Unit III
WATER AND ITS TREATMENT
INTRODUCTION
The nature’s most abundant supply i.e., water is essential for the survival of all the living
beings on earth i.e., man, animals and plants. Water is not only essential for the survival of life, but it is
also used for the operation in a large number of industries as coolant, solvent, for steam
generation, for air conditioning, fire-fighting etc. Water is also used for all domestic purposes like
bathing, drinking, washing, sanitary, irrigation etc.
HARDNESS OF WATER
“Hardness of water is the characteristic of preventing lather formation of water with
soap”. Generally salts like chlorides, bicarbonates and sulphates of Ca2+, Mg2+ and Fe2 make
water hard. This hard water on treatment with soap which is stearic or palmitic acid salts ofsodium or
potassium causes white precipitate formation of calcium or magnesium stearate orpalmitate.
2C17 H35 COONa CaCl2
(C17 H35 COO)2 Ca 2NaCl
Sodium Stearate Hardness Calcium Stearate
(soap) causing (insoluble)
substance
Thus the cause of hardness is the precipitation of the soap and hence prevents lathering at
first. When the hardness causing ions are removed as insoluble soaps, water becomes soft andforms
lather.
TYPES OF HARDNESS
(i) Temporary Hardness is due to the bicarbonates of Ca2+ and Mg2+ and carbonate of
Fe2+. Since bicarbonates readily get precipitated on boiling the water, the temporary
hardness can be easily removed, viz.
Ca HCO3 2 h
eat
CaCO3 H 2O CO 2
insoluble
(ii) Permanent Hardness is due to the presence of chlorides and sulphates of Ca, Mg,Fe,
etc. Permanent Hardness cannot be removed on boiling.
CaCl2 Na2 CO3
CaCO3 2NaCl
(Soda)
EXPRESSION OF HARDNESS AND UNITS OF HARDNESS
Both temporary and permanent hardness are expressed in ppm (parts per million) as
CaCO3. The choice of CaCO3 is due to the fact that its molecular weight is 100 and equivalent
weight is 50 and it is the most insoluble salt in water.
Equivalent of CaCO3
(Mass of hardness producing substance)
(Chemical equivalent of CaCO3 ) Chemical
equivalent of hardness producing substance
Mass of hardness producing substance 50
Chemical equivalent of hardness producingsubstance
Hardness is principally expressed in ppm unit. Other limits include French degree of
hardness, English degree of hardness or Clark, USA degree of hardness and German degree of
hardness.
1. Parts per million (ppm): It is the number of parts of calcium carbonateequivalent
hardness present in one million parts of water.
1 ppm 1 part of CaCO3 equivalent hardness in 106 parts of water.
2. Milligram per lite (mg/L): It is thenumber of milligrams of Calciumcarbonate equivalent
hardness present in one litre of water.
1 mg/lit. 1 mg of CaCO3 equivalent hardness in 1 litre of water.
3. Degree Clarke (oCl): It is the number of parts of CaCO equivalent
3
hardness present in
70,000 parts of water.
4. Degree French (oFr): It is the number of parts of CaCO equivalent
3
hardness present
5
in 10 (1 Lakh) parts of water.
1oFr = 1 part of CaCO equivalent
3
hardness in 105 parts of water.The
above four units are correlated as given below:
1 ppm = 1 mg/L = 0.07oCl = 0.1oFr
Disadvantages of Hard Water
(a) In domestic uses. For washing and bathing, hard water creates difficulties, since it does not
form lather freely with soap. It also creates sticky precipitates that deposit on bath tub,
body, clothes etc. until all the Ca/Mg salts get precipitated. Thus a lot of soap gets wasted
also.
For cooking hard water creates similar difficulties by producing scum on the bottom of the
vessels. Due to the presence of hardness producing salts in hard water, boiling point gets
elevated and during cooking a lot of fuel is wasted. Pulses etc. do not cook in hard water.
Taste of tea, coffee becomes unpleasant.
Drinking of hard water is also problematic since it affects the digestive system and at the
same time the possibility of deposition of calcium oxalate crystals in the urinary tract is
alarming.
(b) In industrial uses. For textile industry and dyeing industry, hard water causes the usual
problem of deposition of insoluble salts that interfere with the proper dyeing and printing ofthe
fabrics. The stains of iron salts also are undesirable on fabrics. Hard water also hampers the
economy by wastage of soap as it does not form good lather.
For sugar industry, the salts responsible for hardness create difficulties in sugar refiningand
crystallization of sugar and the sugar becomes deliquescent.
Calcium and magnesium salts also interfere with the smooth and glossy finish of the
papers in the paper industry. Iron salts interfere with the colour of the paper.
In laundry, hard water causes wastage of costly soap and also interferes with the
coloration due to the staining of iron salts.
The hydration of cement and final hardening of cement are affected by use of hard
water in concrete making.
Hard water is not suitable for preparing drug solutions in pharmaceutical industry
For steam generation in boilers, hard water creates many problems like:
(i) scale and sludge formation,
(ii) corrosion,
(iii) priming and foaming and
(iv) caustic embrittlement.
BOILER TROUBLES
Scale and Sludge
When hard water is used for boilers, on continuous evaporation, the salts present in thehard
water gets saturated and are finally deposited in the areas where the flow is slow. When the
precipitates formed are loose and slimy in nature, it is called sludge, whereas when the precipitates
formed are hard and adhere strongly on the inner walls of the boiler, they are called scale.
Sludge can be easily removed by scrapping with a brush. Sludge is formed by the presenceof
MgCO3, MgSO4, MgCl2, CaCl2 etc. These salts are more soluble in hot water.
Disadvantages of sludge formation are:
(i) poor heat conduction due to the presence of sludge on the surface;
(ii) difficulty in the operation of the boiler;
(iii) if sludge is formed along with the scale and is trapped within the scale formed and soit is
difficult to remove and
Water
Boiler Hard adhering coating
Wall on inner walls of boiler
(scale)
Loose precipitate
suspended in water
(sludge)
Heat Heat
Figure 1.1: Scale and sludge formation in boilers
Removal of Sludge
Sludge formation can be prevented by:
(i) using soft water for boiler operation
(ii) removing the concentrated salty water from time to time so that deposition of sludge is
prevented. Scales are the hard deposits on the inner surface of the boilers which are
difficult to remove.
This scale formation takes place due to the following reasons:
(a) In low pressure boilers scale formation occurs due to the formation of CaCO3 from
Ca(HCO3)2.
Ca(HCO3 ) 2
CaCO3 H 2 O CO 2
scale
(b) In high pressure boilers this CaCO3 gets converted to soluble Ca(OH)2. But here
CaSO4 forms the hard scale. Since the solubility of CaSO4 decreases with increase in
temperature, and at high temperature the precipitated CaSO4 forms hard scale. Similar
hard scales are formed when SiO2 is present in the hard water. It deposits as CaSiO3 or
MgSiO3. These calcium or magnesium silicate scales are very difficult to remove.
Dissolved magnesium salts also precipitate as Mg(OH)2 forming soft type of scale.
Disadvantages of scale formation are similar to sludge formation but the severity is
more, since its removal is more difficult.
Disadvantages include
(i) Poor heat transfer from boiler to water leading to increase in fuel consumption. The
increase in thickness of the scale from 1.25 mm to 12 mm leads to increase in fuel
consumption from over 50% to 150%.
(ii) Due to the overheating of the boiler, different parts of the boiler become weak and
distorted and so the operation of the boiler becomes unsafe, particularly the high
pressure boilers. The thick scales may sometimes lead to explosion due to sudden
development of high pressure.
(iii) Valves and condensers of the boilers are chocked due to scale formation and Boiler
efficiency decreases.
Removal of scales can be done by:
(i) Wooden scraper or wire brush, suitable for removing loose scales.
(ii) Blow down operations for loose scales. The operation actually involves the removal of
very hard water from a tap at the bottom of the boiler and replenishing the water with
softened water called ‘make up’ water.
(iii) Giving thermal shocks, which involve alternate heating and cooling to make the scales
brittle.
(iv) Chemical treatment with 5-10% HCl for carbonates and EDTAtreatment for Ca/Mg
salts forming complexes.
Differences between Sludge and Scale
S.No. Sludge Scale
1. Loose, slim , non-adherent precipitate Hard, thick , strong adherent precipitate
2. Due to salts like MgSO4, MgCl2 Due to salts like CaSO4, Ca(HCO3)2
3. Due to poor conductance, they decrease Due to poor conductance, they decrease the
the boiler efficiency to lesser extent and boiler efficiency to maximum extent, cause
causing chocking in the pipelines. reduced fuel economy, improper boiling,
boiler explosion etc.,
4. It can be prevented by periodical It can be prevented by special methods like
replacement of concentrated hard water i) External treatment of ion exchange,
by fresh water. This process is known as ii) Internal carbonate, phosphate,
“blow down” method. Calgonconditioning and iii)
Mechanical hard
scrubbing methods.
2. Caustic embrittlement
Definition:Intercrystalline cracking of boiler material
It is a form of stress corrosion takes place in boilers operating at high
temperature(200-250˚ C) and pressure. Caustic embrittlement focus at stressed part of
boilers such as cracks, rivets, bents, joints etc. The boiler fed water usually contains some
residual sodium carbonate (used for softening process). At high temperature and pressure
it undergoes hydrolysis to form sodiumhydroxide.
Na2CO3 + H2O --------- 2 NaOH + CO2
The alkali water sweeps through the minute cracks, crevices between the rivets and joints
by capillary action. Inside the cracks water gets evaporated leaving behind NaOH. The
concentrations of the NaOH gradually increase on these sites due to poor circulation of water.
When concentrations of the NaOH reaches a value of 10% it attacks the metal at the stressed
region dissolving it in the form of sodium ferroate ( Na2FeO2). Sodium ferroate undergoes
hydrolysis-depositing magnetite as follows
3Na2FeO2 + 4H2O ---------- 6NaOH + Fe3O4 + H2
6Na2FeO2 + 6H2O + O2 ---------- 12NaOH + 2Fe3O4
So NaOH is regenerated in the process and its concentration is keep on increasing maintaining a
required environment. Thus corrosion process develops cracks and making themetal brittle by
the deposition of the product.
3. Priming and foaming
Foaming: Bubbles produced in the water due to heat and pass out with the steam. This is called
foaming and it is caused by high concentration of impurities in the boiler water.
Priming: It is the carry over of varying amounts of droplets of water in the steam, whichlowers the
energy efficiency of the steam and leads to the deposit of water droplets of impurities and cause the
damage of boiler.
Prevention: To prevent it, use of chemicals like anti-foaming and anti-priming agents whichprevents
the formation of priming and foaming in the boiler water.
4) Boiler Corrosion
It may be due to three major reasons:
i) Dissolved Oxygen
ii) Dissolved CO2
iii) Dissolved salts like MgCl2
DESALINATION PROCESS
Definition: Process of removing common salt from sea water.. Seawater desalination hasthe
potential to reliably produce enough potable water to support large populations.
Types of process:
Reverse osmosis (RO) and Nanofiltration (NF) are the leading pressure driven
membrane processes. Contemporary membranes are primarily polymeric materials with
cellulose acetate still used to a much lesser degree. Operating pressures for RO and NF arein
the range of 50 to 1,000 psig (3.4 to 68 bar, 345 to 6896 kPa).
Electrodialysis (ED) and Electrodialysis Reversal (EDR) processes are driven by
direct current (DC) in which ions (as opposed to water in pressure driven processes) flow
through ion selective membranes to electrodes of opposite charge.
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 solutemolecules, then the solvent will
tend to diffuse across the membrane from the less concentrated to the more concentrated solution.
This process is called osmosis.
Osmosis is of great importance in biological processes where the solvent is water. The transport
of water and other molecules across biological membranes is essential to many processes in living
organisms. The energy which drives the process is usually discussed in terms of osmotic pressure..
Reverse osmosis
If hydrostatic pressure greater than the osmotic pressure is applied then the flow of
solvent from high concentration to low concenteration occurs. This is called reverse osmosis.
