Technology Of Water

Technology Of Water

Water:

Water is a chemical substance with the chemical formula H₂O.

•Existence on Earth 1. Solid state-ice,

2. Gaseous state (water vapor or steam) 3. Liquid

•Water is not only universal solvent but also an essential material for engineering applications.

•Prime factor in development

•Water on Earth moves continually through a cycle of evaporation or transpiration (evapotranspiration),

Evotranspiration Process

Sources of Water

•The source of water are listed under two categories 1. Surface Water

2. Underground Water A.Surface Waters:

1. Rain Water 2. River Water 3. Lake Water 4. Sea water

B. Underground Water:

1. Spring

2. Well water

Impurities in water:

Dissolved impurities – inorganic salts of Na, K, Ca, Mg, Al, Fe and traces

of other metals as chlorides, sulphates, bicarbonates, etc.

Dissolved gases – oxygen, nitrogen, CO₂, oxides of N and S, NH₃, or H₂S Suspended impurities - sand or clay particles, decayed organic substances.

Microbial and biological impurities – pathogenic microorganisms, biological matter of plant and animal origin.

Surface and ground water are normally used for industrial and domestic purposes.

Hence undesirable impurities should be removed from these water.

Water treatment / water technology - the process of removing all types of impurities from water and make it suitable for industrial and domestic purposes.

Quality of water for different uses

Purpose Quality requirements

Drinking Palatable, clear and free from colour, odour, turbidity and pathogens.

Domestic washing

Soft water producing lather readily.

Textile dyeing Free from colour, turbidity, organic matter, Fe and Mn.

Boiler feed water

Free from dissolved salts, suspended impurities, silica and dissolved gases.

Drugs and

pharmaceuticals

Soft and clear. Free from pathogens, colour, odour and suspended impurities.

Construction Not too hard, should contain less of chlorides.

Paper industry Free from colour, turbidity, organic matter, Fe, Mn, silica and alkalinity

•Types of Hardness

1) Temporary:- or Carbonate Hardness

Water that contains bicarbonate of calcium and magnesium or of both

• removed by boiling

Ca(HCO₃)₂ → CaCO₃ ↓ + CO₂ ↑+ H₂O Mg(HCO₃)₂ → Mg (OH) ₂ ↓ + 2CO₂ ↑ 2)Permanent :- or Non- Carbonate Hardness

Contains chlorides or sulphates of calcium or magnesium or of both

•Can not be removed by boiling

CaCl₂ → Ca⁺² + 2Cl^-1 MgSO₄ → Mg⁺² + SO₄^-2

Water quality standards 1. It should be clear and odourless.

2. It should be pleasant to taste 3. It should be perfectly cooled.

4. It should be free from disease producing bacteria 5. Its turbidity should not exceed 10 ppm.

6. It should be free from dissolved gases like H2S.

7. It should be free from minerals such as Pb, As, Cr and Mn salts.

8. pH should be in the range of 7 – 7.5

9. Chloride and Sulphate content should be less than 250 ppm.

10. Fluoride content should be less than 1.5 ppm

11. Total dissolved solids should not be more than 500 ppm.

Domestic water treatment Screening

Sedimentation Coagulation

Filtration

Sterilization

Sedimentation

The process of allowing water to stand undisturbed for some time in order to facilitate the settling down of co-suspended particles under the action of gravity is called sedimentation

It removes approximately 70-75% of impurities

Shapes of Sedimentation Tank

Filtration

Process of removing the remaining colloidal matter, most of bacteria and other micro organisms by passing the sedimented water through suitable filters.

Filter bed consists of four layers i.e., Fine sand, coarse sand, fine gravel and coarse gravel.

When water is passed through this, all the colour, odour, part of bacteria are removed. The bed needs periodic regeneration.

Commonly used filter: Rapid sand gravity filter Two types of filters

a) Gravity type filters b) b) Pressure type filters

Dichloramine is relatively stable compound and it slowly decomposes with evolution of chlorine. Thus, the addition of ammonia stabilizes the chlorine to provide a

prolonged effect.

Super Chlorination: A large excess of chlorine is added to the water thereby destroying not only the micro-

organisms but also other organic impurities present. This process ensures rapid and complete sterilization and

successfully used for waters derived from wells and rivers. This process is followed by de-chlorination.

Ozonisation:

• Ozone is powerful disinfectant and is readily dissolved in water.

• Ozone being unstable decomposes by giving nascent oxygen which is capable of destroying the Bacteria.

• This nascent oxygen removes the colour and taste of water and oxidizes the organic matter present in water.

O₃ → O₂+ [O]

Estimation Of Hardness 1. EDTA Method

EDTA Method:

•Estimation is by titration method at pH 10

•Using Indicator Eriochrome Black-T

Versenate/disodium salt of EDTA

2. Soap Solution Method

Addition of soap till hard water gives lather

2C₁₇ H₃₅ COO Na + Mg/CaSO₄ → (C₁₇ H₃₅ COO)₂ Ca/Mg ↓ + Na₂SO₄

2C₁₇ H₃₅ COO Na + Mg/Ca(HCO₃)₂ → (C₁₇ H₃₅ COO)₂ Ca/Mg ↓ + 2NaHCO₃

In industrial uses.

• Textile industry and Dyeing industry: hard water causes the usual problem of deposition of insoluble salts that interfere with the proper dyeing and printing of the fabrics. The stains of iron salts also are undesirable on fabrics. Hard water also hampers the economy by wastage of soap as it does not form good lather.

• Sugar industry: the salts responsible for hardness create difficulties in sugar refining and crystallization of sugar and the sugar becomes deliquescent.

•Paper industry: Calcium and magnesium salts also interfere with the smooth and glossy finish of the papers in the paper industry. Iron salts interfere with the colour of the paper.

•In laundry, hard water causes wastage of costly soap and also interferes with the coloration due to the staining of iron salts.

• The hydration of cement and final hardening of cement are affected by use of hard water in concrete making.

•Pharmaceutical industry: Hard water is not suitable for preparing drug solutions.

•Steam generation in boilers, hard water creates many problems like (i) scale formation, (ii) corrosion, (iii) priming and foaming and (iv) caustic embrittlement.

Disadvantages of Hard Water (A) Domestic Uses:

1.Washing 2. Bathing 3. Drinking 4. Cooking 2C₁₇ H₃₅ COO Na + CaSO₄ → (C₁₇ H₃₅ COO)₂ Ca ↓ + Na₂SO₄

(in water) (White Scum) (B) Industrial Uses:

1. Boiler Feed: should not contain nitrates- scale and sludges

2. Paper Mill: should not contain iron and lime- destroy resin of soap 3. Sugar industries: Sulphates and Alkaline carbonates- Deliquescent 4. Dyeing Industries: should not contain iron and hardness

5. Laundries: should be soft

Scale and Sludge

•Troubles met in boiler which are used in steam production.

Scale and Sludge Formation

When water is evaporated in boilers to produce steam continuously

the concentration of the salts present in the water increases progressely

As the concentration reaches a saturation point the salts are thrown

over the inner surface of the boiler.

Scale:

When precipitates are hard and they adhere strongly to the inner surface of the boilers, they are called scale.

Sludge:

During boiling salts form precipitate of loose slimy form, they are called sludge.

Sludge can be easily removed by scrapping with a brush. Sludge is formed by the presence of MgCO³, MgSO⁴, MgCl², CaCl² etc.

These salts are more soluble in hot water.

Disadvantages of sludge formation are:

(i) poor heat conduction due to the presence of sludge on the surface (ii) difficulty in the operation of the boiler

(iii) if sludge is formed along with the scale and is trapped, so it is difficult to remove

(iv) It clogs the pipe lines and other connections to the vessel near the places where water circulation rate is slow.

. Removal of sludge: Sludge formation can be prevented by (i) using soft water for boiler operation

(ii) removing the concentrated salty water from time to time so that deposition of sludge is prevented.

•Scales are the hard deposits on the inner surface of the boilers which are difficult to remove. This scale formation takes place due to the following reasons:

(a) In low pressure boilers scale formation occurs due to the formation of CaCO³ from Ca(HCO³)².

i.e., Ca(HCO³)² CaCO³ + H²O + CO²

Scale

(b) In high pressure boilers this CaCO³ gets converted to soluble Ca(OH)². But here CaSO⁴ forms the hard scale. Since the solubility of CaSO⁴

decreases with increase in temperature, and at high temperature the precipitated CaSO⁴ forms hard scale.

•Similar hard scales are formed when SiO² is present in the hard water. It

deposits as CaSiO³ or MgSiO³. These calcium or magnesium silicate scales are very difficult to remove.

•Dissolved magnesium salts also precipitate as Mg(OH)² forming soft type of scale.

•Disadvantages of scale formation are similar to sludge formation but the severity is more, since its removal is more difficult.

Calcium Sulphate

Forms a hard scale on the heating surfaces Calcium Bi-carbonate

Decomposes at a low temperature when CO₂ is liberated. Remaining Calcium carbonate deposits on the heating surface as a soft scale.

Ca(HCO₃)₂  CaCO₃ + CO₂ + H₂O

Disadvantages of Scale Formation

1. Wastage of fuel 2. Decreases in Efficiency 3. Lowering of boiler safety 4. Danger of Explosion 5. Corrosion

Removal of Scales

1.By scrapping 2. Giving thermal shocks 3. Dissolving by using chemicals e.g CaCO₃ by 5-10% HCl 4. Adding complexing agents e.g CaSO₄ by EDTA

5. Blow down Process

Disadvantages of Sludge Formation 1.Chocking of pipes

2.Low supply of heat 3.Wastage of fuel

4.Get entrapped in scales

Disadvantages include

(i) Poor heat transfer from boiler to water leading to increase in fuel consumption. The

increase in thickness of the scale from 1.25 mm to 12 mm leads to increase in fuel

consumption from over 50% to 150%.

(ii) Due to the overheating of the boiler, different parts of the boiler become weak and

distorted and so the operation of the boiler becomes unsafe, particularly the high

pressure boilers.

The thick scales may sometimes lead to explosion due to sudden development of high

pressure.

(iii) Valves and condensers of the boilers are chocked due to scale formation and boiler

efficiency decreases.

Removal of scales can be done by:

(i)Wooden scraper or wire brush, suitable for removing loose scales.

(ii) Blow down operations for loose scales. The operation actually involves the removal of very hard water from a tap at the bottom of the boiler and

replenishing the water with softened water called .make up. water.

(iii) Giving thermal shocks, which involve alternate heating and cooling to make the scales brittle.

(iv) Chemical treatment with 5-10% HCl for carbonates and EDTA treatment for Ca/Mg salts forming complexes.

These are methods for the removal of the scales, when they are formed.

There are also some methods for the prevention of scale formation by internal or external treatment.

Prevention of Sludge Formation

1.Blow down Process: Withdrawing portion of sludge containing water and replacing with fresh water

2.Softening of water

Method of Preventing Scale Formation 1.External Treatment:

Treatment of water before entering into boiler - Removal of Ca, Mg and Silica

2. Internal Treatment:

Addition of chemicals directly to water in boiler.

-Scale forming substances produces loose precipitate -Blow down process

-Adding complexing agents: to form soluble complex

Caustic embrittlement :

Embrittlement is the name that has been given to boiler failures due to development of certain types of crack resulting from excessive stress and chemical attack.

In steam boiler operation, the chemicals that are believed to be

responsible are NaOH and silica. During softening processes, Na₂CO₃ are added and it gives rise to NaOH at elevated temperature according to the following reaction.

It is the phenomenon in which the material of a boiler becomes brittle due to the accumulation of caustic substances.

Sodium carbonate is used in softening of water by lime soda process, due to this some sodium carbonate maybe left behind in the water.

Na₂CO₃ + H₂O → 2NaOH + CO₂

As Conc. of NaOH increases, water flows into minute hair cracks.

Water get evaporated and NaOH increases further and react with iron of boiler, hence cause Embrittlement.

NaOH attacks and dissolves out iron of boiler forming sodium ferrate. This finally causes the stressed parts like bends, joints, rivets to lead to the boiler failure.

Prevention of Caustic Embrittlement

(i)Addition of sodium phosphate as softening agent instead of Na₂CO₃.

(ii) Addition of sodium sulfate to ensure a weight ratio Na2SO4/NaOH > 2.5, whereby the deposition of Na₂SO₄ prevents the penetration of NaOH into the cracks and stops caustic embrittlement in high pressure boilers.

(iii) Addition of organic agents like tannin, lignin, querbracho etc. also prevents cracking similar to sodium sulfate in low pressure boilers.

(iv) Use of crack-resisting steels: Certain steels containing Al added during manufacture appear to be resistant against caustic cracking.

Priming and Foaming

Priming:- Carry over of varying amounts of water in the steam e.g.

(Foam, mist)

-Leads to deposits of salt crystals -Lowers the energy efficiency

Causes:-

a)Presence of suspended impurities and dissolved salts b)High steam velocity and sudden boiling

c)High water levels d)Faulty boiler design

Preventions:

a)Good boiler design

b)Avoid rapid changes in temp.

c)Maintaining low water level

d)Fitting mechanical steam purifier

Normal Bubble

Carry Over

Foaming: Formation of small bubbles and forth on the surface of water which do not break easily.

-Caused by high conc. of any solids in water Causes:

a)Difference b/w conc. of solute and suspended matters.

b)Surface tension lowering substances c)Oil and grease

Prevention:

a)Adding antifoaming agents e.g. castor oil

b)Removing oily particles using silicic acid and sodium alluminate.

Boiler Corrosion: Decay of material by chemically or electrochemically Causes:

a)Presence of dissolved gases e.g. O₂ , CO₂ b)Caustic Embrittlement

c)Acid formation due to Hydrolysis d)Presence of free acids.

Treatment Methods of Water

A. Internal Treatment B. External Treatment

Internal Treatment Method 1.Phosphate Conditioning:

- Small amount of phosphate ions are added to precipitate Ca ions.

- Chosen depending on the pH conditions of boiler.

2. Colloidal Conditioning:

-Using kerosine, tannin, starch etc

-Get coated over the scale forming particles -Removed by Blow down Process

3. Carbonate conditioning:

-Na₂CO₃ is added to precipitate Ca salts as CaCO₃ -Removed by Blow down Process

-Used in low pressure boilers

•Calgon Conditioning:

-Scale forming salts are converted into soluble complexes.

-E.g. Sodium Hexameta Phosphate (Na₂PO₃)₆ is added…reacts with Ca and forms Calcium Hexameta Phosphate (Ca₂PO₃)₆

-Prevents Scale formation

•Radioactive conditioning:

-Adding radioactive tablets

-Emits radiation energy which prevents Scale formation

•Electrical Conditioning:

-Mercury bulbs placed in boiler -Emits electrical discharge

-Prevents Scale formation

Internal treatment involves addition of chemical to the boiler water either to

(i)precipitate the scale forming impurities in the form of sludges, which can be easily removed or

(ii)convert the impurities to soluble compounds, so that scale formation can be avoided.

Internal treatment

(a) Colloidal Conditioning: Organic substances like kerosene, tannin, agar-agar are

added to form gels and form loose non-sticky deposits with scale-forming precipitates, which can be easily removed by blow-down operations in low pressure boilers.

Important internal treatments involve

(b) Different sodium phosphates like NaH₂PO₄ (highly alkaline waters), Na₂HPO₄ (slightly acidic waters) and Na₃PO₄ (highly acidic waters)are added to high pressure boilers to react with the hardness forming impurities to form soft sludge of calcium and

magnesium phosphates and finally this can be removed by blow down operation.

3CaCl₂ + 2Na₃PO₄ Ca₃(PO₄)₂ + 6NaCl

(c) Carbonate conditioning: Sodium carbonate is added to the water of low pressure boiler whereby the scale forming CaSO4 gets converted to loose sludge of CaCO3, which can be easily removed by blow-down operation.

CaSO₄ + Na₂CO₃ CaCO₃ + Na₂SO₄

(d) Calgon conditioning: Calgon i.e., sodium hexa meta phosphate when added to boiler water, reacts with scale forming CaSO4 and forms soluble complex compound.

Na₂[Na₂ (PO₃)₆] 2Na⁺+ [Na₄P₆O₁₈]^2-

2CaSO₄ + [Na₄P₆O₁₈]^2- [Ca₂P₆O₁₈]^2-+ 2Na₂SO₄

(e) Sodium aluminate conditioning: Sodium aluminate is hydrolysed yielding NaOH and gelatinous Al(OH)₃. The NaOH formed reacts with magnesium salts to precipitate Mg(OH)₂. This Mg(OH)₂ and Al(OH)₃ are flocculent and entraps the colloidal as well as the finely divided impurities like silica in the boiler water and the loose precipitate is finally removed by blow down operation.

NaAlO₂ + 2H₂O NaOH + Al(OH)₃ MgCl₂ + 2NaOH Mg(OH)₂ + 2NaCl

External Treatment Method (or) Water softening Method

•Removal of hardness causing substances from water Methods:

1.Zeolite process

2.Ion Exchange Process 3.Lime Soda Process

Zeolite (or Permutit) Process: are Hydrated sodium alumino Silicate Na₂O. Al₂O₃ X SiO₂ Y H ₂O (X= 2-10, Y= 2-6 )

Natural Zeolites:

1.Natrolite - Na₂O. Al₂O₃ 4SiO₂ .2H₂O 2.Laumontite - CaO. Al₂O₃ 4SiO₂ .4H₂O

3.Harmotome - (BaO.K₂O). Al₂O₃ 5SiO₂ .5H ₂O - Capable of exchanging its Na ions.

A. Natural Zeolite:-

Derived from green sand by washing, Heating, treating with NaOH.

•Non porous in nature.

B. Synthetic Zeolite: -

Prepared from solution of Sodium Silicate and AlOH

•Higher exchange capacity and porous in Nature.

Process:

•Consist of Steel Tank

•Having Thick Layer of Zeolite

•When water pass through it hardness causing ions (Ca, Mg) are retained by Zeolite.

Chemical Reaction:

Ca(HCO₃)₂ + Na₂ Ze → CaZe + 2 NaHCO₃ CaSO₄ + Na₂Ze → CaZe + Na₂SO₄ MgCl ₂ + Na₂Ze → MgZe + 2NaCl Mg (NO₃)₂ + Na₂Ze → MgZe + 2NaNO₃

•Exchange of Na ions continues until Na ions are exhausted Regeneration:

CaZe + 2NaCl → Na₂Ze + CaCl₂ MgZe + 2NaCl → Na₂Ze + MgCl ₂

•CaCl ₂ and MgCl ₂ led to drain and Na₂Ze can be reused.

Advantages 1.Hardness can be completely removed 2.Process can be made automatically 3.Easy operation. No experts required 4.Less time and sludge

5.Small area requires

Disadvantages 1.Only Ca⁺ and Mg⁺ ions can be removed

2.Large amount of Na ions present in treated water.

3.Leaves other acids which causes corrosion

4.Fe²⁺ and Mn²⁺ containing water can not be treated because Fe and Mn Zeolite can not be regenerated

5.Water should be free from suspended impurities to prevent clogging on Zeolite beds.

6.Treated water contains more dissolved solids.

B. Ion Exchange (or) Demineralization (or) Deionization Process.

• mineral ions are removed

•physical process which uses specially-manufactured ion exchange resins which bind to and filter out the mineral salts from water.

Advantage:

1.Produces soft water

2.Can treat highly acidic or alkaline water 3.Regeneration of ion resins are possible 4.Maintenance cost is low

Disadvantage:

1.Cost of equipment is high

2.Highly turbid water can not be treated 3.Expensive chemicals are required.

Ion exchange resins are organic polymers which are crosslinked having microporous structure and the functional groups are attached to the chains which are responsible for the ion exchange properties.

(i) Cation exchange resins (RH⁺) are phenol-sulfonic acid-formaldehyde resin, styrene- divinyl benzene copolymers which exchange their H⁺ ions with the cations present in the water i.e., Ca²⁺and Mg²⁺.

(ii) Anion exchange resins (ROH^-.): The styrene divinyl benzene or amine formaldehyde copolymers contain quaternary ammonium tertiary sulphonium or amino group in the resin. The resin on treatment with NaOH solution is capable of exchanging the OH^-. with different anions of water i.e., Cl^-., SO^42-etc.

Method:The hard water is passed first through cation exchange resin similar to the permutit process whereby the cations like Ca²⁺, Mg²⁺ are removed from the hard water and exchanged with H⁺ as follows:

2RH⁺ + Ca²⁺/Mg²⁺ → R₂Ca²⁺/R₂Mg²⁺ + 2H⁺

After this the hard water is again passed through anion exchange column, which exchanges all the anions like SO₄^2-, Cl^- etc. present in the water with OH^-

ROH^- + Cl^- → R⁺Cl^- + OH^-

These H⁺ and OH^- combine to form water molecule. Thus, the water coming out finally from the two exchangers is ion free and called deionized or demineralized water.

Regeneration: The inactivated or exhausted cation exchange resin is regenerated by dil. H₂SO₄/HCl.

R₂Ca²⁺ + 2H⁺ → 2RH⁺ + Ca²⁺

Similarly, the exhausted anion exchange resin is regenerated by dil. NaOH R₂SO₄^2- + 2OH^- → 2ROH^- + SO₄^2-

The columns are finally washed with deionized water and the washings are discarded.

Lime-soda process

By this process, soluble calcium and magnesium salts are rendered insoluble by adding calculated amount of lime [(CaOH)₂] and soda [Na₂CO₃]. The insoluble

precipitates of CaCO₃ and Mg(OH)₂ are removed by filtration. By this method, both temporary and permanent hardness are removed.

For the removal of temporary hardness the reactions are:

Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ ↓ + 2H₂O

Mg(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ ↓ + MgCO₃ + 2H₂O MgCO₃ + Ca(OH)₂ → Mg(OH)₂ ↓ + CaCO₃ ↓

Hence, to remove equivalent quantities of Ca and Mg hardness the amount of lime necessary is in the ratio of 1:2.

Again for the removal of permanent hardness. The reactions are:

CaSO₄ + Na₂CO₃ → CaCO₃ ↓ + Na₂SO₄ MgSO₄ + Na₂CO₃ → MgCO₃ + Na₂SO₄

MgCO₃ + Ca(OH)₂ → Mg(OH)₂ ↓ + CaCO₃ ↓

Hence, for the removal of permanent hardness due to Ca-salts, lime is not necessary, but it is necessary for Mg salts. 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.

In the actual process the water is thoroughly mixed with the chemicals and allowed to react for sufficient time. Activated charcoal is added as activator, alum etc. are added as coagulants. To avoid after-precipitation of CaCO₃, sludge of the previous operation is added, which supplies the nuclei for the precipitation.

LIME-SODA PROCESS Basic principle:

• Lime [Ca(OH)₂] and soda [Na₂CO₃] are the reagents used to precipitate the dissolved salts of Ca⁺²and Mg⁺² as CaCO₃ and Mg(OH)₂.

• The precipitated CaCO₃ and Mg(OH)₂ are filtered off.

•Lime reacts with temporary hardness, CO₂, acids, bicarbonates and alums.

•Lime cannot remove the calcium permanent hardness which should be removed by soda.

•The precipitation reactions with lime and soda are very slow.

•Only calculated amounts of lime and soda are to be added. Excess amount of lime &

soda causes boiler troubles like caustic embrittlement.

Treatment with lime and Soda

1. Removal of temporary hardness of Ca⁺²

Ca(HCO₃)₂ + Ca(OH)₂ 2CaCO₃ + 2H₂O L 2. Removal of Temporary hardness of Mg²⁺

Mg(HCO₃)₂ + 2Ca(OH)₂ Mg(OH)₂ + 2CaCO₃ + 2H₂O 2L 3. Removal of Permanent hardness of Mg²⁺

MgCl₂ + Ca(OH)₂ Mg(OH)₂ + CaCl₂

MgSO₄ + Ca(OH)₂ Mg(OH)₂ + CaSO4 L+S Mg(NO₃)₂ + Ca(OH)2 Mg(OH)₂ + Ca(NO₃)₂

4. Removal of CO₂

CO₂ + Ca(OH)₂ CaCO₃ + H₂O L

5. Removal of acids

2HCl + Ca(OH)₂ CaCl₂ + 2H₂O L+S H₂SO₄ + Ca(OH)₂ CaSO₄ + 2H₂O

6. Removal of bicarbonates of Na+ and K+

2NaHCO₃ + Ca(OH)₂ CaCO₃ + Na₂CO₃ + 2H₂O L-S 2KHCO₃ + Ca(OH)₂ 2CaCO₃ + K₂CO₃ + 2H₂O

7. Removal of alums

FeSO₄ + Ca(OH)₂ CaSO₄ + Fe(OH)₂

Al₂(SO₄)₃ + 3Ca(OH)₂ 3CaSO₄ + 2Al(OH)3 L+S NaAlO₂ + 2H₂O Al(OH)₃ + NaOH -L

2NaOH + CaCl₂ Ca(OH)₂ + 2NaCl (2 NaOH is eq to Ca(OH)₂ Treatment with soda:

Removal of Permanent hardness of Ca²⁺

CaCl₂ + Na₂CO₃ CaCO₃ + 2NaCl CaSO₄ + Na₂CO₃ CaCO₃ + Na₂SO₄ Ca(NO₃)₂ + Na₂CO₃ CaCO₃ + 2NaNO₃

Desalination

•Processes that remove some amount of salt and other minerals from Water.

•Brackish Water: contain 3.5 % salts & salty taste. Unfit for drinking

•Separation of salts from water by evaporation followed by condensation.

•Freezing method can also be employed

•Commonly used methods:

1.Electrodialysis 2.Reverse Osmosis

A. Electrodialysis:

•Method of separation of ions from salt water by passing electric current.

•Semi permeable membranes are placed

•Consist of three compartments containing

1. Sea Water 2. Pair of electrodes 3. Semi permeable membrane

•As current applied Na ions moves towards cathode and Cl moves towards anode

•As result brine concentration decreases in the middle compartment.

•Pure water is removed from the central compartment.

•Conc. Brines are replaced by fresh brine water.

•Much more effective separation Ion selective membranes are used

3. Mixed Bed Deionization

•This equipment consist of single column which contain mixture of cation and anion exchangers.

•When water pass through bed, it comes in contact several times with both exchangers

•Resultant water contain lesser amount of salt.

Regeneration: by back wash a) anions with dil. NaOH

b) cations with dil. H₂SO₄

Advantages:

•More convenient and more effective

Disadvantages:

•Regeneration cost is more

•Equipment cost is high

Reverse Osmosis (or) Super Filtration (or) Hyper Filtration

•Movement of solvent molecule from concentrated side to diluted side.

•If only pressure higher than that of osmotic pressure

Membranes:

Cellulose Acetate, Polymide, Polymethylmethaacrylate Thickness:

0.0005 to 0.0000002 µm

•Advantages:

1.Removes both ionic and non ionic and colloidal matters 2.Maintenance cost is low

3.Membrane replacement can be done with in few minutes.

Domestic Treatment of Water A.Removal of Suspended Impurities

B.Removal of Micro organisms

Removal of Suspended Impurities 1.Screening:

Floating matters are removed by passing through screens.

2. Sedimentation:

-Big tanks are used -Left undisturbed

-Water settle down due to gravity.

-Supernatent water is drawn with the help of pump -Takes 2-6 hours

3. Filtration:

Sand Filters are used.

4. Sedimentation with Coagulation:

-Addition of Chemicals (Coagulants) to water. E.g. Alum (Potash Alum or Ferric Alum), Sodium Aluminate (NaAlO₂) ,Ferrous Sulphate (FeSO₄)

-On addition they form an insoluble gelatinous, flocculent precipitate which absorbs very fine particles and form bigger flocs.

-Due to this tiny particles which have no charge also come closers.

B. Removal of Micro Organisms 1.Boiling

2.Adding Bleaching Powder:

-1 kg for 1000 KL

-Mixed and Allowed to stand several hours CaOCl₂ + H₂O → Ca (OH)₂ + Cl₂

Cl₂ + HO → HOCl (Hypochlorous Acid) + HCl HOCl + Germs → Germs Killed

HOCl → HCl+ [O] (Nascent Oxygen) [O] + Germs → Germs Killed

Disadvantages:

-Excess addition give bad odour and taste.

-Introduces Ca into water and makes it hard -Disintegrates on storing

3. By Chlorination:

-Chlorine produces Hypochlorous Acid.

-Hypochlorous acid is strong Germicide.

Cl₂ + HO → HOCl (Hypochlorous Acid) + HCl HOCl + Germs → Germs Killed

-Cl may be used in Gaseous or Concentrated Form.

-Filtered water with 0.3-0.5 ppm Cl is sufficient.

-Lower pH is favourable (5-6.5)

Advantages:

1.Effective and Economical

2.Stable and does not deteriorate on storage 3.No impurities such as salts are introduced.

4.Can be used at high as well as low temp.

Disadvantages

1.Excess chlorination produces unpleasant taste. (0.1-0.2 ppm only) 2.More effective at low pH.

Breakpoint Chlorination:

-The addition of chlorine in such amount that it Oxidizes the organic matter, reducing matters and free ammonia in raw water. And leaves free residual Cl which disinfect Water.

Advantages

1.Oxidizes organic compounds, reducing substances and ammonia.

2.Removes colour and taste.

3.Kill germs.

- Excess of Declorination can be done by 1.Filtering through bed of molecular carbon 2.Addition of SO₂ and Na₂SO₃

SO₂ + Cl₂ + 2H₂O → H ₂SO₄+ 2HCl Na₂SO₃ + Cl₂ + H ₂O → Na₂SO₄ + 2HCl

4. By chloramine:

- Using 2 : 3 ratio of Cl₃ and NH₃

ClNH₂+ H₂O → HOCl + NH₃ HOCl + Germs → Germs Killed 5. By Ozonization:

-Ozone and raw water are allowed to come in contact with each other -10 – 15 min. in 2-8 ppm

3O2 → 2O₃ (Highly Unstable) O3 → O₂ + [O]

[O] + Germs → Germs Killed

Advantage: 1. Leaves no residue. Because of unstability 2. Removes odour, colour, Taste.

Disadvantage: Very Expensive 6. By Ultraviolet Radiation:

- Using Electric mercury vapour lamp

International Standards for water -Depends on purpose

-Standards Set By:

1.WHO- World Health Organization 2.USPHD- United States Health Service

3.ICMR- Indian Council of Medical Research

Water should be free from 1.Turbidity, Colour

2.Taste, Odour 3.Microbes

4.Toxicity (Organic and Inorganic Metals)