water treatment ii
TRANSCRIPT
WATER TREATMENT-II
BYDR. LALIT KUMAR SHARMA
Associate ProfessorAdvanced Educational Institutions
Aurangabad(Palwal)
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Treatment of water for domestic use Water used for domestic purposes must be free from germs & bacteria. It should be
free from objectionable dissolved gases like H2S & dissolved salts like, lead arsenic & manganese salts. Generally for domestic supply, surface water is used & it is contaminated with large number of impurities such as, organic matter, suspended impurities etc. therefore to make it safe for drinking purpose, following treatment processes are employed.
(1) Screening :
Screening is the process of removing floating materials from water. Raw water is allowed to pass through a screen having a large number of perforations which removes the large & small floating matter.
(2) Sedimentation :
It is the process of removing insoluble impurities by allowing the water to stay undisturbed for sometime. Water is allowed to settle in big tanks or reservoirs for a number of hours. The suspended materials, other than living organisms which float due to the gases they produce, settle down at the bottom due to the force of gravity. The process of sedimentation is generally carried out in continuous flow type tanks in which water flows continuously in a horizontal, radial or vertical directions at uniform rate.
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(3) Co-agulation :
Co-agulation is the process of removing fine sized particles from water with addition of certain chemicals known as co-agulants. Actually the fine sized particles present in water either do not settle down at all or take a long time. In order to facilitate quick settling of these particles, some chemical agents known as co-agulants are used. The commonly used co-agulants are the salts of iron & aluminium. e.g. alum [ K2SO4 Al2(SO4)3 · 24H2O ], aluminium sulphate Al2(SO4)3 18H2O, ferrous sulphate FeSO4 · 7H2O, ferric chloride FeCl3 etc. These salts react with carbonate & bicarbonate radicals present in water & form coagulable precipitates (flocks) of hydroxide of these metals. The precipitated hydroxide absorbs the suspended impurities, bacteria & other micro-organisms &
causes them to settle down.
(4) Filtration :
When sludge etc. after co-agulation has settled down, filteration is carried out by means of filters. Filtration is the process of removing insoluble, colloidal & bacterial impurities from water by means of filters. The filters used in water filtration consist of several layers of sand particles of different size.
Gravity send filter consists of a large rectangular take made of concrete. The filtering medium in it consists of 3 feet of find send below which 1 feet thick coarser sand which inturn is supported by 8 inches thick bed of graded gravel. Below the graded gravels are drain through which filtered water goes out.
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As the water percolates ( due to gravity ) through the fine sand bed, most of the objectionable materials are absorbed by it & clear water collects in the under drain channel, form where it is drawn out.
(5) Removal of Bacterial & Micro-Organisms :Although most of bacteria & micro-organisms present in water are removed by co-agulation followed by settling & filtering through sand beds, yet water contains a small percentage of pathogenic bacteria. These bacteria must be completely removed particularly from water supplied by municipality for drinking waterpurposes. Otherwise water borne diseases may be produced. The process of destroying these pathogenic bacterial & micro-organisms is known as sterilization or disinfection.
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(i) Chlorination of water :
Chlorine is a commonly used sterilizer in municipal water supply. It can be used directly as a gas or as chlorine water. Its germicidal action is based on it reaction with water. It reacts with water to form hypochlorous acid & nascent oxygen both of which are powerful germicides.
Cl2 + H2O → HOCl + HClHOCl → HCl + [O]
However, excess of chlorine should be avoided because it produces characteristic unpleasant taste & odour & irritating effect on mucous membrane. For filtered water about 0.3-0.5 ppm of chlorine is sufficient. The treated water should not contain more than 0.1-0.2 ppm of free chlorine.(ii) Bleaching powder :
Bleaching powder is a good sterilizer for small water works. In practice about 100 liters of water is mixed well with about 1kg of the powder & the resulting solution allowed to stand for several hours. Hypochlorous acid & nascent oxygen produced by action of water on bleaching powder are powerful germicides.CaOCl2 + H2O → Ca(OH)2 + Cl2
Cl2 + H2O → HOCl + HClHOCl → HCl + [O] www.advanced.edu.in
Water softening Permutit Process
Permutit is also known as Zeolite. They are capable of exchanging ions reversibly.
Thechemical formula for permutit is Na2O. Al2O3.SiO2.6H2O. In short it is written as Na2-P
or Na2-Z. For softening of water by this method, hard water is percolated at a
specified rate through a bed of zeolite kept in a cylinder. The hardness causing ions (Ca++
&Mg++) are retained by the permutit as Ca-Z & Mg-Z. While the out going water contains sodium salts.
1. Natural zeolite: Non porous & amorphous, eg. Natrolite: Na2OAl2 O3 3 SiO2.2 H2O , Laumontite: Na2O.Al2 O3.3 SiO2.2H2O
2. Synthetic zeolite: porous and possess gel structure, possess high exchange capacity as compared to natural zeolite. Sodium zeolite/ permutit - Na2O.Al2 O3 .x SiO2.y H2O where x = 2-10, y = 2-6. They are prepared by heating sodium silicate, Al2(SO4)3, NaAlO2
Permutit undergo following chemical reaction with temporary hardness forming salt.
Ca(HCO3)2 + Na2-Z → Ca-P + 2NaHCO3
Mg(HCO3)2 + Na2-Z → Mg-P + 2NaHCO3
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Permutit undergo following chemical reaction with permanent
hardness forming salt.
CaCl2 + Na2-Z→ Ca-P + 2NaCl
MgCl2 + Na2-Z → Mg-Z + 2NaCl
CaSO4 + Na2-Z → Ca-Z + Na2SO4
MgSO4 + Na2-Z → Mg-Z + Na2SO4
When permutit is completely converted into calcium &
magnesium permutit & it ceases to soften the water. It gets
exhausted. At this stage, the supply of hard water is stopped
& the exhausted permutit is regenerated by treating the bed
withconcentrated 10% NaCl solution.
Ca-Z + 2NaCl → Na2-Z+ CaCl2
Mg-Z + 2NaCl → Na2-Z+ MgCl2
The washing with water is led to remove CaCl2 & MgCl2 & thus obtained regenerated
permutit can be again used.
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Ion exchange method This is most modern method for softening hard water. By using this method almost all salts can
be removed completely from hard water and the water obtained is as good as distill water. In this process two types of resins are used i.e. cation exchange resin & anion exchange resin, cation exchange resin contains (-COOH, -SO3H) function groups and are capable of exchanging their H+ ions with cations. While anion exchange resin contains (-NH2, -OH) functional group and are capable of exchanging OH- ions with anions. In the process two columns, one consist of cation exchange resin & another consist of anion exchange resin are used.The hard water is first allowed to pass through a column containing cation exchange resins. Which remove all the cations like Ca+2, Mg+2 etc. and release H+ ions. Reaction in first column takes place as under.
R-H2 + CaCl2 → Ca-R + 2HCl
R-H2 + MgCl2 → Mg-R -- 2HCl
R-H2 + CaSO4 → Ca-R -- H2SO4
R-H2 + MgSO4 → Mg-R + H2SO4
The anions like chloride & sulphates are converted into acid like HCl &H2SO4. Which is passed
through another column containing anion exchanger i.e. R-(OH)2 resin where the following
reaction takes place.
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R-(OH)2 + 2HCl → R-Cl2 + 2H2O
R-(OH)2 + H2SO4 → R-SO4 + 2H2O
Water thus obtained is free from all cations and anions & is called soft water or distill water.When both resins get fully exhausted then they are regenerated. The acidic resin is regenerated by passing dilute acid solution (in first column). While basic resin is regenerated by passing dilute NaOH solution (in second column), followed by washing with water.
Ca-R + 2HCl → R-H2 + CaCl2 in first
Mg-R + 2HCl → R-H2 + MgCl2 column
R-Cl2 + 2NaOH → R-(OH)2 + 2NaCl in second
R-SO4 + 2NaOH → R-(OH)2 + 2Na2SO4 column
The regenerated acidic and basic resins are
again reused for softening of hard water.
Mixed bed Dimineralization process:
Mixed bed of cationic and anionic resin is taken
in single vessel water is passed through mixed
bed, it comes in contact with two type of resins
several times. Production of deionized water having less than 1ppm of dissolved ions. When resin exhausted
mixed bed backwashed. Cation and anion exchange resin regenerated with NaOH and dil H2SO4
respectively. www.advanced.edu.in
Lime- Soda process:
(a) Removal of temporary, calcium and magnesium hardness:
Ca(HCO3)2 + Ca(OH)2 → 2 CaCO3 + 2H2O
Mg(HCO3)2 + 2Ca(OH)2 → 2 CaCO3 +Mg(OH)2 + 2H2O
(b) Removal of permanent magnesium hardness:
MgCl2 + 2Ca(OH)2 → CaCl2 +Mg(OH)
MgSO4 + Ca(OH)2 → CaSO4 +Mg(OH)2
(c) Removal of dissolved iron , aluminium salts:
FeSO4 + Ca(OH)2 → CaSO4 + Fe (OH)2
H2O + 2Fe (OH)2 + ½ O2 → 2Fe (OH)3
Al2(SO4)3 + 3 Ca(OH)2 → 3CaSO4 + 2Al (OH)3
(d) Removal of dissolved CO2 and H2S:
CO2 + Ca(OH)2 → CaCO3 + H2O
H2S + Ca(OH)2 → CaS + 2H2O
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(e) Removal of free mineral acids H2SO4 + Ca(OH)2 → CaSO4 + 2 H2O 2HCl + Ca(OH)2 → CaCl2 + 2 H2O
Function of soda:CaCl2 + Na2CO3→ 2 NaCl + CaCO3
CaSO4 + Na2CO3→ Na2SO4 + CaCO3
(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.
NaAlO2 + 2H2O → NaOH +Al(OH) 3
Al2 (SO4 ) 3 +3 Ca(HCO3 ) 2 → 2Al(OH) 3 + 3 CaSO4 + 6CO2
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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.
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(b) Hot lime soda process
Involves in treating water with softening chemicals at a temperature of 80 to 150 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.
Hot lime-soda plant consists essentially of three parts (a) a ‘reaction tank’ in which raw water, chemicals and steam are thoroughly mixed; (b) a ‘conical sedimentation vessel’ in which sludge settles down, and (c) a ‘Sand filter’ which ensures complete removal of sludge from the softened water.
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Desalination
Desalination, refers to any of several processes that remove some amount of salt and other minerals from saline water. More generally, desalination may also refer to the removal of salts and minerals,[as in soil desalination.
Salinity
1.Fresh water less than 1000 mg/l
2. Brackish water 1000-35000 mg/l
3. Sea water greater than 35000 mg/l
Reverse Osmosis
Reverse Osmosis works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure. The desalinated water that is demineralized or deionized, is called permeate (or product) water. The water stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject (or concentrate) stream. As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure) the water molecules pass through the semi-permeable membrane and the salts and other contaminants
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are not allowed to pass and are discharged through the reject stream (also known as the concentrate or brine stream), which goes to drain or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it. It is important to understand that an RO system employs cross filtration rather than standard filtration where the contaminants are collected within the filter media. With cross filtration, the solution passes through the filter, or crosses the filter, with two outlets: the filtered water goes one way and the contaminated water goes another way.
Electrodialysis
Electrodialysis is an electrochemical process whereby electrically charged particles, ions, are transported from a raw solution (retentate, diluate) into a more concentrated solution (permeate, concentrate) through ion-selective membranes by applying an electric field. When a salt solution is under the influence of an electric field, as is the case in an electrodialysis module, the charge carriers in the solution come into motion. This means that the negatively charged anions migrate towards the anode and the positively charged cations towards the cathode.
In order to separate salts from a solution, ion-selective membranes, through which only one type of ion can permeate in an ideal case, are arranged in the solution perpendicular to the electric field. Thus negatively charged particles (anions) can pass through an anion exchange membrane on their way to the anode but are selectively retained by the upstream cation exchange membrane. This separation stage results in a concentration of electrolytes in the so-called concentrate loop and a depletion of charge carriers in the so-called diluate loop.
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Thank you
DR. LALIT KUMAR SHARMAAssociate Professor
Advanced Educational Institutions,70 km Milestone,
Delhi-Mathura Road, Dist. Palwal, Haryana-121105 +91–1275–398400, 302222
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