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4 Introduction Impurities in water i) Suspended Impurities Notes: Engg. Chemistry Unit- 1 Water Technology The impurity particles like soil, sand, organic waste in water having size of particles greater than 1000Ao and visible are the suspended impurities. Removal Method- They can be separated from water by simple filtration or the sedimentation or settlement methods. ii) Colloidal Impurities Finely divided organic or inorganic matter of the colloidal particle size 10 to 1000Ao^ makes water turbid and do not settle down. Removal Method - The colloidal particles in water are negatively charged. They are separated by coagulation followed by sedimentation or filtration. The coagulants are like FeSO 4 , alum, puls floors, sodium aluminate and aluminum sulphate. iii) Dissolved Impurities Various metal salts are water soluble. Gases like O 2 , SO 2 , NH 3 , H 2 S, CO 2 etc are soluble in water. Organic solids like sugars alcohols carboxylic acids, urea etc are soluble in water. Under ground and surface water due to contact with soil, rocks, contains cations like Na+, K+, Ca+2, Mg+2, Fe+2, Mn+2,^ Al+ etc. and anions like Cl-, NO 3 - , HCO 3 - , SO 2 -^ etc. Removal Method The inorganic dissolved impurities are removed by chemical treatment. Dissolved gases are removed by warming the water or mechanical deaeration method. The organic dissolved impurities are removed by oxidation or biochemical oxidation. iv) Biological Impurities These impurities in water include bacteria, algae, fungi, and other small size aquatic animals. Removal Method: They are removed by first filtration and killed by sterilization. The sterilization can be done by using chemicals like bleaching powder, sodium hypochlorite, chlorine, chloramines, ozone etc. and by physical methods like boiling of water, UV light. Hardness of water: It is lather forming capacity of water on treatment with soap. It is due to presence of dissolved salts of heavy metals like Ca+2, Mg+2, Fe+2, Zn+2, Al+3, Mn+2^ in the form of their Carbonates, bicarbonates, sulphates, nitrates and chlorides.
The water which does not produce foam/lather with soap readily, is known as hard water. Hard water does not produces foam with soap because it contains impurities of hardness causing salts which form sticky white precipitates. Rain water, distilled water, softened water form foam readily with soap and it is known as soft water. Causes of Hardness: (i) Dissolution of Minerals : When rain water flows or deposits on ground or percolates deep down up to nonporous hard rock, it comes in contact with various salts in the earth crust. Water soluble salts of heavy metals gives the hardness character to water (ii) Action^ of^ Dissolved^ CO 2 : CO 2 in air is acidic gas, comes in contact with rain drops and gets dissolved in rain water. When this water comes in contact with CaCO 3 , MgCO 3 on rocks, the insoluble carbonates are slowly attacked to convert them into soluble and hardness causing bicarbonates. e.g. (iii) Action^ of^ O 2 : Oxygen gas is slightly soluble in water and it can convert some insoluble minerals to soluble and hardness causing salts , On the basis of these salts, hardness is divided in to two main types (1). Temporary Hardness/Carbonate Hardness and (2). Permanent/Non carbonates Hardness
1. Temporary Hardness/Carbonate Hardness: The hardness character in water due to presence of bicarbonate and some soluble carbonate salts of heavy metals, is the temporary hardness. Such salts are Ca (HCO 3 ) 2 , Mg (HCO 3 ) 2 , MgCO 3 , FeCO 3.
- These water soluble salts can be converted to their water insoluble forms by boiling the water and the temporary hardness gets removed.
- This^ hardness^ can^ be^ removed^ by^ boiling^ the^ water^ i.e.^ CO 2 is^ removed.^ It^ is^ also^ called^ as^ alkaline hardness.
Theory: Disodium EDTA reacts quickly with the hardness causing metal ions in water, even in very low conc. of salts. During the reaction H+^ ions are formed & decrease the pH of reaction mixture. Therefore buffer solution of pH about 10 is necessary during titration. So that it will not allow the decrease of pH, otherwise the reaction is reversible. The reaction between disodium EDTA & heavy metal ion, forms cyclic co-ordination complex & hence the titration is known as complexometric titration. Organic dyes such as Erichrome black-T (EBT) (pH range 10), Calcon (pH range 12.5), etc act as indicators for the EDTA titrations. Di Sodium EDTA Procedure : Part I: Standardization of EDTA solution: Fill a burette with disodium EDTA solution & pipette out 25 ml std. MgSO 4. Solution in a conical flask. Add about 15 ml buffer solution of pH about 10 & 4 - 5 drops of EBT indicator in it. Titrate the pink red (Wine red) coloured mixture against the EDTA solution till it changes to sky blue. Let the titration reading be V 1 ml. Part II: Hardness of water sample Take 50 ml water sample in a conical flask. Take about 15 ml of buffer solution of pH 10 & 4-5 drops of EBT indicator solution in it. Titrate this pink-red mixture against the EDTA solution till the color change to sky blue. Let the titration reading be ‘y’ ml. Calculations: Part I: Standardization of EDTA M 1 V 1 =M 2 V 2 EDTA=MgSO 4
Part II: Total hardness of water sample: 𝑦 𝑥 𝑍 𝑥 100 𝑥 1000
Hardness =
ppm CaCO 3 equivalent. Where y = Volume of EDTA Z = Molarity of EDTA V= Volume of Water sample Temporary & permanent hardness by EDTA :
- First total hardness of a water sample is found out, by using above formula.
- Then hardness of the boiled & filtered water sample is found, which contains only permanent hardness.
- Temporary hardness = Total hardness - Permanent hardness. Advantages:
- Suitable indicators available for accurate reading.
- Color change at the end point of titration is sharp.
- The EDTA titration method is convenient, fast, and easy. Q. 100 ml of water requires 18.5 ml M/50 disodium EDTA for end point in titration .100 ml of the same water sample after boiling and filtration takes 10.7 ml of the disodium EDTA for end point in titration .Calculate Temporary & permanent hardness of water sample in pap CaCO3 equivalent. Solution:- Volume of water sample titrated (V) = 100 ml Molarity of EDTA (Z) = M/50 =0.02M Volume of EDTA (y 1 ) = 18.5 ml for Total hardness Volume of EDTA (y 2 ) = 10.7 ml for permanent hardness
Total Hardness =
ppm CaCO 3 equivalent. = (18.5 x 0.02 x 100 x1000) / 100 = 370 ppm CaCO 3 equivalent
Permanent hardness Hardness =
ppm CaCO 3 equivalent. = (10.7 x 0.02 x 100 x1000) / 100 = 214 ppm CaCO 3 equivalent. Temporary hardness = Total hardness - permanent hardness = 370 - 214 = 156 ppm CaCO 3 equivalent.
Phenolphthalein alkalinity = 𝑃 = 𝑉 1 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 Total or Methyl orange alkalinity = 𝑀 = 𝑉 2 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 Types of alkalinities: The possible combinations of alkalinities in water are : i) Only OH–^ ii) Only CO 3 -^2 ii) Only HCO 3 -^ iv) OH–^ & CO 3 -^2 together v) CO 3 -^2 & HCO 3 -^ together. OH–^ & HCO 3 -^ cannot exist together as they form CO 3 -^2 & water. The types & amounts of alkalinities are calculated from the relation between value of P & M Alkalinity Quantity of OH-^
**- 2 Quantity of CO 3
Quantity of HCO 3** P =0 0 0 M P =1/2M 0 2P 0 P = M P 0 0 P < 1/2M 0 2P M-2P P > 1/2M (2P-M) 2(M-P) 0 Numericals: Q. 50 ml of a water sample requires 9.2 ml of N/50 HCl up to phenolphthalein end point and total 13. ml of the acid for complete neutralization. Find the types and amount of alkalinity in the water sample. Soln: V 1 = 9.2 ml, V 2 = 13.1 ml, Z= N/50= 0.02 N, V= volume of water sample= 50 ml Phenolphthalein alkalinity = 𝑃 = 𝑉 1 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 = 9.2 X 0.02 X 50 X 1000/ = 184 ppm CaCO3 equivalent Methyl orange alkalinity = 𝑀 = 𝑉 2 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 = ( 13.1X 0.02 X 50 X 1000)/ = 262 ppm CaCO 3 equivalent As P >1/2 M, Type of alkalinity present are OH–^ & CO 3 -^2 ∴ Quantity of [OH]-^ = (2P-M) = 106 ppm CaCO 3 equivalent. And quantity of [CO 3 ]-^2 = 2(M-P) = 156 ppm CaCO 3 equivalent. Q. 50 ml of water requires 3.7 ml of 0.025 N H 2 SO 4 upto phenolphthalein end point and further 4.8 ml upto Methyl orange end point. Calculate the types and amounts of alkalinities in the water sample. (Ans: P = 92.5, M = 212.5; Carbonate = 185 ppm; Bicarbonate = 27.5 ppm)
Ill effects of hard water on boiler: Large scale use of boiler water in industries is mainly for: i) Steam generation ii) coolant Depending upon the operating pressure of boiler, the feed water should satisfy the following hardness: Types of boiler, Steam pressure Permitted hardness of feed water Low pressure : Below 15 Kg/cm^2 25 - 50 ppm^ CaCO 3 equivalent Medium pressure: 15 - 30 Kg/cm^2 10 - 25 ppm^ CaCO 3 equivalent High pressure : Greater than 30 Kg/cm^2 0 - 10 ppm CaCO 3 equivalent Modern boilers require high quality water for safe and efficient production of steam. But due to use of hard water some problems are created known as ill effect. Which are discussed below. There are main four adverse effects occurring on boiler; A) Boiler Corrosion B) Priming & foaming C) Sludge & scale formation D) Caustic embrittlement
Foaming : “It is the formation of continuous foam or bubbles on the surface of water.” Causes: 1. High conc. of dissolved salts in boiler feed water.
- Presence of oil droplets and alkalies.
- Presence of Finely dispersed and suspended impurities.
- Violent agitation of boiler feed water Disadvantages of priming and foaming:
- Because of foaming actual height of water level cannot be judged well.
- Because of priming, the salts present in the droplets enter in the part of machineries where steam is being used, thereby decreasing life of machineries.
- The dissolved salts in droplets of wet steam on evaporation get deposited, which reduces life and efficiency of machine parts.
- Foaming causes wet steam formation. Prevention of Priming :
- Priming can be prevented by use of well softened and filtered water
- Maintaining low water level in boiler prevent priming
- Rapid changes in steam rate should be avoided.
- Steam purifier can be used. Prevention of foaming :
- Foaming can be prevented by adding antifoaming agents like castor oil.
- Addition of chemicals like sodium aluminate to remove oil from boiler feed water.
- Blow down operations can be carried out time to time. C) Sludge & scale formation Definition: Sludge: The loose & slimy deposit of precipitated salts in boiler tube, depositing at the bends and valves, affecting free flow of water, is known as sludge. Sludge formation / causes : In boiler water evaporate continuously & the concentration of salt left behind, goes on increasing after the saturation point they get precipitated. The ppt. remains in boiler tubes as loose and slimy matter. These are generally formed at cooler portions of boiler and the parts of boiler where flow rate is slow. Disadvantages:-
- They waste some portion of heat.
- It disturbs working of boiler and sometimes may choke up the pipe
- It reduces the flow rate of water in boiler. Prevention:-
- Use of water containing very low quantity of total dissolved solids.
- Frequently making blow down operation.
Scale:- Definition: The hard and strong coating formed inside boiler tube by chemical reactions, which is bad conductor of heat, is called as scales. **Causes-
- Decomposition of bicarbonates** : At high temp, bicarbonates decompose into sticky water insoluble material. Ca(HCO3)2 → CaCO 3 ↓ + H 2 O + CO 2 ↑ Mg( HCo 3 ) 2 → Mg (OH ) 2 + 2CO 2 ↑ 2) Hydrolysis of magnesium salts : at higher temperature, magnesium salt undergoes hydrolysis, to form sticky. MgCl 2 + 2 H 2 O → Mg (OH) 2 ↓ + 2 HCl ↑ 3) Presence of silica: Silica in the form of colloidal particles can deposit as calcium silicate or magnesium silicate as strongly adhered material. 4) Decreased solubility^ of^ CaSO 4 : CaSO 4 has lesser solubility at higher temperature. Hence at high temp. CaSO 4 present in boiler will precipitate as hard scale forming material. Disadvantage of Scales:
- Wastage of Fuel : Scales are bad conductors of heat & result in the reduction of heat transfer to the boil.
- Over heating of boiler : Scale reduces transfer of heat from boiler to boiler water, hence overheating is required to keep the required steam pressure. 3) Boiler Safety: Overheating of boiler is done due to scale formation. To maintain constant steam supply with required pressure boilers are overheated. Overheating makes boiler metal soft & weak which causes distortion of boiler tube & becomes dangerous in high pressure boiler. 4) Danger of explosion: When thick scale cracks due to uneven expansion, the water comes in contact with overheated boiler metal which causes large amount of steam formation & develops sudden high pressure. Due to this boiler metal may burst with explosion. Removal of scales:
- The scale can be dissolved by addition of suitable chemicals like EDTA, sodium phosphate, calgon, etc & removed by blow down operation.
- Thin scales can be removed by use of scrapper or wire brush.
- Thick scales can be removed by hammer & chisel.
- To remove hard & brittle scale thermal shocks techniques is used i.e. heating empty boiler & cooling suddenly with cold water, which causes the contraction of boiler & scales get cracked. Prevention of scales :
- Use of softened water.
- Adding sodium phosphate to the water. (Phosphate conditioning).
- Frequent blow down operations to remove the sludge & precipitate rich water from boiler.
- Adding sodium aluminates, which can trap the scale forming particles.
- Adding organic chemicals like tannin which forms coating on the scale forming particles. This matter becomes easily removable by blow down operation.
Methods for Water Treatment: A) Internal treatment of boiler water: Principle: - “Chemicals added for internal treatment either does not allow the scale formation to take place or they attack on the previously formed scales to convert them into sludge. Or dissolve the scales.” Two methods of internal treatment are a) Calgon conditioning b) Phosphate conditioning c) Colloidal Conditioning a. Calgon conditioning: Scale forming salts like CaSO 4 , Mg(HCO 3 ) 2 etc in the boiler water can be converted into highly soluble complexes by addition of small amount of calgon in boiler water. Calgon is sodium hexa metaphosphate (NaPO 3 ) 6 or Na 2 [Na 4 P 6 O 18 ). This substance forms highly soluble coordination complexes with Ca++^ Mg++, Fe++^ and thus scale formation is prevented. The optimum pH required for complex formation is 9.0 to 10. Na 2 [Na 4 P 6 O 18 ) 2Na+^ + [Na 4 P 6 O 18 ]^2 - Ca+2^ + [Na 4 P 6 O 18 ]-^2 9 - 10.5 2Na+^ + [CaNa 2 P 6 O 18 ]-^2 (Soluble complex) b. Phosphate Conditioning: Principle: Hard adherent scales are converted into their phosphate by treatment with sodium phosphate and are removed as sludge by blow down operation. Chemical in scale Sod. Phosphate Sludge Orthophosphates: NaH 2 PO 4 = Sodium dihydrogen phosphate Na 2 HPO 4 = Di Sodium hydrogen phosphate Na 3 PO 4 = Tri Sodium phosphate Na 4 P 2 O 7 = Sodium pyrophosphate NaPO 3 = Sodium metaphosphate Sodium pyrophosphate, Sodium metaphosphate in water gets hydrolysed into orthophosphates. Na 4 P 2 O 7 + H 2 O 2 Na 2 HPO 4 NaPO 3 + H 2 O NaH 2 PO 4 Reactions: Sodium orthophosphate can attack scales to convert them to sludge. When scale is formed in boiler proper quantity of sodium phosphate is added in feed water. 3CaCO 3 + 2NaH 2 PO 4 Ca 3 (PO 4 ) 2 + Na 2 CO 3 + 2H 2 O + 2CO 2 3Mg(OH) 2 + 2 Na 3 PO 4 Mg 3 (PO 4 ) 2 + 6 NaOH 3CaSO 4 + 2 Na 3 PO 4 Ca 3 (PO4) 2 + 3 Na 2 SO 4 Depending upon the quality of feed water choice of particular orthophosphate is made. Use of such treatment combined with blow down operation frequently, helps even high pressure steam boilers work without scale formation. Phosphate conditioning treatment converts i) Scale inside the boiler to sludge ii) Hard water to soft water c. Colloidal conditioning: In low pressure boilers, scale formation can be avoided by adding organic substances like tannin, agar-agar gel etc. These substances get coated on scale forming ppt particles, which do not stick and remain as loose deposits in boiler i.e. sludge. Sludge can be removed by blow down operation.
B) External treatment of water: i) Zeolite process Principle : Sodium Zeolite has the property of capturing the heavy metal ions from water & in exchange released the Na+^ due to removal of heavy water ions ( Ca ++, Mg++, etc ) from water, the hard water is converted into soft water,by zeolite. Zeolites are hydrated complexes of metal silicates and metal aluminates, also called as ‘Permutits’, they have crystalline structure like honey comb in which Na+^ ions are loosely held. Heavy metal cations exchanged by Na+^ and thus Na+^ are released in water. Zeolite structure representation: Na 2 O. Al 2 O 3. xSiO 2 .yH 2 O Process: The zeolite softener consists of cylindrical pot in which powder of sodium zeolite is placed over a performed plate. There is an inlet on the top for or hard water & an outlet for collecting soft water from bottom. There is another inlet for adding 10%NaCl solution to regenerate the exhausted Zeolite bed. Fig: Zeolite exchanger When hard water percolates downwards through the zeolite bed, the hardness causing Ca++, Mg++, etc. Cation is retained in the bed & Na+^ ions are released in exchange. The soft water obtained and it contains equivalent quantity of sodium salts. There is reaction during treatment : Na 2 Ze + Mg++^ → MgZe + 2 Na+ Na 2 Ze + Ca ++^ → CaZe + 2 Na+ Regeneration: When a large quantity of hard water is passed through the zeolite bed , the sodium zeolite bed completely gets changed to calcium & magnesium zeolite & there is no exchangeable Na+^ ions in it. Such zeolite is said to be exhausted & need’s regeneration. The regeneration can be done by passing suitable volume of 10% NaCl (Brine) solution. MgZe + 2 NaCl → Na 2 Ze + MgCl 2 CaZe + 2 NaCl → Na 2 Ze + CaCl 2 Advantages:
- Clean and quick process to operate.
- Low cost operating process.
- Process adjusts with water of any hardness & there is no need to analyze water before softening.
- The zeolite bed is whether active or exhausted, can be easily tested with soap solution.
- The softener occupies small space.
3 ii) Demineralization process / Ion Exchange Process: Principle : When water containing cations & anions, is passed through the resins, cation exchange resin captures all cation & anion exchanger resin captures all anions, to give pure & all ions free water. There are two synthetic resins used (in separate vessels) i.e. a cation exchanger resin and an anion exchanger resin, a) Cation exchanger resin : i) This is a polymer having carboxylated / sulphonated aromatic rings attached to the chain; ii) The H+^ ions are loosely held on – SO −^ or – COO−^ groups and are easily exchanged with cations from water. b) Anion exchanger resin : i) This is a polymer having aromatic rings linked to the polymer chain and the rings are with quaternary ammonium group; ii) The −^ OH ions are loosely held and are easily exchangeable with all the negative ions from water. Process: The flow sheet of the treatment is shown in fig.
i) The water to be treated first enters in porous cation exchanger resin, where all cations are captured by the resin & H+^ is released in exchange. e. g. H 2 R+ Na+^ → Na 2 R + 2H+ H 2 R + Ca ++^ → Ca R + 2 H+ ii) Then the acidic output water from first resin enters in the porous anion exchanger resin where all anions in water are captured & - OH is released in exchange. e.g.. R’(OH) 2 + 2 Cl-^ → R’Cl 2 + 2 - OH R’(OH) 2 + CO 3 -^2 → R’CO 3 + 2 - OH Regeneration: i) The exhausted cation exchanger is regenerated by washing with dil. HCl solution. Na 2 R + 2 HCl → H 2 R + 2 Na+ Ca R + 2 HCl → H 2 R + CaCl 2 ii) Cation exhausted anion exchanger resin is generated by washing with NaOH solution. R’ Cl 2 + 2 NaOH → R’(OH) + 2NaCl R’ SO 4 + 2 NaOH → R’(OH) 2 + 2Na 2 SO 4 Thus the water flowing out from second exchanger resin contains equivalent amount of H+^ & OH-^ ions which combines to form H 2 O & there is no ionic impurity in the final water. This method is also called as deionization or demineralization process. Advantages:
- The method gives water of zero hardness & no ionic impurities.
- If^ the^ output^ water^ is^ passed^ throw de-gassifier,^ then the gaseous^ impurities like^ O 2 ,^ CO 2 also get expelled, to get water of ‘distilled water’ standard.
- Equipment occupies small space.
- Process is easy to operate, with negligible running cost. Limitations : i) Initial investment high. ii) Process can be operated only for small scale purification of water. Desalination of brackish water: The water contains high concentration of dissolved salts is called brackish water. Desalination is the process of removing common salts (mostly NaCl) from water Commonly used Membrane techniques for Desalination are i) Electrodialysis ii) Reverse Osmosis Electrodialysis: - The process of removing ionic pollutants from water using membranes and electric field is known as Electro dialysis. Construction and working:- An Electro dialysis cell consists of a large number of paired sets of plastic membranes. The membranes are ion selective. E.g. Cation selective membrane will allow only cation to pass through it. (as this membrane consists of negatively charged fixed groups which repel anions and do not allow going it.)
Principle: “The flow of solvent from higher concentration to lower concentration solution through semi permeable membrane by applying external pressure slightly higher than osmotic pressure on higher conc. Solution, is known as Reverse osmosis” This is the reverse process than normal osmosis. In normal osmosis, solvent flows from lower conc. to higher conc. through semi permeable membrane. In the reverse osmosis, we apply external pressure on the higher conc. solution, thus solvent (water) flows from higher conc. solution to lower conc. solution and impurities remained behind. Thus water gets separated from the impurities. Diagram of RO: Process: Semi permeable membrane made up from polymeric materials like acrylics, polyamides, aramids etc. Pore size is around 600 nm. So that only water molecules will pass through membrane. Hard water or sea water is filled in reverse osmosis cell. When pressure 200 psi is applied on it, the water passed through Semi permeable membrane and impurities remains on Semi permeable membrane. Pure water collected from lower outlet. Advantages:
- Ionic, colloidal, non-ionic impurities remove from water.
- Pure water for high pressure boiler can be obtained.
- It can be used to obtain mineral water for drinking by using specific membranes.
- Simple to operate.
- Low cost process.
