By Kareem Bakhsh Ansari
Sequence for water conditioning at WTP
i)Removal of suspended solids
• Coagulation & flocculation(carried in settling tank)
• Filteration(carried in sand filters)
ii)Removal of dissolved solids
• Demineralization(carried ion exchangers).
iii) Removal of dissolved gases
• Dearteation(carried in deareator)
• Chemical scavenging(carried in boiler)
Canal water :
• Due to less dissolved impurities in it is preferred
over well water.
Clarification of Canal Water:
• Normally, turbidity is removed by adding a
coagulant prior to the sedimentation process.
• Alum is added at moga pit and in second screen
pit for thorough and uniform mixing of coagulant
• Canal water from a nearby canal flows through a
moga and concrete pipes to a 73x40x17 feet
settling tank(dimensions for DHCL Plant).
• Mud settles effectively in a two hours residence
time settling tank. Settled mud is removed by
• Remove of colloidal impurities from water by
conglomeration of small colloidal particles into
bigger particles having enough mass to settle
by the action of gravity.
• process used to de stablize the colloidal
• Due to like charges , colloidal particles repel
each other and can not spontaneously
conglomerates into larger particles.
• Coagulation is affected by doing of stimulants
that neutralize the charge of the colloidal
• These neutralized particles coagulates on
collision with similar other neutralized particles.
• These coagulated particles grow in size and
become too large to outweigh gravity by
buoyant force exerted upon them by water.
• They settle on bottom of the settling tank as
gelatinous and porous flakes.
CHEMICALS as coagulants/flocculants :
• Aluminum sulphate.
• Ferrous/ferric sulphate
• Ferric chloride
• Polyelectrolytes :polyacrylamide are mostly used
• The process of coagulation is very sensitive to
a)For Al-Coagulant it should be 5-7
b)For Fe-Coagulant it should be 8-10
It is necessary to maintain some degree of
alkalinity in the water being treated with coagulant
because the (H ions) are liberated due to hydrolysis
of metal ions that reduce the alkalinity
Bridging of a large ,and more efficient ,agglomerate
• Suspended particles tend to have a surface charge,
the charge on the polymer is quite important in
• If the fouling material has a negative charge as in
the case with mud, silt and biological matter, a
cationic polyelectrolyte is used to neutralize the
charge resulting in the formation of a floc.
• This floc, which is of low density, will remain in
solution and can be removed by blow down or
•Flocculants will bridge smaller colloids together using
charge and molecular weight.Polyacrylamides are used as
How flocculants work????
• STOKE’S LAW predicts that spherical particles
suspended in a fluid medium settle at a rate
proportional to the fourth power of the radius of
• Thus large particles settle much faster than
• It is the process of clarification of water by
passing it through a filter bed composed of a
porous material that retain coarse suspended
solids on its surface and in the pores.
• Raw water with turbidity less than
approximately 30 NTU (Nephlometric Turbidity
units) is pumped to raw water sand filters.
• Different numbers of filters are used depends
upon the quantity of water to be filtered at the
• Raw water, well or canal is stored in a STORAGE
TANK after passing through these sand filters,
turbidity is decreased upto 5 NTU.
• The driving force of filtration is the pressure drop
across the filter bed.
• Pressure drop through the filtering bed depends
upon the following factors;
1)Height of the filtering bed
2)Rate of filtration.
3)Particle size ( i.e. grain diameter of the filtering
4)Degree of contamination of the filter bed by
• Each filter to have to filter certain GPMs of water.
• Each filter has filtering bed composed of charcoal,
sand, granite and gravel.
When the filter bed backwashed???
• Backwashing is reversal in flow of wash water.
• When pressure drop across the filter increases or
turbidity at outlet increases the filtering bed is
• Before backwash is surface washed to loosen the
packed mud layer.
Removal of Dissolved Gasses
• Dissolved gasses in water can cause corrosion in boiler
condensate and feed water system.
• Dissolved impurities are removed by two methods
• A deaerator is a device widely used to expel the
dissolved gasses, in particular oxygen , from the
water, prior to its use in steam generating boilers.
• This also reduces the need for oxygen scavengers.
• Deaeration of boiler feed water is usually
accomplished by steam heating.
• In particular, dissolved oxygen in boiler feed
waters will cause serious corrosion damage in
steam systems by attacking to the walls of metal
piping and other metallic equipment and forming
• Water also combines with any dissolved carbon
dioxide to form carbonic acid that causes further
• The typical horizontal tray-type deaerator has a
vertical domed deaeration section mounted above
a horizontal boiler feedwater storage vessel.
• Boiler feedwater enters the vertical deareation
section above the perforated trays and flows
downward through the perforations.
• Low-pressure deareation steam enters below the
perforated trays and flows upward through the
• The steam strips the dissolved gas from the boiler
feedwater and exits via the vent at the top of the
• The deaerated water flows down into the
horizontal storage vessel from where it is pumped
to the steam generating boiler system.
• Low-pressure heating steam, which enters the
horizontal vessel through a sparger pipe in the
bottom of the vessel, is provided to keep the
stored boiler feedwater warm.
• External insulation of the vessel is typically
provided to minimize heat loss.
• Close deareating heaters of such type operate at
105 C & GENERALLY lower the oxygen content to
below 0.01 PPM.
• Any of remaining traces of the oxygen can then be
chemically combined by using an oxygen
scavenger such a;
Sodium sulfite or hydrazine hydrate.
• Such complete deoxygenation is desireable to
minimise corrosion in the modern high
temperature high pressure boilers
Removal of Dissolved solids
• Several method are available for the removal
of unwanted dissolved salts in the boiler feed
selection of the method depends upon the
nature of source and post-treatment
Dissolved salts are generally removed from
Water softening methods.
Cold lime-soda softening
Hot-lime soda process
Sodium zeolite softening
Cation exchange resins
Anion Exchange Resins
Cold soda lime process
• Now a days ,organic polyelectrolytes are used
instead of these tradititional inorganic
coagulants,because the latter can result in
carry over of aluminium and iron which in turn
may cause problems in downstream
equipments. Most objectionable feature of
this process is large volume of sludge
formed,disposal of which is troublesome &
Hot lime-sada process
• This process is carried out near the boiling point
• The reaction involved are same as in cold lime-
• Partial removal of dissolved oxygen also take
• High temperature : 80-150 C
• Process completion time: 15 mint.
• No coagulants used.
• Water with 15-30 PPM hardness is removed.
• Substantial amount of silica is removed as
• Hardness is further reduced due to low
solubility of calcium carbonate and nagnesium
hydroxide in hot water.
• Old technique ,Used for conditioning of BFW.
• It operates at BP of water,so,reactions proceed
faster,coagulation & precipitation are facilitated
• Analysis of raw water.
• Heating of raw water with steam.
• Mixing & proportioning of lime & soda ash in
conformance with the raw water analysis.
• Pumping of lime slurry & soda.
• Reaction of lime and soda, facilitated by mixing
with or with or without previous heating
coagulation or release of the “super saturation “by
variation method , such as slow agitation or
contact with “seeds” by sludge recirculation
• Setting or removal of the precipitate with or
without filtration pumping away of the softened
water periodic washing away of the sludge from
the cone tank bottom (and from the clarifying
• Demineralization means the removal of cations
of cal. & mag.of salts dissolved in water.
Ion EXCCHANGE method
• Raw water dissolved salts are removed by ion
exchange method using cationic and anionic
• Each train has one cation, one primary anion and
one secondary anion vessel.
• Cation resin removes cationic part of salt and
anion resin removes anionic part .
• Raw water enters cation bed, cations are
removed by chemical reaction of salt with resin
• Ion exchange is a chemical reaction in which
mobile hydrated ions are exchanged,equivalent
for equivalent,for ions of like charge in solution.
• The solid has open fishnet like structure,and
mobile ions electrically neutralize charged,or
potentially charged,groups attached to the
solid matrix,called the ion exchanger.
• Ion exchange resins are synthetic polymers
having exchangeable positive or negative ions
depending upon the type of resin.
CLASSIFICATIONS OF ION EXCHANGE RESINS
• Ionizable groups attached to the resin bead
determine the functional capability of the resin.
Industrial water treatment resins are classified
into four basic categories:
• Strong Acid Cation (SAC)
• Weak Acid Cation (WAC)
• Strong Base Anion (SBA)
• Weak Base Anion (WBA)
Cation exchange resins
• They have fixed negatively charged sites and
exchangeable positive ions are associated with
• These sites take positive ions from water and give
up their ion to water.
Anion exchange resins
They have fixed positive charged sites and
exchangeable negative ions are associated with
These sites take negative ions from water and give up
their negative ion to water.
• From cation the effluent water with certain FMA
and nil hardness leads to Degasser where
dissolved CO2 is degassified.
• It is the removal of carbon dioxide present in
• Before anion exchangers mechanical
decarbonation is applied to reduced the size of
• Counter current flow of air and water is achieved
through a packed bed with rasching ring packing.
• Combination of strong acid cation and strong
base anion resins are applied to produced high
purity water for 1500 psig steam generation.
For 1500 psig steam, boiler feed water should have
Dissolved solids 0.1 ppm maximum
Silica 0.005 ppm maximum
Conductivity 1.0 micro-mhos maximum
Out put of mixed bed is our final outlet product
Microscopic view of cellular resin
beads (20-50 mesh) of a sulfonated
styrene-divinylbenzene strong acid
cation exhcanger. (Courtesy of Rohm
and Haas Company.)
Ion Exchange resin
• An ion-exchange resin or ion-exchange polymer
is an insoluble matrix (or support structure)
normally in the form of small (1–2 mm diameter)
beads, usually white or yellowish, fabricated from
an organic polymer substrate. The material has
highly developed structure of pores on the
surface of which are sites with easily trapped and
• Most typical ion-exchange resins are based on
crosslinked polystyrene. The required active
groups can be introduced after polymerization, or
substituted monomers can be used.
• Ion-exchange devices consist of a bed of plastic
(polymer) beads covalently bound to anion
groups, such as -COO-. The negative charge of
these anions is balanced by Na+ cations attached
to them. When water containing Ca2+ and Mg2+ is
passed through the ion exchanger, the Ca2+ and
Mg2+ ions are more attracted to the anion groups
than the Na+ ions. Hence, they replace the Na+
ions on the beads, and so the Na+ ions (which do
not form scale) go into the water in their place.
Exchange Capacity :
• The total capacity of an ion exchange resin is
defined as the total number of chemical
equivalents available for exchange per some unit
weight or unit volume of resin. The capacity may
be expressed in terms of milliequivalents per dry
gram of resin or in terms of milliequivalents per
dry gram of resin or in terms of millequivalents
per milliliter of wet resin.