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WATER
 Hard water:
Water which does not produce lather with water is known as hard water. It is due to
some of the salts dissolved into the water. When we treat the water with soap, it gets
precipitated in the form of insoluble salts of calcium and magnesium.
CaCl2
+
2C17 H35 COONa
→
(Soap)
(C17 H35 COO)2 Ca +2NaCl
(Insoluble precipitate)
 Types of Hardness:
1.
Temporary Hardness: It is due to the presence of bicarbonates of calcium and
magnesium. It can be easily removed by boiling
Heating
Ca (HCO3)2
→
CaCO3 + H2 O + CO2
Heating
Mg (HCO3)2
→
Ma (OH)2 + 2CO2
2. Permanent Hardness: This type of hardness can not be removed by boiling.
This is due to the presence of chlorides and sulphates of calcium and
magnesium. The hardness can be removed by the addition of some agents.
 Units of Hardness: 1. Parts per million (ppm)
2. Milligrams per litre (mg/L)
3. Degree French (°Fr)
4. Degree Clark (°Cl)
 Relationship: 1ppm = 1mg/L = 0.1 ºFr = 0.07 ºCl
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 Determination of Water Hardness
Hard water is due to metal ions (minerals) that are dissolved in the ground water. These
minerals includeCa2+, Mg2+, Fe3+, SO42-, and HCO3-. Rain water moving through the vast
amount of limestone, CaCO3 and rocks, due to these the water becomes hard. This is why we
measure hardness in terms of CaCO3. The determination of water hardness is a useful test that
provides a measure of quality of water. When hard water is heated, CaCO3 precipitates out,
which then clogs pipes and industrial boilers.
The degree of hardness of the water is classified in terms of its calcium carbonate concentration
as follows:
Hardness
rating
Concentration of Calcium
Carbonate (mg/L)
Concentration of Calcium Carbonate
(grains/US gallon)
Soft
0 to <75
0 to <5.2
Medium hard
75 to <150
5.2 to <10.5
Hard
150 to <300
10.5 to <21
Very hard
300 and greater
21 and greater
 Determination of Water Hardness
a)
Complexometric Titration:
EDTA
Method :
Permanent hardness is usually determined by titrating it with a standard solution of ethylene
diamine tetra acetic acid (EDTA). The EDTA is a complexing or chelating agent used to capture
the metal ions. This causes the water to become softened, but the metal ions are not removed
from the water. EDTA simply binds the metal ions to it very tightly.
It is frequently used in soaps and detergents because it forms complexes with calcium and
magnesium ions. These ions which are in hard water are bound to the EDTA and cannot
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interfere with the cleaning action of the soap or detergent.
EDTA grabs all the metal ions in the water, not just the Ca2+ ions. This gives us a value that is not
truly the concentration of Ca2+ ions. This causes an experimental error of about 1% which is
acceptable in this type of titration.
1) Indicator : Eri-chrome Black – T
2)
3)
4)
5)
TITRE : Hard Water Sample
Titrant : Ethylene Diamine Tetra Acetic Acid (EDTA)
Buffer : Mixing of NH4OH and NH4Cl
Preparation of Reagents :
a) Hard Water : Weigh out 0.5 gms of Magnesium Sulphate (MgSO4.7H2O) ,Transfer it
into 100 ml Volumetric Flask and make up the flask with distilled Water up to the
mark. Shake the solution vigorously to get homogeneous solution.
b) EDTA : Weigh out 3.723 gms of Ethylene Diamine Tetra Acetic Acid ,Transfer it into
1000 ml Volumetric Flask and make up the flask with distilled Water up to the mark.
Shake the solution vigorously to get homogeneous solution.
c) EBT : Weigh out 0.5 gms of Eri-chrome Black – T ,Transfer it into 100ml Volumetric
Flask and make up the flask with Ethyl/methyl alcohol up to the mark. Shake the
solution vigorously to get homogeneous solution.
d) Buffer : Weigh out 62.7 gms of NH4Cl and Transfer it into 570 ml NH4OH ,Shake the
Solution vigorously to get homogeneous solution.
Procedure : Take 20ml of Hard water sample In a conical flask , to this add 3 drops of
EBT and 2 ml of Buffer it turns into Wine red colour solution.
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( Blue )
this red color solution is titrated against EDTA which is in Burette with continuous
stirring , at a some stage the wine red is converts to blue color and that is taken as an
end point of the titration. Repeat the titration until we get concurrent values.
From then values we calculate the amount of Hardness of Water
(Write the Calculation Part from Lab observation book).

Boiler Troubles: The dissolved salts causing effects for steam boilers.
The following are the troubles.
1) Priming and foaming
2) Scale and sludge’s
3) Caustic embrittlement
4) Boiler corrosion
1) Priming and foaming
a) Priming: It is the carryover of varying amounts of droplets of water in the steam (foam
and mist), which lowers the energy efficiency of the steam and leads to the deposit of
salt crystals on the super heaters and in the turbines. Priming may be caused by
improper construction of boiler, excessive ratings, or sudden fluctuations in steam
demand. Priming is sometimes aggravated by impurities in the boiler-water.
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b) Foaming: It is the contamination of the steam with boiler-water solids. Bubbles or froth
actually build up on the surface of the boiler water and pass out with the steam. This is
called foaming and it is caused by high concentration of any solids in the boiler water. It
is generally believed, however, that specific substances such as alkalis, oils, fats, greases,
certain types of organic matter and suspended solids are particularly conducive to
foaming. In theory suspended solids collect in the surface film surrounding a steam
bubble and make it tougher. The steam bubble therefore resists breaking and builds up
foam. It is believed that the finer the suspended particles the greater their collection in
the bubble
Prevention :
a) The most common measure to prevent foaming and priming is to maintain the
concentration of solids in the boiler water at reasonably low levels.
b) Avoiding high water levels, excessive boiler loads, and sudden load changes also helps.
Very often contaminated condensate returned to the boiler system causes carry-over
problems. In these cases the condensate should be temporarily wasted until the source
of contamination is found and eliminated.
c) The use of chemical anti-foaming and anti-priming agents, mixtures of surface-active
agents that modify the surface tension of a liquid, remove foam and prevent the carryover of fine water particles in the stream, can be very effective in preventing carry-over
due to high concentrations of impurities in the boiler-water.
d) Hardness salts are the cause of scale inside a boiler; if they are not prevented or removed
regularly they will cause localised overheating. This can lead to tube failure (explosion risk)
and/or a reduction in the heat transfer properties of the transfer surfaces.
2) Boiler Scale & Sludge
a) Sludge’s : sludges are soft ,loose,non-sticky precipitates produced due to higher
concentration of dissolved salts.
b) Scales : These are hard,stickly deposites formed on the inner walls of the boiler.
Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer
surfaces or by suspended matter in water settling out on the metal and becoming hard and
adherent. The evaporation in the boiler causes impurities to concentrate.
In untreated boiler water, the formation of scale is like a "back to nature" movement. As
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minerals are deposited out from water they from many types of crystalline and rock-like
structures. The most common scale in boilers is due to carbonate deposits caused by hardness.
Carbonate scale is usually granular and sometimes very porous. A carbonate scale can be easily
identified by dropping it in a solution of hydrochloric acid. Bubbles of carbon dioxide will
effervesce from the scale.Sulphates scales are harder and more dense. A sulphate deposit is
brittle and does not effervesce when dropped in acid. Silica scales resemble porcelain. This
scale is very brittle, is not soluble in acid, and dissolves slowly in alkali.Iron deposits are very
dark colored. The are either due to corrosion or iron contamination in the water. They are
soluble in hot acid giving a dark brown solution.
Problems Caused by Scale
The biggest problem caused by scale is overheating and failure of boiler tubes. The thermal
conductivity of porous boiler scale is similar to insulating brick. The scale acts as an insulating
layer and prevents an efficient transfer of heat through the tubes to the circulating water. The
reduction in thermal conductivity means lower boiler efficiency which in turn leads to hot acid
giving a dark brown solution.
overheating and may result in the softening, bulging or even fracturing of the boiler tubes. Boiler
scale can also cause plugging or partial obstruction of circulating tubes in a water tube boiler,
which again causes starvation and overheating of the tubes.
Another important aspect is that corrosion may occur under the boiler scale. In general, boiler
scale causes
a.
b.
c.
d.
e.
increased fuel bill by decreasing the operating efficiency
thermal damage
unscheduled down-time
increased cleaning time and cleaning costs
reduced working life of a boiler.
3) Caustic embrittlement :
It is the appearance of cracks inside the boilers particularly at joints and bends due to high
concentration of alkali’s leading to the failure of the boiler.
The boiler water containing Carbonates and bicarbonates of alkali metals Purified by limesoda process. During the process lime reacts with dissolved salts and soda is remains same in
water. The lime reacts with water and form as caustic soda (NaOH)
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Na2CO3 + H2O
-------→
2NaOH + CO2
The water in which NaOH is present is flows through Small pits and hair cracks present on the
inner walls of the boilers. This soda forms concentration cells as
( + )Metal at bends / Conc.NaOH // Dil.NaOH / Metal at Plane Surface ( - )
( Anode)
(cathode)
The metal at plane surfaces surrounded by dil.NaOH becomes anodic, where as the metal at
bends surrounded by Conc.NaOH becomes cathodic which consequently corroded. Due to this
inbalance in the same metal cracks will be developed , that cracks will be appeared as brittle
nature. This is called as Caustic Embrittlement.
Preventions :


By using Sodium Phosphate, disodium hydrogen phosphate as softening reagents, they
forms Complexes with Ca+2,Mg+2 .
By using sodiumsulphate( Na2SO4 ) and sodium hydroxide ( NaOH ) in 1:1 ratio with
boiled water.
4) Boiler Corrosion :
The decay of boiler material by the attack of chemical or electrochemical environment is called
as Boiler Corrosion. The reasons for the boiler corrosion is mainly due to
a) Dissolved Oxygen
b) Dissolved Carbon di Oxide
c) Dissolved Acids
Dissolved Oxygen :
O2 is the most corroding environment .At 300c water contains 8cc of O2 per litre and the
Moisture Contains 8mg/l of O2.
Source : It is Naturally present
Damages : At high temperatures oxygen attack the boilers creating serious corrosion
2 Fe +2 H2O + O2
2 Fe (OH)2 + O2
----- 2 Fe (OH)2
----------- >
2 Fe2 O3 .3 H2O (rust)
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Removal Methods :
a) Addition of sodium sulphide removes O2 by converting it into sodium sulphate.
Na2S + O2
---
Na2SO4
b) Addition of sodium sulphite removes O2 by converting it into sodium sulphate.
Na2SO3 + O2 ---
Na2SO4
c) Addition of Hydrazene removes O2 by converting it into Nitrogen
NH2-NH2 + O2 --- N2 + 2 H2O
 Disslved CO2 :
Dissolved CO2 has a slow corrosive effect on boiler metals.
Source : The bicarbonaes present in water ,at high pressure & Temperature
decomposes in to CO2 .
Ca(HCO3)2 -----
CaCO3 + CO2 ↑ + H2O
Mg(HCO3)2 ----- Mg (OH)2 + 2 CO2 ↑
Damages : It react with water and Producing Carbonic Acid
CO2 + H2O ---
H2CO3
Removal Methode : By the addition of Calculated amount of Ammonia
NH4OH + CO2 - (NH4)2 CO3
Dissolved Acids :
Source : The dissolved Magnesium salts which undergo hydrolysis to produce
Acids
MgCl2 + 2 H2O -- Mg (OH)2 +
HCl
Damages : Acids reacts with metal of the Boilers produce the decay of the
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Metal.
Fe + 2 HCl --- 2 FeCl2 + H2 ↑
FeCl2
+ H2 O
2 Fe (OH)2 + O2
----
Fe (OH)2 + 2 HCl
----------- >
2 Fe2 O3 .3 H2O (rust)
Removal Methodes :
a) Softening of of the boiler water to remove MgCl2 from water.
b) By frequent blow down operation i.e., removal of water , concentrated
with dissolved salts and feeding the boiler with fresh water.
 Purifications Methods :
1)To remove temporary hardness
(a) Lime-soda process : In this method, the soluble calcium and magnesium in water are
chemically converted into insoluble compounds, by adding calculated and specific
amounts of lime [Ca(OH)2] and soda [Na2CO3].
Calcium carbonate [CaCO3] and magnesium hydroxide [Mg(OH)2], these are precipitated
and then filtered off.
This lime soda process is of two types namely Cold lime soda and Hot lime soda :
(i) Cold lime soda process: In this method, calculated quantity of chemical (lime and
soda) is mixed with water at room temperature.
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At room temperature, the precipitates formed are finely divided, so they do not settle down
easily and cannot be filtered easily.
Consequently, it is essential to add small amounts of coagulanfs (like alum, aluminium sulphate,
sodium aluminates, etc.), which hydrolyze to flocculent, gelatinous precipitate of aluminum
hydroxide, and it then entraps the fine precipitates.
Use of sodium aluminate as coagulant also helps the removal of silica as well as oil, if present in
water.
Cold L-S process provides water, containing a residual hardness of 50 to 60 ppm.
NaAlO2 + 2H2O ---> Na0H + Al(0H)3
Al2(S04)3 + 3Ca(HC03)2 ---> 2Al(0H)3 + 3CaS04 + 6C02
The procedure for Cold Lime soda is as follows :
Raw water and calculated quantities of chemicals (lime + soda + coagulant) are fed from the top
into the inner vertical circular chamber, fitted with a vertical rotating shaft carrying a number of
paddles.
As the raw water and chemicals flow down, there is a vigorously stirring and continuous mixing,
whereby softening of water takes place.
As the softened water comes into the outer co-axial chamber, it rises upwards. The heavy sludge
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(or precipitated floc) settles down in the outer chamber by the time the softened water reaches
up.
The softened water then passes through a filtering media that's usually made of wood fibres to
ensure complete removal of sludge. Filtered soft water finally flows out continuously through the
outlet at the top.
Sludge settling at the bottom of the outer chamber is drawn off occasionally.
(ii) Hot lime-soda process : It involves treating the water with softening chemicals at a
temperature of 80 - 150 C.
Since hot process is operated at a temperature close to the boiling point of the solution, so
following things takes place :
(a) The reaction proceeds faster.
(b) The softening capacity of hot process is increased to many fold.
(c) The precipitate and sludge formed settle down rapidly and hence, no coagulants are needed.
(d) Much of the dissolved gases like C02 and Air driven out of the water.
(e) Viscosity of softened water is lower, so filtration of water becomes much easier. This in turn
increases the filtering capacity of filters.
(f) Hot lime-soda process produces water of comparatively lower residual hardness of 15 to 30
ppm.
Hot lime-soda plant consists essentially three parts :
(a) A 'reaction tank' in which raw water, chemicals and steam are thoroughly mixed
(b) A 'conical sedimentation vessel' in which sludge settles down.
(c) A 'sand filter' which ensures complete removal of sludge from the softened water.
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 To Remove Permanent Hardness
(a) Zeolite or permutit procees : Chemical structure of sodium zeolite may be represented as:
Na2O.Al2O3.xSiO2.yH2O or Na2Al2O3.xH2O.
where x = 2 - 10 and y = l - 5.
Zeolite is hydrated sodium alumino silicate, capable of exchanging reversibly its sodium ions for
hardness-producing ions in water.
Zeolites are also known as pemutits. Zeolites arc of two types :
(i) Natural Zeolites are non-porous. For example, natrolite, Na2O.Al203.4Si02.2H2O.
(ii) Synthetic zeolites are porous and possess gel structure.They are prepared by heating together
china clay, feldspar and soda ash. Such zeolites possess higher exchange capacity per unit weight
than natural zeolites.
figure (3)
The proces is as follows :
For softening of water by zeolite process, hard water is percolated at a specified rate through a
bed of zeolite, kept in a cylinder .
The hardness-causing ions like Ca+2, Mg+2 etc. are retained by the zeolite as CaZe and MgZe
while the outgoing water contains sodium salts. Reactions taking place during the softerring
process are :
Na2Ze + Ca(HCO3)2 ---> CaZe + 2NaHCO3
Na2Ze + Mg( HCO3)2 ---> MgZe + 2NaHCO3
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Na2Ze + CaCl2 (or) CaSO4 ---> CaZe + 2NaCl ( or Na2SO4)
Na2Ze + MgC l2 (or MgS04) ---> MgZe + 2NaCl ( or Na2SO4)
(Zeolite)................(Hardness)
Regeneration : After some time, the zeolite is completely converted into calcium and magnesium
zeolites and it ceases to soften water, it means it tets exhausted.
At this stage, the supply of hard water is stopped and the exhausted zeolite is reclaimed by
treating the bed with a concentrated (10%) brine (NaCl) solution.
CaZe (or MgZe) + 2NaCl ---> Na2Ze + CaCl2 or MgCl2
(Exhausted zeolite)...........(Brine)....(Reclaimed Zeolite)...........(Washings)
The washings that containing CaCl2 and MgCl2 are led to drain and the regenerated zeolite bed
thus obtained is used again for softening Purpose.
(b) Ion Exchange (de-ionization or de-mineralization) :
The ion exchange resins are insoluble, cross-linked, long chain organic polymers with a
microporous structute, and the "functional groups" attached to the chains are responsible for the
ion-exchanging.
Resins containing acidic functional groups (COOH, -SO3H, etc.) are capable of exchanging their H+
ions with other cations, which comes in their contact whereas containing basic functional groups
( -NH2 = NH as hydrochloride) are capable of their anions with other anions, which comes in their
contact.
The ion-resins may be classified as:
(i) Cation exchange resins (RH ) are mainly styrene-diainyl benzene co polymers, which ion or
carboxylation, become capable to exchange their hydrogen ions within the water.
(ii) Anion exchange resins (R'OH ) are stytene-divinyl benzene or amine formaldehyde
copolymers, which contains amino or quaternary ammonium or quaternary phosphonium or
tertiary sulphonium groups as an integral part of the resin matrix. These, after treatment with dil.
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NaOH solution, become capable to exchange their OH anions with anions in water.
Process : The hard water is passed first through cation exchange column, which removes all the
cations (like Ca , Mg , etc.) from it, and equivalent amount of H ions are released from this
column to water.
Thus:
2RH+ + Ca+2 -----> R2Ca+2 + 2H+
2RH+ + Mg+2 ------> R2Mg+2 + 2H+
After cation exchange column, the hard water is passed through anion exchange column, which
removes all the anions like S042-, Cl , etc. present in the water and equivalent amount of OH- ions
are released from this column to water. Thus :
R'OH + Cl
-----> R'Cl + OH
2R'OH + SO42- -----> R2' S042- + 20H
2R'OH + CO42- -----> R2' CO32- + 2OH
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H and OH ions that are released from cation exchange and anion exchange columns
respectively together get combined to produce water molecule.
H+ + OH
-----> H2O
Thus, the water coming out from the exchanger is free from cations as well as anions.
Ion free water, is known as deionized or demineralised water.
Treatment of municipal water (Drinking Water) and its treatment:
Municipal water is that water that should be portable to supply to humans thus should be fit for
drinking purposes.
And it should be Treated as follows :
1. Screning process
2. Aeration
3. Sedimentation.
4. Fitration
5. sterelisation & Disinfactantion
6. Storage & supply
Purification :
1. Screening : The raw water is passed through screens, having large number of holes, when
floating matters are retained by them.
2. Aeration :
2. Sedimentation : It is a process of allowing water to stand undisturbed in big tanks, about 5 m
deep, when most of the suspended particles settle down at the bottom, due to the force of
gravity.
The clear supernatant water is then drawn from tank with the help of pumps.
The retention period in a sedimentation tank ranges from 2-6 hours.
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When water contains fine clay particles and colloidal matter, it becomes necessary to apply
sedimentation with coagulation for removing such impurities.
Sedimentation with coagulation is the process of removing fine suspended and colloidal
impurities by the addition of requisite amount of chemicals called coagunlants to water before
sedimentation.
Just 4 Idea (don’t draw as it is )
Next, the water flows to a tank called a sedimentation basin where gravity causes the
flocs to settle to the bottom. Large particles settle more rapidly than small particles. It
would take a very long time for ALL of the particles to settle out and that would mean we
would need a VERY large sedimentation basin. So the clarified water, with most of the
particles removed, moves on to the filtration step where the finer particles are removed.
(3) Filtration : It is the process of removing colloidal matter and most of the bacterias, micro
organisms, etc. just by passing water through a bed of fine sand and other proper sized granular
materials.
Filtration is carried out by using sand filter.
(a) The sand filter consists of a thick top layer of finely placed over coarse sand layer and gravels.
It is provided with an inlet for water and an under drain channel at the bottom for exit of filter
water.
(b)Sedimented water entering the sand filter is uniformly distributed over the entire fine sand
bed.
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During filtration, the sand pores get clogged, due to retention of impurities in the pores.
When the rate of filtration becomes slow, the working of filter is stopped and about 2cm - 3cm of
the top fine sand layer is scrapped of and replaced with clean fine sand and the filter is put back
into use again.
The scrapped sand is washed with water, dried and stored for reuse at the time of next scrapping
operation.
(c) Micro organisms :
Water after passing through sedimentation, coagulation and filtration, operations still contains a
small percentage of pathogenic bacteria which is disease producing. Consequently, water used,
particularly for drinking or municipal purposes, must be freed from these disease producing
bacteria, micro-organisms, etc.
The process of destroying or killing the disease producing bacteria, micro-organisms, etc., from
the water and making it safe for use, is called disinfection.
The clemicals or substances which are added to water for killlng the bacteria, etc. are known as
disinfectants.
The disinfection of water can be carried out by following methods :
(1) By boiling water for 10-15 minutes, all the disease producing bacterias are killed and water
becomes safe for use.
(2) By adding bleacing powder : In small water works about 1kg of bleaching powder per 1,000
kilolitres of water is mixed and water allowed to stand undisturbed for several hours.
The chemical action produces hyypochlorous acid which is a powerful germicide.
(3) By Chlorination : Chlorine produces hypochlorous acid.
Cl2 + H2O - HOCl + HCl
HCl + Bacterias ----> Bacterias killed.
It was earlier believed that the disinfecting action of chlorine was due to the nascent oxygen
liberated which oxidised harmful bacteris etc.
After long experimentation it was reported that the death of micro organisms bacteria, etc.,
results from chemical reaction af hypochlorus acid (HOCl) with the enzymes in the cells of the
organisms, etc.
Since enzyme is essential for the metabolic proceses of the micro organisms so death of micro
organisms results due to inactivation of enzyme (in th cells of organisms) by hypochlorous acid.
(4) By Chloramine (CINH2) : When chlorine and ammonia are mixed in the ratio 2 : 1 by volume a
compound chloramine is formed.
Cl2 + NH3
- NH2Cl + HCl
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Chloramine is much more lasting than chlorine alone and consequently it's a better bactericidal
than chlorine.
NH2Cl + H2 O -- HOCl + NH3
Electro Dialysis :
Electro Dialysis (ED) is a membrane process, during which ions are transported through semi
permeable membrane, under the influence of an electric potential.
The membranes are cation- or anion-selective, which basically means that either positive ions or
negative ions will flow through. Cation-selective membranes are polyelectrolytes with negatively
charged matter, which rejects negatively charged ions and allows positively charged ions to flow
through.
By placing multiple membranes in a row, which alternately allow positively or negatively charged
ions to flow through, the ions can be removed from wastewater.
In some columns concentration of ions will take place and in other columns ions will be
removed. The concentrated saltwater flow is circulated until it has reached a value that enables
precipitation. At this point the flow is discharged.
This technique can be applied to remove ions from water. Particles that do not carry an electrical
charge are not removed.
Cation-selective membranes consist of sulphonated polystyrene, while anion-selective
membranes consist of polystyrene with quaternary ammonia.
Sometimes pre-treatment is necessary before the electro dialysis can take place. Suspended
solids with a diameter that exceeds 10 µm need to be removed, or else they will plug the
membrane pores. There are also substances that are able to neutralize a membrane, such as
large organic anions, colloids, iron oxides and manganese oxide. These disturb the selective
effect of the membrane.
Pre-treatment methods, which aid the prevention of these effects are active carbon filtration
(for organic matter), flocculation (for colloids) and filtration techniques.
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Reverse osmosis system
Reverse osmosis (RO) is a filtration method that removes many types of large molecules and ions
from solutions by applying pressure to the solution when it is on one side of a selective
membrane. The result is that the solute is retained on the pressurized side of the membrane and
the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not
allow large molecules or ions through the pores (holes), but should allow smaller components of
the solution (such as the solvent) to pass freely.Reverse osmosis is most commonly known for its
use in drinking water purification from seawater, removing the salt and other substances from
the water molecules. This is the reverse of the normal osmosis process, in which the solvent
naturally moves from an area of low solute concentration, through a membrane, to an area of
high solute concentration. The movement of a pure solvent to equalize solute concentrations on
each side of a membrane generates a pressure and this is the "osmotic pressure." Applying an
external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis.
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