UNIT IV - Water & Water Treatment

Hardness
The soap consuming capacity of water is known as the hardness of water.

Or

Water samples that do not readily produce lather with soap, or deposit scale on the walls of the container when
there is appreciable change in temperature, are called hard water.

Hardness is generally caused by calcium & magnesium ions present in water.

• Other divalent metallic cations that cause hardness are Fe+2, Mn+2and Sr+2 but these are usually present
in small amounts.

• The cations react with soap (Sodium or Potassium salts of fatty acids ) to form precipitates:

2 C17H35COO Na + M+2 (C17H35COO)2M + 2Na+

Sodium Stearate(Soap) Ppt..

Types of hardness
• Temporary hardness –

Due to bi carbonates and carbonates of Ca+2 and Mg+2 ions and can be removed by boiling.

• Permanent Hardness –

Due to sulphates , chlorides and nitrates of Ca+2 and Mg+2 ions.

The hardness in water can be determined by EDTA method.

Units of Hardness –

 mg/l
 Ppm
 0
Cl
 0
Fr
 1 mg/l = 1 ppm = .070Cl = 10 0 Fr

Numericals –

1. 100 ml of water sample has a hardness eq of 12.5 ml of .08 N MgSO4. What is the hardness in ppm.
( Ans : 500 mg/l )

2. A water sample contains 408 mg CaSO4 /lit. Calculate hardness in CaCO3 eq.

3. 50 ml sample water consumed 15 ml of .01 M EDTA before boiling & 5 ml of the same after boiling.

Calculate total hardness.

Boiler Feed Water –
Water is mainly used in boilers for the generation of steam. It should correspond with the following composition-

1. Its hardness should be below .2 ppm.

2. Its caustic alkalinity (OH ions) should lie b/w .15 and .45 ppm.

3. Its soda alkalinity (due to sodium carbonate) should be .45 to 1 ppm.

Excess impurities in water results in the following problems-

BOILER TROUBLES:
In treatment of water complete elimination of all the impurities is not possible.

• The impurity that gives rise to certain troubles will be removed to certain extent. In modern pressure boilers and
laboratories, water purer than the distilled water is required.

• Some of the boiler troubles caused by the use of unsuitable water are..

1. Carryover – Priming and Foaming

2. Scale formation

3. Boiler corrosion and

4. Caustic embrittlement

Priming –

It may be defined as the violent or rapid boiling of water occurring in the boiler which results in carrying out of water with
steam in the form of a spray. When a boiler is producing steam rapidly, some particles of liquid water are carried along
with the steam. This process of wet steam formation is called priming.

• Priming mainly caused due to very high water level. The presence of large amount of dissolved solids, high steam
velocities, sudden increase in steam production rate.

Sudden steam demand which leads to sudden boiling, presence of excessive foam filling the foam spare, and due to faulty
boiler design.
priming can be controlled by proper boiler design, fitting mechanical steam purifier, avoiding rapid change in
steam-rate, proper evaporation & adequate heating surfaces, uniform distribution of fuel and providing anti priming pipes,
keeping the water level low & avoid sudden steam demands, efficient softening & filtration of the boiler feed water.

Foaming –

Foaming is the formation of small but stable bubbles above the surface. The main reason for foaming is being presence of
fatty acids and other impurities.

Foaming can be controlled by using anti-foaming chemicals, removal of concentrated boiler water and
replacing it by fresh feed water. Removing oil from boiler water by adding compounds like sodium aluminate .

SLUDGE –
 • Sludge is a soft, loose and slimy precipitate formed within the boiler. It is formed at comparatively colder portions
of the boiler and collects in the area where flow rate is slow. These are formed by substances which have greater
solubility in hot water than in cold-water.

Eg. MgCO3, MgCl2, CaCl2, MgSO4.

SCALES –

Scales are hard, adhering precipitates formed on the inner walls of the boilers. They stick very firmly on to the inner wall
surface and are difficult to remove with chisel and hammer. Causes of scale formation:

a. Decomposition of calcium bicarbonate:

Ca(HCO3) 2 CaCO3 + H2O + CO2

• In low pressure boilers, CaCO3 causes scale formation.

• In High pressure boilers, CaCO3 becomes soluble. CaCO3 + H2O - Ca(OH)2 + CO2

b. Decomposition of calcium sulphate:

The solubility of CaSO4 in water decreases with rise of Temperature. In super heated water CaSO4
is insoluble. This is the main cause in high-pressure boilers.

c. Hydrolysis of Magnesium salts:

Dissolved Magnesium salts undergo hydrolysis forming Mg(OH)2 precipitate.

• MgCl2 + 2H2O Mg(OH)2 + 2 HCl

• Mg(OH)2 so found by hydrolysis of magnesium salts is a soft type of scale.

d. Presence of Silica:

Silica present in small quantities deposits as silicates like CaSiO3 and MgSiO3. These are very difficult to remove.

• Disadvantages:

a. Danger of Explosion: The hot scale cracks because of expansion and water suddenly comes in contact with
overheated Iron plates. This causes in formation of large amount of steam suddenly. This results high pressure
causing boiler to burst.
 b. wastage of fuel: The scale formation causes decreases of heat transfer. As a result over heating is required this
causes consumption of fuel.

Boiler Corrosion:

The chemical or electro chemical eating away of metal by its environment in a boiler is known as boiler corrosion.
The main reason for this problem is the presence of excess of oxygen in water. It can be prevented by mechanical
deaerator, pre-heating and chemical treatment

a. Dissolved oxygen: Water usually contains about 8 ml of dissolved oxygen per liter at room temperature.
Dissolved oxygen in water, in presence of prevailing high temperature, attacks boiler material:
• 2Fe + 2H2O + O2 -----------------> 2Fe(OH)2
• 4Fe(OH) 2 + O2 -----------------> 2[ Fe2O3.2H2O]

• Removal of dissolved oxygen:

By adding calculated quantity of sodium sulphite or hydrazine or sodium sulphide.
2Na2SO3 + O2 -----------------> 2Na2SO4
N2H4 -----------------> N2 + 2H2O
Na2S + O2 -----------------> Na2SO4
Hydrazine is an ideal internal treatment chemical for the removal of dissolved oxygen. It reacts
with oxygen, forming nitrogen and water. Nitrogen is harmless. Recently Azamina 8001-RD a poly organic
compound, has been employed for degassing of water.

b. Dissolved carbon dioxide: carbon dioxide is carbonic acid,

CO2 + H2O H2CO3
This has a slow corrosive effect on the boiler material. Carbon dioxide is also released inside the boiler, if water
used for steam generation contains bicarbonate,
Mg(HCO3) 2 Heat MgCO3 + H2O + CO2
 Removal of carbon dioxide:
1. By adding calculated quantity of ammonia,
2NH4OH + CO2 (NH4) 2CO3 + H2O
2. By mechanical – aeration process along with oxygen.

c. Acids from dissolved salts: Water containing dissolved magnesium salts liberates acids on hydrolysis.
MgCl2 + 2H2O Mg(OH) 2 + 2HCl
The liberated acid reacts with iron in chain – like reactions producing HCl again and again.
Fe + 2HCl FeCl2 + H2
FeCl2 + 2H2O Fe(OH) 2 + 2HCl
Consequently, presence of even a small amount of MgCl2 will cause corrosion of iron to a large extent.

Caustic Embrittlement:
The formation of brittle and in crystalline cracks in the boiler shell is called caustic
embrittlement. It is a type of boiler corrosion and the main reason for this, is the presence of alkali-metal
carbonates and bicarbonates in feed water and also the presence of sodium sulphate.
In lime-soda process, it is likely that, some residual Na2CO3 is still present in the softened
water. This Na2CO3 decomposes to give NaOH and CO2, due to which the boiler water becomes “Caustic”.
Na2CO3+ H2O -----------------> NaOH + CO2
 This caustic water flows inside the boiler and causes some minutes hair-cracks, by capillary action. On evaporation
of water, the dissolved caustic soda increases its concentration which attacks the surrounding area, thereby
dissolving Iron of boiler as Sodium ferroate. This causes embrittlement of boiler parts such as bends, joints, reverts
etc, due to which the boiler gets fail.
Therefore, caustic embrittlement can be prevented.
1. By maintaining the pH value of water and neutralization of alkali.
2. By using Sodium Phosphate as softening reagents, in the external treatment of boilers.
3. Caustic embrittlement can also be prevented by adding Tannin or Lignin or Sodium sulphate which
prevents the infiltration of caustic-soda solution blocking the hair-cracks.

Water softening –
• Internal Treatment- Colloidal conditioning, Carbonate conditioning, Phosphate conditioning , Calgon
conditioning, Sodium Aluminate Treatment.

• External Treatment- Zeolite Process , Lime soda process, Ion exchange process.

• Internal Treatment-
Internal treatment of boiler water is carried out by adding proper chemicals to precipitate the scale
forming impurities in the form of sludge and to convert the scale forming chemicals into compounds
which will stay in dissolved form in water. This process is mainly used as a corrective treatment to
remove the slight residual hardness and also sometimes to remove the corrosive tendencies in water.

1. Colloidal conditioning –

The addition of organic substances such as Kerosene, tannin, Gel etc., to the surface in low pressure
boilers may prevent the scale formation. These substances gets coated over the scale forming
precipitates and gives a loose and non-sticky precipitates which can be removed by using blow-down
operation.

2. Phosphate conditioning –

The addition of sodium phosphate in hard water reacts with the hardness causing agents and gives
calcium and magnesium phosphates which are soft and non-adhere and can be removed easily by blow-
down operation. In this way, scale formation is removed in high pressure boilers.

3CaCl2 + 2 Na3PO4 Ca3(PO4)2 + 6NaCl

3. Carbonate conditioning –

In low-pressure boilers, scale-formation can be avoided by adding sodium carbonate to boiler water,
when CaSO4 is converted into calcium carbonate in equilibrium.

CaSO4 + Na2CO3 CaCO3 + Na2SO4

Consequently, deposition of CaSO4 as scale doesn’t take place and calcium is precipitated
as loose sludge of CaCO3 which can be removed by blow-down operation.

4. Calgon conditioning –
 Involves in adding calgon to boiler water. It prevents the scale and sludge formation by forming soluble
complex compound with CaSO4.

• Calgon = Sodium hexa Meta phosphate = (NaPO3)6

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

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

5. Sodium Aluminate Treatment –

Sodium aluminate gets hydrolyzed yielding NaOH and a gelatinous precipitate of aluminum hydroxide.

NaAlO2 + 2H2O NaOH + Al (OH)3

The sodium hydroxide, so-formed, precipitates some of the magnesium as Mg (OH)2, i.e.,

MgCl2 + 2NaOH Mg (OH)2 + 2NaCl

The flocculent precipitate of Mg (OH)2 plus aluminum hydroxide, produced inside the boiler, entraps
finely suspended and colloidal impurities, including oil drops and silica. The loose precipitate can be
removed by pre-determined blow-down operation.

EXTERNAL TREATMENTS

1. Zeolite Process( Permutitt Method)-

Most conventional water-softening devices depend on a process known as ion-exchange in which "hardness"
ions trade places with sodium and chloride ions that are loosely bound to an ion-exchange resin or a zeolite
(many zeolite minerals occur in nature, but specialized ones are often made artificially.) These are mainly of
two types-
• Natural Zeolites- Thomsonite,(Na2O.CaO)Al2O32SiO2.2.5H2O
Laumonite, CaO.Al2O3.4SiO2.4H2O etc.
 Synthetic Zeolites- These are prepared from sodium silicate and sodium aluminates.

Zeolite Water Softener-

The zeolite softener consist of a steel tank packed with a thick layer of loosely packed zeolite over a layer of sand
and gravel. The water enters at the top and passes through the bed of zeolite.

• Principle – It operates on the principle involving alternate cycle of softening run and regeneration run. The water is
softened by passing it through the zeolite bed, where Ca ion and Mg ions removed from the water by zeolite and
simultaneously releasing equivalent amount of Na ion in exchange.
• Process – Hard water passes at a specified rate through a bed of active granular sodium zeolite, present in a
zeolite softener , when the Ca ions and Mg ions are taken up by zeolites as CaZ and MgZ,while outgoing water
contains eq amount of sodium salts.

The chemical reactions taking place in zeolite softner are-

• CaSO4 + Na2Z CaZ + Na2SO4
• Ca(HCO3) 2 + Na2Z CaZ + 2 Na2CO3
 The illustration depicts a negatively-charged zeolite to which [positive] sodium ions are attached. Calcium or
magnesium ions in the water displace sodium ions, which are released into the water.

• It removes hardness almost (about 10ppm hardness only). It requires less time for softening.
• Herein lies one of the drawbacks of this process: most of the salt employed in the regeneration process gets
flushed out of the system and and is usually released into the soil or drainage system— something that can have
damaging consequences to the environment, especially in arid regions.

2. Ion exchange or deionization or Demineralization Process –

Ion exchange resins are insoluble , cross – linked , long chain organic polymers with a micro-porous structure,
and the ‘functional groups’ attached to the chains are responsible for the ion-exchanging properties. Resins
containing acidic functional groups ( -COOH, SO3H, etc ), are capable of exchanging their H+ with other cations,
whereas those containing basic functional groups (-NH2 etc ) are capable of exchanging their anions with other
anions.
Anion exchange Resins – Anion exchange resins are styrene-divinyl benzene or amine formaldehyde copolymers, which
contains amino, quaternary ammonium or quaternary phosphonium or tertiary sulphonium groups as an internal parts of
the resin matrix. These after the treatment with NaOH solution, become capable to exchange their OH ions with anions in
water. These have basic functional groups like –NH2 .

Cation Exchange Resins: These are mainly styrene divinyl benzene co-polymers, which on sulphonation or carboxylation.
These are capable of exchanging their hydrogen ions with cations in water.

2RH+ + Ca+2/Mg+2 R2 Ca+2/Mg+2 + 2H +

Cation
Exchange resin

2R’OH+ + Cl-/CO3-2/SO4-2 R2 Cl-/CO3-2/SO4-2 + 2OH -

Anion
Exchange resin

Revival of exhausted ion exchange resins –

R2 Ca+2/Mg+2 + 2H + 2RH+ + Ca+2/Mg+2

R2 Cl-/CO3-2/SO4-2 + 2OH – 2R’OH+ + Cl-/CO3-2/SO4-2

How It Works - In ion-exchange process, hard water is allowed to pass through cation exchange resins, which remove
Ca+2 and Mg+2 ions and exchange equivalent amount of H+ ions. Anions exchange resins remove bicarbonates, chlorides
and sulphates from water exchange equivalent amount of OH-. A typical ion exchange system consists of an enclosed
container filled with millions of small resin beads. The contaminated water is pumped through the container and flows
between the beads.

The perchlorate ion is attracted to the resin, where it attaches and displaces a chloride ion. The chloride ion replaces the
perchlorate ion in the groundwater.

 The amount of chloride ions introduced to the groundwater depends on the levels of perchlorate and competing
ions, like nitrate in the groundwater being treated.The concentrations of these ions are low, so very little chloride
will be added to the groundwater during the ion exchange process.
  The treated water passes through a screen small enough to hold back the resin beads before the water exits the
container and is discharged back into the aquifer using reinjection wells. Treated water is sampled frequently to
confirm the ion exchange resin is effectively removing the perchlorate.
 Over time, perchlorate and competing ions will saturate the ion exchange resin, requiring it to be changed for new
resin. Used resin will be treated and/or disposed of in accordance with environmental regulations.
Advantage and Disadvantage-

 This process can be used to soften highily acidic and alkaline waters and produces water of very low hardness.
 However the equipment is very costly and chemicals are very expensive.

Lime Soda Process –

• Chemical precipitation is one of the more common methods used to soften water. Chemicals normally used are
lime (calcium hydroxide, Ca(OH)2) and soda ash (sodium carbonate, Na2CO3). Lime is used to remove chemicals
that cause carbonate hardness. Soda ash is used to remove chemicals that cause non-carbonate hardness. When
lime and soda ash are added, hardness-causing minerals form nearly insoluble precipitates.

• Calcium hardness is precipitated as calcium carbonate (CaCO3). Magnesium hardness is precipitated as magnesium
hydroxide (Mg(OH)2). These precipitates are then removed by conventional processes of coagulation/flocculation,
sedimentation & filtration. Because precipitates are very slightly soluble, some hardness remains in the water-
usually about 50 to 85 mg/l (as CaCO3). This hardness level is desirable to prevent corrosion problems associated
with water.
5. MgSO4 + Ca(OH)2 Mg(OH)2+CaSO4
6. CaSO4 + Na2CO3 CaCO3 + Na2SO4

From the above reactions it is clear that for each molecule of calcium bicarbonate hardness removed, one
molecule of lime is used. For each molecule of magnesium bicarbonate hardness removed, two molecules of lime
are used. For each molecule of non-carbonate calcium hardness removed, one molecule of soda ash is used. For
each molecule of non-carbonate magnesium hardness removed one molecule of lime plus one molecule of soda
ash is used.
Lime soda process may be classified into following groups : Cold Lime Soda Process
& Hot Lime Soda Process
• Cold Lime Soda Process –
It includes mixing of calculated quantity of lime and soda with water at room temp. A small amount of
coagulant is required to add to trap the fine precipitate formed.
The following reaction takes place depending upon the hardness.

If it is Temporary hardness and due to calcium salt

Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O
Slimy suspended ppt

If it is due to Magnesium salt

Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 + 2H2O
Slimy suspended ppt

If it is permanent hardness and due to calcium salt

Ca2+ + Na2CO3 CaCO3 + 2Na+ (soda)
Slimy suspended ppt
If it is due to Magnesium salt
Mg2+ + Ca(OH)2 Mg(OH)2 + Ca2+ (lime)
Slimy suspended ppt

Ca2+ + Na2CO3 CaCO3 + 2Na+ (soda)

Slimy suspended ppt

The precipitates CaCO3 and Mg(OH)2 are very fine and forms sludge like precipitates in the boiler water and are
difficult to remove because it does not settle easily making it difficult to filter and the removal process. Finally
reduces the efficiency of the boiler.

When coagulants are added flocculation takes place followed by the formation of flocculants.
NaAlO2 + 2H2O NaOH + Al(OH)3
Coagulant Flocculent- Gelatinous
precipitate which entraps
the fine precipitates of
CaCO3 and Mg(OH)2
Al2(SO4)3 + 3 Ca(HCO3)2 2Al(OH)3 + CaSO4 + CO2
Aluminium Hard water sample Flocculent- Gelatinous
Sulfate precipitate which entraps
the fine precipitates of
CaCO3 and Mg(OH)2

The Al(OH)3 formed by the addition of coagulants initiates the process of flocculation and entraps the fine
precipitates and becomes heavy. The heavier flocs then settles at the bottom and filtered off easily.
Types-
Intermittent type
Continuous type
Continuous cold lime soda softener

Hot Lime Soda Process

 Rules
1. If Ca(HCO3)2 and Mg(HCO3)2 are considered as ions (Ca2+ + 2HCO3-) and (Mg2+ + 2HCO3-) respectively then the
calculation result will be the same based on the ability of the ions to take up bicarbonate ions
2. If treated water found to contain excess of OH- and CO32- ions these are formed from excess equivalent each of
Ca(OH)2 and Na2CO3 and hence these excess amounts should be added to the calculation (in temp. hardness and
perm. hardness)
3. When the impurities are given as CaCO3 and MgCO3 present in water it should be considered as due to
bicarbonates of calcium and magnesium respectively
4. Substances like NaCl, KCl, Na2SO4, SiO2, Fe2O3 etc do not contribute to hardness and therefore, they do not
consume any soda or lime and hence if these present need not be taken in to consideration during calculation.
5. Soda (Na2CO3) neutralizes only permanent hardness
 Lime required –
74/100[Temp hardness of Ca ions+ 2(Mg ion Temp Hardness) + permanent Mg hardness] volume of water
• Soda required –
106/100(permanent hardness) volume of water

Desalinization- It, refers to any process that removes some amount of salt and
other minerals from water.

TECHNOLOGIES FOR DESALINATION

PROCESS

 Reverse Osmosis (Pressure membrane

process)
 Electrodialysis membrane process
  When two solutions of unequal concentration are separated by a semi-permeable membrane, flow of solvent
takes place from dilute to concentration side, due to increase in osmostic pressure, which is termed as osmosis.

 However, when a hydrostatic pressure in excess of osmotic pressure is applied on the concentrated side, the
solvent flow is reversed from concentrated side to dilute side, across the membrane. This principle is termed as
reverse osmosis.

 The semi-permeable membrane (in reverse osmosis) is selective in not permitting the passage of dissolved
solute particles such as molecules, ions, etc.) It permits only the flow of water molecules (solvent) from the
concentrated to dilute side.

 Cellulose acetate, polyamide, etc., are used as membrane

 Reverse osmosis process requires only mechanical force to generate the required hydrostatic pressure.

 Hydrostatic pressure generated is in the order of 15-40 Kg m-2

Advantages of Reverse Osmosis

1. Nearly all contaminant ions and most dissolved non-ions are removed
2. Suitable for small systems with a high degree of seasonal fluctuation in water demand
3. Insensitive to flow and TDS levels
4. Operates immediately without any minimum break-in period
5. Possible low effluent concentrations
6. Removes bacteria and particles
 7. Simplicity and automation operation allows for less operator attention which makes them suitable for small
system applications.

Limitation of RO

1. High operating costs and capital

2. Potential problem with managing the wastewater brine solution
3. Pretreatment at high levels
4. Fouling of membranes

Principle –Electrodialysis
 Electrodialysis is an electrochemical process whereby electrically charged particles, ions, are transported from a
raw solution (retentate, diluate) into a more concentrated solution (permeate, concentrate) through ion-
selective membranes by applying an electric field.

Theory of Electrodialysis
• Electrodialysis chamber comprises of sheet like barriers made out of high-capacity, highly cross-linked ion
exchange resins that allow passage of ions but not of water.

• There are two types : (a) Cation exchange and (b) Anion exchange membranes

• Cation exchange membranes consists of an insoluble matrix and mobile cation reside in the pore space that
allows the pass through of only cations.

• Anion exchange membranes consists of an insoluble matrix and mobile anion reside in the pore space that
allows the pass through of only anions.

• Cation- and Anion- exchange membranes are installed alternatively in the tank.

• By impressing electricity on the electrodes, the positive anode attracts negative ions in solution, while the
negative cathode attracts positive ions in the solution.

Advantages of Electrodialysis
 1. All the contaminant ions and many of the dissolved non-ions are removed
2. Insensitive to flow and TDS levels
3. Possible low effluent concentrations

Limitations of Electrodialysis

1. Operating costs and capital are high

2. Level of pretreatment required is high
3. Twenty to ninety percent of feed flow is rejected stream
4. Replacement of electrodes