Water Treatment Sem i Full

8/18/2019 Water Treatment Sem i Full

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WATER TREATMENT


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Contents

• Introduction

• Hardness of Water

• Determination of Hardness• Water Softening Techniques

• Boiler Feed Water and Boiler Problems


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Introduction• Water is essential for life

• solvent of great importance

• Water is used for industrial purposes and for municipal supply

• Water is used in for the production of electricity

• Water is also used in chemical plants, paper industries,

pharmaceutical industries, textile industries, steel industries, food

industries as well as in atomic reactors

• used in irrigation for agricultural purposes

• widely used in domestic uses such as drinking, bathing, washings,

sanitary etc.

• Although water is nature’s most wonderful and abundant compound

but only less than 1% of the world’s water resources is available for

ready use.


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Sources of Water

Sources of

water

Rain Water Surface Water

River Water Lake Water Sea Water

Ground Water


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Impurities in Water Dissolved impurities:

(a) Inorganic salts:Cations: Ca2+, Mg2+, Fe2+, Al3+, Mn2+, Na+, K +, Zn2+, Cu2+, Sr 3+ etc.

Anions: HCO3-, Cl-, SO42-, NO3

-, CO32-, F- etc.

(b) Organic constituents:Aldrin, benzene, carbon tetrachloride, chlordane, tetrachlorethane,trichlorethane, chloroform, lindane, methoxychlor etc.

(c) Gases:SOx, NOx, CO2, H2S etc.

Suspended impurities: :

(a) Inorganic impurities: Sand, Clay etc(b) Organic impurities: Oil globules, vegetable and animal matter

(c) Colloidal impurities: Finely divided silica and clay, organic waste products

Microorganism:Algae, Fungi, bacteria etc.


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Hardness of Water

• Hardness is that characteristics of water by which it can prevent the

formation of lather / foam with soap solution.”

• also defined as the soap consuming capacity of water.

• Presence of certain salts of Ca, Mg and other heavy metal ions like

Al3+, Fe3+ and Mn2+

• Typical reactions of soap with water(CaCl2 and MgCl2) does not

produce lather but for insoluble white scum or precipitate

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

2C17H35COONa + MgCl2 → (C17H35COO)2Mg↓ + 2NaCl


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– Hard Water

• Does not produce lather

• Dissolved salts of Ca and Mg

– Soft Water

• Lather forms easy

• Doesn’t contain dissolved Ca and Mg salts in it

Types of Water: Hard Water and Soft Water


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• Temporary Hardness

• Presence of dissolved bicarbonates of calcium and magnesium

and other heavy metals

• Removed by mere boiling

• Carbonate hardness or alkaline hardness

Types of Hardness: Temporary hardness and Permanent hardness

Ca(HCO3)2 CaCO3 + H2O + CO2

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


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• Permanent Hardness

• Presence of dissolved chlorides and sulphates of calcium and

magnesium, iron and other heavy metals

• Not removed by boiling

• Non-Carbonate hardness or non-alkaline hardness


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• Molecular mass is 100

• Most insoluble salt that can be precipitated in water

treatment

• Most common substance in hardness

Degree of Hardness

Hardness of water is conveniently expressed in terms of

equivalent amount of CaCO3

33

tan

tan CaCOof weight Molecular xcesubs producinghardnessof weight Molecular

cesubs producinghardnessof Amount Equivalent CaCO

33

tan

tanCaCOof weight Equivalent x

cesubs producinghardnessof weight Equivalent

cesubs producinghardnessof Amount Equivalent CaCO


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Degree of Hardness

oCl

mg/L

oFr

ppm

Units of

hardness

It is the parts of calcium carbonate equivalent hardness per a particular

number of parts of water depending upon the unit used


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Determination of Hardness

• By complexometric titration

• Permanent hardness is usually determined by titrating it

with a standard solution of EDTA by using Eriochrome

Black-T

• The EDTA is a complexing, or chelating agent used to

capture the metal ions

• This causes the water to become softened, but the metal ions

are not removed from the water

• EDTA simply binds the metal ions to it very tightly.


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(1) Metal + Indicator Metal-Indicator complex

Ca++ + EBT (blue dye) Ca-EBT complex(Unstable wine red color)

(2) Metal-Indicator complex + EDTA Metal-EDTA complex

+ Indicator

Ca-EBT + EDTA Ca-EDTA + EBT (blue dye)

(Unstable wine red complex) (Stable colorless complex)


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Hardness rating Concentration of Calcium

Carbonate (mg/L)

Soft 0-50

Moderately soft 50-100

Slightly hard 100-150

Moderately hard 150-200

Hard 200-300Very hard >300



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Latin word Permutare = To exchange

Na2O.Al2O3.xSiO2.yH2O

X = 2-10 (5-13); y = 2-6 (3-4)Natrual Zeolites:

Natrolite: x = 1; y = 1

Thomsonite: x = 3; y = 2 Analcine: x = 4; y = 3

REMOVAL OF HARDNESS BY

ZEOLITE (PERMUTIT) PROCESS


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ZEOLITE (PERMUTIT) PROCESS


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ZEOLITE PROCESS REACTIONS

REGENERATION OF ZEOLITE


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ION EXCHANGE RESINS


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ION EXCHANGE PROCESS


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ION EXCHANGE PROCESS REACTIONS

REGENERATION OF ION EXCHANGE RESIN


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Cold Lime Soda Treatment


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Hot Lime Soda Treatment


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• Alkalinity is the name given to the

quantitative capacity of water to neutralize

an acid.

• Due to OH−, CO3−2, HCO3

− ions

OH− + H+ → H2O

CO3−2 + 2H+ → CO2 + H2O

HCO3− + H+ → CO2 + H2O

Alkalinity


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Lime Soda Process Reactions


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S. NO. Result of Titration OH- ion CO32- ion HCO3

- ion

1 P = 0 Nil Nil M

2 P = M P or M Nil Nil

3 P = ½ M

(V1=V2)

Nil 2P Nil

4 P > ½ M

(V1>V2)

2P-M 2(M-P) Nil

5 P < ½ M

(V1


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Desalination of Water

• Brackish water or briny water is water

that has more salinity than fresh water, but

not as much as seawater.

• Dissolve salts like NaCl, KCl etc.• Fresh water: < 1000 ppm

• Brackish water: 1000-3500 ppm

• Sea water (Saline water): > 3500-5000 ppm• Brine water: > 5000 ppm


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DESALINATION OF BRACKISH WATER BY

ELECTRODIALYSIS PROCESS

Pressure: 5-6 Kg/m2


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DESALINATION OF BRACKISH WATER BY

REVERSE OSMOSIS PROCESS (RO)

Pressure: 15-40 Kg/cm2


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BOILER FEED WATER AND

ITS TREATMENTS


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Contents

• Boiler Feed water

• Major Problems in Boiler

ScalingBoiler corrosion

Caustic embrittlement

Foaming & priming


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Boiler Feed Water

• A boiler is a device for generating steam

• Consists of two main parts:• Furnace and Boiler proper

Furnace provides heat, usually by burning afuel

Boiler proper, a device in which the heatchanges water into steam

• The steam or hot fluid is then recirculatedout of the boiler for use in variousprocesses in heating applications


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• Proper treatment of boiler feed water is an

important part of operating and maintaining aboiler system.

• As steam is produced, dissolved solids becomeconcentrated and deposits inside the boiler.

• This leads to poor heat transfer and reduces theefficiency of the boiler.

• Dissolved gases such as oxygen and carbondioxide will react with the metals in the boiler system and lead to boiler corrosion.

• In order to protect the boiler from thesecontaminants, they should be controlled or removed, through external or internaltreatment

Boiler Feed Water


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• In boiler, water is continuously converted into

steam• Increasing the concentration of salts.

• Concentrates changes into loose and slimyprecipitates called sludge and

• if these are become hard and adherent, calledscales.

• Scales are hard deposits, which stick very firmly tothe inner surface of the boiler.

• The type of scale will depend upon the chemicalcomposition of the concentrated water. Scale due to calcium and magnesium

Scale due to iron oxide

Scale due to copper

Scale due to silica

Problems in Boiler Scaling


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• The thermal conductivity of boiler scale is verylow (similar to insulating brick). The scale acts asan insulating layer and prevents an efficienttransfer of heat through the tubes to thecirculating water. The reduction in thermalconductivity means lower boiler efficiency which

in turn leads to overheating and may result in thesoftening, bulging or even fracturing of the boiler tubes.

• Boiler scale can also cause clogging or partialobstruction of circulating water tubes in a boiler,which again causes overheating of the tubes.

• It increases the maintenance and operating costs.• Scales also cause corrosion (serious problems in

boiler operation)

ScalingEffect of Scale or Sludge

Formation


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• Scale/sludge is prevented by introducing the kerosene,colloidal substances such as tannins, lignosulphonates,

polyacrylates, maleic acrylate copolymer, maleic styrenecopolymer, polystyrene sulphonates etc. these colloidsget coated over the scale and forming particles and aresuspended in water which can easily blown-off from theboiler.

• Internal treatment using chemical conditioning

carbonate conditioningphosphate conditioning

calgon conditioning

into boiler feedwater to keep the scale forming materialsin dissolved form.

ScalingPrevention of Scale or Sludge

FormationExternal pretreatment of feed water with water softeners, demineralizers, or reverse osmosis to

remove minerals before enter into boiler


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• Sodium carbonate is added to boiler water.

• Scales react with it and form insoluble

calcium carbonate

CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4• Used only for low pressure boilers

• In high pressure boilers the excess of

sodium carbonate get converted intosodium hydroxide due to hydrolysis and

causes caustic embrittlement

Prevention of Scale or Sludge Formation

Carbonate Conditioning

Process


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• Calgon [sodium hexametaphosphate,

(NaPO3)6] is added to boiler water

• forms the sludge [Na4P

6O

18]2- and

prevents the scale formation by

forming the soluble complex

compounds

Na2[Na4(PO3)6] → 2Na+ + [Na4P6O18]

2-

[Na4P6O18]2- + 2CaSO4 → [Ca2P6O18]

2- +

2Na2SO4

Prevention of Scale or Sludge Formation

Calgon Conditioning Process


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• Destructive attack or decay of boiler

material or metal by chemical or

electrochemical reaction with itsenvironment

• Main responsible for corrosion in boiler:

Dissolved Oxygen

Dissolved Carbon dioxide

Acids from dissolved salts

Problems in Boiler

Boiler Corrosion


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• Water usually contains about 8 ml D.O/L at

room temperature

• D.O in water and presence of hightemperature attacks boiler material

2Fe + 2H2O + O2 → 2Fe(OH)2 ↓

4Fe(OH)2 ↓ + O2 → 2[Fe2O3.2H2O] ↓Ferrous hydroxide (Rust)

BoilerCorrosion

Dissolved oxygen (D.O)


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• By mechanical deaeration• Chemically, oxygen is removed by adding

oxygen scavenger or absorbing chemicals in acalculating amount such as sodium sulphite or

hydrazine or sodium sulphide If only small quantities of oxygen is present, the

addition of sodium sulphite is practical , in largequantities causes foaming

Hydrazine is used in large utility boilers to remove

dissolved oxygen but not recommended for heatingboilers because it must be closely controlled becauseof explosive nature of hydrazine and if used excess,it decomposes to produce ammonia which againcause corrosion

BoilerCorrosion

Removal of Dissolved oxygen

(D.O)


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• Carbon dioxide in presence of water forms

carbonic acid (H2CO3)

CO2 + H2O → H2CO3• Carbon dioxide is also released inside the

boiler, if water used for steam generation

contains bicarbonates

Mg(HCO)3 → MgCO3 + H2O + CO2

• Slow corrosive effect on the boiler material

BoilerCorrosion

Dissolved carbon dioxide


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• By adding calculated quantity of ammonia

2NH4OH + CO2 → (NH4)2CO3 + H2O

• By mechanical deaeration process, reduces CO2concentration to 5-10 ppm

BoilerCorrosion

Removal of Dissolved Carbon

Dioxide


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• Water containing dissolved magnesium

salts liberate acids on hydrolysis

MgCl2 + 2H2O → Mg(OH)2 ↓ +2HCl

• The liberated acids reacts with iron (of theboiler) producing HCl again and again

BoilerCorrosion

Acids from dissolved salts


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• Caustic embrittlement is a type of boiler corrosion caused by using highly alkalinewater in the boiler.

• During softening process by lime-sodaprocesses, free sodium carbonate ispresent

• sodium carbonate decomposes to givecarbon-dioxide and sodium hydroxide(caustic)

Na2CO3 + H2O → 2NaOH + CO2

Problems in Boiler

Caustic Embrittlement


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• sodium hydroxide containing water flows intothe minute hair-line cracks by capillary action

• water evaporated and the dissolved caustic

soda concentration increases gradually

• caustic soda attacks the surrounding area,

thereby dissolving iron of boiler as sodium

ferrate

• This causes embrittlement of boiler parts

Caustic embrittlement


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• By using sodium phosphate as softeningagent instead of sodium carbonate

• By adding tannin or lignin to boiler water

• By adding sodium sulphate to boiler water,blocks hair cracks in the boiler

• By adjusting the pH of boiler water to 8.0– 8.5

Causticembrittlement

Prevention of Caustic

Embrittlement


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• When a boiler is producing steam veryrapidly some particles of the water carriedalong with the steam. This process of ‘Wet

Steam’ formation is called priming.• It is caused by:

• presence of large amount of dissolved solidssuch as alkali sulphate and chlorides

• sudden boiling

• improper boiler design

• sudden increase in steam-production rate

Problems in Boiler

Priming and Foaming

Priming


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• Reduces the efficiency of boiler

• Decrease the life of the machinery part

• maintenance cost increases

Priming

Disadvantage of Priming


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• By improving the designing of boiler

• By addition fitting of mechanical steam

purifiers

• By maintaining low water level in the

boilers.

• By using soft water.

• By decreasing the amount of dissolved

salts present in the feed-water.

Priming

Prevention of Priming


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• Foaming is the persistent formation of

bubbles or foam in the boiler

• which do not break easily.

• It is due to the presence of oily substances

in water.

• These oily substances reduce the surface

tension of water in boiler

Problems in Boiler

Foaming


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• Actual height of the water column cannot

be judge.

• Dissolved salts in water carried by the wetsteam may damage the machinery parts

especially turbine blades.

• Boiler pressure cannot be maintained

Foaming

Disadvantage of Foaming

F i


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• By the addition of anti-foaming agents

such as castor oil, Gallic acid, tannic acid

etc.

• By adding compounds like sodium

aluminate for removing oil from boiler water.

Foaming

Prevention of Foaming

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Water Treatment Sem i Full