eliminates anionic components as well, but because of its reduced basicity, it requires less caustic during the regeneration cycle. In general, a Type II strong base anion exchange resin is indicated when the concentration of silica in the treated effluent is not as critical, as well as when the raw water has a relatively high chloride and/or sulphate content. Strong Base Type I anion resin is FFIP, and Strong Base Type II anion resin is NIP. These resins are isoporous, which means they won’t be contaminated by organic materials found in most surface waters.
Weak base anion exchange resin
Weak base anion exchange resin is used to remove strong acids like hydrochloric and sulphuric acid in the field of deionization. Weak base anion exchange resin, unlike strong base anion exchange resin, cannot remove carbon dioxide or silica, but it has a significantly better capacity for removing chlorides and sulphates.
The resin’s structure can be gel or macroporous. The gel type weak base resin is utilized to treat water that does not have any organic fouling issues. Macroporous weak base anion resins are preferred for the treatment of water containing organic pollutants. The macroporous weak base anion resin INDION 850 is an example.
PROS OF ION EXCHANGE PROCESS
Boundary conditions
Ion exchanger contamination can be split into two types: soluble and insoluble chemicals. Undissolved particles is filtered through the resin. During the rinse and/or regeneration phases, these contaminants leave the resin. A maximum suspended matter level of 10 mg/l is an example of a useful number. Dissolved matter forms a link with the resin and is only released when the resin is regenerated. Iron, oils, microbiological matter, and organic compounds are the main contaminants.
Effectiveness
Cations (Cu, Ag, Au, Cd, Zn, Ca, Mg, etc.) and anions (nitrate, sulphate, chlorides, etc.) are removed using ion exchangers. Because ion-specific resins are frequently used, the yield ranges from 80 to 99 percent. The yield is partly determined by the matrix.
Support aids
Ion exchange process is also used as a regeneration fluid. It aids the removal of harmful particles in water that can cause corrosion to industrial structures like pipes and vessels.
Costs
On the one hand, the investment cost for an ion-exchange system is divided into column and piping costs, and on the other side, resin costs. The ion concentrations in the to-be-treated flow determine the operating expenses. The higher the concentration, the more frequent cleaning is required.
Complexity
Simple principles govern ion exchange. However, effluent quality must be monitored carefully, and rinse frequency must be properly set.
Level of automation
It is simple to automate ion exchange process.
CONS OF ION EXCHANGE PROCESS
Calcium Sulfate Fouling
Sulfuric acid is the most popular regenerant (chemical used to replenish the resin) for cation resin. When calcium combines with the regenerating sulfuric acid to generate calcium sulphate as a precipitate during the regeneration process, it forms calcium sulphate as a precipitate. This precipitate can clog the vessel’s pipes and dirty the resin beads.
Iron Fouling
The subterranean water bores’ feed water contains soluble iron in the form of ferrous ion. Ion exchange softeners remove a little bit of this iron, however if this supply water comes into touch with air before treatment, the ferrous ions are changed to ferric ions. After interacting with water, these ferric ions precipitate as ferric hydroxide. This substance has the potential to clog resin beads and reduce resin efficiency. The softener column may possibly fail as a result of this.
Adsorption of Organic Matter
The amount of dissolved organic matter in lake and river feed water is usually quite considerable. The dead foliage and other organic debris contained in this feed water give it a yellow or brown tint. These organic compounds could become permanently adsorbent within the resin beads, lowering resin efficiency. As a result, the treated water’s quality deteriorates. Prior to treatment with resin, these organic impurities can be eliminated by treating the feed water with alum to precipitate the organic debris.
Organic Contamination from the Resin
Organic contamination can sometimes be caused by the ion exchange resin itself. Organic components are frequently found in fresh ion exchange resin beads after manufacture. This type of resin pollution can be addressed by running purified water through an ultrafiltration treatment facility.
Bacterial Contamination
Bacteria and other microbes are not removed from the feed water by ion exchange resins, but they can occasionally help them flourish. The organic ingredients used to construct the resin beds offer bacteria with nourishment, allowing them to thrive. Heat, UV irradiation, or extremely fine filtering should be employed to treat the demineralized water produced by the ion exchange treatment plant when sterile water is required following treatment. Formaldehyde or other disinfectants can be used to disinfect ion exchange resin beds, but not heat or chlorine, as these can destroy the resin.