Download Water Technology: Treatment, Purification, and Disinfection Processes and more Study notes Analytical Chemistry in PDF only on Docsity!
Module-1 (Water Analysis)
Lecture-
1. INTRODUCTION 1.1.Water technology The process of removing all types of impurities from water and making it fit for domestic or industrial purpose is called water technology. Water is the most useful, wonderful and abundant compound on earth. It’s a vital component of life forms. One can live without food for many numbers of days but one cannot live without water. Water is the important constituent of all the body fluids, without which all the cells or organisms are in crystalline or dead state. (For example, the human body contains 70%, land plants 50–75% and aquatic plants 95–99%). Water is not only essential for the lives of animals and plants, but also occupies a unique position in industry.
1.2. Learning Objectives To describe the chemical compositions of natural waters, and explain how and why these compositions vary. To describe the main sources of water pollution, the main types of pollutant and how each type may be controlled. To identify the criteria for drinking water acceptability and outline the processes used to treat water for a public water supply To outline how sewage may be treated before discharge to the environment. 1.3. Sources of Water The chief sources of water supply for industrial use are: (a) Rainwater: The purest form of water, collected on the roofs. Yet this method is seldom adopted in industry. (b) Surface waters: Flowing water such as rivers, streams etc. Still water such as lakes, ponds etc. (c) Ground Water: Water from springs. Water form shallow wells. (d) Seawater: Its use is very limited as its uses entail very great problems of chemical engineering.
1.4. Uses of Water Water is used in power generation, as steam in steam turbines and as coolants. Blasting and water jet cutters. Very high pressure water guns are used for precise cutting. It is also used in the cooling of machinery to prevent over-heating, or prevent saw-blades from over- heating. Industry requires pure water for many applications and utilizes a variety of purification techniques both in water supply and discharge. Water plays many critical roles within the field of food science. Water is widely used in the production of steel, rayon, paper, atomic energy, textiles, chemicals, ice and for air- conditioning, drinking, bathing, sanitary, washing, irrigation and fire-fighting etc.
1.5. Impurities of Water Source and nature of impurities of water Water in the form of vapour in clouds is said to be pure. Yet when it condenses as rain and flows on the ground it takes many impurities from atmosphere and ground. The major situations in which water gets impure are as followed: Dissolved gases: during raining water absorbs much of the gases like oxygen, carbon dioxide, hydrogen sulphide etc. from atmosphere. The resultant water will be slightly acid. Soluble crystalloids: water when flow over the surface of the land (like rivers, streams), dissolved soluble minerals. The most of the soluble minerals include chlorides, sulphates, bicarbonates of sodium, calcium, magnesium and iron. Where as in sea water 3.5% of dissolved salts are seen in which 2.5% include sodium chloride. Organic matter: This is derived from the decomposition of plants and small particles of sand and rock in suspension. 1.6. Characteristics imparted by impurities of water: As such pure water is odourless, colourless, tasteless, etc., but the presence of impurities imparts various physical and chemical changes to water nature which are as followed. Colour: The metallic substances like humus, tannins, peat, algae, weeds, protozoa, industrial effluents and salts of iron, manganese in the colloidal form imparts color to water. For example yellowish tinge is due to the presence of chromium or appreciable amount of organic matter, yellowish–red colour due to the presence of iron, while reddish brown is due to the presence of peat. (The intensity of the colour indicates these verity of toxic nature of water).
Lecture-
- Hard water does not give ready and permanent lather with soap. Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.
2.1 Hardness of Water: Hardness o f w a t e r i s defined as the soap consuming capacity of a water sample. Soap is the sodium salt of higher fatty acids, e.g. sodium stearate. The sodium salt is soluble in water, but the corresponding calcium and magnesium ions are insoluble in water. When soap is added to soft water, it dissolves and lathers r e a d i l y.
On adding soap solution to a sample of hard water which contains calcium or magnesium ions, soap is precipitated as insoluble salts which prevent the formation of lather. This reaction causes the loss of soap. No lather is obtained until all the ions are removed. So, large amount of soap is consumed unnecessarily before lather is formed.
2C 17 H 35 COONa + CaCl 2 (C 17 H 35 COO) 2 Ca + 2NaCI (Sodium stearate) (Calcium stearate) 2.2. Types of Hardness The hardness of water is of two types: (i) Temporary Hardness , and (ii) Permanent Hardness.
(i) Temporary Hardness Temporary Hardness is due to the presence of calcium and magnesium bicarbonates [Ca(HCO 3 ) 2 and Mg(HCO 3 ) 2 ]. It is also called carbonate hardness (CH). It can be removed by Boiling. During boiling, the soluble Mg(HCO 3 ) 2 is converted into insoluble Mg(OH) 2 and Ca(HCO 3 ) 2 is changed to insoluble CaCO 3.
These insoluble precipitates can be removed by filtration. Filtrate thusobtained will be soft water.
Mg(HCO 3 ) 2 → Mg (OH) 2 + 2CO 2 Mg(HCO 3 ) 2 → Mg (OH) 2 + 2CO 2
(ii) Permanent Hardness Permanent hardness is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water (CaCl 2 , CaSO 4 , MgCl 2 and MgSO 4 ). Permanent hardness is not removed by boiling. It is also called non-carbonate hardness (NCH).
Total Hardness: Total hardness is the summation of temporary and permanent hardness. It is also expressed as the sum of the concentration of calcium and magnesium ions. Total Hardness = Temporary Hardness + Permanent Hardness Total Hardness = [Ca2+] + [Mg2+] 2.3 Units of hardness: The following are the common units used in hardness measurements. (i) Parts per million (ppm): It is defined as the number of parts by weight of CaCO 3 present in million parts by weight of water. 1 ppm = 1 part of CaCO 3 equivalent hardness in 10^6 parts of water
1ppm =
Both temporary and permanent hardness is expressed in ppm as CaCO 3 equivalent. The choice of CaCO 3 is due to the fact that it is the most insoluble salt in water and the molecular mass of CaCO 3 is 100. (ii) Milligram per litre: It is defined as the number of milligrams of CaCO 3 present in one litre of water. 1 mg/l = 1 mg of CaCO 3 equivalent hardness in one litre of water Since weight of 1 litre of water = 1 kg = 1000 g = 1000 x 1000 = 10^6 mg 1 mg/l = 1 mg of CaCO 3 per 10^6 mg of water = 1 part of CaCO 3 per 10^6 parts of water = 1 ppm Thus, mathematically both units are equal. Hardness is usually expressed in terms of equivalent of CaCO 3 in order to simplify the calculations in water analysis. Hence, all the hardness causing impurities are first converted in terms of their respective weights equivalent to CaCO 3 by using the relation
(Or)
Lecture- 3.1. Sludge and Scale Formation in Boilers: In a boiler, water is continuously converted into steam. Due to this continuous evaporation of water, the concentration of soluble matters increases progressively. Then the salts start separating out from the solution in the order of their solubility, the least soluble one separating out first. Some of the salts like MgCI 2 , CaCl 2 and MgSO 4 separate in the body of the liquid in the form of soft deposits which can be flushed out easily (Figure 1.1 a). Such soft and non-adherent deposits are known as sludges. Highly alkaline water may cause deposition of precipitates and sludges in boiler tubes and pipes. Some salts like CaSO 4 and Mg(OH) 2 form hard adherent deposits on the inner walls of the boiler. They are called scales (Figure 1.1 b). Scales are so hard and adherent that they are difficult to remove even with the help of hammer and chisel.
Figure 1.1 : (a) Sludge in boiler and (b) scale in boiler
3.2 Disadvantages of scale formation: The following are the disadvantages of scale formation [1] Scale is a poor conductor of heat and thus it decreases the evaporative capacity of the boiler. So, the consumption of fuel is much more than usual. [2] Since the scale acts as a heat insulator, the boiler metal is overheated. Due to overheating, the metal expands until the scale on it cracks. When thick scales crack, the water suddenly comes in contact with overheated boiler metal. This causes the formation of a large amount of steam suddenly. This results in the development of high pressure inside the boiler which may lead to a dangerous explosion. [3] Due to scale formation, heat available to water is reduced and hence more heat is required to produce steam. This causes overheating of boiler plates and tubes and thus their life is reduced.
3.3 Caustic Embrittlement: It is the phenomenon during which the boiler material becomes brittle due to the accumulation of caustic substances. It is a very dangerous form of stress corrosion occurring in mild steel boiler metals exposed to alkaline solution at high temperatures, resulting in the failure of the metal. Stressed
parts like bends, joints and rivets are severely affected. Boiler water usually contains a small proportion of Na 2 CO 3. In high pressure boilers, this breaks up to give NaOH and makes the boiler water more alkaline:
Na 2 CO 3 + H 2 O → 2NaOH + CO 2 This very dilute alkaline boiler water flows into the minute hair cracks and crevices by capillary action. There the water evaporates and the concentration of caustic soda increases progressively. The concentrated alkali dissolves iron as sodium ferroate in crevices, cracks, etc. where the metal is stressed. This causes brittlement of boiler parts particularly stressed parts like: bends, joints and rivets causing even failure of the boiler. Highly alkaline water may lead to caustic embrittlement. Caustic embrittlement can be avoided (a) by using sodium phosphate as softening reagent instead of Na 2 CO 3 and (b) by adding tannin or lignin to boiler water which blocks the hair cracks.
3.4. Boiler Corrosion: It is the decay of boiler material due to chemical or electrochemical attack of its environment. The corrosion in boilers is due to dissolved oxygen, dissolved CO 2 and acids produced by the hydrolysis of dissolved salts. Therefore, removal of these prevents the corrosion of boilers.
3.5. Removal of Dissolved O 2 : Dissolved oxygen in water is mainly responsible for the corrosion of a boiler. The dissolved oxygen in water attacks the boiler material at high temperatures. 2Fe + 2H 2 O + O 2 → 2Fe(OH) 2 4Fe(OH) 2 + O 2 + 2H 2 O → 2[Fe 2 O 3 ·3H 2 O] (rust) Dissolved oxygen can be removed from water by chemical and mechanical means. Sodium sulphite, hydrazine, etc. are some of the chemicals used for removing oxygen. 2Na 2 SO 3 + O 2 → 2Na 2 SO 4 N 2 H 4 + O 2 → N 2 + 2H 2 O Hydrazine is found to be an ideal compound for removing dissolved oxygen since the products are water and inert N 2 gas. It removes oxygen without increasing the concentration of dissolved salts. 3.5.1 Mechanical deaeration method Dissolved O 2 can also be removed from water by mechanical deaeration (Figure 1.2). In this process, water is allowed to spray slowly by the perforated plates fitted inside the tower. Vacuum is applied to this tower and the sides of the tower are also heated. High temperature and low pressure reduce the quantity of dissolved O 2 in water.
Lecture- Softening of hard water: The process of removing hardness producing salts from water is known as softening of water. There are two mode of softening the hard water i. External Treatment ii. Internal Treatment The three important industrial methods used for softening of water externally are:
- Lime – Soda process
- Zeolite or permutit process
- Ion exchange or demineralization process
- Lime – Soda process : In this process, the soluble calcium and magnesium salts are chemically converted into insoluble compounds by adding calculated amounts of lime [Ca(OH) 2 ] and soda [Na 2 CO 3 ]. Calcium carbonate [CaCO 3 ] and magnesium hydroxide [Mg(OH) 2 ] so precipitated are filtered off. By this method both temporary and permanent hardness are removed. For removing temporary hardness the reactions are: Ca(HCO 3 ) 2 + Ca(OH) 2 → 2CaCO 3 + 2H 2 O Mg(HCO 3 ) 2 + Ca(OH) 2 → CaCO 3 + MgCO 3 + 2H 2 O MgCO 3 + Ca(OH) 2 → Mg(OH) 2 + CaCO 3 Hence to remove equivalent amount of Ca and Mg hardness the amount of lime requirement is in the ration 1:2. Again for the removal of permanent hardness the reactions are: CaSO 4 + Na 2 CO 3 → CaCO 3 + Na 2 SO 4 MgSO 4 + Na 2 CO 3 → MgCO 3 + Na 2 SO 4 MgCO 3 + Ca(OH) 2 → Mg(OH) 2 + CaCO 3 Hence for the removal of permanent hardness due to Ca-salts, lime is not necessary, But for Mg salts it is required. Extra addition of Ca(OH) 2 causes hardness. So, calculated quantities of lime and soda are to be added after the determination of actual hardness. 1. LIME-SODA PROCESS: In this Method, the soluble calcium and Magnesium salts in water are chemically converted into insoluble Compounds by adding calculated amounts of lime Ca (OH) 2 and soda Na2CO3 calcium carbonate CaCO and magnesium hydroxide Mg (OH) 2. So Precipitated, are filtered off.
A) COLD LIME-SODA PROCESS:
In this Method, Calculated quantity of chemical (lime and soda) is mixed with water at room temperature. At room temperature, the precipitates formed are finely divided, so they do not settle down easily and cannot be filtered easily. Consequently, it is essential to add small amounts of coagulants (Like Alum, aluminum sulphate, sodium aluminate, Etc.)Which hydrolyse to flocculent, gelatinous precipitate of aluminium hydroxide, and entraps the fine precipitates. Use of sodium aluminate as coagulant, also helps the removal of silica as well as oil, if present in water. Cold L-S process provides water, containing a residual hardness of 50 to 60 ppm. NaAlO 2 + 2H 2 O → NaOH +Al (OH) (^3) Al 2 (SO 4 ) 3 + 3 Ca(HCO 3 ) 2 → 2Al(OH) 3 + 3 CaSO 4 + 6CO 2
Method: Raw water and calculated quantities of chemicals ( Lime+soda+coagulant) are fed from the top into the inner vertical circular chambers, fitted with a vertical rotating shaft carrying a number of paddles , As the raw water and chemicals flow down there is a vigorous stirring and continuous mixing, whereby softening of water takes place. As the softened ware comes into the outer chamber of the lime the softened water reaches up. The softened water then passes through a filtering media (usually made of wood fibers) to ensure complete removal of sludge. Filtered soft water finally a flow out continuously through the outlet at the top sludge settling at the bottom of the outer chamber is drawn off occasionally.
HOT LIME-SODA PROCESS:
Involves in treating water with softening chemicals at a temperature of 80 to 150o^ C. Since hot process is operated at a temperature close to the boiling point of the solution, so (a) the reaction proceeds faster; (b) the softening capacity of hot process is increased to may fold; (c) the precipitate and sludge formed settle down rapidly and hence, no coagulants are needed;(d) much of the gases (Such as CO2 and air) Driven out of the water;(e) Viscosity of softened water is lower, so filtration of water becomes much easier. This in-turn increases the filtering capacity of filters, and (f) Hot Lime-Soda Produces water of comparatively lower residual hardness of 15 to 30ppm.
**Lecture-
- ZEOLITE PROCESS:** Chemical structure of sodium zeolite may be represented as Na 2 O 3 , SiO 2 , YH 2 O where x=2-10 and y=2-6. Zeolite is hydrated sodium alumino silicate, capable of exchanging reversibly its sodium ions for hardness, producing ions in water Zeolite are two types, (i) Natural zeolites are non porous for Ex; Natrolite Na 2 Al 3 O 3 .4SiO 2 , 2H 2 O (ii) Synthetic zeolites posses gel structure. Synthetic Zeolites posses higher exchange capacity than natural Zeolites Process : - For Softening of water by Zeolite process, hard water is percolated at a specified rate through a bed of zeolite; kept in a cylinder. The Hardness causing ions (Ca+2, Mg+2^ etc.) are retained by the zeolite as CaZe and MgZe; while the outgoing water contains sodium salts. Reactions taking place during the softening process are Na 2 Ze + Ca(HCO 3 ) 2 → CaZe +2NaHCO 3 Na 2 Ze + Mg(HCO 3 ) 2 → MgZe +2NaHCO 3 Na 2 Ze +CaCl 2 (or CaSO 4 ) → CaZe +2NaCl( or Na 2 SO 4 ) Na 2 Ze +MgCl 2 (or MgSO 4 ) → MgZe +2NaCl( or Na 2 SO 4 )
REGENERATION : After Some time the zeolite is completely converted into calcium and magnesium Zeolites and it ceases to soften water i.e.; it gets exhausted. At this stage the supply of hard water is stopped and the exhausted zeolite is reclaimed by treating the bed with a concentrated NaCl solution. CaZe ( or MgZe) + 2NaCl Na 2 Ze + CaCl 2 (or MgCl 2 ) The washings are led to drain and the regenerated zeolite bed thus obtained is used again for softening process
Limitations : (i) If the supply of water is turbid in will clog the pores of zeolite led. (ii) Water contains large quantities of colored ions such as Mn+2^ and Fe+2^ they may be removed first because these ions produce Mn and Fe Zeolites, which can’t be easily regenerated. (iii) Mineral acids destiny the zeolite bed.
ADVANTAGES : (i) If removes the hardness almost completely
(ii) Equipment occupying a small space (iii)Requires less time (iv) It is quite clean
DISADVANTAGES : (i) Treated water contains more sodium salts than in time soda process. (ii) The method only replaces Ca+2^ and Mg+2^ ions by Na+^ ions leaves all the acidic ions.
3) ION EXCHANGE PROCESS: Ion exchange resins are insoluble, cross linked long chain organic polymers with micro porous structure, and the functional groups attached to the chains are responsible for the Ion – exchanging properties resins containing acidic functional groups (-COOH,-SO 3 H etc,) are capable of exchanging their H+^ ions with other cations, which comes in their contact where as those containing basic functional groups(-NH 2 =NH, hydrochloride) are capable of exchanging their anions with other anions, which comes in their contact Ion exchange resins may be classified as (i) Cation exchange resin(RH+) are mainly styrene-divinyl benzene copolymers, which on sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cat ions in water. (ii) Anion exchange resins (ROH) are styrene-divinyl benzene or amine-formaldehyde, copdymers, which contains amino or quaternary ammonium or quaternary phosphonium or tertiary sulphonium groups as an integral part of the resin matrix these after treatment with dilute .NaOH solutions become capable to exchange their OH‒^ anions with anions.
PROCESS: The Hard water is passed first through cat ion exchange column, which removes all the cat ions like Ca+ etc, from it and equivalent amount of H+ ions released from this column to water, thus 2RH+^ + Ca2+^ R 2 Ca2+^ + 2H+ 2RH+^ + Mg2+^ R 2 Mg2+^ + 2H+
After Cat ion exchange column, the hard water is passed through anion exchange column which removes all the anions like SO 4 , Cl-^ etc present in the water and equivalent amount of OH-^ ions are released from this column to water thus: R’OH-^ + Cl-^ R’Cl-^ + OH- 2R’OH-^ + SO 4 2-^ R’ 2 SO 4 2-^ + 2OH- 2R’OH-^ + CO 3 2-^ R’ 2 CO 3 2-^ +2OH-
Lecture-
S.NO
LIME-SODA PROCESS PERMUTIT/ZEOLITE ION-EXCHANGE/RESIN
1 Water treatment plant occupies more area or place.
Water treatment plant occupies less area.
Water treatment plant occupies less area.
2 Water after treatment has lesser dissolved solids.
Water after treatment has much more dissolved solids.
Water after treatment free from dissolved solids
3 This method of water treatment plants is not much expensive and material used is cheap.
This method of water treatment plants is more expensive and material used for softening is expensive.
This method of water treatment plants is more expensive and material used for softening is expensive.
4 Operation expenses are higher
Operation expenses are low Operation expenses are higher
5 The cost incurred on softening of water is high.
The cost incurred on softening of water is low.
The cost incurred on softening of water is high 6 It cannot operate in under pressure.
It can even operate in under pressure.
It can even operate in under pressure. 7 It can be used for treating acidic water also.
This method of softening the water is not used for treating acidic water.
It can be used for treating acidic and alkaline water also.
8 There is a problem of settling, coagulation and removal of sludge.
There is no problem of settling, coagulation and removal of sludge.
There is problem of turbidity.
9 It is not possible. This method can be made automatic.
This method can be made automatic.
10 In order to meet the changing hardness of incoming water, frequent control and adjustment of reagents is needed.
Control test comprises only in checking the hardness of treated- water.
Control test comprises only in checking the hardness of treated- water.
11 Residual hardness is low about 15 to 50 ppm
Residual hardness is low about 10 to 15 ppm
Residual hardness is low about 0 to 2 ppm
12 It is not good for boilers It is not good for boilers It is very good for treating water for use in high pressure boilers. 13 Skilled persons required It required less skill for maintenance as well as operation
It required less skill for maintenance as well as operation
INTERNAL TREATMENT: In this process; an ion is prohibited to exhibit its original character by converting it into other more soluble salt by adding appropriate reagent. An internal treatment is accomplished by adding a proper chemical to the boiler water either to precipitate the scale forming impurities in the form of sludge, which can be removed by blow down operations, or to convert them into compounds, which will stay in dissolved form in water and they do not cause any harm. Important Internal treatment methods are : (i) Colloidal conditioning: In low pressure boilers, scale formation can be avoided by adding organic substances like Kerosene, tannin ,agar-Agar etc; which get coated over the scale firming precipitates, there by yielding coated non sticky and loose deposits (ii) Phosphate conditioning : In High pressure boilers, scale formation can be avoided by adding sodium phosphate which reacts with hardness of water forming non- adherent and easily removable soft sludge 3CaCl 2 +2Na 3 PO 4 Ca 3 (PO 4 ) 2 + 6NaCl The main phosphates employed are (a) NaH 2 PO 4 (b) Na 2 HPO 4 (c) Na 3 PO 4
Osmosis is the phenomenon by virtue of which flow of solvent takes place from a region of low concentration to high concentration when two solutions of different concentrations are separated by a semi- permeable membrane. In this process pure water is separated from salt water. 15-40 kg/cm2 pressure is applied for separating the water from its contaminants. The membranes used are cellulose acetate, polymethylmethacrylate and polyamide polymers. The process is also known as super or hyper filtration.
Advantages:
➢ It is simple and reliable process & Capital and operating expenses are low.
➢ The life of the semi-permeable membrane is about two years and it can be easily replaced within a few minutes, thereby nearly uninterrupted water supply can be provided. Electrodialysis process:
Electrodialysis ( ED ) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference.
Electro dialysis is used whenever solutions, which contain dissolved electrically charged ions, have to be concentrated or diluted. The electrolyte separation of solutions which contain electrical uncharged molecules, like e.g. sugar, proteins and so on, is also possible. The electro dialysis process is driven by the electrical potential. For conditioning, the solutions are pumped through the membrane stack and at the electrodes, a direct current voltage will be applied. The positively charged salt ions (cations) follow the electrical field and flow to the cathode, the negatively charged ions (anions) flow to the anode. A membrane stack for electro dialysis consists of a series of cells, which are separated from eac h other by alternating anions and cations exchange membranes. The stack is completed by a anode and a cathode and fixed in a clamping device which looks similar to a filter press. The positively charged ions are able to pass the cations exchange membrane, but they cannot pass the following anions exchange membrane and are retained. Vice versa the negatively charged ions are able to pass the anions exchange membrane and are retained at the cations exchange membrane. Hence, ions are concentrated (concentrate) in every second cell whereas the ions of the neighboring cells are removed (diluate). By supply and drain connections at the cell frame, the diluate and concentrate flow can be pumped through the cells of the membrane stack. Field of application of electrodialysis
Acid recovery High concentration of brines
Water conditioning
Desalination of brackish water Nitrate removal of potable water Reduction of operating costs of ion exchanger plants by upstream desalination with electro dialysis Conditioning of reverse osmosis concentrate to reach a plant recovery of more than 95% Concentration of Sea water for table salt production
