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Questions and Answers
What is the characteristic of hard water when using soap?
Which type of hardness can be removed by simple boiling?
What are common ions that cause water hardness?
Ca2+, Mg2+, Fe2+
Name the process used to soften water by adding lime and soda.
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What is the general formula for Zeolite?
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Zeolite can be regenerated easily using brine solution.
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What is the residual hardness of water typically achieved using the Zeolite process?
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Which of the following processes is NOT used for water softening?
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The method that uses ion-exchange resins is known as __________.
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What type of hardness is caused by the presence of dissolved bicarbonate salts of calcium and magnesium?
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Which method is effective for removing permanent hardness in water?
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What is the primary compound formed when magnesium bicarbonate is treated to remove hardness?
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What type of ions do cation-exchange resins primarily exchange?
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Which of the following functional groups is responsible for cation exchange?
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Which method typically employs zeolites for water softening?
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What is a characteristic outcome of treating hard water with soap?
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Which ions typically need to be removed to achieve the softening of hard water?
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Which of the following ions contributes to water hardness and cannot be removed by boiling?
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What is the role of a mixed-bed deionizer?
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In which process are calcium carbonate and magnesium hydroxide precipitated out to soften water?
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What is the effect of treating exhausted resins with strong mineral acid or alkali?
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How does hard water interact with a cation exchanger?
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Which type of ion-exchange resin can exchange OH- ions with anions in water?
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What type of polymers are cation-exchange resins typically made from?
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What is the primary product when hydrogen ions are exchanged with cations like Ca2+?
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What is the primary function of reverse osmosis in water treatment?
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Which of the following best describes brackish water?
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What is the key principle behind effective corrosion control according to Michael Henthorne?
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Which material selection is recommended for corrosion resistance?
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What should be avoided in the design of structures to minimize corrosion?
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In the context of the corrosion control, what is the role of anodic and cathodic materials?
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What is the primary purpose of using corrosion inhibitors?
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What describes the process of desalination?
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What is the primary advantage of using a mixed bed exchanger for water treatment?
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Which chemical is used to regenerate the cation exchange resin during the backwashing process?
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What is a necessary condition for water quality to use the ion-exchange process effectively?
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What is the role of hydrostatic pressure in the reverse osmosis process?
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Which of the following materials can be used for membranes in reverse osmosis?
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What is a disadvantage of the ion-exchange water treatment process?
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What quality of water can be achieved with the ion-exchange process?
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Which of the following is a prerequisite before treating water with ion-exchange process?
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What is one of the main advantages of the Zeolite process for water softening?
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Which ion exchanges occur in the Zeolite process?
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Which material is primarily used to make artificial zeolite?
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What limits the use of the Zeolite process in water softening?
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What byproduct is formed during the regeneration of zeolite?
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Which of the following is NOT a characteristic of the Zeolite process?
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What is the general structural formula of common zeolite?
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Which of the following is true regarding colored or impure water in relation to the Zeolite process?
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Study Notes
Water Hardness
- Water hardness is caused by dissolved salts like chlorides, bicarbonates, and sulfates of calcium, magnesium, and iron
- Hard water prevents lather formation with soap due to the precipitation of calcium or magnesium stearate or palmitate
- Hardness is categorized into temporary (carbonate) and permanent (non-carbonate) hardness
- Temporary hardness is caused by dissolved bicarbonate salts of calcium and magnesium
- Permanent hardness is caused by chlorides and sulfates of calcium, magnesium, and other heavy metals
Water Softening Methods
- Lime-soda process uses lime [Ca(OH)2] and soda [Na2CO3] to chemically convert soluble calcium and magnesium salts into insoluble compounds
- Zeolite (permutit) process uses sodium aluminium silicate (Na2OAl2O3.xSiO2.yH2O) which exchanges Na+ ions for Ca2+ and Mg2+ ions
- The Zeolite process involves regeneration using brine solution to remove calcium and magnesium ions from the zeolite
- Ion-exchange process uses insoluble, cross-linked, long chain organic polymers with functional groups that facilitate ion exchange
- Cation exchange resins exchange cations like calcium and magnesium for other ions
Zeolite Process Details
- Sodium zeolite (Na2Ze) is used to remove hardness-causing ions
- Na2Ze reacts with calcium and magnesium salts to form calcium or magnesium zeolite and sodium salts
- Regeneration of zeolite requires a brine (NaCl) solution to displace the calcium and magnesium ions from the zeolite
Zeolite Process Advantages
- Residual hardness is about 10 ppm
- Equipment is compact and manageable
- Softening process is quick
- No sludge formation, clean process
- Easy regeneration with brine solution
- Removal of all types of hardness
Zeolite Process Disadvantages
- Colored water or water with suspended impurities require filtration prior to softening
- Acidic water destroys zeolite and cannot be softened
Ion-exchange Process
- Ion-exchange resins are insoluble, cross-linked, long chain organic polymers
- They have a microporous structure and functional groups responsible for ion-exchange properties
Cation Exchange Resin
- Cation exchange resins are important for water softening
- Cation exchange resins exchange cations like calcium and magnesium for other ions
- They are synthetically made with specific functional groups
- They are highly effective for removing hardness from water
Anion Exchange Resin
- Anion exchange resins are also used for water purification
- They remove anions like chloride, sulfate, and nitrate.
Reverse Osmosis (RO)
- Reverse osmosis uses a semi-permeable membrane to separate water from dissolved salts
- The pressure applied is higher than osmotic pressure
- RO is an effective method for removing all types of contaminants from water
Other Water Treatment Methods
- Coagulation and Flocculation remove suspended solids
- Filtration removes particulate matter
- Disinfection kills bacteria and viruses
- Aeration removes dissolved gases
- Water softening is essential for industrial and domestic purposes.
- Hard water can cause scaling and damage to pipes and appliances
Hardness of Water
- Hardness is a characteristic of water that prevents the formation of lather with soap.
- It is caused by the presence of dissolved salts like chlorides, bicarbonates, and sulfates of calcium, magnesium, and iron.
- When hard water is treated with soap, white precipitate forms due to the formation of calcium or magnesium stearate or palmitate.
- This precipitation prevents lathering.
- The "hardness" is actually the precipitation of the soap.
- When the hardness-causing ions are removed, the water becomes soft and forms lather.
Types of Hardness
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Temporary Hardness (or) Carbonate Hardness:
- Caused by dissolved bicarbonate salts of calcium, magnesium, and other heavy metals like iron.
- Can be removed easily by boiling.
- The reaction of bicarbonate salts with heat leads to the formation of insoluble carbonates and the release of carbon dioxide.
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Permanent Hardness (or) Noncarbonate Hardness:
- Caused by the presence of chlorides and sulfates of calcium, magnesium, and other heavy metals.
- Cannot be removed by simple boiling.
- Can be removed by zeolites, lime-soda, or ion-exchange processes.
Water Softening Methods
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Lime-Soda Process:
- Involves chemically converting soluble calcium and magnesium salts into insoluble compounds by adding calculated amounts of lime (Ca(OH)2] and soda (Na2CO3).
- Calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2) precipitate out and are filtered off.
- Can be performed as a batch process or a continuous process (either cold or hot).
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Zeolite (Permutit) Process:
- Zeolite is a hydrated sodium aluminum silicate with the general formula Na2OAl2O3.xSiO2.yH2O.
- Zeolites exchange Na+ ions for Ca2+ and Mg2+ ions, softening the water.
- Common zeolites include natrolith (Na2OAl2O3.3SiO2.2H2O), gluconites, green sand, and Permutit (artificial zeolite).
- The effectiveness of zeolites in water softening depends on their porous and glassy structure, which allows for greater cation exchange capacity compared to other materials.
- Zeolites can be regenerated by treatment with brine solution (NaCl).
Zeolite Process: Method of Softening
- This is the chemical reaction that removes calcium and magnesium from water:
- Na2Ze + Ca(HCO3)2 --> 2 NaHCO3 + CaZe
- Na2Ze + Mg(HCO3)2 --> 2 NaHCO3 + MgZe
- Na2Ze + CaSO4 --> Na2SO4 + CaZe
- Na2Ze + CaCl2 --> 2 NaCl + CaZe
Zeolite Process: Regeneration of Zeolite
- The zeolite is regenerated by treating with a brine solution:
- CaZe (or) MgZe + 2 NaCl --> Na2Ze + CaCl2 or MgCl2
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Ion-Exchange Process:
- Ion-exchange resins are insoluble, cross-linked, long-chain organic polymers with a microporous structure.
- Functional groups attached to the polymer chains determine the ion-exchange properties.
- Cation exchange resins exchange cations with H+.
- Anion exchange resins exchange anions with OH-.
- Examples of functional groups:
- Acidic: -COOH, -SO3H (exchange H+ for cations)
- Basic: -NH2, =NH (exchange OH- for anions)
Ion-Exchange Process: Cation Exchange Resins (RH+)
- Common cation-exchange resins are styrene divinyl benzene copolymers.
- Sulphonation or carboxylation of these polymers makes them capable of exchanging hydrogen ions with cations in the water.
Ion-Exchange Process: Anion Exchange Resins (R’OH-)
- Common anion-exchange resins are prepared from styrene-divinyl benzene or amine-formaldehyde copolymers.
- These resins contain amino, quaternary ammonium, quaternary phosphonium, or tertiary sulfonium groups.
- After treatment with dilute NaOH solution, they can exchange their OH- ions with anions in the water.
Ion-Exchange Process: The Process of Ion-exchange
- This is how cation and anion exchange work:
- 2 RH+ + Ca2+/Mg2+ --> R2Ca2+/R2Mg2+ + 2 H+ (Cation exchange)
- R’OH- + Cl- --> R’+ Cl- + OH- (anion exchange)
- 2 R’OH- + SO42- --> R’2 SO42- + 2 OH- (anion exchange)
- 2 R’OH- + CO32- --> R’2 CO32- + 2 OH- (anion exchange)
- Finally, H+ + OH- --> H2O
Ion-Exchange Process: Regeneration of Exhausted Resins
- The exhausted resins are regenerated by treatment with strong mineral acid (for cation exchange resins) or alkali (for anion exchange resins).
Ion-Exchange Process: Mixed-Bed Process
- In a mixed-bed deionizer, cation and anion exchange resins are mixed together in a single pressure vessel.
- The mixed bed process allows multiple contacts between water and both types of resins, resulting in a higher quality of water.
- Mixed-bed exchangers can produce water with hardness less than 1 ppm.
- Regeneration: The mixed bed is first backwashed to separate the cation and anion exchange layers.
- The layers are then washed with NaOH (for the anion exchanger) and H2SO4 (for the cation exchanger).
- Finally, compressed air is used to mix the resins again.
- The softened water can be further purified by passing through a mixed bed system.
Advantages of Ion-Exchange Process:
- Can be used for highly acidic or alkaline water.
- Can achieve very low residual hardness levels (as low as 2 ppm).
- Good for treating water for high pressure boilers.
Disadvantages of Ion-Exchange Process:
- Expensive equipment and chemicals.
- Turbidity of water should be less than 10 ppm, or the output will reduce.
- Requires skilled labour.
Osmosis and Reverse Osmosis
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Osmosis: The movement of solvent molecules from a region of lower solute concentration to a region of higher solute concentration across a semipermeable membrane.
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Reverse Osmosis: The process of reversing the flow of solvent molecules by applying hydrostatic pressure greater than the osmotic pressure to the concentrated side of the membrane.
Examples of Membranes Used in Reverse Osmosis:
- Cellulose acetate
- Polysulfone
- Polysulfone amide
- Polyamide
- Poly-acrylonitrile
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Reverse Osmosis Advantages:
- Removes ionic, non-ionic, colloidal, and high molecular weight organic matter.
- Removes colloidal silica.
- Low maintenance cost (membrane lifespan ~ 2 years).
- Easy membrane replacement.
- Low operating cost and high reliability.
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Reverse Osmosis Filters:
- Have a pore size of about 0.0001 micron.
- The water passing through these filters is essentially pure water.
- This process removes most minerals, organic molecules, and viruses from the water.
- Reverse osmosis removes monovalent ions, effectively desalinating the water.
Brackish Water
- Brackish water contains dissolved salts, giving it a salty taste.
- It is less salty than seawater.
Desalination of Brackish Water
- The process of removing salt from water.
- Common methods include:
- Electrodialysis
- Reverse osmosis
Corrosion and Corrosion Control
- Corrosion is the deterioration of a material due to chemical reactions with its environment.
- Because corrosion is impossible to completely eliminate, effective engineering focuses on controlling it.
Corrosion Control Methods
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Material Selection:
- Choose metals as pure as possible, as impurities enhance the rate of corrosion.
- Consider using noble metals, which are highly resistant to corrosion.
- Avoid contact between dissimilar metals in a corrosive environment.
- If two dissimilar metals must be used, they should be as close as possible to each other in the electrochemical series.
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Proper Designing:
- When anodic and cathodic materials are used together, the area of the anodic material should be larger.
- The anodic part should not be painted or coated to avoid localized corrosion due to any coating damage.
- When direct joining of dissimilar metals is unavoidable, use an insulating fitting to prevent electrical contact.
- Avoid sharp corners, angles, and edges during construction to minimize the accumulation of impurities.
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Corrosion Inhibitors:
- Chemicals added to a corrosive environment to slow down the corrosion process.
- Anodic inhibitors act by forming a protective film on the anodic surface, slowing down oxidation.
- Cathodic inhibitors act by interfering with the cathodic reaction, slowing down the reduction process.
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Description
Explore the concepts of water hardness, including the causes and types of hardness. Learn about various water softening methods such as the lime-soda process, zeolite process, and ion-exchange process to remove hardness from water.
