Download Softening of hard water

UNIT 1: WATER TECHNOLGY
Introduction:Water is the major creation of the nature. It covers about three-fourth part of the earth’s
surface out of this 99.4% is locked up into seas, oceans, glaciers and polar ice caps. However
about 1% is available for human requirements, plants and animals. Also water is essential
materials in the industries for steam generation, power generation, construction materials and
textile.
Source of water: The main sources of water are as follows.
1) Ground water or Surface water: - It includes Rain water, River water, lake water and sea
water.
a) Rainwater: It is the purest form of natural water. Still it contains impurities such as CO2,
NO2, SO2, etc.
b) River water: It contains soluble minerals of the soil such as chlorides, sulphates,
bicarbonates of sodium, calcium, magnesium and iron. It also contains organic matters and
suspended impurities.
c) Lake water: It contains much lesser amount of dissolved minerals but the quantity of
organic matter is higher.
d) Sea water: It is the most impure from of natural water. It contains about 2.6% sodium
chloride and other salts like sodium sulphate, bicarbonates of potassium, magnesium and
calcium etc.
2) Underground water or well water: It contains more amounts of dissolved salts. Hence
hardness is more than surface water. It is of high organic purity and clear in appearance.
Impurities in water: The impurities present in water may be broadly classified as follows.
1) Suspended impurities: These are due to presence of suspended particles of clay and organic
matter which can be removed by sedimentation process.
2) Colloidal impurities: These are due to presence of very fine suspended particles of clay and
organic matter which can be removed by coagulation process.
3) Mineral impurities: These are due to presence of dissolved mineral salts of Ca, Mg & other
heavy metals which can be removed by special chemical treatment process like lime-soda,
zeolite or ion-exchange process.
4) Biological impurities: These are due to presence of Micro-organisms, bacteria and viruses
etc. these pathogens are harmful to human being, which can be removed by sterilization of
water such as boiling, using chemicals like bleaching powder, UV-rays, ozonization.
Hardness of Water:
Definition of hardness:
It is a property of water which prevents lathering of soap or it is the soap consuming
capacity of water.
The property of hardness in water is due to presence of certain salts of calcium,
magnesium and other heavy metals in dissolved form. When a sample of hard water treated with
soap (sodium or potassium salt of higher fatty acids), it does not produce lather. On the other
hand, it forms a white scum or precipitate. This is due to formation of insoluble soaps of calcium
& magnesium as following.
2C17H35COONa + CaCl2
(C17 H35 COO)2Ca ↓+ 2NaCl
Sodium stearate
Calcium stearate
(Insoluble)
(C17 H35 COO)2Mg↓ + Na2SO4
2C17H35COONa + MgSO4
Magnesium stearate
(Insoluble)
It seems from above reactions that soap forms white precipitate or scum instead of lather. Thus,
soap is wasted. The water, which does not produce lather readily with soap but forms a white
scum, is called hard water. The hard water contains more than 60 ppm hardness. The water
which produces lather easily with soap is called soft water. Usually it has hardness upto 60 ppm.
Types of hardness: Hardness may be divided into two types.
1) Temporary or carbonate hardness:
It is caused by the presence of dissolved bicarbonates of Ca, Mg and other heavy metals
and the carbonate of iron. It can be removed by mere boiling. On boiling, bicarbonates convert
into insoluble carbonates or hydroxides. The reactions are as follow.
Ca(HCO3)2
Mg(HCO3)2
CaCO3↓+ H2O+ CO2↑
Mg(OH)2↓+ CO2↑
2) Permanent or Non- carbonate Hardness:
It is caused due to the presence of sulphates, chlorides and nitrates of Ca, Mg, Fe and
other heavy metals. It is also known as Non alkaline hardness. It can not be removed by simple
boiling. It can be removed by softening methods such as lime soda, zeolite or ion exchange
process.
Units of Hardness
1) Parts per million (ppm):
It is the part of CaCO3equivalent hardness per 106 parts of water.
2) Milligrams Per liter (mg/dm3):
It is the no of milligrams of CaCO3 equivalent hardness per liter of water.
1mg/L =1 mg CaCO3 equivalent per 106 mg of water
3) Degree Clarkes (°Cl):
It is the no. of grams of CaCO3 equivalent per gallon of water.
1°Cl = 1 part of CaCO3 per 70,000 parts of water.
4) Degree French (°Fr):
It is the part of CaCO3equvalent hardness per 105 parts of water.
1°Fr = 1 part of CaCO3equvalent hardness per 105 parts of water.
Inter-conversion between various units of hardness:
1ppm = 1mg/L = 0.1°Fr = 0.07°Cl =0.02meq/L.
Determination of Hardness:
Hardness of water can be determined by any of the following methods.
1) O’Hehner’s method
2) Soap titration method
3) EDTA Method.
EDTA Method or Complexmetric method:
Principle:
In this method EDTA acts as a complexing agent. It forms complex with various metal
ions present in hard water. Hence this method is also known as complexometric method. The
method includes the titration of standard solution of EDTA (disodium salt of EDTA) with hard
water sample using Erichrome Black–T (EBT) indicator. EBT forms unstable wine-red complex
with metal ions Ca2+ and Mg2+ in hard water at 10 PH.
M2+ + EBT
(M2+= Ca2+ or Mg2+)
[M-EBT]
red complex (Unstable)
During the course of titration, EDTA decomposes the above complex. It forms stable
complex with metal ions and EBT becomes free.
[M-EBT] + EDTA
Wine red complex
(Unstable)
[M- EDTA] + EBT
Blue complex
(Stable)
Thus end point is indicated by the change of wine-red colour to blue colour. Knowing the
volume of EDTA, hardness can be determined.
The structure of disodium salt of EDTA and M-EDTA complex is as follows.
Requirement of Solution:
In this process, following solutions are required.
1) Standard Hard Water (SHW): It is prepared by dissolving 1.0g of pure and dried CaCO3 in
concentrated HCl. This solution is heated to obtain precipitate by removal excess of HCl. The
precipitate is then dissolved into distilled water to make1 liter solution.
1 ml of this solution contains 1mg CaCO3 (1ml SHW = 1 mg CaCO3)
2) EDTA solution: It is prepared by dissolving 4 g of pure EDTA + 0.1g Mg Cl2 in 1 liter of
distilled water.
3) EBT indicator solution: It is prepared by dissolving dissolve 0.5g of Eriochrome black-T in
100 ml of alcohol.
4) Buffer solution: It is prepared by dissolving 67.5g of NH4Cl in 570 ml of concentrated
ammonia solution and dilute to 1 liter with distilled water.
Procedure:
1) Standardization of EDTA solution:
Take 50 ml of standard Hard water (SHW) in a conical flask with the help of pipette. Add
10-15 ml of buffer solution and 4-5 drops of EBT indicator. Titrate with EDTA solution till
wine-red colour changes to deep blue. Let volume of EDTA required is V1 ml.
2) Titration of Unknown hard water:
Titrate 50 ml of unknown water sample as above. Let volume of EDTA required is V2 ml.
3) Titration of boiled water:
Take 250 ml of hard water sample in a large beaker. Boil it till the volume remains upto
50 ml. Filter obtained precipitate from the water. Finally make the volume of filtered water
upto 250 ml by distilled water. Titrate 50 ml of this water sample as above. Let volume of
EDTA required is V3 ml.
Calculations: The hardness of water is calculated as follow.
1) Standardization of EDTA:
V1 ml of EDTA = 50 ml of SHW
V1 ml of EDTA = 50 mg of CaCO3
( 1ml SHW = 1 mg CaCO3)
50
1 ml of EDTA = 𝑉1 mg of CaCO3
2) To find total hardness:
50
1 ml of EDTA = 𝑉1 mg of CaCO3
 V2 ml EDTA =
𝑉2×50
𝑉1
mg of CaCO3
We know that, V2 ml of EDTA = 50 ml of hard water sample
 50ml of hard water =
𝑉2×50
𝑉1
mg of CaCO3
𝑉2
 1000 ml of given hard water = 1000× ( 𝑉1 ) mg of CaCO3
 Total hardness of water = 1000× (
3) To find permanent hardness:
𝑉2
𝑉1
) mg/L or (ppm)
50
1 ml of EDTA = 𝑉1 mg of CaCO3
 V3 ml EDTA =
𝑉3×50
𝑉1
mg of CaCO3
We know that, V3 ml of EDTA = 50 ml of boiled water sample
50ml of Boiled water =
𝑉3×50
𝑉1
mg of CaCO3
1000 ml of given Boiled water = 1000×(
Permanent hardness =1000 × (
𝑉3
𝑉1
𝑉3
𝑉1
) mg of CaCO3
) mg/L or (ppm)
Temporary hardness = (Total  permanent) Hardness
𝑉2
𝑉3
= 1000 𝑉1 − 𝑉1 𝑝𝑝𝑚
=
1000(𝑉2−𝑉3)
𝑉1
𝑝𝑝𝑚
Advantages of EDTA method: In comparison with other methods, this method is 1) Very
accurate, 2) Convenient, and 3) more rapid. Hence this method is most commonly used for the
determination of hardness.
Softening of hard water:
The process of removing hardness producing salts from water is known as Softening of
hard water. The hardness causing salts can be removed from the water by two methods.
1) Internal treatment:
This process is carried out inside the boiler. It includes calgon, phosphate, carbonate and
aluminate conditioning.
2) External treatmentThis process is carried out outside the boiler. It includes lime soda process, Zeolite
process and ion-exchange process.
Lime-soda process:
Principle:
In this process, required amount of Lime (Ca(OH)2) and Soda (Na2CO3) are added to the
hard water. The hardness causing salts react with lime and soda. During the reactions, insoluble
precipitates of carbonate and hydroxides (e.g. CaCO3, Mg(OH)2 etc. are formed. These
precipitates may be removed by settling and filtration. It involves following chemical reactions.
i) Lime removes temporary hardness
Ca (HCO3)2 + Ca(OH)2
Mg (HCO3)2 + 2Ca(OH)2
2 CaCO3
+ 2H2O
Mg (OH)2 + 2 CaCO3 + 2H2O
ii) Lime removes permanent magnesium hardness
MgCl2 + Ca (OH)2
Mg (OH)2
+ CaCl2
MgSO4 + Ca (OH)2
Mg (OH)2
+ CaSO4
Mg (NO3)2 + Ca (OH)2
Mg (OH)2
+ Ca(NO3)2
Mg2+ + Ca (OH)2
Mg (OH)2
+ Ca2+
iii) Lime removes dissolved iron and aluminium salts
FeSO4 + Ca (OH)2
Al2 (SO4)3 + Ca(OH)2
Fe(OH)2
2Al(OH)3
+ CaSO4
+ 3 CaSO4
iv) Lime removes free mineral acids
2HCl + Ca (OH)2
CaCl2 + 2H2O
H2SO4 + Ca (OH)2
CaSO4 + 2H2O
i.e., 2H+ + Ca (OH)2
Ca2+ + 2H2O
v) Lime removes dissolved CO2 and H2S
Ca (OH)2 + CO2
CaCO3 + 2H2O
Ca (OH)2 + H2S
CaS
vi) Lime also reacts with bicarbonate ions
+ 2H2O
2HCO3 + Ca (OH)2
CaCO3 + 2H2O + CO3
vii) Soda removes permanent hardness
CaCl2 + Na2CO3
CaCO3
+ 2NaCl2
CaSO4 + Na2CO3
CaSO4
+ Na2SO4
Ca2+ + Na2CO3
CaCO3
+ 2Na2+
Role of Coagulant:
During the softening of hard water by L.S. Process, various insoluble compounds (such
as magnesium hydroxide and calcium carbonate) are formed. In order to increase the rate of
precipitation of these compounds as well other impurities (clay particles, etc.), the coagulants are
added to water for the quick settling. Some of the commonly used coagulants are FeSO4.7H2O,
NaAlO2, K2SO4.Al2 (SO4)3.24H2O (alum), etc. The coagulants react with water and form the
precipitate of hydroxide as follows.
NaAlO2 + H2O
Al2 (SO4)3 + 3Ca (HCO3)2
Al(OH)3 + NaOH
2Al(OH)3 + CaSO4 + 6CO2
As shown in above reactions the precipitate of aluminium hydroxide is formed. It is flocculant
and gelatinous. It helps for settling of dust, oil, etc.
Cold lime-soda process:In this method, calculated quantity of chemicals (lime + soda) is mixed with water at room
temperature. The small amount of coagulant (like alum, sodium aluminate or aluminium
sulphate) is added for the easy settling of precipitates. Sodium aluminate removes silica and oil
also. Hardness can be removed upto 50-60 ppm. This process can be carried out in the following
ways:
a) Batch process or intermittent process:It consists of a set of two tanks, which are used alternately. Each tank is provided with inlets
for raw water and chemicals, outlets for softened water and sludge and a mechanical stirrer. Raw
water and calculated quantities of chemicals are added through inlet. They are mixed thoroughly
with the help of mechanical stirrer. The chemicals react with the salts present in water. The
precipitate of metal hydroxide and carbonates are formed. It is allowed to settle. Thus clear
softened water is collected through a float pipes which is further sent to filtering unit.
b) Continuous process:It consists of inner vertical circular chamber, fitted with paddle stirrer. Raw water and
calculated quantities of chemicals (lime + soda + coagulant) are fed from the top into the inner
chamber. Due to continuous mixing by paddle stirrer, softening reactions take place. The heavy
sludge (precipited) settles down in outer chamber and the clean softened water reaches upwards.
It passes through the filtering media (wood fiber). Thus filtered soft water is collected through
the outlet at top. Sludge is removed from the bottom at the intervals.
Fig:- Cold lime-soda process
Hot lime soda process:In the process the raw water is treated with chemicals (lime + soda) at a temperature of
80-1500C. It is maintained by passing hot stream. Hot lime soda plant consists of three parts.
a) A reaction tank- in this tank raw water, chemicals and stream are thoroughly mixed.
b) Sedimentation vessel- It is conical in shape at the bottom of the plant. Sludges settle down in
this tank.
c) A sand filter- It is the separate chamber in which filter is arranged (gravels, course sand and
fine sand).
Thus softening reactions takes place easily in reaction tank. Sludge settles at the
sedimentation vessel. Soft water is passed through the filter. Thus clean soft water is obtained.
Diagram: Hot lime soda continuous process
Advantages hot lime soda process:
1) In hot lime-soda process, the hardness is removed upto 15-30 ppm.
2) Due to high temperature reaction takes place easily and the process becomes fast.
3) No coagulants are needed in this process.
4) Stirrer is not required because the mixing of chemicals take place by blowing steam.
Advantages of L-S process:1) It is most economical.
2) Lime and soda increases pH value of water therefore corrosion of pipes is reduced.
3) Fe and Mn ions are also removed to certain extent.
4) Coagulants are not needed in hot L-S process.
5) Amount of pathogenic bacteria’s also reduce due to alkaline nature of treated water.
Disadvantages of L-S process:1) Hardness can be removed up to 15 ppm only.
2) Careful operation and skilled supervision is required.
3) Disposal of large amount of sludge poses problem.
4) Softener requires a large space.
Hot L.S. Process
In this process, reactions proceed faster.
Cold L.S. Process
In this process, reactions proceed slowly.
Stirrer is not needed, because mixing of water Stirrer is needed for the mixing of water and
and chemical take place by blowing steam.
chemicals.
Coagulants are not needed, as the precipitate Coagulants are needed, for quick steeling the
settles rapidly at high temperature.
sludge/ precipitate.
Dissolved gases such as CO2, air, etc. are Dissolved gases are not removed in this
removed.
process.
Hardness of water is removed upto 15-30 ppm
Hardness of water is removed upto 50-60ppm.
Zeolite or Permutit process:
Principle:
In this process hard water is passed through zeolite bed. When zeolite comes in contact
with hard water, its sodium ions are exchanged by hardness causing metal ions (Ca2+, Mg2+, etc.)
from hard water. Thus hardness of water is removed by using zeolite.
Zeolite is hydrated sodiumaluminosilicate i.e. Na2OAl2O3xSiO2yH2O (where x is 2−10
and y is 2−6). Zeolite is also known as permutit. It is represented as Na2Ze. There are two types
of Zeolite,
1) Natural Zeolite:
They are obtained from natural rocks, for example natrolite ( Na2OAl2O34SiO22H2O)
2) Synthetic zeolite:
They are prepared by heating together china clay, feldspar and soda ash. They possess high
exchange capacity than natural zeolite.
Process:
Zeolite softner consists of vertical cylindrical tank. It is packed with a bed of zeolite
(Na2Ze). The hard water is allowed to pass through zeolite bed at specified rate.
The hardness causing ions (Ca2+, Mg2+, etc.) are exchange with Na+ ions of zeolite. Thus,
Ca2+, Mg2+, etc. ions are retained by zeolite and Na+ ions flow in outgoing water. Following
reactions take place during the softening process.
Na2Ze + Ca(HCO3)3
Na2Ze + Mg(HCO3)3
CaZe + 2NaHCO3
MgZe + 2NaHCO3
Na2Ze + CaCl2
CaZe + 2NaCl
Na2Ze + MgCl2
MgZe + 2NaCl
Na2Ze + CaSO4
CaZe + Na2SO4
Na2Ze + MgSO4
MgZe + Na2SO4
Diagram: Zeolite process
Regeneration of exhausted zeolite:
When zeolite is completely exhausted, it can be regenerated by washing with 10% NaCl
solution (brine Solution). Following chemical reactions take place during regeneration
CaZe + 2NaCl
Na2Ze + CaCl2
MgZe + 2NaCl
Na2Ze + MgCl2
Thus zeolite bed can be reused for further softening.
Advantages of zeolite process:
1) Hardness of water removed upto 10 ppm.
2) The equipment is compact and occupies less space.
3) The process does not produce any sludge and hence it is clean process.
4) It requires less time for softening.
5) It is for operation and maintenance.
Disadvantages of zeolite process:
1) The softened water contains more amounts of sodium salts.
2) Acidic water destroys the zeolite bed.
3) If water is highly turbid, suspended particles are deposited on the zeolite bed. Thus, it clogs
the pores of zeolite bed and affects softening process.
4) Anions like HCO3−, CO3−, Cl, SO42, etc. in hard water do not exchange with zeolite bed.
These anions remain in soft water in the form of sodium salts. It causes alkalinity to soft
water. They hydrolyze into NaOH at high temperature in boilers and causes caustic
embrittlement.
5) The hardness water is removed upto 10 ppm only.
6) The cost of equipment and materials is expensive.
Limitations of zeolite process:
1) If the raw water is turbid, the suspended matters block the pours of zeolite bed. Hence these
impurities must be removed before treatment.
2) The coloured ions like Fe2+, Mn2+, etc. react with zeolite. Such zeolite cannot be easily
regenerated. Hence these ions must be removed before treatment.
3) Mineral acids destroy the zeolite bed. Hence they must be removed with the help of soda
before treatment by zeolite process.
Ion Exchange or de-ionization or de-mineralization process:
Principle:
In an ion exchange process, both cations (Ca2+, Mg2+, etc.) and anions (Cl, SO42, etc.)
are removed from hard water by using ion exchange resins. These resins are cross-linked, long
chain organic polymers with microporous structure. They are of two types.

Cation Exchange resins (RH):
These resins are co-polymers of styrene-divinylbenzene containing acid groups like –SO3H, -
COOH, etc. They are represented by RH or RH+. They are capable of exchanging their H+ ions
with cations like Ca2+, Mg2+, etc. from hard water.

Anion Exchange resins (ROH):
These resins are co-polymers of styrene-divinylbenzene or amine-formaldehyde containing
basic groups like amino (NH3+), quaternary ammonium [(CH3)3N+], etc. in hydroxide form.
They are represented by ROH or ROH.
Process:
In practice, cation exchange resins are packed in separate columns. Raw water is first
passed through cation exchange column. In this column, cations Ca2+, Mg2+, etc. from hard water
are exchanged with H+ ions of resin. The reactions are as follow.
2RH + Ca2+
R2Ca + 2H+
2RH + Mg2+
R2Mg + 2H+
After the removal of cations from the hard water, it is then passed through anion
exchange column. In this column, all the anions (Cl, SO42, etc.) from hard water are exchanged
with OH ions of resin. The reactions are as follow.
ROH + Cl
RCl + OH
R2SO4 + 2OH
2ROH + SO42
H+ and OH ions released during above reactions, combine to produce water.
H+ + OH
H2O
Thus, water obtained by this process is free from cations as well anions which are
responsible for hardness. Hence this soft water is known as de-ionized or de-mineralized water.
Diagram: Ion exchange resins
Regeneration of ion exchange resins:
Both cation and anion exchangers lose their ion exchange capacity after working for
certain period. At this stage, they are said to be exhausted. The exhausted cation exchange resin
is regenerated by passing dilute HCl or dilutes H2SO4. During the regeneration, cation exchanger
gets back its H+ ion. The reactions are as follow.
R2Ca + 2HCl
2RH + CaCl2
R2Mg + 2HCl
2RH + MgCl2
The exhausted anion exchange resin is regenerated by passing dilute NaOH solution.
During the regeneration, anion exchanger gets back its OH ion. The reactions are as follow.
RCl + NaOH
ROH + NaCl
R2SO4 + 2NaOH
2ROH + Na2SO4
After the regeneration, both columns are washed with de-ionized water and washing is
passed to sink or drainage.
Advantages of ion exchange resins:
1) Hardness of water is removed up to 0 to 2 ppm. Hence this soft water is fit for use in high
pressure boilers.
2) Both acidic and alkaline water can be softened by this process.
3) No coagulants are required.
4) All types of ions can be removed by this process.
Disadvantages of ion exchange resins:
1) Cost of equipment is very high.
2) More expensive chemicals are needed.
3) Turbidity of raw water must be below 10 ppm. If it is more, it has to be removed by
coagulation, filtration, etc.