Water treatment

Water treatment is any process that
makes water more acceptable for a specific end-use.
The end use may be drinking, industrial water supply,
irrigation, river flow maintenance, water recreation or
many other uses including being safely returned to the
environment. Water treatment removes contaminants or
reduces their concentration so that the water becomes fit
for its desired end-use.

The main sources of water are:
(i) Surface water: It includes flowing water (streams and
rivers) and still water (lakes, ponds and reservoirs).
(ii) Underground water: It includes water from wells and
springs.
(iii) Rain water
(iv) Sea water.

Moorland Surface
Drainage
Lowland Surface
Drainage
Deep well water
SOURCES FOR
INDUUSTRIAL PURPOSE

(a)Moorland Surface Drainage –
 Fairly constant in composition
 Generally clear and coloured brown.
 Slightly acidic in nature
 Hardness of water is low
(b) Lowland Surface Drainage –
 Vary widely in composition
 Generally not coloured but may contain fine mud in
suspension
 Hardness is usually high
(c) Deep Well Waters –
 Fairly constant in composition
 Usually colourless and clean with sparkling
 The hardness is purely alkaline hardness
 Sulphate content is low

The impurities present in water may be categorised into following
categories:
(A) Dissolved Impurities
(a) Dissolved gases: O2 , CO2 , H2S etc.
(b) Inorganic salts:
a) Cations: Ca++, Mg++, Na+ , K+ , Fe++, Al+++ etc.
b) Anions: CO3
– , Cl– , SO4
– , NO3
– etc.
(c) Organic salts
(B) Suspended Impurities
(a) Inorganic: Clay and sand.
(b) Organic: Oil globules, vegetables, and animal material.
(C) Colloidal Impurities: Finally divided clay and silica Al(OH)3 ,
Fe(OH)3 , organic waste products, colouring matter, amino acids etc.
(D) Microscopic Matters :Bacteria, algae, fungi etc.

Following are the sources of impurities in water:
(i) Gases (O2 , CO2 etc.) are picked up from the atmosphere
by rainwater.
(ii) Decomposition of plants and animals remains introduce
organic impurities in water.
(iii) Water dissolves impurities when it comes in contact with
ground, soil or rocks.
(iv) Impurities are also introduced in water when it comes in
contact with sewage or industrial waste.

River water contains dissolved minerals like chlorides, sulphates,
bicarbonates of sodium, magnesium, calcium and iron. It also contains
suspended impurities of sand, rocks and organic matter. The composition
of river water is not constant. The amount of dissolved impurities in it
depends on its contacts of the soil. Greater the duration of contact, more
soluble is the minerals of soil in it.
Lake water has high quantity of organic matter present in it but lesser
amount of dissolved minerals. Its chemical composition is also constant.

Rain water is obtaining as a result of evaporation from the surface water.
Probably it is the purest form of natural water. But during its downward journey
through the atmosphere it dissolves organic and inorganic suspended particles and
considerable amount of industrial gases like (CO2 , NO2 , SO2 etc.). Rain water is
expensive to collect and is irregular in supply.
Underground water is free from organic impurities and is clearer in appearance
due to the filtering action of the soil. But it contains large amount of dissolved
salts.
Sea water is very impure due to two reasons:
1. Continuous evaporation increases the dissolved impurities content,
which is further increased by the impurities thrown by rivers as they
join sea.
2. It is too saline for most industrial uses except cooling.

“Hardness in water is that characteristics, which prevents the
lathering of soap”. In other way we may define it as “soap consuming
capacity of water”.
Cause of Hardness:
Hardness is due to presence of certain salts of Ca2+, Mg2+ and
other heavy metal ions like Al3+, Fe3+ and Mn2+ in water.
Mechanism of cause of hardness:
It can be explained by the reaction of soap in soft and hard water.

Reaction of soap in soft water
When soft water is treated with soap, lather is produced according to the
following reaction:
C17H35COONa + H2O C17H35COOH + NaOH
Soap (Sodium stearate) Stearic acid
C17H35COONa + C17H35COOH Lather Soap
(Sodium stearate) Stearic acid
Actually hardness is due to presence of Cl– , SO4
– –, HCO3
– and CO3
– of Ca++,
Mg++ and other heavy metal ions like Fe++, Al+++ and Mn++. The presence of
CO2 also breaks up Na or K soaps into free fatty acids and does not allow lather
to be formed.

Reaction of soap in hard water
A sample of hard water, when treated with soap (sodium or
potassium salt of higher fatty acid like oleic, palmitic and stearic acid), does
not produce lather, but on the other hand forms insoluble white scum or
precipitate which do not possess any detergent action. This is due to the
formation of insoluble soap of calcium and magnesium. Typical reaction of
soap (sodium stearate) with calcium chloride and magnesium sulphate are
shown below.

Types of Hardness
It is of following types:
1. Temporary Hardness
(a) Temporary hardness is caused by the presence of dissolved bicarbonates of
calcium, magnesium and other heavy metals and the carbonates of iron and
other metals also. Thus, the main salts responsible for temporary hardness are
Ca(HCO3)2 and Mg(HCO3)2 .
(b) Temporary hardness can be largely removed by more boiling of water, when
bicarbonates are decomposed, yielding in soluble carbonates or hydroxides,
which are deposited as a crust at the bottom of vessel.
(c) Temporary hardness is also known as carbonate hardness or alkaline
hardness.
(d) Alkaline hardness is due to the presence of bicarbonates, carbonates and
hydroxides of the hardness producing metal ions. This is determined by
titration with HCl using methyl orange as indicator.

2. Permanent Hardness
(a) It is due to the presence of dissolved chlorides and sulphates of
calcium, magnesium, iron and other heavy metals. Hence, the salts
responsible for permanent hardness are CaCl2 , MgCl2 , CaSO4 ,
MgSO4 , FeSO4 , Al2(SO4)3 etc.
(b) Unlike temporary hardness, permanent hardness is not destroyed on
boiling.
(c) It is also known as non-carbonate or non-alkaline hardness.
(d) The difference between total hardness and alkaline hardness gives
the nonalkaline hardness.

Degree of Hardness
• Although hardness of water is never present in the form of the calcium carbonate,
because it is insoluble in water, hardness of water is conveniently expressed in terms of
equivalent amount of CaCO3.
• The reason for choosing CaCO3 as the standard for reporting hardness water is the ease
in calculation as its molecular weight is exactly 100. Moreover, it is the most insoluble
salt that can be precipitate in water treatment.
Equivalents of CaCO3=[Amount of hardness producing substance] × [Chemical equivalent
of CaCO3 (= 50)] × 2/[Chemical equivalent of hardness producing substance]× 2
= ([Amount of hardness producing substance] / [Chemical equivalent of
hardness producing substance × 2]) × 100
= [Amount of hardness producing substance] × (Multiplication factor) in ppm.

Units of Hardness :
Unit Definition
1 ppm 3 eq. hardness in 106 parts of water. “It is the number of parts by weight of
CaCO3 equivalent hardness present per million parts of water”.
1 mg/lit. Mg of CaCO3 eq. hardness in 1 lit. of water. “It is the number of mg of CaCO3
equivalent hardness present per lit of water
“1 mg/lit = 1 ppm”.
°Fr Parts of CaCO3 eq. hardness in 105 parts of water. “It is the parts of CaCO3
equivalent hardness present per 105 parts of water.”
°Cl Parts of CaCO3 eq. hardness in 70,000 parts of water. “It is the number of Grains
(1/7000 lb)*of CaCO3 equivalent hardness present per gallon (10 lb) of water or
parts of CaCO3 equivalent hardness present per 70,000 parts of water”.
1 meq/lit. Meqs of CaCO3 eq. hardness in 106 parts of water. “It is the number of
milliequivalents of CaCO3 equivalent hardness present per lit of water” 1
meq/lit. = 50 ppm.

Relation between various units of hardness:

SEDIMENTATION
 It is the next step in conventional filtration plants. The purpose of sedimentation is to
enhance the filtration process by removing particulates.
 Sedimentation is the process by which suspended particles are removed from the
water by means of gravity or separation.
 In the sedimentation process, the water passes through a relatively quiet and still
basin. In these conditions, the floc particles settle to the bottom of the basin, while
“clear” water passes out of the basin over an effluent baffle or weir. Figure illustrates
a typical rectangular sedimentation basin.
 The solids collect on the basin bottom and are removed by a mechanical “sludge
collection” device. The sludge collection device scrapes the solids (sludge) to a
collection point within the basin from which it is pumped to disposal or to a sludge
treatment process.

COAGULATION
 Finely divided silica, clay and organic matter does not settle down easily and hence
cannot be removed by simple sedimentation. Most of these are in colloidal form and
are generally negatively charged. These are removed by adding certain chemical
which produces ions of right charges that neutralize colloidal particle. This process is
termed as coagulation.
 Aluminium sulphate is the most common coagulating agent used for removing clay
particle and is generally called filter alum. When this is added to natural waters,
hydrolyses to form colloidal aluminium hydroxide and an eq. quantity of sulphuric
acid as follows :
Al2(SO4)3 + 6H2O 2Al(OH)3 + 3H2SO4
 The Al(OH)3 so formed acts as floc or coagulant, which has an enormous surface
area per unit volume and removes the finely divided and colloidal impurities by
neutralisation.

FILTRATION
 Filtration is commonly the mechanical or physical
operation which is used for the separation of solids
from fluids (liquids or gases) by interposing a
medium through which only the fluid can pass.
 The fluid that passes through is called the filtrate.
Oversize solids in the fluid are retained, but the
separation is not complete; solids will be
contaminated with some fluid and filtrate will
contain fine particles (depending on the pore size and filter thickness).
 Filtration is also used to describe some biological processes, especially in water
treatment and sewage treatment in which undesirable constituents are removed
by absorption into a biological film grown on or in the filter medium as in slow
sand filtration.

 Slow Sand Filtration-
Slow sand filtration have a number of
unique qualities:
1. Unlike other filtration methods,
slow sand filteration use biological
processes to clean the water, and
are non-pressurized systems. Slow
sand filters do not require
chemicals or electricity to operate.
2. leaning is traditionally done by use of a mechanical scraper, which is usually driven
into the filter bed once the bed has been dried out. However, some slow sand filter
operators use a method called "wet harrowing", where the sand is scraped while
still under water, and the water used for cleaning is drained to waste.
3. For municipal systems there usually is a certain degree of redundancy, since it is
desirable for the maximum required throughput of water to be achievable with one
or more beds out of service.
4. Slow sand filters require relatively low turbidity levels to operate efficiently. In
summer conditions and in conditions when the raw water is turbid, blinding of the
filters occurs more quickly and pre-treatment is recommended.

 Rapid-Gravity Filtration
1. Rapid-gravity filters are capable of producing potable waters at flow rates
as high as 100 gal/sq. ft./hour. This is achieved by using carefully graded
quartz sand and collecting the filterate as evenly as possible over the entire
bottom area of the sand bed which avoids undesirable channeling.
2. Rapid-Gravity filteration has the following advantages-
a) The filter bed and the quantity of the filterate can be easily inspected.
b) Filter is unaffected by pressure variations on either the inflow or draw-
off sides.
c) Large reinforced concrete filters can be constructed at relatively low
cost.

 Rapid-Pressure Filtration
1. Rapid-pressure filters are much more widely used than rapid-gravity
filtration.
2. Filtration rates are of order of 80 to >200 gallons/sq.ft./hour.
3. This filters are manufactured in Horizontal and Vertical types

STERILIZATION OF WATER
 Sterilization is a term referring to any process that eliminates or kills all forms
of life and other biological agents (such as viruses which some do not consider
to be alive but are biological pathogens nonetheless), excluding prions which
cannot be killed, including transmissible agents (such
as fungi, bacteria, viruses, prions, spore forms unicellular eukaryotic organisms
such as Plasmodium, etc.) present in a specified region.
 Chlorine is most common sterilizing agent in water treatment. It may be added
in form of bleaching powder or directly as a gas or in the form of concentrated
solution in water.

 Softening is the chemical processes in which hardness causing ions ( Ca 2+ ,
Mg2+ ) are removed from water either completely or partially.
 Softening may be a chivied by chemical precipitation using the Lime- Soda Ash
method or by ion exchange.
 In the chemical precipitation method the objective is to produce CaCO3 and
Mg(OH)2 :
Ca2+ + CO3
2- 2CaCO3(s)
Mg2+ + 2OH- Mg(OH)2(s)
 These two reactions are achieved by the addition of Lime [Ca(OH)2] and Soda
ash [ Na2CO3] as will be shown.

 A common water treatment goal is to provide a water with a hardness in the
range of 75 to 120 mg/L as CaCO3.
 To precipitate CaCO3 and Mg(OH)2 we need to raise the pH to 10.3 by the
addition of Lime [Ca(OH)2]. The addition of the OH- will convert HCO3- to
CO3
2-.
 To precipitate Mg(OH)2 we need to raise the pH to 11 by the addition of
Soda ash [ Na2CO3]. This will add the CO3
2- ion needed to react with the
remaining Ca2+ .
 Some of the added lime [Ca(OH)2] is consumed to remove CO2 which is
necessary to raise the pH.

Boiler water treatment
Boiler water treatment is a type of industrial water
treatment focused on removal or chemical modification of substances
potentially damaging to the boiler. Varying types of treatment are used at
different locations to avoid scale, corrosion, or foaming. External treatment
of raw water supplies intended for use within a boiler is focused on removal
of impurities before they reach the boiler. Internal treatment within the
boiler is focused on limiting the tendency of water to dissolve the boiler,
and maintaining impurities in forms least likely to cause trouble before they
can be removed from the boiler in boiler blowdown.
The treatment of boiler water can be put in to two parts. These are
(1) internal treatment and
(2) external treatment.

The internal treatment is for boiler feed water and external
treatment is for make-up feed water and the condensate part of the system.
Internal treatment protects against feed water hardness by preventing
precipitating of scale on the boiler tubes. This treatment also protects against
concentrations of dissolved and suspended solids in the feed water without
priming or foaming. These treatment chemicals also help with the alkalinity
of the feed water making it more of a base to help protect against boiler
corrosion.

BOILER TROUBLES
• The major boiler troubles caused by the use of unsuitable water are –
 Carry Over: Priming and Foaming
 Scale formation
 Corrosion
 Caustic embrittlement
• All these troubles increases with increase of operating pressure of the
boilers.
• In treatment of boiler feed water, total elimination of all the impurities is
generally not attempted.

Water treatment

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