WATER & ITS IMPURITIES
:
Water is never found in a pure state in nature because it is
an extemely good solvent. As it falls
through the atmosphere in the form of rain, it dissolves gases, in particular
oxygen, carbon dioxide, sulphur dioxide and other acid gases. This means that when the rain reaches the
ground, it is in fact a mixture of dilute acids.
At ground level the acids can have a dissolving action on
the rocks, thus taking mineral matter into solution. The amount and type of minerals dissolved
depends upon the variety of the rock encountered. In limestone or chalk areas the principal
minerals are calcium and magnesium carbonates.
These are acted upon by the acids in the rain to produce principally
calcium and magnesium bicarbonates and calcium and magnesium sulphates. Lesser amounts of the chlorides and nitrates
are also found. Other minerals taken
into solution include iron, aluminium, sodium and become alkaline. In areas where the principal rock type is
granite, the solvent action of the rain is much less. The acids in the rain are only partially
neutralised and thus the water remains on the acid side of neutrality. The amount of mineral matter is then usually
small. Silica levels, however, can be
similar to those found in waters from limestone areas.
Thus essentially the raw water essentially contains salts
consisting of following ions :
CATIONS ANIONS
Calcium Sulphates
Magnesium Carbonates
Sodium Bicarbonates
Iron Chlorides
Potassium Nitrates
Barium Flourides
Strontium Phosphates
Manganese Carbondioxide
Ammonia Silica
Depending on the various application and use the water has
to be essentially treated :
Various treatment process involve :
1) Filtration
2) Activated
Carbon Filtration
3) Chlorination
4) Softening
5) De-mineralisation
6) Reverse
Osmosis
7) Ultraviolet
Disinfection
FILTRATION
Water filtration essentially
consists of :
1) Coagulation
2) Clarification
3) Filtration
Coagulation :
The suspended solids in water acquire a negative charge
which prohibits these particles from coming together to form a large mass for
easier settleability. Coagulants such as
Alum, Ferrous Sulphate, Ferric Chloride which have positive charged ions like
Aluminum, Iron, etc. are then dosed into the water to hasten the settling of
the suspended particles. Suitable dosing
systems are employed for this
purpose. Coagulated particles are known
as “Flocs”.
Clarification :
The purpose of clarification is to bring the flocs together
to form a larger mass. The larger the
mass of flocs, the heavier the particles and easier settleability.
Operation :
Raw water is pumped to a chamber outside the flash-mixer
tank. Coagulant (Alum) is dosed into
this chamber through a dosing pump.
Water then enters the flash mixing chamber. Mixing of coagulant with water takes place
with the help of motorised mixer and then it enters the flocculation
chamber. Slow speed agitator mounted on
this chamber rotates at gentle speed, which coalesces the floc. The overflow of flocculator tank passes into
the clarifier chamber. Clear water moves up and is collected in the outlet
launder.
PRESSURE QUARTZ FILTER :
The Pressure Quartz Filter is a rapid flow filter using very
Fine Silex Quartz as Filter Media.
It is ideal for filtration of water having very fine suspended matters
like mud, rust, particles, dirt, etc.
A Filter is a bed of granular material which physically
removes suspended matter from the water passing through it. The only change
in water quality resulting from filtration is the reduction of suspended
solids. Raw Water flows downwards through the filter bed and the turbidity
and suspended matter is retained on the quartz surface. Filtered water is evenly collected by an
under drain system in the bottom of the vessel and flows through the outlet to
service.
When the pressure drop increases to a given level, the
filter is clogged and requires cleaning by backwashing.The fine suspended
impurities are removed from the filter bed by backwash at very high velocity..
CHLORINATION :
The use of chlorine is the oldest and most common
disinfection method for private water supplies.
Chlorine is inexpensive and readily available, reliable, easy to use and
monitor, and effective against most pathogenic bacteria, virus and cyst
organisms. It also kills non-pathogenic
iron, manganese and sulfur bacteria.
Chlorine is also a strong oxidizing agent which causes a
problem mineral such as soluble iron and manganese to change to an insoluble
precipitate so it can be filtered from the water.
Chlorination may be done in many ways. Chlorine may beused continously in the dry or
liquid form that is dropped or injected into the well water using a chemical
feed pump. For periodic or shock water
treatment, chlorine can also be poured in or fed in solution using a hose.
ACTIVATED CABON FILTRATION :
The Activated Carbon
Filter is used for effective removal of colour, odour and organic contamination
in the water. It also removes dissolved
chlorine in the water. It is a rapid
flow filter using water treatment grade of activated carbon granules supported
by very fine quartz filter media. It is
ideal for filteration of water from underground sources having either colour,
odour or organic contamination which is not accepted for potable applications.
Regular backwashing is adequate to loosen up the bed and
expose fresh surfaces of activated carbon granules to trap the dissolved
impurities in the water. When the carbon
granules are exhausted, it is either replaced with new activated carbon
granules or is thermally reactivated.
The water flows downwards through the carbon filter bed, the
colour, odour and organic contamination is trapped by adsorption on the surface
of activated carbon granules. Water is evenly collected by an under drain
system in the bottom of the vessel and flows through the outlet to service. It is highly recommended to feed filtered
water to Activated Carbon filter to avoid fouling of activated carbon.
SOFTENER:
Hardness & Scale Formation :
Water containing susbtantial quantities of caclium and
magnesium compounds is called hard water.
It is hardness which is largely responsible for scale and
deposit formation in boilers and cooling water systems. Hardness is often classified for convenience
into temporary or alkaline hardness and permanent or non-alkaline
hardness. Temporary hardness arises from
the presence of calcium and magnesium bicarbonates. When water containing these substances is
boiled, the soluble bicarbonates decompose to the insoluble carbonates which
form scale. This can be represented
chemically as follows :
Softening offers a simple means of removing the undesirable calcium
and magnesium scale forming salts from the water. A diagramatic form of the plant is reproduced
below :
The softening process may be simplified as indicated below
:-
2NaR + CaSO4 Na2SO4 + CaR2
2NaR + MgSO4 Na2SO4 + MgR2
R = Resin
The calcium and magnesium cations have a strong affinity for
the active sites incorporated into the resin structure and hence become
associated to the resin at the expense of the sodium ions, which then pass
freely into the flowing water at a rate in proportion to the removal rate of
the calcium and magnesium cations.
When all the sodium sites on the resin have become occupied
by hardness cations (a situation which in practice should not arise), the resin
is termed “exhausted” and requires “regeneration”. The regeneration cycle involves exposing the
resin to a strong solution of brine whereby the sodium ions, due to the grossly
excessive numbers, liberate the hardness cations off the resins and to waste.
Regeneration process :
CaR2 CaCl2
+ 2 NaCl 2 NaR +
MgR2 MgCl2
The base exchange softening process should be capable of
producing a water quality containing 0 - 5 ppm of residual hardness.
DEMINERALISATION PLANT :
The process of producing demineralised or deionised water by
ion-exchange is basically a two stage process.
The raw water to be deionised passes through a two stage deioniser
comprising of Cation Exchanger followed by an Anion Exchanger.
In the cation exchanger the water is passed through a column
of cation exchange resin charged with mobile replaceable hydrogen ions. The cations in the raw water essentially
consisting of Ca, Mg and Na get adsorbed on the resin surface which in turn
releases hydrogen ions in the water.
Hence, the water coming out of cation exchanger will contain acid salts
such as Hydrochloric acid, Sulphuric acid, Carbonic acid. The composition is as follows :
CATIONS ANIONS
Carbonates
Bicarboantes
Hydrogen Chlorides
Sulphates
Hence, the pH of Decationised water will be about 3 to
3.5. This water will be fed to Anion
exchanger, in which it will pass through a column of anion resins charged with
mobile replaceable hydroxyl ions. The
anions in decationised water essentially consist of carbonates, bicarbonates,
chlorides, sulphates and silica and get adsorbed on the anion resin surface,
which in turn release hydroxylions in the water. The composition of water after anion
exchanger is as follows :
H
+ OH H2O (Pure Water)
The absence of cations and anions in the water at the outlet
of anion exchanger will be indicated by low electrical conductivity of DM
water.
After the hydrogen charge on the cation resin gets
exhausted, a solution of hydrochloric acid is passed through the cation resin
bed. This process will regenerate the
cation resin and restore the hydrogen charge and the cation exchanger will be
ready for the next cycle of operation.
Similarly after the hydroxyl charge on the anion resins gets
exhausted, a solution of Sodium Hydroxide (Caustic Soda) is passed through the
anion resin bed. This process will
regenerate the anion resins and restore the hydroxyl charge and the anion
exchanger will be ready for next cycle of operation.
Normally, the cation and anion exchanger columns will allow
a small slippage of both cations and anions through the respective beds. Hence,
the conductivity of D. M. Water at the outlet of anion exchanger will be upto
10 ms/cm equivalent to approximately upto 10 ppm of dissolved
solids in D. M. Water.
This D. M. Water is further polished by passing through a
Mixed Bed Polisher Unit, in which the remaining cations and anions are removed
to get D. M. Water confirming to IP Standards for deionised water.
The Mixed Bed Unit consists of a mixture of both cation and
anion exchange resins which remove the traces of remaining cations and anions
from the D. M. Water.
REVERSE OSMOSIS:
Osmosis is a natural process involving fluid flow across a semipermeable
membrane barrier. It is selective in the
sense that the solvent passes through the membrane at a faster rate than the
dissolve solids. The difference of
passage rate results in solvent solids sparation. The direction of solvent flow is determined
by its chemical potential which is a function of pressure, temperature and
concentration of dissolved solids.
Pure water in contact with both sides of an ideal
semipermeable membrane at equal pressure and temperature has no net flow across
the membrane because the chemical potential is equal on both sides. If a soluble salt is added on one side, the
chemical potential of this salt solution is reduced. Osmotic flow from the pure water side across
the membrane to the salt solution side will occur until the equilibrium of
chemical potential is restored (Figure 1a).
Equilibrium occurs when the hydrostatic pressure differential resulting
from the volume changes on both sides is equal to the osmotic pressure. This is a solution property independent of
the membrane.
Application of an external pressure to the salt solution
side equal to the osmotic pressure will also cause equilibrium. Additional pressure will raise the chemical
potential of the water in the salt solution and cause a solvent flow to the
pure water side, because it now has a lower chemical potential. This phenomenon is called reverse Osmosis
(Figure 1b).
A semi-permeable membrane is selective in that certain
component of a solution, usually the solvent, can pass through it, while
others, usually the solute (dissolved solids) cannot. Osmotic flow from the pure water side to salt
solution side will occur across the membrane until equilibrium is reached, at
which chemical potential on both sides of membrane is equal (fig 1a & 1b).
Reverse Osmosis is a membrane separation process in which the solvent
(water) molecules from a pressurised solution flow through appropriate
semi-permeable membrane. The mebrane
acts as a barrier to the flow of solute (dissolved solids) molecules, thereby
separating solvent from solute. RO process is generally used for desalination
of water. The permeate (the liquid
flowing through the membrane or purified water) which generally emerges at near
atmospheric pressure, is reduced in salt content, while the feed solution which
is pressurised on the other side of the membrane increases in salt
content. The only other side of the
membane increases in slat content. The
only energy input required is that for pressurising the feed. Unlike thermal desalination, membrane
desalination operates at ambient temperature and without phase change, and
hence energy consumption for membrane desalination is lower than that for
thermal desalination. This process is at
times also also referred as Hyperfiltration.
ULTRAVIOLET LIGHT DISINFECTION :
Ultraviolet light is a method of disinfecting private water
systems. Ultraviolet radiation adds
nothing to the water and does not produce any taste or odour. The UV light is produced by a low pressure
mercury vapor lamp which produces a disinfecting dose rated in
microwatt-seconds per square centimeter (Mws/cm²). Values of 30,000 Mws/cm² will kill most types
of pathogenic bacteria. However, viruses
are more resistant and variable and may need upto 45,000 Mws/cm².
An Ultraviolet water treatment device is quite simple. The most common design consists of a
stainless cylindrical chamber with a cylindrical mercury are lamp located in
it. Water enters one end of the
chamber, flows through the chamber around the lamp and exits the other end
within a few seconds.
To be effective as a disinfection treatment, ultraviolet
radiation must pass through every particle of water. The thinner the water film and the slower the
water flow, the more effective the system will be. Also, the water cannot have any turbidity,
suspended soil particles, or organic matter.
As a consequence, ultraviolet light treatment should only be attempted
on clear water. A prefilter is recommended
on ultraviolet systems as is periodic inspection and lamp cleaning.
DRINKING WATER - SPECIFICATION
AS PER IS 10500:91
Sr.No.
|
Particulars
|
Desirable Limit
|
Essential Characterstics
|
||
1
|
Colour, Pt-Co
|
5
|
2
|
Odour
|
Unobjectionable
|
3
|
Taste
|
Agreeable
|
4
|
Turbidity, NTU
|
5
|
5
|
pH Value
|
6.5 to 8.5
|
6
|
Total Hardness (as CaCO3) mg/l
|
300
|
7
|
Iron (as Fe), mg/l
|
0.3
|
8
|
Chlorides (as Cl), mg/l
|
250
|
9
|
Residual free chlorine
|
0.20
|
Desirable Characterstics
|
||
10
|
Dissolved solids, mg/l
|
500
|
11
|
Sulphates (as SO4), mg/l
|
200
|
12
|
Nitrate (as NO3), mg/l
|
45
|
13
|
Alkalinity, mg/l
|
200
|
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