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
:
Ca(HCO3)2 CaCO3
+ CO2 +
H2O
Mg(HCO3)2 MgCO3
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
7 Silica
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
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
|
|
|
|
GENERAL
STANDARD FOR BACTERIOLOGY
|
|
|
|
|
1
|
Total
Plate Count / ml
|
100
|
|
2
|
MPN
Coliform / 100 ml
|
1
|
|
