WATER TREATMENT

Contents

• Introduction• Hardness of Water• Determination of Hardness• Water Softening Techniques• Boiler Feed Water and Boiler Problems

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.

Sources of Water

Sources of water

Rain Water Surface Water

River Water Lake Water Sea Water

Ground Water

Impurities in Water Dissolved impurities:

(a) Inorganic salts: Cations: Ca2+, Mg2+, Fe2+, Al3+, Mn2+, Na+, K+, Zn2+, Cu2+, Sr3+ etc.Anions: HCO3-, Cl-, SO4

2-, NO3-, CO3

2-, 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.

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

– 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

• 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

• 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

• Molecular mass is 100

• Most insoluble salt that can be precipitated in water

treatment

• Most common substance in hardness

Degree of HardnessHardness of water is conveniently expressed in terms of

equivalent amount of CaCO3

33 tantan CaCOofweightMolecularx

cesubsproducinghardnessofweightMolecularcesubsproducinghardnessofAmountEquivalentCaCO

33 tantan CaCOofweightEquivalentx

cesubsproducinghardnessofweightEquivalentcesubsproducinghardnessofAmountEquivalentCaCO

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

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.

Determination of Hardness

(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)

Determination of Hardness

Hardness rating Concentration of Calcium Carbonate (mg/L)

Soft 0-50

Moderately soft 50-100Slightly hard 100-150Moderately hard 150-200Hard 200-300Very hard >300

Determination of Hardness

Latin word Permutare = To exchangeNa2O.Al2O3.xSiO2.yH2OX = 2-10 (5-13); y = 2-6 (3-4)Natrual Zeolites: Natrolite: x = 1; y = 1Thomsonite: x = 3; y = 2Analcine: x = 4; y = 3

REMOVAL OF HARDNESS BY ZEOLITE (PERMUTIT) PROCESS

REMOVAL OF HARDNESS BY ZEOLITE (PERMUTIT) PROCESS

ZEOLITE PROCESS REACTIONS

REGENERATION OF ZEOLITE

REMOVAL OF HARDNESS BY ION EXCHANGE RESINS

ION EXCHANGE PROCESS

ION EXCHANGE PROCESS REACTIONS

REGENERATION OF ION EXCHANGE RESIN

Cold Lime Soda Treatment

Hot Lime Soda Treatment

• 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

Lime Soda Process Reactions

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<V2)

Nil 2P M-2P

Relation of Phenolphthalein & Methyl Orange readings with the possibility of alkalinity producing ions

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

DESALINATION OF BRACKISH WATER BY ELECTRODIALYSIS PROCESS

Pressure: 5-6 Kg/m2

DESALINATION OF BRACKISH WATER BY REVERSE OSMOSIS PROCESS (RO)

Pressure: 15-40 Kg/cm2

BOILER FEED WATER AND ITS TREATMENTS

Contents

• Boiler Feed water• Major Problems in Boiler

ScalingBoiler corrosionCaustic embrittlement Foaming & priming

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 a

fuel Boiler proper, a device in which the heat

changes water into steam• The steam or hot fluid is then recirculated

out of the boiler for use in variousprocesses in heating applications

• Boiler receives feed water in the form of• Recovered condensed water (return water)• Fresh water (make up water: purified in varying

degrees or natural water in its raw state)• Feed-water composition therefore depends

on the quality of the make-up water and theamount of condensate returned to the boiler.

• Steam which is coming out from the boiler,contains liquid droplets and gases.

• Water remaining in the liquid form at thebottom of the boiler picks up all the impuritiesfrom any form of water.

• The impurities must be blown down by thedischarge of water from the boiler to thedrains.

Boiler Feed Water

• Proper treatment of boiler feed water is animportant 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 boilersystem and lead to boiler corrosion.

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

Boiler Feed Water

• In boiler, water is continuously converted intosteam

• Increasing the concentration of salts.• Concentrates changes into loose and slimy

precipitates called sludge and• if these are become hard and adherent, called

scales.• Scales are hard deposits, which stick very firmly to

the inner surface of the boiler.• The type of scale will depend upon the chemical

composition 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 BoilerScaling

• 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 whichin turn leads to overheating and may result in thesoftening, bulging or even fracturing of the boilertubes.

• 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

• 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 conditioningcarbonate conditioningphosphate conditioningcalgon conditioning

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

ScalingPrevention of Scale or Sludge FormationExternal pretreatment of feed water with watersofteners, demineralizers, or reverse osmosis toremove minerals before enter into boiler

• 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 andcauses caustic embrittlement

Prevention of Scale or Sludge Formation

Carbonate ConditioningProcess

• Calgon [sodium hexametaphosphate,(NaPO3)6] is added to boiler water

• forms the sludge [Na4P6O18]2- andprevents the scale formation byforming the soluble complexcompounds

Na2[Na4(PO3)6] → 2Na+ + [Na4P6O18]2-

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

Prevention of Scale or Sludge Formation

Calgon Conditioning Process

• Destructive attack or decay of boilermaterial or metal by chemical orelectrochemical reaction with itsenvironment

• Main responsible for corrosion in boiler: Dissolved Oxygen Dissolved Carbon dioxide Acids from dissolved salts

Problems in Boiler

Boiler Corrosion

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

Boiler Corrosion

Dissolved oxygen (D.O)

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

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

Boiler CorrosionRemoval of Dissolved oxygen

(D.O)

• Carbon dioxide in presence of water formscarbonic acid (H2CO3)

CO2 + H2O → H2CO3

• Carbon dioxide is also released inside theboiler, if water used for steam generationcontains bicarbonates

Mg(HCO)3 → MgCO3 + H2O + CO2

• Slow corrosive effect on the boiler material

Boiler Corrosion

Dissolved carbon dioxide

• By adding calculated quantity of ammonia

2NH4OH + CO2 → (NH4)2CO3 + H2O

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

Boiler CorrosionRemoval of Dissolved Carbon

Dioxide

• Water containing dissolved magnesiumsalts liberate acids on hydrolysis

MgCl2 + 2H2O → Mg(OH)2 ↓ +2HCl

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

Boiler Corrosion

Acids from dissolved salts

• Caustic embrittlement is a type of boilercorrosion 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 BoilerCaustic Embrittlement

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

• water evaporated and the dissolved causticsoda concentration increases gradually

• caustic soda attacks the surrounding area,thereby dissolving iron of boiler as sodiumferrate

• This causes embrittlement of boiler parts

Caustic embrittlement

• By using sodium phosphate as softening agent 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

Caustic embrittlementPrevention of Caustic

Embrittlement

• When a boiler is producing steam veryrapidly some particles of the water carriedalong with the steam. This process of ‘WetSteam’ formation is called priming.

• It is caused by:• presence of large amount of dissolved solids

such as alkali sulphate and chlorides• sudden boiling• improper boiler design• sudden increase in steam-production rate

Problems in BoilerPriming and FoamingPriming

• Reduces the efficiency of boiler

• Decrease the life of the machinery part

• maintenance cost increases

Priming

Disadvantage of Priming

• 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

• Foaming is the persistent formation ofbubbles 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 BoilerFoaming

• Actual height of the water column cannotbe judge.

• Dissolved salts in water carried by the wetsteam may damage the machinery partsespecially turbine blades.

• Boiler pressure cannot be maintained

Foaming

Disadvantage of Foaming

• By the addition of anti-foaming agentssuch as castor oil, Gallic acid, tannic acidetc.

• By adding compounds like sodiumaluminate for removing oil from boilerwater.

Foaming

Prevention of Foaming