Chemistry Water Technology PDF

# 1) WATER TECHNOLOGY ## Introduction ### Sources of water 1. **Rain water:** * Purest form of water. * Dissolved gases are present in it: CO2, SO2, NO2, Cl2, H2S. 2. **Surface water** * **River water:** All types of impurities are present. * **Sea water:** Most impure f...

# 1) WATER TECHNOLOGY ## Introduction ### Sources of water 1. **Rain water:** * Purest form of water. * Dissolved gases are present in it: CO2, SO2, NO2, Cl2, H2S. 2. **Surface water** * **River water:** All types of impurities are present. * **Sea water:** Most impure form of water. * Contains high % of salts & organic matter. * **Lake water:** Constant composition of impurities. * More amount of organic matter. 3. **Underground water** * Rain water percolates → Soil → No. of salts dissolved. * It is free from organic impurities. * Sources: Well, Bore well, Springs. # Impurities in Water ### A) Suspended impurities * Ex: Clay, mud, organic matter, large sized particles. * Removal method = Filtration. ### B) Colloidal impurities * Ex: Very fine clay, mud particles. * Cannot be removed by filtration. * Removal method = Coagulation. * Coagulating material = Alum, Sodium aluminate. ### C) Biological impurities * Ex: Microscopic algae, Fungi, Bacteria, viruses * Removal method = Sterilization. * **Methods of Sterilization** * **Physical Method** * Ex: Boiling * UV light * Ozone * **Chemical Method** * Ex: liquid Cl2 * Na-hypochloride ### D) Dissolved impurities * It is of 2 types: * **Dissolved Gases** * Ex: CO₂, Cl2, H2S, SOx, NOx * **Dissolved salts** * Bicarbonates of Ca, Mg * Sulphates of Ca, Mg * Chlorides of Fe, Al * Nitrates of Na, K * Removed: Boiling, Distillation. # Hardness of Water ## 1) Types of Water | Hard Water | Soft Water | | :--------------------------------------------- | :---------------------------------------------------------- | | Not produce lather with soap solution. | Easily forms lather with the soap. | | Contains Salts | Absence of salts | | Bicarbonates, chlorides, sulphates, nitrates of Ca, Mg, Fe, Al, Mn | - | | Ex: CaCO3, MgCO3, Ca(HCO3)2, Mg(HCO3)2, FeSO4, CaSO4, MgSO4 | - | ## 2) Temporary Hard Water 1. Hard water → Soft water 2. How: Boiling & Filtration 3. Why: As it contains bicarbonates of Ca, Mg, Fe, Al, Mn 4. Reac? ``` Ca(HCO3)2 △ CaCO3 + H2O + CO2↑ Mg(HCO3)2 △ Mg(OH)2 + 2CO2↑ ``` 5. Other Name: Carbonate hardness ## 2) Permanent Hard Water 1. Hard water ↛ Soft water 2. Not possible by boiling 3. Chlorides, sulphates, nitrates of Ca, Mg, Fe, Al, Mn. 4. - 5. Non-carbonate hardness # 3) Degree of Hardness * Hardness is due to various salts. * Ex: Ca(HCO3)2, Mg(HCO3)2, CaCl₂, MgCl2, CaSO4, MgSO4 * CaCO3 is selected for expression of degree of hardness because: * CaCO3 mol. wt = 100 ... Calculations becomes easy * CaCO3 is most insoluble salt in water. ## Formula ### CaCO3 Equivalent = (Wt. of salt * Mol.wt of CaCO3) / (mg/L * Mol.wt of salt) # Units of Hardness 1. mg/L: 1 mg/L = 1mg of CaCO3 equivalent in 1 liter of water. 2. ppm: 1ppm = 1 part of CaCO3 equivalent in million parts of water 10^6 3. Degree clarke (°ci) 4. Degree French (°fr) # Salaries Which Do Not Cause Hardness * NaCl, Fe2O3, Silica, Turbidity # 5) Numerical & Basics ## Formula Required 1. Total Hardness = Temporary Hardness + Permanent Hardness 2. Temporary Hardness = Carbonate (CO3^2-, HCO3-) 3. Permanent Hardness = Non-carbonate (Cl-, SO4^2-, NO3-) ## CaCO3 equivalent = (Wt. of salt * Mol. wt. of CaCO3) / (mg/L * Mol. Wt. of Salt) 4. Total Hardness = (X * M * 100 * 1000) / Y mg/L @ ppm 5. Permanent Hardness = (X * M * 100 * 1000) / Z mg/L Where: * Y = Volm of EDTA * Z = Volm of water * X = Volm of boiled water * M = Molarity of EDTA | Atoms | Atomic Weight | Salt | Molecular Weight | | :---- | :------------- | :--------------- | :---------------- | | H | 1 | Ca(HCO3)2 | 162 | | C | 12 | Mg (HCO3)2 | 146 | | N | 14 | CaCl2 | 111 | | O | 16 | MgCl2 | 95 | | S | 32 | CaSO4 | 136 | | Ca | 40 | MgSO4 | 120 | | Mg | 24 | Ca(NO3)2 | 164 | | Fe | 56 | Mg(NO3)2 | 148 | | | | CaCO3 | 100 | | | | MgCO3 | 84 | # 6) Determine Hardness of Water **Q.** Calculate temporary and total hardness of water sample containing: * Mg(HCO3)2 = 7.3 mg/L * Ca(HCO3)2 = 16.2 mg/L * MgCl2 = 9.5 mg/L * CaSO4 = 13.6 mg/L * NaCl = 5 mg/L * Silica = 15.0 mg/L | Salt Name | Salt/Impurity Mol. wt. | Quantity in ppm(mg/L) | CaCO3 Equivalent | Type of Hardness | | :-------- | :--------------------- | :------------------ | :------------------ | :----------------- | | Mg(HCO3)2 | 146 | 7.3 | 5 | Carbonate | | Ca(HCO3)2 | 162 | 16.2 | 10 | Carbonate | | MgCl2 | 95 | 9.5 | 10 | Non-carbonate | | CaSO4 | 136 | 13.6 | 10 | Non-carbonate | | NaCl | - | 5.0 | - | No hardness | | Silica | - | 15.0 | - | No hardness | * Temporary Hardness = Mg(HCO3)2 + Ca(HCO3)2 (Carbonate) = 15 mg/L * Permanent Hardness = MgCl2 + CaSO4 (Non-carbonate) = 20 mg/L * Total Hardness = Temporary + Permanent = 35 mg/L # Determination of Hardness of Water by EDTA Method. ## Theory * Hardness in water is mainly due to the presence of bicarbonates, sulphates, chlorides, nitrates of Ca, Mg, & other heavy metals. * EDTA: **Ethylene Diamine Tetra Acetic Acid** * Long form: ``` HOOC-CH2 | N-CH2-CH2-N | HOOC-CH2 ``` * Structure of EDTA: ``` [-CH2-CH2-]2 | [-N-]4 | CH2-COOH CH2-COOH ``` * **Type of Titration:** Complexometric Titration * **Complexing Agent:** EDTA (Eriochrome Black-T) * **Indicator:** EBT * **Buffer Solution:** (NH4OH + NH4Cl) pH = 10. * EDTA reacts with cations like Ca2+, Mg2+ & other metals to give complex structure (i.e. Cheate or Closed ring structure). * **End point of titration:** Wine Red to blue. ## Principle * Ca2+ & Mg2+ present in water forms complex with EBT. * This complex is wine-red in color. * When EDTA is added to it, EDTA take away Ca2+ & Mg2+ ions from wine-red complex & forms a colorless, stable complex. * When all Ca2+ & Mg2+ ions combine with EDTA, EBT becomes free giving its original blue color. * Reactions : 1. Ca2+ + EBT → Ca-EBT (Wine-Red) 2. Mg2+ + EBT → Mg-EBT (Unstable Complex) 3. Ca-EBT + EDTA → Ca-EDTA + EBT (Blue Color) 4. Mg-EBT + EDTA → Mg-EDTA + EBT (Blue Color) # Procedure: ## Step 1: Standardisation of EDTA * Fill 1st burette with (0.01 M ZnSO4) sol * Fill 2nd burette with 0.01 M EDTA sol. * Take 25 mL ZnSO4 in a conical flask (CF). * Add 5mL ammonical buffer sol' in CF. * Add 4-5 drops of EBT indicator in CF. * Color of the solution is Wine Red. * Now, start the addition of Na2EDTA from burette till color changes to Blue. * Note down the reading as V1 mL. ## Step 2: Determination of total hardness of water * Fill 1st burette with (hard water). * Fill 2nd burette with 0.01 M Na2EDTA. * Take 25 mL hard water in a conical flask (W). * Add 5mL ammonical buffer sol' in CF. * Add 4-5 drops of EBT indicator in CF. * Color of the solution is Wine Red. * Now, start the addition of Na2EDTA from burette till color changes to Blue. * Note down the reading as X mL. ## Step 3: Determination of Permanent Hardness * Fill 1st burette with (Boiled Water). * Fill 2nd burette with 0.01 M Na2EDTA. * Take 25mL boiled water in conical flask (W2). * Add 5mL buffer & EBT indicator. * Start addition of EDTA till color changes to Blue. * Note down the reading as Y mL. # Calculations 1. **To calculate Exact Molarity of EDTA Solution.** * ZnSO4 = EDTA ... [M = Molarity, V = volume] * M1V1 = M2V2 * M1V1 = M2V2 * V2 = M2 = Molarity of EDTA. 2. **To calculate Total Hardness of Water.** * Total Hardness = (X * M2 * 100 * 1000) / Y **in ppm.** 3. **To calculate Permanent Hardness of Water.** * Permanent Hardness = (Y * M2 * 100 * 1000) / Z **in ppm** 4. **Total Hardness = Temporary Hardness + Permanent Hardness** 5. **Temporary Hardness = Total Hardness - Permanent Hardness** # Numericals **Calculate temporary, permanent and total hardness of water in ppm from the following determination.** * 20 mL of 0.05 M solution of standard hard water (SHW) required 40 mL of EDTA for titration. * 20 mL of hard water sample consumed 30 mL of the same EDTA and 20 mL of hard water after boiling, filtering required 20 mL of EDTA for titration. 1. **To calculate Molarity of EDTA Solution.** * Water = EDTA * M1V1 = M2V2 * M1 = 0.05 = Molarity of water in M * V1 = 20 = Volume of water in mL * M2 = M2 = Molarity of EDTA = ? * V2 = 40 = Volume of EDTA in mL * M2 = (M1V1) / V2 * M2 = (0.05 * 20) /40 * M2 = 0.025 M 2. **Total Hardness** * Total Hardness = (X * M * 100 * 1000) / Y * Total Hardness = (30 * 0.025 * 100 * 1000) / 20 * Total Hardness = 3750 ppm 3. **Permanent Hardness** * Permanent Hardness = (Y * M * 100 * 1000) / Z * Permanent Hardness = (20 * 0.025 * 100 * 1000) / 20 * Permanent Hardness = 2500 ppm 4. **Temporary Hardness** * Temporary Hardness = 3750 - 2500 * Temporary Hardness = 1250 ppm # Determination of Alkalinity of Water ## Theory * Alkalinity due to the presence of HCO3^2-, CO3^2-, OH- of Na, K, Ca & Mg. * **Alkalinity** * Bicarbonate: HCO3- Ex: Ca(HCO3)2 * Carbonate: CO3^2- Ex: CaCO3 * Hydroxide: OH- Ex: NaOH, KOH, Ca(OH)2, Mg(OH)2 ## Principle * The type and amount of alkalinity present in a water sample may be determined by titrating water sample with a Standard acid using phenolphthalein & methyl orange indicator. * The determination is based on the following reactions. 1. OH- + H+ → H2O 2. CO3^2- + H+ → HCO3- 3. HCO3- + H+ → H2O + CO2 ## Alkalinity * Phenolphthalein alkalinity (P) * Methyl Orange alkalinity (M) * Phenolphthalein end point (V1) * Methyl Orange end point (V2) * Completion of i) & iⅱ) Reaction * Completion of i), ii), iii) Reaction * Complete neutralization of OH- * Complete neutralization of OH-, CO3^2-, HCO3- * Half neutralization of CO3^2- * Yellow to orange. * End point: Pink → colorless * P = OH- + 2CO3^2- * M = OH- + CO3^2- + HCO3- # Procedure 1. Fill 1st burette with alkaline water. 2. Fill 2nd burette with Acid (Ex: HCl). 3. Take 25mL water in a conical flask. 4. Add 1-2 drops of phenolphthalein indicator. * Color of the solution turns Pink. 5. Add acid in a conical flask dropwise, till color disappears. (Colorless). * [End Pt: Pink to colorless] 6. Note down this reading as V1 mL. 7. In the same conical flask (without throwing colorless sol') add 2-3 drops of 2nd indicator i.e. Methyl Orange indicator. * Color of the solution turns yellow. 8. Now, continue the addition of acid till color changes to orange. 9. Note down the burette reading as V2 mL. # Calculations ## A) Calculations of phenolphthalein alkalinity (P). * 1 mL 1'N acid = 50 mg CaCO3 equivalent. * V mL Z'N acid = ? (V1 x Z x 50) mg If 'V' is the volume of water taken for titration, then 'V' mL of water contains (V1 x Z x 50) mg CaCO3 equivalent. 1000 mL water contains = (V1 x Z x 50 x 1000) / V mg CaCO3 equin. * = (V1 x Z x 50 x 1000) / V mg/L * P = (V1 x Z x 50 x 1000) / V ppm ## B) Calculations of Methyl orange alkalinity (M) * M = (V2 x Z x 50 x 1000) / V ppm Where: * V1 = Phenolphthalein end point. * V2 = Methyl Orange end point. * V = Volume of water taken for titration * Z = Exact normality of acid # Relation between 'P' & 'M' Used to calculate the type & amount of OH^-, CO3^2- & HCO3- | Sr No. | Relation between P & M | Hydroxide Alkalinity (OH-) | Carbonate Alkalinity (CO3^2-) | Bicarbonate Alkalinity (HCO3-) | | :------ | :--------------------------- | :------------------------- | :------------------------------- | :------------------------------------ | | 1 | P = 0 | - | - | - | | 2 | P = M | P OR M | - | - | | 3 | P = 1/2 M | - | 2P or M | - | | 4 | P > 1/2 M | 2P - M | 2(M-P) | - | | 5 | P < 1/2 M | - | 2P | M - 2P | # Numericals **100 mL water sample requires 6 mL of N/50 H2SO4 for neutralization upto phenolphthalein end point. Another 16 mL of the same acid was needed for further titration to methyl orange end point. Determine the type and amount of alkalinities.** Given: * V = Volume of water taken for titration = 100mL * V1 = Volume of acid for phenolphthalein end pt. = 6mL * V2 = Volume of acid for methyl orange end pt = 16mL = 6+16 = 22mL * Z = Normality of acid = N = 0.02 N * P = Phenolphthalein alkalinity * P = (V1 x Z x 50 x 1000) / V * P = (6 * 0.02 * 50 * 1000) / 100 * P = 60 ppm * M = Methyl orange alkalinity * M = (V2 x Z x 50 x 1000) / V * M = (22 * 0.02 * 50 * 1000) / 100 * M = 220 ppm **Relation between P & M: Here P<1/2M** ∴ CO3^2- & HCO3^2- are present. * CO3^2- alkalinity = 2P = 2 x 60 = 120 ppm * HCO3- alkalinity = M - 2P = 220 - 120 = 100 ppm # Ill effects of Hard water in boiler. 1. **Priming** * Defn: Violent or vigorous boiling which leads to formation of wet steam is called \...liquid H2O + gas H2O i.e steam * Causes: 1. Presence of excessive salts. 2. High steam velocity. 3. Sudden Tin temp. 4. Improper boiler design. * Prevention: 1. Use soft water. 2. Avoid rapid change in steam rate. 3. Slowly increase the temperature 4. Maintain low water level. 5. Mechanical steam purifiers can be fitted. * Disadvantages: 1. Wet steam carry dissolved Salts which get deposited on turbine blades & on heaters, which reduces their efficiency. 2. Dissolved salts may enter the parts of other machinery. ∴ shorting the life of machinery. 2. **Foaming** * Production of persistent foam or bubble on the surface of water in boilers which do not break easily is called foaming. * Causes: 1. Presence of oil, soaps in H2O 2. Presence of alkaline salts. * Prevention: 1. Add antifoaming agents like Castor oil.. 2. Remove oil impurities by adding Na-aluminate (Na-AlO2). * Disadvantages: 1. Actual height of the water column cannot be judged properly. ∴ Boiler maintenance becomes difficult. # 3) Boiler Corrosion * **Defn:** The destruction of boiler metal by a chemical or electrochemical attack by its environment. ## Causes 1. **Dissolved Salts** * Magnesium salts (MgCl2) dissolved in H2O; liberates acid on hydrolysis. * Reach: MgCl2 + H2O → Mg(OH)2↓ + 2HCl(Acid) * This liberated acid reacts with Fe metal of boiler & produces acid again. * Reach: Fe +2HCl → FeCl2 + H2↑ * Reach: FeCl2 + 2H2O → Fe(OH)2↓ + 2HCl (Acid) * Hence, even a small amount of Mg-Salts can cause considerable corrosion of boiler metal. 2. **Dissolved Gases** * Dissolved O2 , Dissolved CO2 * Reach: 2Fe + 2H2O + O2 → 2Fe(OH)2↓ * Reach: 4Fe(OH)2 + O2 → 2(Fe2O3 H2O) * Reach: H2O + CO2 → H2CO3 (Carbonic Acid) ## Prevention 1. **Dissolved salts** * Remove Mg-salts by zeolite or ion-exchange process * Add small amount of Alkali externally to neutralize Acid. * HCl + Alkali → Neutralization 2. **Dissolved gases** * Removal of O2 * Add reducing agent * 2 Na2SO3 + 02 → 2 Na2SO4 (Sodium Sulphite) * Na2S + 02 → Na2SO4 (Sodium sulphide) * N2H4 + 02 → N2↑ + 2H2O (Hydrazine) * By Mechanical de-aeration * Removal of CO2 * Add liq. NH3 * 2 NH4OH + CO2 → (NH4)2CO3 + H2O (liquid ammonia) * Mechnical de-aeration # 4) Scale * **Defn:** When hard & adherent deposits are produced at the inner wall of the boiler is called scale. * **Solubility:** Scales are formed by the salts which are less soluble in hot water but may be soluble in cold water. * **Location:** Scales are formed at hot parts of the boiler. * **Salts responsible for formation (Causes):** * Bicarbonates decomposition * Mg-salt hydrolysis * Silica is present in water * CaSO4: less soluble in hot water * **Removal Method:** * EDTA Treatment * Scrapers * Wire brush * Thermal Shock Method * Empty hot boiler is treated with cold water. Due to sudden expansion & contraction, scales develop cracks & become loose. * **Prevention:** Use soft water * **Disadvantages:** * **Fuel costage:** * Scales have lower thermal conductivity. ∴ Rate of heat transfer from boiler to inside H2O is greatly reduced. * Overheating is done to provide steady supply of heat. ∴ increase in fuel consumption. * **Boiler safety ves:** * Overheating is done for supply of steam. * Boiler material becomes softer & weaker. ∴ Boiler becomes unsafe to bear high pressure. * **Boiler efficiency test:** * Scales may deposit in valves, condensers of the boiler & choke them partially. ∴ Decrease in boiler efficiency. * **Danger of explosion:** * Due to overheating boiler metal undergo expansion. * Scales undergo cracking. * Through these cracks boiler metal come in contact with boiled water. High P explosion. * **Fuel wastage:** Sludges are poor conductor of heat so they will waste a portion of heat generated. * **Entrapped in scales:** If sludges are formed along with scales, then they get entrapped in scales & get deposited as scales. * **Excessive sludge formation:** Decreases the efficiency of the boiler. * **Sludges settle down in the regions of poor water circulation such as pipe connection plug opening:** Causing choking of pipes. # 5) Sludge * **The soft & non – adherent precipitate** is formed at the bottom of boiler is called as sludge. * **Solubility:** Less soluble in cold water * **Location:** Cooler Part # 6) Caustic Embrittlement * **Defn:** Brittlement (boiler corrosion) of boiler occur due to use of highly alkaline water at high pressure. ## Causes * During softening process by lime-soda, free soda (Na2CO3) is used as a softening reagent. * Usually, a small portion of Na2CO3 is present in softened H2O * If such water is used in high pressure boilers, soda undergoes decomposition & gives NaOH & CO2. * Reach: Na2CO3 + H2O → 2NaOH + CO2↑ * This means that in the boiler, Soda (Na2CO3) turns into Caustic Soda (NaOH) and Carbon Dioxide (CO2) * Formation of NaOH makes boiler water highly alkaline. * NaOH containing water flows into the minute cracks. always present in the inner walls of the boiler by capillary action. * As boiler is steaming rapidly water from cracks evaporates & dissolved NaOH get deposited in the cracks. * The concentration of caustic Soda increases progressively and slowly attacks surrounding areas, thereby dissolving boiler metal. * NaOH reacts with boiler metal Fe; forming sodium ferroute. * Reach: 2 Fe + 2 NaOH + O2 → 2 NaFeO2 + H2 * The NaFeO2 & H2 penetrate along grain boundaries. * ∴ Brittlement of the boiler parts takes place. ## Prevention 1. Use soft water 2. Use Na-phosphate as softening agent instead of Na2CO3. 3. Boiler walls are treated with Lignin or Tannin which blocks the cracks of boiler. ∴ Preventing accumulation of NaOH.. # Treatment ## External * Treatment in which scale forming & corrosive impurities are removed from water before it enters the boiler. * **Zeolites:** Two types; Hydrates sodium alumino silicate * **Natural:** Obtained from green sand. Green wash Heat NaOH Sand. Non-porous is nature. More durable. * **Synthetic:** Prepared in laboratory. China + Feldspar + soda △ Cool clay + Na + Al → Granulation. Silicate aluminate supHate ## Internal * Treatment in which various substances are added to boiler to remove residual non-carbonate hardness to prevent scale formation. * Zein = Boiling, lithos = stone # 1) Zeolite / Permutit Method ## Principle * Zeolite crystals held hold sodium ions loosely and can easily exchange their sodium ions with other multivalent cations like Ca2+, Mg2+ etc. from water. ## Process * Zeolite crystals are packed in the column. * Hard water containing dissolved Ca & Mg salts are passed over a bed of No-Zeolite. [Na2Z] * The exchange of Ca & Mg with Nation takes place as follows: * Reach: Na2Z + CaCl2 → 2NaCl + CaZ. * Reach: Na2Z + MgSO4 → Na2SO4 + MgZ. * Reach: Na2Z + Ca(HCO3)2 → 2NaHCO3 + CaZ * Thus, when hard is passed through No-Zeolite bed the hardness causing cations (ex. Ca2+, Mg2+) are exchanged with Nations of zeolite. * ∴ Salts of Na are releaved in water and Ca2+ & Mg2+ ions remains on a zeolite bed. ## Regeneration Process. * After some time when all sodium ions of zeolite gets got exchanged with Ca2+, Mg2+ (multivalent cations) from water, zeolite is unable to exchange as it got exhausted. * To regenerate exhausted zeolite, it is treated with brine solution or NaCl. * Reach: CaZ + 2 NaCl → Na2Z + CaCl2 * Exhausted Zeolite + Brine → Regenerated Zeolite ## Advantages 1. Equipment used is compact & requires less space. 2. Hardness of water is completely removed. (Soft H2O < 10 ppm) 3. No precipitation impurities. 4. It is clean process. ∴ No danger of sludge formation 5. It requires less time for softening. 6. Low cost process. ## Disadvantages 1. Turbid water should not be directly used because the pores of zeolite bed will be clogged. 2. Neutral water (ie pH = 7) is used otherwise excess acidity or alkalinity may destroy zeolite crystal. 3. Water with Fe2+ & Mn2+ can not be used, as zeolite regeneration is not so easy with brine. 4. The treated water is always associated with more Na+. ∴ may cause caustic embattlement in high pressure boilers. # 2) Ion-Exchange Method ## Principle * When hard water is passed through ion-exchanger, cation exchangers removes all the cations while anion exchangers removes all the anions * ∴ water becomes free from all the anions as well as anions which is called as Deionised water or Demineralised water. ## Process * When hard water is first passed through cation exchanger, it removes all cations like Ca2+, Mg2+, Na+ and there is release of H+ ions. * **Reaction in cation exchanger** * Reach: RH2 + Ca(HCO3)2 → RCa + 2 H2CO3 * Reach: RH2 + Ca2+ → RCa + 2H+ * Reach: RH2 + Mg2+ → RMg + 2H+ * Reach: 2Na2+ → RNa + 2H+ ## Regeneration * The water collected from cation exchanger is free from all cations, but is acidic. * After this, the acidic hard water is passed through an anion exchanger; which removes all the anions like SO4^2-, Cl-, etc. & an equivalent amount of OH- are released. ## Reaction in Anion Exchanger * Reach: ROH + HCl → R-Cl + OH- * Reach: ROH + HCl → R-Cl + H2O * Reach: 2ROH + SO4^2- → R2SO4 + ōn * Reach: 2ROH + CO3^2- → R2CO3 + ōn * In this way, the water coming out from anion exchanger is free from cations & anions. * The H+ (coming from cation exchanger) and OH- (coming from anion exchanger) combine to produce water molecule, as * Reach: H+ + OH- → H2O * The water is finally passed through a degasifier to remove dissolved gases from water. * Low pressure and high temperature reduces the quantity of dissolved gases. * In the process, the sequence of water flow is important. * Always cation exchange resign need to be used first because if water is passed 1st through anion exchange resin, alkali is produced which can harm the cation exchange resin. ## Regeneration of exhausted resins. * The capacity of these ion - exchange resins to exchange ions from hard water is based on their ion-exchange capacities. * When their ion-exchange capacities are lost, they are said to be exhausted. * When the resins are exhausted, the supply of water is stopped. * The exhausted cation exchanger is regenerated by passing dilute HCl or H2SO4 solution. ## Advantages 1. The process is used for highly acidic or alkaline H2O. 2. The process produces water of low hardness (~ 2 ppm) 3. It is used for high pressure boilers. ## Disadvantages 1. The cost of equipment is high. 2. Turbid water cannot be used, as it ↓ output flow rate. 3. Large space required for equipment. # Purification of Water ## 1) Reverse Osmosis (RO) ### Desalination * Sea water or blackish water contains 3.5% dissolved salts. * Sea water can be converted to fresh water or potable water (drinking water) after removing dissolved salts. * This process is called as desalination * Commonly used technique for desalination: Distillation, Freezing, Electro-dialysis, Reverse-Osmosis * Expensive * Uses membranes. ### Osmosis * Low solvent → High conch * The movement of solvent from lower concentration to that of higher concentration through semi-permeable membrane is called as osmosis. ### Reverse Osmosis * High solvent ← Low conch * The movement of solvent from high concentration to that of low concentration through semi-permeable membrane under the influence of pressure is called as Reverse osmosis (R.O.). ## Principle of RO * When the direction of normal osmotic flow of water across the membrane is reversed by applying pressure slightly greater than osmotic pressure over the solution having high concentration is known as R.O. * When the solvent flows from the compartment containing higher concentrations of the solute that having lower concentration, which results in high concentration of solute in one compartment and pure water in other compartment. * R.O. is known to be the finest hyperfiltration technique which can remove particles as well as dissolved individual ions from solution. ## Process