Water supply system

(soft water, clarified water & DM water)

31 October 2019 1
 Why water treatment?
• Raw water contains many dissolved minerals and
organic materials. Bacteria can cause sickness and
severe digestive issues/health problems
• At high temperature certain minerals leave scaling on the
tube metal of the boiler and cause permanent damage.
• Some dissolved minerals lead to corrosion of tube
metals. Some lead to foaming.
• At high pressure and temperature an element, silica may
be carried away with steam, especially during drum level
high, causing damage to turbine blades.
• A Thermal Power Station needs water of varying quality
for different processes and hence the requirement.
• The performance and life expectancy of the station
greatly
31 October 2019 depends on water chemistry compliance. 2
 Sources of water
Various sources of water can be broadly classified
as:

1. Rain Water
2. Surface waters (Rivers, Streams, Ponds, Lake
and Reservoirs)
3. Ground water (Springs, Shallow wells & Deep
wells)

31 October 2019 3
 Type of water treatment
• The type of demineralization process chosen for a
power plant depends on three main factors :
1. The quality of the raw water.
2. The degree of de-ionisation i.e. treated water
quality
3. Selectivity of resins.

31 October 2019 4
Different water qualities in use at Power Plants.
Type of water used

1 Condenser cooling Raw water/ Clarified & post-chlorinated water.

2 BA flushing Raw water

3 Boiler feed water DM water

4 ESP / Channel flushing Raw/ Clarified water

5 Other aux. cooling Clarified water/ DM water

6 H2 generation plant DM water.

7 Drinking water….. Clarified Filtered water & post-chlorinated water.

8 DM plant operation Clarified Filtered water

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 Pretreatment

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 Pre treatment process
Pretreatment plant removes the suspended solids such
as clay, silt, organic & inorganic matter.
• The coarse components, such as sand, silt etc, are
removed from the water by simple sedimentation
• The finer particles (non-separable solids /colloids ) do
not settle down in any reasonable time and are
floculated to produce large particles which settle easily.
• Coagulation: To bring together the suspended particles
by the addition of chemicals. The mixing of chemicals
is done in Flash Mixer.

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 Pre treatment process…. cont
• Flocculation : Coagulated particles grow in size by
attaching to each other.
• Flocculation is obtained by gentle and prolonged
mixing which converts the submicroscopic
coagulated particle into discrete, visible &
suspended particles. These particles settle rapidly
under the influence of gravity and are removed.
• Proper pre treatment reduces loading on anion and
cation exchanger.

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 If pre-treatment is not done effectively?
Pre-treatment not done Consequences
effectively
1 SiO2 may escape with water This will increase the Anion loading

2 Escaping of organic matter May cause organic fouling in the Anion

exchanger bed. Hence an optimum
quantity of Chlorine di-oxided added.
3 Addition of Lime (CaOH2) in Cation loading may increase
excess for raising pH for maximum
floc formation.
4 If less than calculated amount of Proper pH will not be obtained and silica
(CaOH2) is added may escape to DM section thereby
increasing the loading of Anion bd.

31 October 2019 9
 Steps of treatment process
• Mechanical Filtration
• Aeration of raw water
• Adding chemicals for bacteria removal (before Clarifloculator)
• Adding chemicals for sedimentation of suspended
particles (before Clarifloculator)
• Flocculation (takes place in Clarifier)
• Filtration (in filter bed and followed by ACF)
• Ion Exchange process (in acid/base cation beds)

Water treatment process is generally made up of two sections :

1. Pretreatment section
2. De-mineralisation section
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 Mechanical Filtration
➢Raw water contains large,
heavy debris such as wood,
trees, branches, water hyacinth
(जल कुम्भी), rocks, sand,
pebbles, plastic, animal,
floating object and trash
(kachra/foreign matter).

Trash Racks (strong, rugged and durable

metal structure):
➢ Used to provide protection
from above impurities.
➢Provide uniform flow/suction
to the CW pump
31 October 2019 11
 Trash rack cleaning machine

https://youtu.be/XL1AQMvGNO8

31 October 2019 12
 Travelling water screen
It catches small pieces of coal, sand, gravel, wood, plastic,
herbs, leaves, which can go into the impeller and may
choke/damage the pump.

31 October 2019 13
 The working & cleaning operation of Travelling water screen

https://youtu.be/j59KYOGbaR0

31 October 2019 14
 The pre treatment process
Aeration
• It is the process in which water is brought in
intimate contact of air so as to remove the
following:
➢ Dissolved gases like CO2 , H2S etc.
➢ to remove undesirable organic mater.

31 October 2019 15
 The pre treatment process cont …..
• The simplest type of
Aerator employs a vertical
riser that discharges water
by free fall into a basin.
• The riser usually operates
on the available head of
water.
• The efficiency of aeration is
improved as the fall
distance is increased.
• Steps or shelves are
added to break up the fall
and spread the water into
thin sheets or films, which
increases contact time and
aeration efficiency.
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The following chemicals are added before the water enters
CLARIFLOCULATOR.

➢ Alum: suspended solids are brought together (Coagulation).

Coagulated particles grow in size by attaching to each other
(Floculation). Thus suspended solids become heavy and settle down.

Water + alum = Aluminium Hydroxide + H2SO4.

H2O + Al2(SO4)3 = Al(OH)3 + H2SO4

Al(OH)3 helps in floc formation and thus suspended solids settle down.

➢ Chlorine dioxide (ClO2): kills bacteria/germs and organic matter like

algae (काई) . (Use of chlorine has become obsolete because of
hazardous nature of chlorine gas).

➢ Lime Ca (OH)2 :
➢ Mixing of chemicals takes place in Flash Mixer.
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Water from the Flash Mixer enters into
the flocculating zone of the clarifier
through the R.C.C Central shaft
opening.
The clarifier is fitted with a rotating
bridge driven by a electric motor. Here
mechanical flocculation is done by the
motor operated stirrer mounted on the
bridge.
Preliminary settlement of flocs takes
place in the bottom zone.

Water then enters into the clarifying

zone through bottom openings. The
sludge in settling zone and other
clarification zone is scrapped towards
the central pit by means of scrappers
linked with the rotating bridge. Sludge is
then removed from this pit with the help
of sludge pump.
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 Clarifloculator

Floculation zone

Clarifier Zone

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 Raw water to Clarified water
Aerator
Raw water bay

RW make up
pumps

Clarified Clariflocculator Chlorine dioxide dosing

water Pershall
flume
Lime dosing

Alum dosing

Flash
31 October 2019 mixer 20
 Clarified water to Filtered water

GF Bed
Clarified water Sand filters

Filtered water Drinking water

sump sump
Clari-flocculator
Cl2 Post-
chlorination
DW
Cl2 House Filtered pumps
water
pumps

Filtered water
To Township for
DM water
DM PLANT drinking water
for Plant2019
31 October P 21
purpose Turbidity & Free Cl2 check
 Pre-treatment of water
(HOW WATER TREATMENT PLANT WORKS -
COMPONENTS & WORKING PROCESS)

https://www.youtube.com/watch?v=NsES4Jkb27A

31 October 2019 22
 Demineralization plant

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 What is Demineralization?
The process of removal of cations & anions present in water is
called Demineralization.
Also called Ion Exchange process or De-ionization.

In this process, ion exchange resins are used. These resins are
insoluble, cross linked (stable, strong, durable) which are porous.

Suppose impurity present in hard water is CaSO4.

Cation Ca + + and Anion SO4 - -

The cations (Ca + + )will be removed in cation bed & Anions

(SO4 - - ) in Anion bed.

31 October 2019 24
 Deionization or demineralization:

➢It means the removal of ions.

➢Ions are electrically charged atoms or molecules found in water that has
either a net negative or positive charge. These ions are considered
impurities and must be removed from the water.

➢Ions with a positive charge are called “Cations” and ions with a negative
charge are called “Anions”.

➢ Ion exchange resins are used to exchange non-‐desirable cations and

anions with hydrogen and hydroxyl, respectively, forming pure water (H20),
which is not an ion.

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 Types of Resins
1. Cation Exchange Resin
2. Anion Exchange Resin

➢Cation exchange resin.

In this resin Acidic group is attached with any polymer network.
Denoted by R H + ( R denotes polymer network of resin & H +
denotes acidic group.
Example of Cation Exchange Resin: Poly styrene cross linked
with divinyl benzene.
➢Anion exchange resin:
In this resin basic group is attached with any polymer network.
Denoted by R’ OH-
Example of Anion Exchange Resin: Tertiary Amine or
Quanternary Ammonium group.

31 October 2019 26
Filtered water Demineralization Plant
ACF WAC SAC

Degasser

WBA SBA MB

CST

31 October 2019 PMI Revision 00 27

 • Activated carbon Filter (ACF)

• Activated carbon acts on principle of adsorption which

is a surface active phenomenon
• Removes residual turbidity (<2 NTU) of water to its 1/10
level.
• Removes organic molecules to control colour and
odour.
• Removes free residual chlorine present in filtered
water(0.5 ppm →Nil)

31 October 2019 28
 List of ions commonly found in water

Cations Anions
Removed by cation Resins Removed by Anion Resins

Ca + + Cl -
Mg + + SO4 - -
Mn + + CO3 - -
Na + Sio2 -
H+ OH -

•Cation Resin has a -ve functional group and so attracts +vely charged ions.
•Anion Resin has a +ve functional group and so attracts –vely charged ions

31 October 2019 29
The stream is first treated with a weak acid
cation (WAC) followed by strong acid cation
(SAC) resin.

The resin captures the dissolved cations, and

releases hydrogen (H+) ions in exchange.

Example:

Cations like Al + + + , Na +, Ca + + , Mg + + in
water are exchanged.

H+ ions in resin are released.

2R H+ + CaCl2 = R2 Ca + 2HCl
or
2RH + + Ca + + = R2 Ca + + + 2H +

31 October 2019 30
 Working of Degasser/Degassifier
• After cation exchange, the bicarbonate and carbonate (if any) ions are converted
to carbonic acid, or carbon dioxide. CO2 is soluble in water, but it tends to
escape into the air.
• A degasser is used to remove CO2. This reduces the ionic load on the strong
base anion resin, and the consumption of caustic soda is thus lower.
• The ex-cation water is trickled in fine streams from top of a tall tower packed with
raschig rings, and compressed air is passed from the bottom through degasser
blower.
• Carbonic acid break into CO2 and water. Carbon dioxide escapes into the
atmosphere. Water is pumped back to anion exchanger bed .
• Before cation exchange, the water is containing bicarbonate. After it, the cations
in water (Ca++, Mg++ and Na+ principally) are converted to H+ ions, which combine
with the HCO3— bicarbonate anions to produce carbonic acid (H2CO3 = CO2 +
H2O).
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 The process of Degasification
The water coming out of the cation
exchange unit is introduced into a column
where it is sprinkled over a bed of filling
material, often polypropylene shaped as
saddles to leave a maximum volume of
voids in the bed.

Air is introduced at the bottom of the column

by a blower, and escapes at the top, loaded
with carbon dioxide from the water.
The solubility of CO2 in pure water is high
(about 1.5 g/L or more). When the water is
broken into small droplets and air is blown
through the "rain", the gas tends to move
into the air because the partial pressure of
CO2 in air is much below the equilibrium
pressure.
The residual CO2 after an atmospheric
degasifier is typically 10 mg/L as CO2.
31 October 2019 32
The solution is now passed through:

Weak Base anion (WBA) followed by strong

base anion (WBA) resin.

Anions like SO4 - - , Cl - , HCO3-, CO3 - -

present in water are exchanged with OH-
present in resin.

Outlet of SBC is OH-

R' OH- + Cl - = R' Cl- + OH -

Because of unlike charge, H + ions & OH-

ions have strong affinity and combine to
form H2O.

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 Mixed Bed or Polishing unit:

• Ex-anion water is fed to the mixed bed exchanger containing

both cationic resin and anionic resin.
• This bed takes care of the following very effectively:
➢ Silica slip from anion exchanger.
➢ Sodium slip from cation exchanger.
• The final output from the mixed bed is an extra-ordinarily
pure water having the following specifications:
➢ Conductivity < 0.2 μ℧ (micro mho)
➢ pH 7
➢ Silica < 0.02 ppm.

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 Internal Treatment
• D.M.Water has a good affinity to absorb carbon dioxide
and oxygen, and both are extremely harmful to metal
surfaces for their destruction like corrosion,
• water is finally de-oxidized by a suitable doze of hydrazine,
at the point after deaerator.
• The pH of feed water is maintained at around 9.0

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 Back washing of Gravity Sand Filter
➢The process of reversing the flow of water, back through
the filter media to remove the entrapped solids is called
Backwashing.

➢Flow of water in this filter from top to bottom ie water to be

cleaned flows through sand, Anthracite (pure carbon) and
then through pebbles in that order.

➢Backwashing is done using filtered water against the

direction of flow (bottom to top).

➢Water source: filtered water from overhead tank.

➢Time taken: 5 to 10 minutes approx.

31 October 2019 PMI Revision 00 36
 Backwashing of rapid sand filter

https://youtu.be/HSlxQ-nLvrY?t=5

31 October 2019 37
 Back washing of Activated Carbon Filter

➢Backwashing is done using filtered water against the

direction of flow (bottom to top).

➢Water source: filtered water from overhead tank &

pump.

➢ACF is regenerated by backwashing the bed with air-

water.

. 31 October 2019 38
 Regeneration of ion exchanger

• During water treatment any deviation from the

specified quality in the output means that the resins
in mixed bed have got exhausted and need
regeneration.
• Regeneration of the mixed bed first calls for
suitable back washing and settling, so that the two
types of resins are separated from each other.

31 October 2019 39
 Regeneration of resin in WAC & SAC bed
▪ Weak Acid cation bed is backwashed (whenever regeneration is to be done) to
remove suspended solids & de-compact the bed. But SAC bed is
backwashed during every 10th cycle only.
• WAC and SAC are regenerated by counter current regeneration with
4% HCl with a down flow of DM water to avoid lifting of SAC bed.
Acid is first injected from bottom of SAC bed and its outlet goes to
WAC bed which is further taken to Neutralization pit and finally to
Effluent Treatment plant. The same may be used for de-ashing
purpose.

31 October 2019 40
 Regeneration of resin in WAC & SAC bed

31 October 2019 PMI Revision 00 41

 Regeneration of resin in WBA & SBA bed
•WBA and SBA are regenerated by counter current regeneration with 4% caustic
with a down flow of DM water.
•The procedure is same as that used for WAC & SAC bed.

•Resins of MB is first separated by air scouring.

31 October 2019 42
 Mixed bed regeneration
The steps for backwashing of mixed bed are as under:

medium used
1 Backwashing of resin bed to separate Water from SBA outlet
cation & anion resins. Cation resin being or DM water (Not the
heavier will settle at bottom. Anion resin raw water).
comparatively lighter will settle at the top.
(This can be seen through glass window.
(Anion of faded colour and cation of
golden colour).
2 Inject NaOH from top & HCl from bottom dilute NaOH (4 to 5%).
(counter current flow). Outlet of excess Dilute HCl (4 to 5%)
acid & base shall be from middle.

3 Rinse the mixed bed with water through

respective injecting lines. This is called
slow rinsing.
4 Rinse the mixed bed from top (Fast
Rinsing) & drain from bottom.

5 Carry out air scouring with compressed air

so as to ensure intimate mixing of both
the resins.

31 October 2019
(Commercial NaOH/HCL has 47%/ 30% concentration approx respect). 43
 Regeneration of ion exchanger
• After regeneration, the effluent comes out from the system
is treated in a neutralizing pit for discharging through normal
drain.

31 October 2019 44
Hard water:

Water that contains an appreciable quantity of dissolved

minerals (like calcium and magnesium).

Soft water:
Treated water in which the only ion is sodium.

When hard water comes in contact with the resin beads,

the calcium and magnesium are pulled out of
the water and replaced with sodium or potassium,
leaving your water soft.

Used in laundry.

31 October 2019 45
 THANK YOU

31 October 2019 46