Water Technology PDF

Water Technology Introduction:  Water is nature’s most wonderful, abundant, and useful compound.  Without food, human can survive for a few days, but water is such an essential that without it one cannot survive.  Occupies a unique po...

Water Technology Introduction:  Water is nature’s most wonderful, abundant, and useful compound.  Without food, human can survive for a few days, but water is such an essential that without it one cannot survive.  Occupies a unique position in industries, it’s most important use as an engineering material, is in steam generation.  used as coolant in power and chemical plants.  Water is widely distributed in nature. About 75% matter on the earth’s surface consists of water.  large amount of water under earth to an average depth of over three kilometers.  The air consists of 12 to 15% of volume of water vapor.  Water is found in living things. The body of human being consists of about 60% of  Plants, fruits and vegetables contain 90 to 95% of water. Sources of water: Natural sources of water: Rainwater, Oceans, Rivers, Lakes, Streams, Ponds and Springs Man-made sources : Dams, Wells, Tube wells, Hand-pumps, Canals, Reservoirs etc. For industrial purposes, natural waters are broadly divided into: 1. Surface waters: i) Flowing waters e.g., streams and rivers (Moorland surface drainage) ii) Still waters e.g., ponds, lakes and reservoirs (Low land surface drainage) 2. Underground supplies: i) Deep well waters &springs: ii) Water from shallow wells 3. Rainwater 4. Estuarine and Sea water The three major sources of water for industrial use are: a) Moorland surface drainage b) Low land surface drainage c) Deep well waters Impurities in various sources of water: River water:  contains dissolved minerals like chlorides, sulphates, bicarbonates of sodium, magnesium, calcium and iron.  It also contains suspended impurities of sand, rocks and organic matter.  The composition of river water is not constant. Lake water:  has high quantity of organic matter present in it but lesser amount of dissolved minerals.  Its chemical composition is also constant. Rainwater:  the purest form of natural water, obtained because of evaporation from the surface water.   Consists of dissolved organic and inorganic suspended particles and considerable amount of industrial gases like CO2, NO2, SO2 etc. (Collects during its downward journey through the atmosphere).  Rainwater is expensive to collect and is irregular in supply. Underground water:  free from organic impurities and is crystal clear (due to the filtering action of the soil). it contains large amount of dissolved salts. Effect of Water on Rocks and Minerals: 1. Dissolution: Mineral rocks such as NaCl and CaSO4.2H2O gets readily dissolved in water 2. Hydration: Disintegration of rocks due to hydration of some minerals Example: CaSO4 → CaSO4.2H2O (33% expantion) Anhydrate gypsum Mg2SiO4 → Mg2SiO4.XH2O Olivine Serpentine 3. Effect of dissolved oxygen : leads to oxidation & hydration Fe3O4 → Fe2O3 → 3Fe2O3.2H2O Magnatite haematite Limonite 4. Effect of dissolved CO2: Dissolved CO2 convert the insoluble carbonates of Ca, Mg and Fe into their relatively soluble bicarbonates. CaCO3+H2O+CO2 → Ca (HCO3)2 Sea water: is very impure due to two reasons:  Continuous evaporation increases the dissolved impurities content; increased by the impurities thrown by rivers as they join sea.  It is too saline for most industrial uses except cooling.  About 3.5% of dissolved salts, out of which about 2.6% is sodium chloride. Other salts present are sulphate of sodium, bicarbonates of potassium, magnesium and calcium etc. Impurities in water: An undesirable substance contained in water in any form is called impurities in water. Natural water is usually contaminated by different types of impurities, which impart some properties to the water. Types of Impurities present in water: Based on the characteristics of impurities: Three types. They are 1. Physical impurities 2. Chemical impurities 3. Biological impurities 1. Physical impurities: Which impart  Colour: in water is caused by metallic substances like salts of Fe, Mn.  Turbidity: is due to the colloidal, extremely fine suspensions such as insoluble substances like clay, slit, and micro-organisms. Taste: presence of dissolved minerals. Eg: Bitter – due to presence of Fe, Mn, Al Salts, excess lime; Soapy – sodium bicarbonates; Brackish – Unusual amounts of salts etc.  Odor: is undesirable for domestic as well as industrial purposes. Grassy – Iron sulphur bacteria, Earthy – Finely divided clay, sand etc. 2. Chemical impurities: Which impart acidity - due to presence of freeCO2, mineral acids induce corrosive reactions – presence of dissolved O2 Hardness and Alkalinity - presence of mineral content such as Inorganic chemicals: Cations (Al+3, Ca+2, Mg+2, Fe+2, Zn+2, Cu+2 Na+, K+) Anions (Cl−, SO4−2, NO3−, HCO3−, F−, NO2−) Organic chemicals: dyes, paints, petroleum products, pesticides, detergents, drugs, textile Materials, other organic related materials Sources of chemical impurities in water 1. Gases absorbed from the atmosphere by rain, 2. Dead animals and plants discovered near streams, rivers, and lakes, 3. Sewage and wastewater from industry, 4. River water contains high levels of calcium, iron, magnesium, sodium, and chlorides, 5. Organic compounds derived from agricultural and household industrial waste. 6. Inorganic compounds emitted by medical waste and equipment systems. Industrial point of view Hardness and alkalinity are important. 3. Biological Impurities: impart fouling as well as corrosion.  Biological impurities are Algae, pathogenic bacteria, fungi, viruses, pathogens, parasite- worms.  Source: discharge of domestic and sewage wastes, excreta (man, animals, birds). Based on the state of presence, impurities may be classified as: A) Suspended impurities - Coarse particle Size Colloidal impurities - tiny particles B) Dissolved impurities - all dissolved metals and its salts. C) Biological impurities - pathogens, algae, fungi, Viruses A) Suspended Impurities:  remain in suspension due to the same specific gravity as that of water.  make the water turbid.  Can be removed by filtration & sedimentation.  Example: sand, silt, clay, algae, fungi, etc. Colloidal Impurities  Small, nonvisible with the naked eye, and electrically charged particles, which remain in continuous motion.  Their size is 10-3mm to 10-6mm.  Ex: Organic matters containing bacteria, finely divided sand, clay etc.  Removed by neutralizing charge by the addition of chemicals.  Cause the color in water & these impurities cannot be removed by the ordinary filter. B) Dissolved Impurities:  Not visible.  Large in amount because water is a very good solvent & can dissolve all the salts with which it comes in contact.  May be organic compounds, inorganic salts, liquids or gases, minerals, etc.  Make bad taste, hardness & alkalinity.  Can be removed by precipitation, adsorption, distillation, chemical method, or aeration method.  The concentration of dissolved impurities is measured by weighing the residue after evaporation of the water sample from a filtered sample. It is measured in ppm or mg/L. Effects of Impurities on Natural waters: Properties Impurities Removed by Colour dissolved or colloidally dispersed organic Coagulation, settling, matter (metals & metallic salts of iron absorption, filtration ,manganese, peat ,algae) &sometimes chlorination. Yellowish tinge Indicate the presence of chromium and appreciable amount of organic matter. Yellowish red Presence of Iron Red-Brown Peaty matter Turbidity Finely divided insoluble impurities: Inorganic: Sedimentation followed by clay, slit. silica, etc or coagulation and filtration/ Organic: finely divided vegetable or animal coagulation &settling/ matter, oils, fats, microorganisms etc coagulation, settling and filtration Taste Dissolved minerals Bitter Fe, Al, Mn, SO4-2 or excess of lime Soapy taste Sodium bicarbonate Brackish Unusual amounts of salts Palatable Dissolved gases(CO2) and minerals(nitrates) Odour Living organisms, decaying Organic tastes & odours are vegetation(including algae, fungi, removed by activated carbon, weeds),sewage/industrial effluent aeration, aeration followed by Grassy Presence of Algae(liberation of traces of oil) activated carbon Offensive Peaty Iron & Sulphur bacteria Earthy Colloidal vegetable matter Finely divided clay &sand Hardness Dissolved mineral matter such as Ca+2,Mg+2, HCO3-,Cl-, SO4-2etc Alkalinity HCO3-,SiO3-2,HSiO3-,some times CO3-2 World Health Organization (WHO) Guidelines: 1. Guidelines for Drinking-Water Quality (GDWQ):  Covers various aspects of water quality, including microbial, chemical, and physical parameters.  Some key points include:  Acceptable levels of various contaminants such as bacteria, viruses, heavy metals, and chemical pollutants.  Recommendations for monitoring and testing water quality.  Guidelines for the management of water supply systems to ensure safe drinking water. 2. Guidelines for Drinking-Water Quality, Fourth Edition (2017):  Latest edition of WHO's GDWQ and contains the most up-to-date information and standards for drinking water quality. 3. Emergency Response to Water Quality Incidents:  WHO provides guidelines for responding to water quality incidents, including outbreaks of waterborne diseases. 4. Water Safety Planning:  WHO promotes the development and implementation of water safety plans to ensure the continuous provision of safe drinking water. Indian Council of Medical Research (ICMR) Guidelines:  ICMR is an Indian research body that provides guidelines and recommendations for various health-related issues, including drinking water quality.  Some key guidelines:  ICMR Guidelines for Drinking Water Quality (2017): These guidelines provide standards and recommendations specific to India, considering the local context and water quality challenges in the country.  Desirable Limits of Inorganic Chemical Substances in Drinking Water: ICMR sets permissible limits for various inorganic chemical substances commonly found in drinking water sources.  Guidelines for Testing and Evaluation of Water Purification Materials: ICMR provides guidelines for testing and evaluating water purification materials and devices to ensure their effectiveness in improving water quality.  Guidelines for Surveillance of Waterborne Diseases: ICMR offers guidelines for the surveillance of waterborne diseases, which are essential for monitoring the health impact of drinking water quality.  ICMR Manual for Quality Control of Water Quality Testing Kits: This manual outlines the quality control procedures for water quality testing kits used in the field. Note: Both WHO and ICMR guidelines are periodically updated to reflect the latest scientific research and knowledge about water quality. Local and national authorities may also have their own regulations and standards for drinking water quality, which should be followed in addition to these international and national guidelines to ensure safe and clean drinking water for the population. pH : According to the different standards proposed by WHO, ICMR, CPCB, BIS (listed in Table ),  the range of pH lies between 6.5 to 8.5.  If the pH is less than 6.5, it discontinues the making of vitamins and minerals in the human body.  More than 8.5 pH values cause the taste of water more salty.  causes eye irritation and skin disorder for pH of more than 11.  The rainwater which has no minerals useful for human body has a pH of 5.5–6 and not harmful on used as drinking purpose.  pH in the range 3.5–4.5 affects the aquatic life (Adarsh and Mahantesh, 2006, Leo and Dekkar, 2000). Turbdity:  The increase of turbidity of water results in interference of the penetration of light.  This will damage aquatic life and deteriorate the quality of surface water.  In the season of monsoon heavy soil erosion and suspended solids from sewage increased the turbidity which has an effect on the river and aquatic life (Verma et al., 1984).  High values of turbidity minimize the filter runs which cause pathogenic organisms to be more hazardous to the human life.  Due to this reason the WHO, ICMR and BIS (Table ) proposed a maximum range of 2.5, 5 and 5 NTU respectively depending upon the processes used for treatment of waste water (Sawyer et al., 1994, Burden et al., 2002, De, 2003). Total Dissolved Solids (TDS):  TDS is determined for measuring the amount of solid materials dissolved in the water (surface, ground).   High TDS values causes harmful effect to the public health such as the central nervous system, provoking paralysis of the tongue, lips, and, face, irritability, dizziness.  The presence of synthetic organic chemicals even in small concentrations imparts objectionable and offensive tastes, odors and colors to fish and aquatic plants (Chang, 2005).  The range of TDS falls between 500–1500 mg L−1 are prescribed by the US EPA (1997), and ICMR, WHO and BIS (Table 1) (Sawyer et al., 1994, Leo and Dekkar, 2000). Bio-chemical Oxygen Demand (BOD):  BOD is used for determination of requirement of oxygen for stabilizing household and industrial wastes (De, 2003).  The effluents disposed by domestic and industries into the surface and ground water contaminate the quality of the water which can be assessed by BOD determination (Sawyer et al., 1994).  According to WHO drinking water standard, BOD should not exceed 6 mg L−1 (De, 2003). Nitrate-Nitrogen (NO3-N):  Different agricultural activities yield in the increase of nitrate concentration in ground and surface water (Nas and Berktay, 2006).  Increase in the amounts of Nitrate-Nitrogen in surface water causes different problems such as level of oxygen in the water, decrease of O2 effects aquatic life, plants and algae (Davie, 2003).  Blue baby syndrome disease in human body occurred due to reaction of nitrite and iron in with red blood cell create methemoglobin which stops oxygen level.  The children under age of 1 year suffered most due to consumption water contaminated with nitrate.  The range of Nitrate-Nitrogen prescribed by ICMR, WHO, BIS are 20, 45, 45 mg L−1 respectively (Nyamangara et al., 2013). Hardness of water:  The water which does not produce lather immediately with soap is called hard water.  hardness in water is the characteristic, which “prevents the lathering of soap”.  the water which produces lather easily on shaking with soap solution, is called soft water.  hardness of water is caused by the presence of dissolved salts such as bicarbonates, sulphates, chlorides and nitrates of divalent metal ions like calcium and magnesium.  Soap is sodium or potassium salt of higher fatty acids like stearic, oleic and palmetic acids.  When soap is mixed with soft water lather is produced due to stearic acid and sodium stearate.  When soap comes in contact with hard water, sodium stearate react with dissolved calcium and magnesium salts and produce calcium stearate or magnesium stearate which is white precipitate. The different types of water are commercially classified on the basis of degree of hardness as follows; Hardness Name of water 0-70 mg/L Soft water 70-150 mg/L Moderate hard water 150-300 mg/L Hard water >300 Very hard water Types of Hardness: The hardness of water is two types; 1. Temporary hardness. 2. Permanent hardness. 1. Temporary hardness or Carbonate hardness or Alkaline hardness:  Due to presence of dissolved bicarbonate salts Ca(HCO3)2 and Mg(HCO3)2.  The hardness is called temporary because, it can be removed easily by boiling.  During boiling, bicarbonates are decomposed to yield insoluble carbonates or hydroxides. 2. Permanent hardness or non-carbonate hardness or non-alkaline hardness:  Due to the presence of dissolved chlorides, sulphates and nitrates of calcium and magnesium.  These salts are CaCl2, MgCl2, CaSO4, MgSO4, Ca(NO3)2, Mg(NO3)2.  It cannot be removed easily by boiling.  These salts don’t thermally decompose on heating, but the concentration of these salts increases, as boiling removes only water (in the form of vapor). Therefore, permanent hardness can’t be removed by boiling.  Removed by chemical treatment. (by ion exchange resin treatment or a water softener such as calcium hydroxide). Total Hardness = Temporary hardness + Permanent hardness Expression of hardness:  The hardness of water is expressed in terms of calcium carbonate equivalents.  The weights of different salts causing hardness are converted to weights equivalent to that of CaCO3.  CaCO3 was selected for expression of the degree of hardness because:  The Molecular Weight of CaCO3 is 100 (Equivalent weight is 5 which is easy for calculation.  It is an insoluble salt, and all the dissolved salts of calcium are precipitated as CaCO3. Hardness causing salt Mol.wt 120 parts by weight of MgSO4 would react with the same Ca(HCO3)2 162 amount of soap as 100 parts by weight of CaCO3. Mg(HCO3)2 146 i.e 100parts of CaCO3 == 120 parts of MgSO4 CaSO4 136 == 136 parts of CaSO4 MgSO4 120 == 162 parts of Ca (HCO3)2 MgCl2 95 == 146 parts of Mg (HCO3)2 CaCl2 111 And so on ….. Ca(NO3)2 164 Mg(NO3)2 148 Equivalents of CaCO3 = Amount of the hardness causing salt x 100 Molecular weight of hardness causing salt Units of hardness: 1. Parts per million. 2. Milligrams per litre. 3. Degree Clark. 4. Degree French. 5. meq per litre. WATER TECHNOLOGY 1. Parts per million (ppm): It is the parts of calcium carbonate equivalent hardness per 106 parts of water. i.e. 1ppm = 1 part of CaCO3 eq hardness in 106 parts of water. 2. Milligrams per litre (mg/L): It is the number of milligrams of calcium carbonate equivalent hardness present per litre of water. Thus, 1 mg/L = 1 mg of CaCO3 eq hardness per1 L of water But 1 L water = 1 kg = 1000g = 106 mg 1 mg/L = 1 mg of CaCO3 eq per 106 mg of water = 1 part of CaCO3 eq per 106 parts of water = 1 ppm 3. Degree Clark (0Cl): It is the number of grains of CaCO3 equivalent hardness per gallon of water. (or) It is the parts CaCO3 equivalent hardness per 70,000 parts of water. 10Cl = 1 grain of CaCO3 eq hardness per gallon of water 10Cl = 1 part of CaCO3 eq hardness per 70,000 parts of water 4. Degree French (0Fr): It is the parts of calcium carbonate equivalent hardness per 105 parts of water. i.e. 10Fr = 1 part of CaCO3 eq hardness in 105 parts of water. 5. Milliequivalent per litre: (meq/L): It is the number of milliequivalents of hardness present per litre. Thus 1meq/L = 1 meq of CaCO3 per L of water = 10-3 x 50g of CaCO3 eq per L = 50mg/L = 50ppm Relationship between various units of hardness: 1ppm = 1 mg/L = 0.1 0Fr = 0.07 0Cl = 0.02 meq/L Units of hardness No. of parts of CaCO3 Abbreviation &their inter equivalent hardness per relationship: Unit Parts per million 106 parts of water ppm Milligrams per litre One litre of water mg/L Degree Clarke 70,000 parts of water 0 Cl Degree French 105 parts of water 0 Fr Milli equivalents per litre Milli equivalents of meq/L hardness per litre. Disadvantages of hard water:  In domestic use: i) Washing: causes wastage of soap as it doesn’t give lather until the hardness causing ions gets precipitated. The quality of the cloth gets spoiled. ii) Bathing: cleansing quality of soap is lowered so wastage of soap. iii) Cooking: boiling point of water is elevated due to dissolved hardness producing salt. Hence more fuel & time is required for cooking. iv) Drinking: causes bad effects on the digestive system. The possibility of formation of calcium oxalate crystal in urinary tract is increased.  Industrial use: i) Textile industry: water containing Fe & Mn salts causes colored spots on fabrics. An exact shade of the color doesn’t get due to presence of Ca & Mg salts.  ii) Sugar industry: crystallization of sugar is effected by water containing sulphates, nitrates, alkali carbonates etc. iii) Dyeing industry: due to formation of undesirable precipitate impure shades and spots are formed on dyed fabric iv) Concrete making: water with chlorides and sulphates affects the hydration of cement and final strength of hardened concrete. v) pharmaceutical industry: certain undesirable products are formed at the time of preparation of drugs and ointments  Steam generation in boilers causes troubles such as scale & sludge formation, corrosion, Priming & foaming and caustic embrittlement. CONCENTRATIONS FOUND CONCENTRATIONS FOUND IN GROUND WATER IN SURFACE WATER Total Hardness 300 - 400 ppm 75 - 200 ppm Alkalinity 250 - 350 ppm 45 - 250 ppm Dissolved Oxygen near 0 2 - 14 ppm Carbon Dioxide 1 - 10 ppm low Calcium Hardness high sometimes high,usually low Magnesium Hardness tends to be high sometimes high, usually low [Minerals and impurities are normally present in very small concentrations and are measured in parts per million (ppm) (how many parts of impurities in a million parts of water) or milligrams per liter (mg/l). Some contaminants can also be measured in parts per billion (ppb) or micrograms per liter (μg/l)] Boiler troubles  A boiler feed water should consist of the following composition: Hardness should be below 0-2ppm. Caustic alkalinity (due to OH-) should lie in between 0.15-0.45ppm Soda alkalinity (due to Na2CO3) should be 0.45-1.0ppm Major boiler troubles caused by the use of unsuitable water are:  Carry over: Priming & Foaming  Sludge & Scale formation  corrosion  Caustic embrittlement Sludge: Soft, loose and slimy precipitate formed within the boiler. Formed at colder portions of the boiler and are collected at the bends. Formed by substances which have greater solubilities in hot water than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc Easily removed with wire brush Disadvantages of sludge formation Sludges are poor conductors of heat, so they tend to waste a portion of heat generated.  Excessive sludge formation disturbs the working of the boiler.  By using well softened water 2) by blow down operation i.e. drawing off a portion of the concentrated water. Scale: Hard deposits firmly sticking to the inner walls of the boiler. Difficult to remove, even with the help of hammer & Chisel. Formed due to: i) Decomposition of Calcium bicarbonate: Ca (HCO3)2 CaCO3 + H2O+CO2 Scale (Soft, formed mainly in low pressure boilers) In high pressure boilers, CaCO3 is soluble due to formation of Ca (OH)2 CaCO3 + H2O Ca (OH)2 + CO2 ii) Deposition of CaSO4:  Solubility of CaSO4 decreases with increase in temperature. Completely insoluble in super-heated water. Hard scale formation in high pressure boilers iii) Hydrolysis of Magnesium salts: Soft scale formation due to hydrolysis of Mg salts in high pressure boiler. MgCl2 + 2H2O Mg (OH)2 + 2HCl iv) Presence of silica: Deposits of Ca/Mg silicates, which adheres firmly to the inner boiler walls. It is difficult to remove. Disadvantages of scale formation: i) Wastage of fuel: Rate of heat transfer is greatly reduced due to poor conductivity of scales Overheating is required for steady supply of heat hence fuel consumption increases. Wastage of fuel depends on thickness and nature of scale: Thickness : 0.325 0.625 1.25 2.5 12 Wastage of fuel : 10% 15% 50% 80% 150% ii) Lowering of boiler safety: To supply steady heat overheating is required, which makes the boiler material weak &soft. Results in distortion of boiler tube & makes the boiler unsafe to bear the high pressure. iii) Decrease in efficiency of boiler: Deposition of scales in valves and condensers choke them partially& decreases the efficiency of boiler iv) Danger of Explosion: Due to uneven expansion the thick scales gets cracked, results in formation of large steam & develops high pressure. It may cause explosion of boiler. Removal of scales:  Mechanical/chemical method  Loosely adhering scales are removed with the help Scraper/wire brush  Brittle scales are removed by giving Thermal shocks  Loosely adhering scales are removed by frequent blow down operation (frequently removing precipitates)  Adherent & hard scales are removed by dissolving them by adding chemicals CaCO3 scales - 5-10% HCl CaSO4 scales - EDTA Prevention of scales formation: includes External treatment: It involves removal of hardness causing ions from water before entering the boiler i.e efficient ‘softening of water’. Internal treatment (Sequestration): It consists of adding chemicals directly to the water in the boilers for removing dangerous scale forming salts that can’t be completely removed in external treatment.  Either to precipitate the scale forming impurities in the form of sludge or to convert them into compounds, this will stay in the dissolved form. Generally followed by blow down operation Prevention of scales formation: Internal treatment (Sequestration): involves either precipitating the scale forming impurities in the form of sludge or converting them into compounds, by adding chemicals. It is followed by a blow down operation. Important internal conditioning/treatment methods: Colloidal conditioning: In low pressure boilers, scales can be avoided by adding organic substances like kerosene, tannin, agar-agar, which forms slime loose precipitates. easily removed by blow down operation. Phosphate conditioning: In high pressure boilers scale formation can be avoided by adding sodium phosphate React with hardness of water forms non adherent, easily removable soft sludge of Ca / Mg phosphates. 3CaCl2 +2Na3PO4 Ca3 (PO4)2 + 6Nacl Carbonate conditioning: In low pressure boilers scale formation can be avoided by adding sodium carbonate to boiler water, when CaSO4 is converted into calcium carbonate in CaSO4 +Na2CO3 CaCO3 +Na2SO4 Consequently, deposition of CaSO4 as scale doesn’t takes place and calcium is precipitated as loose sludge of CaCO3 which can be removed by blow down operation. Calgon conditioning: The process involves the addition of calgon to boiler water. It prevents scale and sludge formation by forming soluble complex compounds with CaSO4. Calgon = sodium hexa meta phosphate Na2 [Na4(PO3)6] 2Na+ + [Na4P6O18]2- 2CaSO4 + [Na4P6O18]2- [Ca2P6O18]2- + 2 Na2SO4 Boiler corrosion: The chemical or electro chemical eating away metal by its environment in boiler is known as boiler corrosion. The main reason for this is  Dissolved oxygen  Dissolved gases like carbon dioxide  Acids from dissolved salts i Dissolved oxygen: At high temperatures it attacks boiler material, leading to boiler corrosion. 2Fe+ 2H2O +O2 2Fe (OH) 2↓ 4Fe (OH)2 +O2 2[Fe2O3.2H2O] ↓  Prevented by mechanical deaerator, adding reagents like sodium sulphite, Hydrazine or sodium sulphide Na2SO3+O2 →2Na2SO4 N2H4+O2 → N2+ 2H2O ii Dissolved carbon dioxide: Dissolved CO2 in raw water and CO2 obtained by decomposition of bicarbonates in water are the two main sources. CO2 +H2O H2CO3 Removal of Co2: Co2 can be removed by adding required quality of ammonia. NH4OH +CO2 (NH4)2CO3 +H2O By mechanical de-aeration. iii Acids from dissolved salts: If water contains magnesium salts, they liberate acids on hydrolysis. This acid dissolves iron leading to an increase in further corrosion. MgCl2 + 2H2O Mg (OH) 2 + 2HCl Fe + 2HCl Fecl2+ H2 Fe + 2H2O Fe (OH)2+2Hcl The presence of dissolved salts like MgCl2 will increase rate of corrosion. Caustic embrittlement: The formation of brittle and crystalline cracks in the boiler shell is called caustic embrittlement. It is a type of boiler corrosion, caused by highly alkaline water in the boiler. In lime-soda process, it is likely that, some residual Na2CO3 is still present in the softened water This Na2CO3 decomposes to give NaoH and CO2 due to which the boiler water becomes “Caustic”. Na2CO3+ H2O → NaOH + CO2 This caustic water flows inside the boiler and causes some minute hair cracks, by capillary action. on evaporation of water, the dissolved caustic soda interacts with boiler (iron) material and attacks the surrounding area, there by dissolving iron of boiler as sodium ferrate which causes embrittlement of boiler parts such as bends joints, reverts etc, due to which the boiler gets fail. Joints &bends | High conc of NaoH| low conc. (dil)NaoH|Plane surface Anodic site + | |Cathodic site - Prevention methods: By using sodium phosphate as softening reagent in the external treatment of boiler water. By maintaining pH value of water and neutralization of alkali. By adding Tannin or lignin or sodium sulphate to block the hair cracks thereby preventing the infiltration of caustic soda solution Water softening method: Ion exchange or deionization or demineralization process: An important water softening method. Principle: A reversible exchange of ions takes place between the stationary ion exchange phase and the external liquid mobile phase. Ion exchange resins are insoluble, cross linked, high mol.wt., organic polymers and the functional groups attached to the chains are responsible for the ion exchange properties. These are of two types Cation exchange resins Anion exchange resins Cation exchange resins (RH+): These are capable of exchanging H+ ions with the cations. It is mainly styrene divinyl benzene copolymer which on sulphonation or carboxylation becomes capable of exchanging its hydrogen ions with cations in the water. They have acidic functional groups like -SO3H, -COOH or –OH Anion exchange resin (ROH-) These are capable of exchanging OH- ions with the anions. It is nothing but a copolymer of styrene divinyl benzene or amine formaldehyde copolymers which contain basic functional groups such as quaternaryamino (or) quartenary ammonium ion or quartenary phosphonium ion or Tertiary sulphonium ion groups. On treatment with dil NaOH they have capability to exchange its OH- ions with anions in the water Process: The hard water is passed first through cation exchange column, which removes all the cations like Ca+2, Mg+2 from it and equivalent amount of H+ ions are released from the column to water. 2RH+ + Ca2+ R2Ca2+ + 2H+ 2RH + Mg + 2+ R2Mg2+ + 2H+ The hard water is now passed through anion exchange column, which removes all the anions like SO4-2, Cl- from it and equivalent amount of OH- ions are released from the column to water. ROH- + Cl- RCl- + OH- 2ROH- + SO42- R2SO42- + OH- 2ROH + CO3 - 2- R2CO32- + OH- H+ and OH- ions are released from the cation exchange and anion exchange columns respectively get combined to produce water molecule. H+ + OH- H2O Water coming out from the exchanger is free from cations and anions and is known as deionized or demineralized water. Regeneration of exhausted beds: When the beds are getting exhausted (capacity to exchange H+ & OH- ions are lost) then they are regenerated The exhausted cationic exchanger is regenerated by washing with acids like HCl, H2SO4 etc R2Ca2+ + 2H+ 2RH+ + Ca2+ The exhausted anion exchanger is regenerated by washing with bases like NaOH R2SO42- +2OH - 2ROH- + SO42- The columns are washed with deionized water and washings are passed to sink or drain. Now they are ready for softening process. Advantages and disadvantages of Ion exchange process. Produce water of very low hardness (2ppm ) The process can be used to soften highly acidic or highly alkaline waters. Water produced by this process water can be used in high pressure boilers Disadvantages High capital cost and chemicals & equipment are costly If water contains turbidity efficiency of the process decreases  Desalination The process of removing common salt (sodium chloride) from the water is known as desalination. Commonly employed methods for desalination of brackish water are: i) Electro dialysis ii) Reverse osmosis i) Electrodialysis: It is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential. Sea water Sea water Sea water Cathode - +Anode Cl- Na+ pos Membrane Membrane Concentrated brine pure water Concentrated brine Line diagram of electro dialysis When direct electric current is passed through saline water, the Na+ ions moves towards cathode (-ve pole) and the chloride ions moves towards anode (+ve pole) through membrane. The concentration of brine decreases in the central compartment, where as the concentration in outer compartments increases. Desalinated brine (pure water) is removed from time to time, while concentrated one is replaced by fresh brine. Ion selective membranes are employed for more efficient separation, which has permeability for one kind of ions. Cation selective membrane (possess functional groups such as RSO3-) is permeable to cations only Anion selective membrane (possess functional groups such as R4N+Cl-) is permeable to anions only Electrodialysis cell: It consists of a large number of paired sets of rigid plastic membrane. Saline water is passed at a pressure of 5-6 kg m-2 between membrane pairs. Electric field is applied to the direction of water flow Fixed +ve charges inside the membrane repel +ve charge ions (Na+) and permits –vely charged ions. Fixed -ve charges inside the membrane repel -ve charge ions (Cl-) and permits +vely charged ions.  Alternative streams of pure water and brine water are obtained. Advantages of desalination:  It is an compact unit  It is economical.  Best suited if electricity is easily available. Reverse osmosis. (R.O) Reverse osmosis is one of the membrane filtration processes. The process is used to remove salts and organic micro pollutants from water. In this process When it is subjected to a hydrostatic pressure greater than the osmotic pressure on the concentrated side solvent passes through a semi permeable membrane in a reverse direction. This phenomenon is called reverse osmosis. In this process pure solvent (water) is separated from its contaminates, rather than removing contaminants from water. Sometimes it is also called super/hyper filtration. Method: Pressure (of the order 15-40 kg cm-2) is applied to the sea water/impure water. The membrane consists of very thin films of cellulose acetate/ superior membrane made of polymethacrylate and polyamide polymers, affixed to either side of the perforated tube. Advantages:  Removes ionic and non-ionic colloidal particles and high molecular weight organic matter.  It removes colloidal silica  The life time of semi permeable membrane is high.  Membrane can be replaced within few minutes.  Low capital and operating cost & high reliability  Municipal water treatment Potable water: means safe to drink. Essential requirements:  Clear  Colorless and Odorless  Pleasant in taste  It should not have turbidity (not exceed 10 ppm)  pH should be in the range of 7.0-8.5  Free from gases like H2S & minerals like Pb, As, Cr & Mn salts  Total hardness should be less than 500 ppm  Free from disease producing bacteria Sources for municipal water supply: Rivers, Wells, Lakes Various steps involved in the treatment of municipal water: Type of impurity Process to be employed Floating matter( leaves, Screening wood pieces) Suspended (Clay, Sand) Plain sedimentation Fine suspended Sedimentation with inorganic matter coagulation Micro organisms & Filtration colloidal impurities Pathogenic bacteria Disinfection Sedimentation with coagulation:  Plain sedimentation can’t remove finely divided silica, clay and organic matter.  Sedimentation with coagulation is a process of removing fine particles by addition of chemicals (coagulants) before sedimentation.  These impurities are in colloidal form, may carry -ve charge and don’t coalesce (come together) due to mutual repulsion.  Commonly used coagulants are Alum (K2SO4 Al2 (SO4)3.24H2O), Sodium aluminate (NaAlO2) etc.  Due to neutralization of charge on the particles/ mechanical entrainment by coagulants, the colloidal particles become closure, forms bigger particles and coalesce together. This process is called coagulation.   Coagulation process permits the particles to aggregate together until denser particles are formed, which falls through still water at a reasonable rate and is called flocculation.  Explanation with example: Aluminium (most common coagulant) gets hydrolysed to form aluminium hydroxide and sulphuric acid. Al2 (SO4)3 + H2O 2Al (OH)3 + 3 H2SO4 Al(OH)3 acts as flocculent (enormous surface area) and removes the impurities either by neutralizing the charge or by adsorption and mechanical entrainment. In alkaline water (lime/Na2CO3 is added if it is not sufficiently alkaline) the reaction is Al2 (SO4)3 + 3Ca (HCO3)2 → 2Al (OH) 3↓ + 3CaSO4 + 6CO2 Al2 (SO4)3 + 3 Na2CO3 + 3H2O → 2Al (OH) 3↓ + 3Na2SO4 + 3CO2 [Acidic waters are treated with sodium aluminate or in conjugation with Al2 (SO4)3. NaAlO2 + 2 H2O Al (OH) 3↓ + NaOH 6 NaAlO2 + Al2 (SO4)3 + 12 H2O 8Al (OH) 3↓ + 3Na2SO4]  Coagulant aids (lime, fuller’s earth, poly electrolytes) are added to increase the efficiency of the process.  Generally, coagulants are added in solution form with the help of mechanical flocculators, for through agitation.  Substantial reduction of bacteria also takes place during this process. (O2 i.e. released by some coagulants destroys some bacteria, breaks up some organic compounds, partial removal of color &taste producing organisms.) Removal of microorganisms: Removal of pathogenic (Disease causing microorganism) is known as disinfection. Chemicals or substances ( Bleaching powder, Chlorine) added to water for killing bacteria are known as disinfectants.  The disinfection of municipal water is a crucial step in the process of providing safe drinking water to the public.  Several methods are available for disinfection, and two commonly used techniques are Breakpoint Chlorination and UV (Ultraviolet) Ray Treatment. Disinfection by Chlorine – (Chlorination) Addition of liquid chlorine or gaseous chlorine to water produces hypochlorous acid, which kills microorganisms. Cl2+H2O HCl + HOCl Kills Germs Chlorine is good disinfectant at a pH of 6.5 (lower pH). HOCl is about 80 times more destructive to bacteria than OCl- (hypochlorite ion cannot inactivate the enzyme action). Break point chlorination or Dip point (Free residual chlorine): It involves addition of sufficient amount chlorine to oxidize: Organic matter, reducing substances, free ammonia, leaving behind free chlorine which possess disinfecting action against pathogenic bacteria. The addition of chlorine at the dip or break is called as Breakpoint chlorination. At the dip or break point chlorination, free residual chlorine is present which removes pathogenic bacteria. A graph between dosage of applied chlorine and residual chlorine is as shown in the Fig. From this fig it is shown that: i) At lower dosages of Cl2 (initially) all the chlorine added is consumed for complete oxidation of reducing substances like H2S. No residual chlorine. ii) With the increase of amount of chlorine dosage, a steady increase in amount of residual chlorine is observed. It is due to formation of chloro organic compounds without oxidizing them. iii) when the dosage of chlorine is further increased, oxidation of chloro organic compounds takes place and the free residual chlorine decreases and reaches a dip where oxidative destruction is complete. iv)Here after any addition of chlorine is not used in any reaction and the residual chlorine is increased. It acts as disinfectant and removes pathogenic bacteria. Sufficient chlorine i.e added at dip is known as break point chlorination or Free residual chlorination. It ensures complete destruction of organic compounds which gives bad taste and color. Advantages:  It oxidizes completely organic compounds, ammonia and other reducing compounds.  Removes color in water, due to presence of organic matter.  Destroy disease producing (100%) bacteria.  Removes bad taste and odor   Prevents growth of any weed. Disinfection of Municipal Water by UV Ray Treatment  Sterilization of municipal water is a critical step in the delivery of safe drinking water to the public.  One of the emerging techniques is UV (Ultraviolet) ray treatment.  This method uses UV rays to inactivate or kill microorganisms present in the water, making it safe for consumption.  The Process of UV Ray Treatment: 1. UV Lamp Source:  The key component of a UV water treatment system is the UV lamp source. These lamps emit UV-C (short-wave ultraviolet) rays with a wavelength of 254 nanometers, which is highly effective in disinfecting water. 2. Water Flow:  Municipal water is directed through a UV chamber, where UV lamps are installed. The water flows around the lamps in a controlled manner. 3. UV Exposure:  As water passes through the UV chamber, it is exposed to the UV-C rays. These rays penetrate the microorganisms present in the water and disrupt their DNA, preventing them from reproducing or rendering them harmless. 4. Microbial Inactivation:  UV treatment primarily targets bacteria, viruses, and other pathogens in the water. It effectively inactivates or kills these microorganisms, ensuring the water is safe for consumption. 5. Continuous Monitoring:  UV water treatment systems typically include sensors to monitor the intensity of UV radiation and the flow rate of water. This ensures that the required UV dose is maintained for effective disinfection. Significance of UV Ray Treatment in Municipal Water: 1. High Effectiveness:  UV treatment is highly effective in eliminating a wide range of microorganisms, including chlorine-resistant pathogens like Cryptosporidium and Giardia. 2. No Chemical Byproducts:  Unlike traditional water disinfection methods that use chemicals like chlorine, UV treatment leaves no chemical byproducts in the water. It is an environmentally friendly option. 3. Taste and Odor Preservation:  UV treatment does not alter the taste or odor of the water, which can occur with chemical disinfection methods. 4. Minimal Maintenance:   UV systems are relatively low maintenance. The lamps need periodic replacement, and the system requires routine cleaning and monitoring. 5. Fast Disinfection:  UV treatment works instantly, providing rapid disinfection without the need for contact time, as required by some chemical disinfection processes. Challenges and Considerations: 1. Pre-Treatment:  Turbidity, suspended solids, and organic matter in water can reduce the effectiveness of UV treatment. Therefore, pre-treatment processes may be necessary to ensure water clarity. 2. Electrical Power:  UV systems require a stable source of electrical power, which may be a challenge in some areas. 3. Monitoring:  Regular monitoring of UV system performance and maintenance is essential to ensure continuous disinfection. Conclusion:  Sterilization of municipal water by UV ray treatment is a modern, effective, and environmentally friendly method.  Offers a solution to many of the challenges associated with traditional chemical disinfection methods.  Plays a vital role in the provision of safe and clean drinking water to the public. Breakpoint Chlorination UV Ray Treatment Chemical disinfection method that uses A non-chemical disinfection method that uses Chlorine to treat water ultraviolet light to kill microorganisms in water. Chlorine is added to the water, typically in the Water is exposed to UV-C rays, typically at a form of chlorine gas, sodium hypochlorite, or wavelength of 254 nanometers, inside a UV calcium hypochlorite. chamber. Chlorine reacts with organic and inorganic No chemical byproducts are formed during matter and microorganisms present in the disinfection. water, forming various chlorine compounds. The chlorine dosage is carefully controlled to The water is effectively disinfected without the achieve the breakpoint, which is the point use of chemicals where the chlorine demand is satisfied, and free chlorine residual is maintained in the water. The free chlorine is responsible for UV-C rays penetrate the microorganisms, disinfection, as it reacts with and kills bacteria, disrupting their DNA and rendering them viruses, and other pathogens. harmless. Chlorine can react with organic matter to Preserves the taste and odor of the water. produce undesirable tastes and odors.