Water Technology

Objectives

Characteristics of Water

Hardness of water

Estimation hardness in Water

Alkalinity of water

Estimation of alkalinity of water

Water : An Engineering materialWater : An Engineering material

� As a transporter of energy: in steam As a transporter of energy: in steam raising plant (boiler), cooling raising plant (boiler), cooling systems and heating circuits systems and heating circuits (Coolant). (Coolant).

� As an integral part of a process: in As an integral part of a process: in brewing and soft drinks brewing and soft drinks manufacture, chemicals production manufacture, chemicals production and the food industry and the food industry

� As a generator of power: As a generator of power: Hydroelectric powerHydroelectric power

Uses of Water

Consumptive Uses

• Domestic

• Agriculture

• Industry

• Power generation: Hydroelectric power

Non-consumptive uses: Recreation...

Sources of water

Surface Water: not safe for consumption

Underground water

Rain

� Purest form of natural water

�Contains dissolved ind. gases, solid (org, inorg)suspended particles

River

�Fed by rain and spring

�Contains dissolved and suspended (org, inorg) impurities

Lake

�Contains less amount of dissolved minerals

�High quantity of org. matter

Sea

�Most impure form

�Dissolved salt (3.5%)

�Suspended matter (org)

Spring and well

�Rain water penetrates into the earth

�Due to filtering action of the soil it appears clear

�Contain dissolved salts

�High organic purity

Impurities Present in waterImpurities Present in water

Dissolved impurities Suspended Impurities Bacterial impurities

Inorganic salts

Cations: Ca, Mg, Na, K, Fe, Al, Trace of Zn, Cu

Anions Cl,SO4, NO3, HCO3,F, NO2

Gases

CO2, O2, oxides of N, NH3, H2S

Organic salts

humic acid, fulvic acid, tannins.

Inorganic Clay and sand

Organic Oil, vegetable and animal matter

pathogenic bacteria

Bacteria, other micro organism, bacteria, algae, fungi

Characteristics of waterCharacteristics of water� ColourlessColourless� OdourlessOdourless� TastelessTasteless� Universal solventUniversal solvent� High specific heat capacity:High specific heat capacity: It absorbs large amount of It absorbs large amount of heat energy before it begins to get hot and it releases this heat energy before it begins to get hot and it releases this energy when cool down. So it regulates earth’s climate and energy when cool down. So it regulates earth’s climate and body temp of organismbody temp of organism

� The solid form of water, ice, is less dense than its liquid form.The solid form of water, ice, is less dense than its liquid form.� HardnessHardness: Soap consuming capacity of water. It prevents : Soap consuming capacity of water. It prevents lather formation with soap.lather formation with soap.

� AlkalinityAlkalinity: Buffering capacity of water: Buffering capacity of water� pH:pH: Pure water is neutral but rain water is acidic due to Pure water is neutral but rain water is acidic due to dissolved oxides of C, S and Ndissolved oxides of C, S and N

� Dissolved OxygenDissolved Oxygen: Important for aquatic life: Important for aquatic life� Chloride contentChloride content: Over 250ppm impart peculiar taste. : Over 250ppm impart peculiar taste.

Classification of Water1) Hard Water

• Water which does not produce lather with soap solution, but produces white precipitate is called hard water.

• This is mainly due to the presence of calcium and magnesium salts.

2C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl

2) Soft Water

• Water, which produces lather readily with soap solution is called Soft water.

• This is due to the absence of calcium and magnesium salts

Hardness of water • Soap consuming capacity of water

• Dissolved mineral salts result in hardness of water

• It prevents lathering of soap due to the presence salts of calcium (Ca2+), magnesium (Mg2+) ions and other metals(Fe,Zn, Cu,Al) and other dissolved compounds such as bicarbonates, sulfates,chlorides and silicates.

• Calcium usually enters the water as either calcium carbonate (CaCO3), in the form of limestone and chalk, or calcium sulfate (CaSO4), in the form of other mineral deposits.

• The predominant source of magnesium is dolomite (CaMg(CO3)2).

• Ground water contains more mineral salts than surface water and its hardness is higher than surface water

Types of Hardness

• Hard water is categorized by the ions found in the water. They are‘Temporary' and ‘Permanent' hard water.

1)Temporary Hardness or Carbonate Hardness (CH) or Alkaline Hardness

• This is due to the presence of bicarbonates of calcium, magnesium, Iron and other metals.

It can be removed by

• boiling the water

• adding lime to the water

The bicarbonates are converted into insoluble carbonates and hydroxides, which can be removed by filtering.

Ca (HCO3)2 CaCO3 ↓ + H2O + 2 CO2

Mg (HCO3)2 Mg(OH)2 + 2CO2

contd----

2) Permanent hardness or Non-carbonate Hardness (NCH) or Non-alkaline Hardness:

• Permanent hardness is hardness (mineral content) that cannot be removed by boiling.

• It is usually caused by the presence of

calcium,magnesium, iron and other metal sulfates

and/or chlorides in the water, which become more soluble as the temperature rises.

• Despite the name, permanent hardness can be either

removed or controlled by the addition of chemicals

(Internal Treatment) and by large-scale softening with

zeolite and ion exchange resins (External Conditioning).

Expression of Hardness

Hardness is expressed in terms of CaCO3 Equivalents because

• It is most insoluble salt obtained in water treatment

• Its molecular weight is 100

The equivalent of calcium carbonate =

Mass of Hardness producing substance x Equivalent wt of CaCO3

• ---------------------------------------------------------------

• Equivalent wt of hardness producing substance

ESTIMATION OF HARDNESS BY EDTA:

• Ca and Mg ions form complex with the chelating agent, EDTA.

• EBT forms a Meta stable complex with the metal ions, which is wine red in colour. On treating the EBT complex with EDTA, a more stable metal –EDTA complex is formed setting the EBT free. This reaction proceeds with a colour change of wine red to steel blue.

Unit of hardness

1) Miilgrams per litre(mg/L) : No of mg of calcium carbonate

equivalent hardness per litre

2) Parts per million ( ppm): no of parts of CaCO3 equivalent hardness present per106 parts of water

3)Clark degrees (°Clark)/English degrees (°E)No of part of calcium carbonate in one Imperial gallon(70,000 parts) of water

4) French degrees (°Fr) No of part of calcium carbonate in 105 parts of water.

5) Milliequivalent per litre(meg/L): No of milli equivalents of hardness present per litre

1ppm =1mg/L =0.1oF =0.07oCl

Disadvantages of hard waterDisadvantages of hard waterI) I) DomesticDomestic� wastage of soapwastage of soap� produces“ soap scum" and 'bathtub ring' that are difficult to produces“ soap scum" and 'bathtub ring' that are difficult to

remove remove � Produces unsightly spots on dishes, glassware and flat ware Produces unsightly spots on dishes, glassware and flat ware � Increases the risk of diaper rash, rough, red hands, and skin Increases the risk of diaper rash, rough, red hands, and skin

irritation and discomfort. irritation and discomfort. � Reduces the taste of ice cubes and boiled foods such as vegetables Reduces the taste of ice cubes and boiled foods such as vegetables � Bad effect on our digestive system- forming Ca.oxalate in urinary Bad effect on our digestive system- forming Ca.oxalate in urinary

tracktrack

II: IndustrialII: Industrial� Textile: Precipitates of Ca and Mg salts adhere to the fabric. Fe Textile: Precipitates of Ca and Mg salts adhere to the fabric. Fe

and Mg salts produce coloured spots on fabricsand Mg salts produce coloured spots on fabrics� Sugar : Cause difficulties in crystallization of sugar due to the Sugar : Cause difficulties in crystallization of sugar due to the

presence of sulphates, nitrates, alkali carbonates, etc.,presence of sulphates, nitrates, alkali carbonates, etc.,� Dyeing: Dissolved Ca, Mg, Fe Salts react with costly dye, ppt and Dyeing: Dissolved Ca, Mg, Fe Salts react with costly dye, ppt and

give shades and spotsgive shades and spots� Paper: Ca and Mg salts react with chemicals which are empolyed Paper: Ca and Mg salts react with chemicals which are empolyed

to provide a smooth and glossy finish to paper. Fe salts affect the to provide a smooth and glossy finish to paper. Fe salts affect the colour of the papercolour of the paper

Disadvantages of hard waterDisadvantages of hard water� Laundry: Wastage of soaps and Fe salts cause coloration of Laundry: Wastage of soaps and Fe salts cause coloration of

clothcloth� Concrete: Chlorides and sulphates affect the hydration of Concrete: Chlorides and sulphates affect the hydration of

cement and final strength of hardened concretecement and final strength of hardened concrete� Pharmaceutical: Undesirable pdtPharmaceutical: Undesirable pdt� Electrical appliances : Formation of scale that clogs plumbing Electrical appliances : Formation of scale that clogs plumbing

and builds up on the inside of water heaters washing and builds up on the inside of water heaters washing machines, dishwashers, etc., which can increase gas or machines, dishwashers, etc., which can increase gas or electric bills, and may lead to early failure of the heater and electric bills, and may lead to early failure of the heater and decreases life expectancydecreases life expectancy

III) In BoilerIII) In Boiler� Priming and FoamingPriming and Foaming� Scale and sludge FormationScale and sludge Formation� CorrosionCorrosion� Caustic EmbrittlementCaustic Embrittlement

Alkalinity of waterAlkalinity of water► Buffering capacity of water-Buffering capacity of water- Ability of water to neutralize Ability of water to neutralize the acid without changing pH due to carbonate-bicarbonate the acid without changing pH due to carbonate-bicarbonate ions ions

► Soft water has little buffer capacity Soft water has little buffer capacity

Alkalinity is due to presence of Alkalinity is due to presence of

► Caustic alkalinity, OHCaustic alkalinity, OH--, CO, CO332-2-

► Temporary hardness HCOTemporary hardness HCO33--

Advantages of alkalinity of waterAdvantages of alkalinity of water::

Minimum quantity, less than 0.2ppm is Essential to absorb the Minimum quantity, less than 0.2ppm is Essential to absorb the excess H+ ions and protects water from pH fluctuationexcess H+ ions and protects water from pH fluctuation

DisadvantagesDisadvantages► Causes caustic embrittlement in boilerCauses caustic embrittlement in boiler► High content of alkalinity affects aquatic life High content of alkalinity affects aquatic life ► Leads to precipitation of sludge and scales in boilerLeads to precipitation of sludge and scales in boiler

Estimation of alkalinity of water

Acid base titration using phenolphthalein and methyl orange as indicators

OH- + H+ H2O

CO32- + H+ HCO3

-

HCO3- + H+ H2O + CO2

The possible combinations of ions causing alkalinity in water are► As single ions: OH- or CO3

2 - or HCO3-

► As double ions: OH- & CO32- or CO3

2- & HCO3-

The ions OH- & HCO3- cannot exist together because they combine

instantaneously to form CO32-

OH- + HCO3- CO3

2- + H2O

Significances of alkalinity estimation� To control corrosion� For calculating the amount of lime and soda required for softening of

water� To calculate the amount of coagulants like Al sulphate and Fe sulphate

to be added in internal conditioning

PM

Points to RememberPoints to Remember� Salts of Ca, Mg and other metals produce hardness in the waterSalts of Ca, Mg and other metals produce hardness in the water� Hardness prevents water from lather formationHardness prevents water from lather formation� Hard water is categorized by the ions found in the water. They Hard water is categorized by the ions found in the water. They

are‘Temporary' and ‘Permanent' hard water.are‘Temporary' and ‘Permanent' hard water.

1)Temporary Hardness or Carbonate Hardness (CH) or Alkaline 1)Temporary Hardness or Carbonate Hardness (CH) or Alkaline Hardness:Hardness: This is due to the presence of bicarbonates of calcium, This is due to the presence of bicarbonates of calcium, magnesium and other metals. It can be removed by boiling the magnesium and other metals. It can be removed by boiling the water or by adding lime to the waterwater or by adding lime to the water

2) Permanent hardness2) Permanent hardness or Non-carbonate Hardness (NCH) or Non-or Non-carbonate Hardness (NCH) or Non-alkaline Hardnessalkaline Hardness: It is usually caused by the presence of : It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water. calcium and magnesium sulfates and/or chlorides in the water. Permanent hardness can be either removed or controlled by the Permanent hardness can be either removed or controlled by the addition of chemicalsaddition of chemicals (Internal Treatment) (Internal Treatment) and by large-scale and by large-scale softening with zeolite and ion exchange resinssoftening with zeolite and ion exchange resins(External (External Conditioning).Conditioning).

� Hardness is expressed in terms of CaCO3 Equivalents: Hardness is expressed in terms of CaCO3 Equivalents: Commonly used unit is Parts per million ( ppm) Commonly used unit is Parts per million ( ppm)

� Hardness in water alters its properties and makes water Hardness in water alters its properties and makes water harmfulharmful

Boiler Troubles

� Boiler: Transport Heat Energy

� Hardness and other impurities in water: Trouble maker of boiler

� Trouble shooting of boiler

1. Turbine drive

for electric

generating

equipment,

blowers and

pumps

2. Process for

direct contact

with products,

direct contact

sterilization and

non contact for

processing

temperatures

3. Heating and air

conditioning for

comfort and

equipment

Steam Utilization

Problems due to hardness of water in the boiler

Boiler Feed water: Water fed into the boiler for the production of steam

• Priming and Foaming(Carryover): carrying of water by steam along with impurities.

• Priming: Formation of wet steam in the form of spray into the steam outlet. Formation of stable bubbles above the surface of water is called foaming.

• Scale and sludge formation: Evaporation in a boiler causes salts to concentrate and they are precipitated. Soft loose and slimy ppt is called sludge while hard adherent ppt is called scale.

• Boiler Corrosion : Due to presence of dissolved oxygen, CO2, and acids from dissolved salts.

• Caustic embrittlement: Concentration cell corrosion due to variation in conc of NaOH at different parts of the boiler

Priming • When boiler is steaming (producing steam) rapidly, some

particle of the liquid water containing impurities are carried along with the steam

• The process ‘wet steam formation’ (steam containing droplets of water) is called Priming

Caused by

� Sudden boiling : Irregular firing of boiler

� High steam velocities

� Improper boiler design: Steaming above design capacity

� Sudden increase in steam production rate: Variable steam demand or water pressure

� Presence of large amount of non-scaling dissolved salts

Can be avoided by

� Fitting mechanical steam purifiers

� Avoiding rapid change in steaming rate

� Maintaining low water levels in boilers

� Efficient softening and filtration of boiler feed water

Foaming• Chemical vaporous carry-over

• Formation of stable foam or bubbles in boilers which do not break easily

Due to

• Presence of substances like oil, grease which greatly reduce the surface tension of water in boiler water

• High suspended solids

• Other organic impurities

Can be avoided by

�Adding anti-foaming chemicals like castor oil

�Removing oil from water by adding compound like sod. aluminates

Disadvantages of Priming and Foaming

• They occur together

• They are very harmful because

1) Dissolved salts in boiler water are carried by wet steam to the other parts of the machines (heaters and turbine blades) where they get deposited and hence reduce the efficiency.

2) Dissolved salts enter other parts of machinery where steam is being used thereby decreasing the life of machinery

3) Actual height of the water column cannot be judged properly, thereby making the maintenance of boiler pressure becomes difficult

Sludge Formation• The conc. of dissolved salts increases progressively as water

evaporates in the boiler

• The salts start separating out from the water in the order of their saturation point

• Salts like CaCl2, MgSO4 separates in the body of the liquid in the form of soft deposit which can be flushed out easily

• Such soft, slimy and non-adherent deposits are called sludge

• They form at comparatively colder portions of the boiler where the flow rate is slow ex., at bends contd----

Disadvantages and prevention of Sludge formation

• They are poor conductor of heat so they tend to waste a portion of heat generated

• If sludge forms along with scale, then former gets entrapped in the latter and both get deposited as scales

• Excessive sludge settles in the regions of poor water circulation such as pipe-connection, plug-opening thereby causing even choking of pipes

Prevention

• Frequent ‘blow –down operation’ : Partial removal of hard water through tap at the bottom of the boiler when the extent of boiler is so high and addition of fresh softened water to boiler after blow down operation

• Softened water

Scale formation

• Scale is formed by salts like CaSO4, CaCO3, Mg(OH)2, CaSiO3/ MgSiO3 that have limited solubility but are not totally insoluble in boiler water

• These salts reach the deposit site in a soluble form and precipitate in the form of hard adherent deposits on the inner walls of the boiler. They are called scale.

• Scales are so hard so that it is difficult to remove them even with the help of hammer and chisel

They are formed by

• CaCO3 deposit by decomposition of Ca. bicarbonate in low pressure boiler

Ca(HCO3)2 CaCO3 + H2O+ CO2

2) Deposition of calcium sulphate as such in the high pressure boiler

8) Mg(OH)2 deposit by hydrolysis of Mg. salts-

MgCl2 +2H2O Mg(OH)2 + HCl

4) Presence of silica: Even present in small quantities it is ppted as CaSiO3/ MgSiO3. They are very difficult to remove

contd---

Disadvantages of Scale Formation

• Wastage of fuel: Scaled are poor thermal conductor. In order to provide a steady supply of heat to water, excessive or over heating is done and this causes increase in fuel consumption

• Lowering of boiler safety: because of over heating, the boiler material become softer and weaker and makes boiler unsafe to bear the pressure of the steam especially at high pressure

• Decrease in efficiency: Scales may deposit in the valves and condenser of the boiler and choke them partially. This result in decrease in efficiency of the boiler

• Danger of Explosion: When thick scale crack down due to uneven expansion, the water comes suddenly in contact with over heated iron plates. This causes in formation of large amount of steam steadily. The sudden high pressure even cause the explosion of the boiler

contd-----

Removal of Scales

• Physical Method: With the help of scraper or piece of wood or wire brush if they are loosely adhering

• Thermal shocks: Heating the boiler and then suddenly cool with cold water if the scale is brittle

• Chemical method: By dissolving them by adding chemicals if they are adherent and hard

CaCO3 5-10% HCl

CaSO4 EDTA form complex

• Mechanical: By frequent blow down operation

Caustic Embrittlement

• Specific form of stress corrosion, results in the irregular inter crystalline cracking of steel.

• Combination of high stress and high caustic concentrations which eventually led to destructive cracking of the boiler vessel

• During softening process by lime-soda addition excess Na2CO3 is usually added which results in free Na2CO3 in the boiler

• In high press boiler, it decomposes to form NaOH which makes the boiler water caustic

• Na2CO3 + H2O 2NaOH + CO2

• This alkaline water flows into the minute hair-cracks always present in the inner side of the boiler by capillary action

• When water evaporates the conc of dissolved caustic soda conc increases progressively

• It creates conc cells and iron parts which are under stress such as riveted seams, bends and joints act as an anode and gets dissolved to form sod. Ferroate,Na2FeO3

• This causes embrittlement of the boiler parts particularly at stressed parts.(+)Fe at rivets, bends, joints|DilNaOH||Conc NaOH| (-)Fe at plain (under

stress) surface

Contd------

Caustic Embrittlement

Can be avoided by

• Using sod. Phosphate as softening agent instead of sod. Carbonate

• Adding tannin or lignin as these blocks the hair cracks thereby preventing infiltration of caustic soda solution

• Adding NaSO4 as it also blocks hair-cracks, thereby preventing infiltration of caustic soda solutions.

Boiler Corrosion

• Decay of boiler material by chemical or electrochemical attack of its environment

• Main corrosive agents are

1)Dissolved Oxygen

2)Dissolved Carbon dioxide

3)Acids from dissolved salts

contd---

1) Dissolved Oxygen• Water contains about8ml/lt of dissolved O2

• At high temp it leads to rust formation-Mainly pitting type corrosion

• 2Fe+ 2H2O +O2 2Fe(OH)2

• 4Fe(OH)2 +O2 2Fe2O3.2H2O

Removal of dissolved O2

� By Chemical means: By adding calculated quantity of Sod.sulphide or hydrazine or sod. Sulphite which involves addition of chemicals, capable of combining rapidly with the oxygen in aqueous solution.

• 2 Na2SO3+ O2 2 Na2SO4

• N2H4+ O2 N2+ 2H2O

• Na2S + 2O2 Na2SO4

• Hydrazine hydrate is ideal because the reaction products are N2 and water.

• Sod. Sulphide and sulphite form sod.sulphate which are liable to decompose giving SO2

By mechanical deaeration:

Deareater: Spraying water in a perforated plate-fitted tower heated from sides and connected to vacuum pump

High temp, low press and large surface area reduces the dissolved O2 in water

Deaerater Mechanical deaeration: Eliminating oxygen

and other corrosive gases like CO2, NH3from

water

•Gas removal is accomplished by spraying cold

makeup water into a steam environment.

•Heaters in the deaerater release dissolved

gases in the incoming water by reducing it to a

fine spray as it cascades over several rows of

trays.

•The steam that makes intimate contact with

the water droplets then scrubs the dissolved

gases by its counter-current flow.

•The steam heats the water to within 3-5 º F of

the steam saturation temperature and it should

remove all but the very last traces of oxygen.

•The deaerated water then falls to the storage

space below, where a steam blanket protects it

from recontamination.

2) Dissolved Carbon Dioxide

• CO2 +H2O HCO3(carbonic acid)

• It has slow corrosive effect on the boiler material

Generation of CO2: CO2 is released inside the boiler if the water used for steam generation contains bicarbonate

• Mg(HCO3)2 MgCO3 +H2O + CO2

Removal of CO2

1)By adding calculated amount of NH3

2NH4OH + CO2 ( NH4)CO3 +H2O

2)By mechanical deaeration process along with O2

3) Acids From dissolved salts

• Water containing dissolved Mg salts liberate acids on hydrolysis

• MgCl2 +2 H2O Mg(OH)2 +2 HCl

• The liberated acid reacts with Fe in chain like reactions producing HCl again and again

• Fe + 2HCl FeCl2 +H2

• FeCl2 +2H2O Fe(OH)2 +2HCl

• Even the presence of small amount of MgCl2

will cause corrosion of Fe to a large extent

Boiler Corrosion

Requirements of Boiler Feed water

1) Boiler feed water should be free from

b) Turbidity, oil and non-scaling dissolved salts to reduce the tendency for priming and foaming

c) Hardness causing and scale forming constituents like Ca, Mg and other metal salts as the formation of scales would result in wastage of fuel, loss in out put and over heating of the boiler tubes leading to explosion

d) Dissolved O2 and CO2 in order to prevent corrosion in the boiler

e) Caustic alkali to remove caustic embrittlement

2)The limit of tolerance for boiler feed water

Maximum hardness < 20 ppm

Maximum TDS ----150-3500 ppm

Maximum Alkalinity ---- 100-700 ppm

TDS: Total dissolved salt

Boiler water Treatment

• Permanent Hardness can be removed!

• How?

Boiler Water TreatmentPermanent hardness and other impurities can be either removed or controlled by

External Conditioning: Ex-situ method: Removal of hardness producing salts from the water before feeding into boiler

Internal Treatment: In –Situ method : By adding chemicals directly in the boiler to change the character of the ion by complexing it into other more soluble salt

External Conditioning (Softening of Water)

• Removal of hardness producing salts from the water before feeding into boiler. The external treatment can be done by the following methods.

1)Lime Soda process: Insoluble Ca and Mg salts in water are chemically converted into soluble compds by adding calculated amts of lime,Ca(OH)2 and soda Na2CO3

CaCO3 and Mg(OH)2 so ppted are filtered off

2) Zeolite Process: Zeolite is hydrated Na.Al.silicate(Na2O.Al2O3.xSiO2.yH2O) capable of exchanging reversibly its Na ions for hardness producing metal ions in water

3) Ion Exchange Process / Demineralization process / de-ionisation process:

� The process removes all the anions and cations present in the hard water.

� Ion Exchange resins are insoluble cross linked long chain macro polymer with micro porous structure and the functional groups attached to the chains are responsible for the ion exchanging properties.

ION-EXCHANGE PROCESS (ADSORPTION)-Deionisation-dimineralisation of water

�The process of releasing the cation or anion and adsorbing another like ion

�Synthetic resins are used as ion-exchange resin or ion-exchanger in ion-exchange process

�Insoluble cross-linked, long-chain org polymers with the functional gp responsible for ion-exchanging properties

�They posses one adsorbed ion on it

�They release this ion and adsorbs another like ion

TYPES

�Cation exchange resin (R -H +):with acidic functional gp(-COOH, -SO3H, etc) ex. Styrene-divinyl benzene

�Anion Exchange Resin (R+OH-)With basic functional gps(-NH2, =NH as hydrochloride)

•Styrene-divinyl benzene contain amino or quaternary ammonium or quarternary phosphonium or sulphonium gp as an integral part of the resin matrix

Cation Exchanger• Capable of exchanging their cation (H+) with other cations present in the

water.

• Cation exchange resin is represented as R-H+

• Resins containing acidic fn group(- COO-H+, -SO3-H+) are capable of exchanging their H+ ions with other cations of hard water.

• Mainly styrene (divinyl) benzene copolymers which on sulphonation or carboxylation become cabable to exchange their hydrogen ions with cations in water

Process:

• The hard water is first passed through a cation exchange column, which removes all the cations like Calcium, Magnesium from it and equivalent amount of H+ ions are released.

• M+ + R-H+ RM + H+

• CaCl2 +2 RH R2Ca + 2HCl

• NaCl + RH RNa + HCl

Regeneration: when the cation exchange resin is exhausted, it can be regenerated by passing a solution of dil. HCl or H2SO4:

RNa + HCl RH + NaCl

Anion Exchanger

• Resins containing basic functional groups (▬ NH2 ) or quaternary ammonium groups are capable of exchanging their OH- ions with other anions of hard water.

• Anion exchange resin is represented as R+OH-

• Anion exchange resin Styrene-divinyl benzene or amine-formaldehyde copolymers which contain amino or quarternary amm or quarternary phosphonium or quarternary sulphonium gp as integral part of resin matrix

• When water is then passed through an anion exchange column, which removes all the anions like chlorides, sulphates etc. and equivalent amount of OH- ions are released from the column.

• X- + ROH RX + OH -

• SO42- + 2ROH R2SO4 + 2OH-

Regeneration

The exhausted anion exchanger dil.NaOH can be used.

• RCl + NaOH ROH + NaCl

Net result on passing through both cation and anion exchanger: H+ + OH- H2O (Pure water)

Deionisation of water• As the metallic ions (cations) in the water affix themselves to the exchange

material, the latter releases its hydrogen ions on a chemically equivalent basis.

• Because of the relatively high concentration of hydrogen ions, the solution is very acid. deionization process is just half complete.

• While the positive metallic ions have been removed, the water now contains positive hydrogen ions, and the anions originally in the raw water.

• The partially treated water now flows through a second unit, this time an anion exchange material normally consists of replaceable hydroxyl anions and fixed irreplaceable cations.

• The negative ions in solution (the anions) are absorbed into the anion exchange material. Released in their place are hydroxyl anions.

• Contains the positive hydrogen ions released in the initial exchange plus the negative hydroxyl ions released in the second exchange.

• They have combined (positive to negative) to produce water molecules.

• H+ + OH- H2O

• The result of this two-stage ion exchange process is completely free from cations and anions. This water is known as Demineral water or deionised water.

Ion Exchange Process / Demineralization process / de-ionisation process

• Advantages:

• 1) Used to soften highly acidic or alkaline water

• 2) Produces water of very low hardness(2ppm)

Disadvantages:

1) The equipment is costly and expensive

2) If water contains turbidity, then the output of the process is reduced. The turbidity must be below 10ppm.If it is more it has to be removed by coagulation and filtration.

Internal Conditioning

• Internal Treatment or Sequestration or Boiler compounds

• Involves the removal of scale forming substances which are not completely removed by external treatment

• By adding chemicals directly in the boiler to change the character of the ion by complexing it into other more soluble salt

It is achieved by• Converting scale forming impurities into sludge forming ppt which

can be removed by blow down operation

• Converting scale forming impurities into compd which will stay in dissolved form in water and thus do not cause any harm

• Contd-----

Internal Conditioning Methods

• Colloidal Conditioning

• Phosphate conditioning

• Carbonate conditioning

• Calgon conditioning

• Treatment with Coagulant (sod.aluminate)

• !) Colloidal conditioning:

� Scale formation can be avoided by adding sub like kerosene, tanin, agar-agar

� These substances get coated over by the scale forming ppt, thereby yielding non-sticky, loose deposit which can be removed easily by blow-down operation

� Applicable for low press boiler

2) Phosphate Conditioning: � In high press boiler, scale formation can be avoided by adding sod. Phosphate

� It reacts with hardness of water, forming non-adherent and easily removable soft sludge of Ca and Mg which can be removed by blow down operation

� 3CaCO3(Scale) +2Na3PO4 Ca3(PO4)2(sludge) +3 Na2CO3

� 3CaCl2(Scale) + 2Na3PO4 Ca3(PO4)2 ↓(sludge) + 6Cl▬

� The choice of salt depends upon the alkalinity of boiler feed water. The main phosphate employed are

� Acidic: Used for boiler water with too high alkalinity, Sod.hydrogen phosphate NaH2PO4

� Weakly Alkaline: If the boiler water’s alkalinity is moderate, Na2HPO4

� Alkaine: If the alkalinity of boiler water is low, Na3PO4

� Ca cannot be ppted properly below the pH of 9.5 so select a phosphate that adjust pH to optimum value (9.5-10.5)

� Contd----

3) Carbonate Conditioning• In low press boiler, scale formation can be avoided by adding sod.

Carbonate to the boiler water, permanent hardness is converted into temporary hardness

• CaSO4(per)+ Na2CO3 CaCO3 (temp)+ Na2SO4

• CaCO3 is ppted as loose sludge which can be removed by blow-down

operation

4) Calgon Conditioning Involves the addition of calgon, Sod.hexa meta Phosphate (NaPO3)6 or

Na2[Na4(PO3)6]

It prevents the scale and sludge formation by forming soluble compds with CaSO4

• Na2[Na4(PO3)6] ↔ 2Na+ + [Na4P6O18] 2-

• 2CaSO4(scale) + [Na4P6O18]2-

[Ca2P6O18] 2-(aq) + 2Na2SO4Contd-----

5) Treatment with Coagulant (Sod.aluminate ,NaAlO2)

� Used to remove colloidal impurities including oil drops and silica

� Sod.aluminate reacts with water forms gelatinous ppt and NaOH.

� NaAlO2 +2H2O NaOH + Al(OH)3 (gel)

� NaOH+ 2MgCl2 Mg(OH)2(gel) +2NaCl

� The flocculant ppt of Mg(OH)2 and Al(OH)3 produced inside the boiler entraps finely suspended and colloidal impurities including oil drops and silica

� The loose ppt can be removed by pre-determined blow-down operation

Points to remember� Mineral salts and other impurities present in the water are the trouble makers of the

boiler.

� They can be either removed or controlled by

1)External Conditioning: Removal of hardness producing salts from the water before feeding into boiler

Ion Exchange Process / Demineralization process / de-ionisation process:

� The process removes all the anions and cations present by adsorbing them into the ion exchange material.

• Cation exchanger adsorbs cation and releases the positive hydrogen ions while the Anion exchanger releases negative hydroxyl ions by exchanging it with other anions present in the water

• Positive hydrogen ions and negative hydroxyl ions released as a result of this two-stage ion exchange process combine to produce water molecules.

• This water is known as Demineral water or deionised water.

• In mixed-bed deionisers: The cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel so that the whole bed can be regard as an infinite number of anion and cation exchangers in.

• Electrical demineralisation of waterThe rate of diffusion of ion is increased by passing electric current through the electrode placed near , ion-selective membranes membrane

2)Internal Treatment: In –Situ method : By adding chemicals directly in the boiler to change the character of the ion by complexing it into other more soluble salt

It is achieved by

• Converting scale forming impurities into sludge forming ppt which can be removed by blow down operation

• Converting scale forming impurities into compd which will stay in dissolved form in water and thus do not cause any harm

1)Colloidal conditioning: Org sub like kerosene tanin, etc., get coated over the scale forming ppt, thereby yielding non-sticky, loose deposit

2) Phosphate Conditioning: Scales are converted into non-adherent and easily removable soft sludge of Ca and Mg by sod.phosphate

The choice of salt depends upon the alkalinity of boiler feed water.

3) Carbonate Conditioning: Permanent hardness is converted into temporary hardness by adding Na2CO3

4) Calgon Conditioning: Prevents the scale and sludge formation by converting them into soluble compds with calgon (sod.hexa meta phosphate)

5)Treatment with Sod.aluminate (NaAlO2); Sod.aluminate reacts with water and forms flocculant ppt of Mg(OH)2 and Al(OH)3 which entraps finely suspended and colloidal impurities including oil drops and silica

Desalination

• Only about 1% of world’s water resources can be used by man

• How can the large portion(97%) of water locked in the ocean be used to over come the water scarcity of the world

• What are the economically viable techniques used to purify the water

Desalination of Brackish water(Sea Water)

• Brackish water: The water containing salt with peculiar salty (or brackish) taste

• Sea water containing average about 3.5% salt comes under this category

• The process of removing common salt (NaCl) and other salt from water is known as desalination

• Sea water can be converted into fresh water by

1) Membrane Distillation

2) Electro dialysis

3) Reverse Osmosis

Reverse Osmosis

Reverse Osmosis• If two solutions of different concentration are separated by a semi-permeable

membrane which is permeable to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis.

• When a semi permeable membrane separates a dilute solution from a concentrated solution, solvent crosses from the dilute to the concentrated side of the membrane in an attempt to equalize concentrations.

• Reverse osmosis is a membrane separation process for removing solvent from a solution in which solvent will flow from the concentrated to the dilute solution. Pure solvent is separated from its contaminates rather than contaminates from the water

• It is done by applying an opposing hydrostatic pressure to the concentrated solution.

• The magnitude of the pressure required to completely impede the flow of solvent due to osmosis is defined as the "osmotic pressure".

• If the applied hydrostatic pressure exceeds the osmotic pressure flow of solvent will be reversed, that is, solvent will flow from the concentrated to the dilute solution.

• Also called ’Super filtration’ or ‘Hyper filtration’

Procedure• The salt water is pressurized(15-40kg/cm2) on

one side of a semi permeable membrane. The

pressure must be high enough to exceed the

osmotic pressure to cause reverse osmotic flow

of water.

• The membrane is highly permeable to water,

but essentially impermeable to dissolved

solutes, pure water crosses the membrane

• As pure water crosses the membrane, the

concentration of dissolved impurities increases

in the remaining sea water and, as a

consequence, the osmotic pressure increases.

• A point is reached at which the applied

pressure is no longer able to overcome the

osmotic pressure and no further flow of

product water occurs. The membrane becomes

fouled by precipitated

• Membrane consists of very thin films of

cellulose acetate, metha acrylate or polyamide

polymers

Advantages of reverse osmosis

• Removing ionic as well as non ionic colloidal and high mol. Wt org matter

• Removes colloidal silica which is not removed by demineralization

• The maintenance cost is entirely on the replacement of semi permeable membrane

• The life time of membrane is quite high, 2yrs

• Replacement of membrane takes few min thereby providing nearly uninterrupted water supply

• Conversion of sea water into drinking water and for boiler

Drinking water in India

• Microbial contamination of the water supply has led to outbreaks of disease, causing illness and even death.

• How safe is our water?

• How is water treated by Municipality and what’s being done to make it safe?

Requisites of Drinking Water

Drinking or potable water should be

• Clear, colorless and odorless

• Pleasant in taste

• Perfectly cool

• Reasonably soft

• turbidity not > 10 ppm

• Free from objectionable minerals & dissolved gases

• pH ~ 7.5 – 8.0

• TDS < 500 ppm

• Free from disease producing microorganisms

• Free from Pb, As, Cr, Mn, Fe etc.

Where does the water come from for Domestic uses?

– surface waters (lakes, rivers, and reservoirs)

– groundwater (wells).

Domestic water treatment Municipal water

Treatment Objectives �Sources: Lakes and rivers- do not meet all the required specifications of drinking water-contain suspended inorg. and org particles and micro-org�Render water free from these disease producing organisms and toxic substances. �Make it free from color, turbidity, taste, odors�Does not aim at removing all dissolved salt: Does not supply softened water

Process Flow Process Flow

ChartChartRaw WaterRaw Water

ScreeningScreening

sedimentationsedimentation

Sedimentation with CoagulationSedimentation with Coagulation

FiltrationFiltration

DisinfectionDisinfection

Drinking WaterDrinking Water

1) Removal of Removal of

suspended suspended

ImpuritiesImpurities

2) Removal of

micro organism

Drinking water treatment plant

Drinking water treatment plant

1. screening 2.Sedimentation

1.Screening

• To remove floating materials like wood pieces , leaves etc

• Water passed thro’ bar screens to hold back floating matter

2.sedimentation

• In grit chambers & settling tanks

• Gravity pulls the heavy particles to the bottom of the tank & the water is sent to settling tank

2) Sedimentation: Allow water to stand undisturbed in big, deep

tanks. The retention period ranges from 2-6hrs

WATER TOWARDS FILTRATION

Coagulated particles fall, by gravity, through water ina settling tank and accumulate at the bottom of thetank, clearing the water of much of the solid debrisand clear water moves to filtration.

WATER FROM

COAGULATION

Sedimentation

3. Coagulation ( Flocculation) and sedimentation

o Coagulation removes fine suspended and colloidal impurities and other particles suspended in water.

o Alum and other chemicals are added to water to form tiny sticky particles called “floc”“floc”“floc”“floc” which attract the dirt particles.

• Coagulant like alum , or Fe.sulphate provide Al3+ or Fe3+ ion which neutralize the –ve charge on the clay(SiO3)

2- particle

and bring tiny particles together to form bigger particleso The combined weight of the dirt and the alums (floc)

becomes heavy enough to sink to the bottom during sedimentation.

WATER TOWARDS

SEDIMENTATION

Sludge disposal

Coagulation - Reactions

1) Using Alum [K2 SO4 . Al2(SO4)3 . 24 H2O]

Al2(SO4)3 + Ca (HCO3)2 2Al(OH)3 + Ca SO4+ 6CO2 coagulant water >pH 7 ppt 2)Using Sodium Aluminate(NaAlO2)

NaAlO2 + 2H2O Al(OH)3 + NaOH

water <pH 7 gelt . Floc

NaOH + MgSO4 Mg(OH)2 + Na2SO4

ppt

3) Copperas or Ferrous sulphate(FeSO4.7H2O) FeSO4 + Mg(HCO3)2 Fe(OH)2 + MgSO4+ CO2 +H2O

water >pH 7

4Fe(OH)2 +O2 +2H2O 4Fe(OH)3(Heavy floc causes quick sedimentation)

4. FILTRATION: Process of removing colloidal matter and most of bacteria, micro-organism, etc., by passing water through a bed of fine sand and other proper sized granular materials

4)Filtration – Sand trickling filters

Water then flows through large dual media rapid sand filters

made up of layers of gravel, sand, and anthracite coal

Sand Filter

5. Disinfection: Removal of micro-organism

• Process of destroying/killing any disease causing bacteria and microbes to make water for safe

• The chemicals and sub which are used for disinfection are called disinfectant

Methods of disinfection

i) Boiling

ii) Using UV light

iii) Ozone

iv) Bleaching powder

v) Chlorination

vi) Chloramines

1) Boiling• Boil water for 10-15 min

Disadvantages

• Boiling alter the taste of drinking water

• Can kill only existing germ in water at the time of boiling, does not provide any protection against future possible contamination

• It is impossible to employ it in municipal water-works

ii) Using ultraviolet light

• Ultraviolet radiation lies between visible light and X-rays on the electromagnetic spectrum.

• UV light λ = 1000 – 4000A . UV light has a shorter wavelength than visible light

• Earth's atmosphere prevents most UV radiation from space from reaching the ground.

• UV-(short wave) is entirely screened out by

stratospheric ozone at around 35 km altitude. • It is germicidal at 200 and 280 nm. I t is used to

sterilize glassware used in medicine and biological

research.

ULTRAVIOLET RADIATION

SOURCE

�UV light is emitted as a result of

current flow through the mercury vapor

between the electrodes of the lamp.

�The most commonly used UV lamps

(low pressure mercury vapor) produce

the majority of their UV output at

253.7nm, a wavelength which is very

close to the 260 - 265nm wavelengths

which are most effective in killing

microbes.

�The germicidal lamp(200-280nm) is

constructed of UV transmitting quartz;

The advantages of UV disinfection

- disinfection without adding chemicals

- no change to the taste and no removal of beneficial minerals

- minimal maintenance

- immediate treatment without the need for holding tanks

- ideal compatibility with other technologies for complete solutions: carbon filtration, water softeners reverse osmosis

- low power consumption

iii) Using ozone

-OZONE is the Strongest oxidant/disinfectant available.

-More effective against microbes than chlorination

Ozone production

3O2 → 2O3 O2 + [O]

Silent electric

discharge unstable

Nascent oxygen

Kills bacterias &

oxidised org

substances

Advantages

- Does not leave by any residue

Removes simultaneously colour,

odour and taste

-Excess is not harmful

Demerits

- Expensive

- Unstable and can not be stored for

- long time

Unstable

Ozone steriliser

Disinfected water

Raw water

Ozone inlet

tank

Contact time : 10 – 15 mins

Dosage – 2-3 ppm

iv) Bleaching Powder

• About 1kg of bleaching powder per 1000 kilolitres of water is mixed

Reactions

CaOCl2 + H2O → Ca(OH)2 + Cl2

Cl2 + H2O → HCl + HOCl

kill germs by inactivating the enzymes essential for the metabolic

process of micro-organism

Demerits:

•Introduces calcium – hardness

•Deteriorates on storing

•Imparts bad taste /odor if it is added in excess

v) Using chloramines(ClNH2)

• Cl2 & NH3 in the ratio of 2:1

Cl2 + NH3 → ClNH2 + HCl

ClNH2 + H2O → HOCl + NH3

It is preferred because

• Much more lasting than chlorine alone

• Does not produce irritating odor

• Imparts good taste

-

- Inactivates the enzymes essential for the

metabolic process of micro-organism and

and kill them (bacteria and germs)

vi) Chlorination

• Cl2 + H2O HCl + HOCl

• Gas / liquid

-

- Inactivates the enzymes essential for the

metabolic process of micro-organism and

and kill them (bacteria and germs)

Used at low pH values < 6.5

•The acid form, HOCl, is a much stronger disinfectant than

the hypochlorite ion, OCl- (found at higher pH)

•Free chlorine in treated water should not exceed 0.1 – 0.2 ppm

•Efficiency of chlorination depends upon time of contact,

Temp., & pH of water

chlorinator

Chlorination

Advantages

• Effective and economical-The most ideal disinfectant

• Little space for storage

• Stable & do not deteriorate on standing can be used at low & high T.

• Introduces no salt impurities

Disadvantages

• More effective below 6.5 pH & less effective at higher pH values

• Excess chlorine leaves unpleasant odor which produces irritation on muscus membrane

Break point chlorination

Problem during chlorination

• At high pH exists as OCl -

• Presence of NH3 can cause problems as chloramines are

formed which have very little disinfecting power (Chloramines are an effective disinfectant against bacteria but not against viruses)

• Chloramines irritate the skin and eyes and impart unpleasant odour to the water

• Alternatives

• Adding sufficient amounts of chlorine would be an excellent method to oxidise free ammonia along with org. substances and reducing substances present in water

• Excess chlorine would stay as hypochlorous acid

Res

idual

chlo

rine

Chlorine dosage

Break point

Zone IZone II

Zone IIIZone IV

Zone I: Chlorine is reduced to chlorides by reducing compounds in water

Zone II: Chloramines are formed with ammonia

Zone III: Chloramines are broken down and converted to

nitrogen gas which leaves the system (Breakpoint).

Zone IV: Free residual.

Break point chlorination or free

residual chlorination

Produce a residual free available

chlorine (HOCl) with no combined

chlorine

Reactions / Mechanism

• If ammonia is present in the water, then the hypochlorous acid will react to form one of the three types of chloramines depending on the pH, temperature, and reaction time.

• Monochloramine and dichloramine are formed in the pH range of 4.5 to 8.5, however, monochloramine is most common when the pH is above 8.

• When the pH of the wastewater is below 4.5, the most common form of chloramine is trichloramine which produces a very foul odor.

• The equations for the formation of the different chloramines are as follows :

• Monochloramine: NH3 + HOCl -> NH2Cl + H2O

• Dichloramine: NH2Cl + 2HOCl -> NHCl2 + 2H2O

• Trichloramine: NHCl2 + 3HOCl -> NHCl3 + 3H2O

» Contd-----

Zone II

• The final step is that additional free chlorine reacts with the chloramine to produce hydrogen ion, water , and nitrogen

gas which will come out of solution- Zone III

• Thus, added excess chlorine reduces the concentration of chloramines in the disinfection process and the remaining

free chlorine(residual chlorine) forms the stronger

disinfectant, hypochlorous acid- Zone IV

• De-Chlorination: Over chlorinated water may be passed thro bed of activated carbon or by addition of SO2 or Na. Sulphite to remove unpleasant odor/taste

• SO2+ Cl2+ H2O H2SO4 +2HCl

• Na2SO3 +Cl2+ H2O Na2SO4 +2HCl

The advantages of Break point chlorinatiom

• Oxidises completely org. compds

• Removes colour due to org matter

• Destroys 100% pathogenic bacteria

• Removes odour and taste

• Prevents the growth of any weeds in water

Points to remember

• Natural surface water, from lakes and rivers are main sources for municipalities

• Water is purified at different steps to remove suspended particles, org matters and pathogenic bacteria to make it for safe for use

• Suspended particles are removed by screening, settling and sedimentation

• Fine colloidal particles are removed by converting them into floc by adding coagulant followed by sedimentation

• Filtration is done to remove all org matters and micro-organisms

• Complete destroying/killing of pathogenic bacteria and micro-organism is done by disinfection

• Disinfectantants like UV rays, ozone, chloramines, bleaching powder and chlorination are used to kill bacteria, virus and other micro-org present in the water

• UV light wave length ranges from 200 and 280nm is germicidal

• Unstable ozone decomposes and produces nascent oxygen which kills all micro-org and oxidizes all org compds present in the water

» contd---------------

• Bleaching powder, chloramine and liquid/ gaseous Cl2(chlorination) kill bacteria by forming HOCl which inactivates the enzymes essential for the metabolic process of micro-organism and kill them (bacteria and germs)

• Insufficient chlorination of water containing NH3 leads to the formation of mixture of chloramines which irritate the skin and eyes and impart unpleasant odour to the water

• Excess or Sufficient chlorine is added to produce residual free available chlorine in Break point chlorination

• Excess chlorine oxidizes org. substances, reducing substances and free ammonia present in water

• Excess chlorine would stay as hypochlorous acid and destroys all micro-organisms

• Over chlorinated water may be passed thro bed of activated carbon or by addition of SO2 or Na. Sulphite to remove unpleasant odor/taste