DEMINERALISATION OF WATER.ppt

8/13/2019 DEMINERALISATION OF WATER.ppt

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DEMINERALISATION OF WATER

NEED FOR DEMINERALISATION :Raw watercontains various dissolved salts & suspended

solids.

For use as feed in boiler these are to be

removed as they may get deposited in heattransfer zones thereby restricting heat transfer

& may lead to corrosion also.

Typical DM Water will have pH around

6.8 , Conductivity < 0.2 umhos/cm, Silica


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WATER CHEMISTRY-PRE TREATMENT

USE OF WATER IN POWER PLANT

COOLING WATER

FIRE FIGHTING

STEAM GENERATION

HVAC SYSTEM

ALL THE ABOVE MENTIONED USES REQUIRE

DIFFERENT QUALITY

DIFFERENCE IN QUALITY CALLS FOR DIFFERENTTYPES OF TREATMENT


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RAW WATER COMPARISON

SSTP FGPS BTPS RhTPS JHANO

R

SEA

pH 7.4 7.1 7.8 7.8 8.6 7.5-8.4

Cond (s/cm) 94 1294 525 93 210 50000

Turb (NTU) 70 80 37 92 19 NIL

Hard (ppm) 34 310 166 36 102 6000

Alk (ppm) 32 300 172 38.6 113

Chlo (ppm) 5 325 70 6.3 13 18980

Sul (ppm) 8 65 44 7.5 NIL 2650

Silica(ppm) 8 20 9.5 9.7 15 10

Org (ppm) 20 9 2


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Water treatment plant

What is done?Parameter Raw water DM water

Conductivity upto 1800 0.06

Turbidity upto 800 0

TH upto 400 Nil

Silica upto 22 ppm


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Turbidity

Turbidity in the water is suspended

insoluble matter including coarse particles

(mud, sediment, sand etc.) that settle

rapidly on standing.

These materials can be removed bysettling, coagulation and filtration.

Their presence is undesirable because

heating or evaporation produces hard stonyscale deposits on the heating surface & clog

fluid system.


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Impurities of water

Suspended

Dissolved

Gaseous Organic


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Particle diameter

mm m AType of

particle

Settling time

through 1m of

water

10 104 108 Gravel 1 second

1 103

107

Sand 10 seconds10-1 102 106 Fine Sand 2 minutes

10-2 10 105 Clay 2 hours

10-3 1 104 Bacteria 8 days

10-4 10-1 103 Colloid 2 years



SETTLING TIMES FOR VARIOUS PARTICLES


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WATER CHEMISTRY-PRE TREATMENT

FIRST STEP FOR IMPROVING QUALITY IS PRETREATMENT

SCREENING, FLOATING, AERATION

OXIDATION OF ORGANICS

CLARIFICATION

FILTERATION

BENEFITS

BASICALLY REMOVES SUSPENDED MATTER

CHEMICAL ADDITION DURING PRE-TREATMENT CAN HELP

REDUCE COLLOIDS (silica) AND DISSOLVED IMPURITIES(hardness,iron etc.) ALSO

REDUCTION IN ORGANICS


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level of hardness

< 17.1 ppm as CaCO3 - soft

17.1 to 60 ppm- slightly hard

120 to 180 ppm - hard > 180 ppm - very hard


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HARDWATER:

WATER WITH HIGH CONCENTRATION OF Ca2+& Mg2+

TYPES OF HARDNESS:

1. CARBONATE HARDNESS2. NON-CARBONATE HARDNESS

CARBONATE HARDNESS COMPOUNDS:CaCO3, MgCO3, Ca(HCO3)2, Mg(HCO3)2

NON CARBONATE HARDNESS COMPOUNDS:CaSO4, MgSO4, CaCl2, MgCl2


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SOFTENING OF WATER:

REMOVAL OF HARDNESS FROM WATER

REQUIREMENT OF

SOFTENING OF WATER:

SCALE FORMATIONTASTE PROBLEM


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METHOD OF SOFTENING OF WATER:

1. ION EXCHANGE METHOD2. LIME SOFTENING


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REMOVAL OF CARBONATE HARDNESSBY LIME SOFTENING[BY Ca(OH)2]:

AERATION OF WATER:CO2 + Ca(OH)2 CaCO3 + H2O

Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O

(a) Mg(HCO3)2 +Ca(OH)2 CaCO3 +MgCO3 + 2H2O

(b) MgCO3 + Ca(OH)2 CaCO3 + Mg(OH)2

COMPOUNDS: REQUIRED PH:

CALCIUM

MAGNESIUM

9-9.5

10-10.5


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REMOVAL OF NON-CARBONATE HARDNESS[BY, LIME Ca(OH)2 + SODA ASH (Na2Co3)]:

MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4

CaSO4 + Na2CO3 CaCO3 + Na2SO4

COMPOUNDS: REQUIRED PH:

CALCIUM

MAGNESIUM

10-10.5

11-11.5


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WHY SOFTENING PLANT IS REQUIRED?

- Calcium & Magnesium ions are replaced by Sodium ions,

- Soft water in which total hardness (Ca & Mg) < 5 ppmCOC can be increased

-Most of scales formed at heat transfer zones of condenserare carbonates, sulphates & silicates of calcium &magnesium.

-Anions (sulphates, carbonates, silicates) cannot be

controlled at softening plant, so cations i.e. Calcium &Magnesium are controlled to control scale formation.


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Impact of scale

Thickness(mm) scale Loss of fuel(%)

0.75 8

1.5 143.0 20

6.0 50


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Treatment of water

Depending upon the source of water,

different types of treatment may be

required.

Surface water may not need to be

softened, but it may need a filtration and

tannin removal system.

ground water may have hardness and

iron, so softening may be sufficient.


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The process of water treatment can be

broadly classified into 2 stages.1. PT-Stage:

In Pre-treatment all the suspended/colloidal particles like clay, mud, and micro

organisms are removed.2. Demineralisation:

In this step, all dissolved solids areremoved from water with the aid of ion

exchanger resins.


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Clarification

Pre- Treatment of water

Mixing of chemicals with water

Coagulation and flocculation Sedimentation

Filtration


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Coagulants

Aluminium Sulphate, Sodium Aluminate Iron sulphate

Poly electrolytes (long chain amides)

Poly Aluminium Chloride ( PAC )Factors affecting coagulation

pH ( 5.58.0 ) for Al2(SO4)3

Temperature (30- 400C )

Time


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PRE-TREATMENT PROCESS

TO GSF

SLUDGE TO SLUDGE PIT

STILLING

CHAMBR

CLFR-B

LIME & Cl2ALUM & POLYMER

CLARIFIEDWATER

STORA

GETANK

RAW

WATER

CLFR-A

TO CT MAKE

UPSTILLING

CHAMBER


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RESERVOIR CLARIFIER

CHLORINE

/ ClO2

PAC

BLOWDOWN

GRAVITY

FILTERCPH RWFB

PROCESS FLOW CHART-FGPS

FILTER

WATER

SUMP

CT make

up


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Pretreatment


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Pretreatment Done by clarification and filtration

When water is allowed to stand-suspendedimpurities settle down.

Clarification is accelerated by adding coagulants

like Alum Al3(SO4)2,FeSO4 ,NaAlO2

This results in formation of flocs or ppts of

Al(OH)3 which tends to agglomerate colloidal,

suspended or organic impurities.

Impurities which are not settled duringclarification and sedimentation are removed

during filtration.


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Inorganic Coagulants iron and aluminum coagulants are acid salts

that lower the pH of the treated water byhydrolysis.

Depending on initial raw water alkalinity and

pH, an alkali such as lime or caustic must beadded to counteract the pH depression of the

primary coagulant.

Iron and aluminum hydroxide flocs are best

precipitated at pH levels that minimize the

coagulant solubility.


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With aluminum sulfate-coagulation efficiency

and minimum floc solubility normally occur at pH

6.0 to 7.0

Iron coagulants can be used over the pH rangeof 5.0 to 11.0

If ferrous compounds are used, oxidation to

ferric iron is needed for complete precipitation.

This requires either chlorine addition or pH

adjustment.


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pretreatment


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The chemical reactions

Al2(SO4)3 + 6NaHCO3 = 2Al(OH)3-+ 3Na2SO4 + 6CO2aluminum sulfate sodium bicarbonate aluminum hydroxide sodium sulfate carbon dioxide

Fe2(SO4)3 + 6NaHCO3 = 2Fe(OH)3-+ 3Na2SO4 + 6CO2ferric sulfate sodium bicarbonate ferric hydroxide sodium sulfate carbon dioxide

Na2Al2O4 + 4H2O = 2Al(OH)3- + 2NaOH

sodium aluminate water aluminum hydroxide sodium hydroxide


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Oxidation of Ferrous

Chemicals may be added to oxidize

ferrous iron (Fe++), which is relatively high

in some groundwater, to the ferric state

(Fe+++). If pH of the water is above 7(either naturally or by adding lime), the

insoluble compound of ferric hydroxide is

precipitated


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Chemical treatment

Chlorine and other chemicals, such as

alum or lime, are added to the water to

remove impurities, destroy any taste or

odor, raise pH, disinfect, and sometimesremove excess minerals such as iron that

may cause rust or staining problems. The

water is then mixed rapidly to distribute thechemicals evenly.


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Coagulation & sedimentation Coagulation- the process in which suspended particles

are combined by chemical means into sufficiently largemasses to effect rapid settling. adding small, highly

charged cations (aluminium 3+or Fe 3+are usually used)

Flocculation adding small amounts of charge polymer

chains which either form a bridge between theparticulate solids (making them bigger) or between the

particulate solids and the sand

PEL- Some PELs in very small quantities increase the

efficiency of coagulants. Sedimentation- settling


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Polyelectrolytes refers to all water-soluble organic polymers used

for clarification, whether they function ascoagulants or flocculants.

Water-soluble polymers classified as :

anionic-ionize in water solution to formnegatively charged sites along the polymer chain

cationic-ionize in water solution to form positivelycharged sites along the polymer chain

nonionic-ionize in water solution to form veryslight negatively charged sites along the polymerchain

Poly


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Poly Polymeric primary coagulants are cationic

materials with relatively low molecular weights

(under 500,000).

For any given particle there is an ideal

molecular weight and an ideal charge density

for optimum coagulation. There is also an optimum charge density and

molecular weight for the most efficient

flocculant.

Primary Coagulant Polyelectrolytes- Thecationic polyelectrolytes commonly used as

primary coagulants are polyamines.


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Raw waters of less than 10 NTU (NephelometricTurbidity Units) usually cannot be clarified with a cationicpolymer alone.

Best results are obtained by a combination of aninorganic salt and cationic polymer.

waters containing 10 to 60 NTU are most effectivelytreated with an inorganic coagulant and cationic polymer.

a significant portion of the inorganic coagulant demandcan be met with the cationic polyelectrolyte. With turbiditygreater than 60 NTU, a polymeric primary coagulantalone is normally sufficient.

In low-turbidity waters where it is desirable to avoid usingan inorganic coagulant, artificial turbidity can be added tobuild floc. Bentonite clay is used to increase surface areafor adsorption and entrapment of finely divided turbidity.


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Organic polymers vs inorganic coagulants The amount of sludge produced during clarification can be

reduced by 50-90%.

The resulting sludge contains less chemically bound water andcan be more easily dewatered.

Polymeric coagulants do not affect pH. So the need forsupplemental alkalinity, such as lime, caustic, or soda ash, isreduced or eliminated.

Polymeric coagulants do not add to the total dissolved solidsconcentration.

1 ppm of alum adds 0.45 ppm of sulfate ion (expressed asCaCO3). The reduction in sulfate can significantly extend thecapacity of anion exchange systems.

Soluble iron or aluminum carryover in the clarifier effluent mayresult from inorganic coagulant use. Therefore, elimination ofthe inorganic coagulant can minimize the deposition of thesemetals in filters, ion exchange units, and cooling systems.


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Objectives of Biological Control Biological control is the successful inactivation

or removal of organisms so that they do notaffect makeup treatment plant operation orperformance.

Minimize the formation of bacterial slime inpretreatment systems (especially piping,clarifiers, filters and filtered water storagetanks).

Minimize the formation of algae in pretreatmentsystems (particularly open clarifiers, tanks

and filters).


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Removal of organic matter

Organic matter can either be in colloidal or

dissolved form or both. Pre-chlorination of water oxidises dissolved

organic matter and colloidal organic is bestremoved by proper coagulation and clarification.

Ion exchange resins resistant to fouling byorganics present in water should be used.

Any residual organic matter can be removed byNano-filtration (NF) membranes. UF

membranes are not very effective due to the lowmolecular weight of organic matter present insurface waters.


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Chemistry in chlorination of water

When chlorine is added to water, it reacts toform a pH dependent equilibrium mixture ofchlorine, hypochlorous acid and hydrochloricacidCl2 + H2O HOCl + HCl

Depending on the pH, hypochlorous acid partlydissociates to hydrogen and hypochlorite ions:HClO H+ + ClO-

In acidic solution, the major species are Cl2 andHOCl while in alkaline solution effectively onlyClO- is present.


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Chlorine dioxide

Chlorine dioxide is an active oxidising biocide,

has less damaging effects to the environment and human health thanchlorine

It does not form hydrochlorous acids in water;

It exists as dissolved chlorine dioxide, a compound that is a morereactive biocide at higher pH ranges.

Chlorine dioxide is an explosive gas, and therefore it has to beproduced or generated on site, by means of the following reactions:

Cl2 + 2 NaClO2 -> 2 NaCl + 2 ClO2or2 HCl + 3 NaOCl + NaClO2 -> 2 ClO2 + 4 NaCl + H2O.


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Raw water to Clarified water

Raw water bay

RW make up

pumps

Aerator

Cl2 dosing

Alum/ PAC

dosing

Flash

mixer

Clarified

water

Clariflocculator


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Clarification System


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Motion of water in a Clari-flocculator

Flocculation zoneClarification zone


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A view of Clari-flocculator

Flocculation zone

Clarification zone


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SLUDGE TREATMENT

SLUDGE FROM

CLARIFIERS

SLUDGEPIT SLUDGE THICKENERS

DEWATERING

CENTRIFUGES


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DEWATERING CENTRIFUGE

SLUDGE CAKE FOR DISPOSAL


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filtration

The process of removing suspended

matter by passing through a suitable

process material.

Two types

(i) Gravity filters

(ii) pressure filters

Filtering media used is sand ot anthracite.


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Filter MediaTheoretically any inert granular materialcan be used for filtration.

Quarts sand, Silica sand, anthracite coal,garnet may be used for filtration.

Silica sand and anthracite are the typesof filter media which are commonly used.

At FGPS sand is used as filtering

medium and filters are Gravity sandfilters (GSF).


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FILTERED WATER PRODUCTION

GSF-A

GSF-B

GSF-C

FILTERED

WATER

SUMP

C

L

F

W

A

TE

R


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Filter medium layers in GSF

1st layer - 50 mm X 37 mm gravel2ndlayer - 37 mm X 12 mm gravel

3rd

layer12 mm X 6 mm gravel

4thlayer6 mm X 2.5 mm grit

5thlayer0.35 mm X 0.5 mm sand


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Pressure filter


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Clarified water to Filtered water

Clarified water

Clari-flocculator

To Township for

drinking water

Turbidity & Free Cl2check

DM PLANT

Filtered water

DW

sump

FW

sump

FW

pumps

DW

pumps

DM water

for Plantpurpose

GF BedSand filters

Cl2 Post-

chlorination

Cl2House


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Li d ft i


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Lime soda softening Ca and suspended solids can be reduced

Partial reduction of silica also takes place. Use of lime removes-

-alkaline hardness due to the bicarbonates

of Ca and Mg-Non alkaline hardness due to MgSO4-CO2

Use of soda Ash removes non alkalinehardness due to CaSO4.

Reactions


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Reactions Use of lime

CaO + H2O = Ca(OH)2Unslaked lime hydrated lime

Ca(OH)2+ Ca(HCO3)2 = 2CaCO3+2H2O

2Ca(OH)2

+Mg(HCO3

)2

= 2CaCO3

+Mg(OH)2+2H2O

Ca(OH)2+MgSO4 =CaSO4 +Mg(OH)2

Ca(OH)2+MgCl2 =CaCl2 + Mg(OH)2

Ca(OH)2+CO2 = CaCO3 + H2O

Ca(OH)2+ 2NaHCO3 = CaCO3+2H2O +Na2CO3


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Use of soda Ash

Na2CO3+CaSO4 = CaCO3 +Na2SO4

Na2CO

3+CaCl

2= CaCO

3+2NaCl

Use of sodium Aluminate-

Na2Al2O4+ 4 H2O =2NaOH + 2Al(OH)3

Na2Al2O4 +Ca(OH)2= CaAl2O4 +2NaOHCaAl2O4 + 4 H2O =2Al(OH)3 +Ca(OH)2

Wh i i ?


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What are micro-organisms?

All living creatures consist of cells. Cells

are very small basic units of life. They arethe smallest structures capable of basic

life processes, such as taking in nutrients

and expelling waste. Cells can only bemade visible by microscopes.

Micro-organisms are organisms that

usually consist of one single cell. Becauseof this, they are often referred to as

"single-celled organisms".


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Types of microrganismsMicrorganisms are divided up by their cell

characteristics, in the same way as plants andanimals.

There are two kinds of microrganisms. The firstkind is the eukaryotic organism (protista).

Eukaryotic- the cells they consist of containnucleuses and other internal parts, surroundedby membranes.

The second kind of microrganisms is the

Prokaryotic organism (monera). Prokaryoticcells are surrounded by a membrane, but theycontain no nucleus or other internal parts


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Monera-Bacteria Bacteria are very important for other

organisms, because they break downorganic matter. During this processnutrients are formed, which are reused by

plants and animals. Some of the bacteriathat live on earth can cause disease, butmost of them are quite useful as they aidanimals in the decomposition of food in

their bodies. Bacteria differ from othertypes of cells in the fact that they do nothave a nucleus.


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protists -amoebas, diatoms, algae and protozoa These can be a danger to human and animal

health, as certain protists can cause diseases,such as malaria and sleeping sickness. There area wide variety of protists, and they inhabit manydifferent environments; fresh water, seawater,soils, and the intestinal tracts of animals, wherethey perform crucial digestive processes.Many species of protists can produce their ownnutrients by the process of photosynthesisandmany protists can also move around on their own

accord. Protists vary greatly in size and shape;the green alga Nanochlorum is only 0.01 mmlong, but giant kelps can grow to 65 mor more inlength.

f


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Use of microrganisms

Microrganisms can be used to decompose

contaminants in wastewater. This kind of

water treatment is called biological water

treatment.

During biological water treatment

microrganisms break down organic matter,

nitrates and phosphates.

Removal of ammonium and nitrates


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Removal of ammonium and nitrates

The removal of ammonium and nitrates is quite complex. It is awater treatment process that takes both aerobic and anaerobic

conversion to remove the contaminants. In the aerobic conversion stagethere are two bacterial species

involved. Firstly, Nitrosomonas bacteria convert ammonia tonitrite. Secondly, Nitrobacter bacteria convert nitrite to nitrate.These two processes together are commonly known as thenitrification process.

After that, the anaerobic bacteriatake over. These bacteriaconvert nitrate to atmospheric nitrogen gas. This process iscalled denitrification.

Denitrification is accomplished with many anaerobic bacteria,such as Achromobacter, Bacillus and Pseudomonas.

The first stage of denitrification is the reverse of the nitrificationprocess, it converts nitrate back to nitrite. The second stage ofdenitrification converts nitrite to nitrogen gas (N2).

Removal of phosphates


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Removal of phosphates Phosphates can be removed from wastewater by an aerobic

(oxygen-dependent) bacterium, called Acinetobacter. This

bacterium accumulates polyphosphates in the cell tissues. The Acinetobacter can take up a higher amount of phosphatesthan it needs for its cell synthesis. The extra amounts ofphosphates are stored in the cells as polyphosphates.The storage of polyphosphates causes the Acinetobacter to beable to temporarily survive anaerobic circumstances.

When the Acinetobacter resides in an anaerobic zone in thewastewater, it takes up fatty acids to store them as sparesubstances. During this process, polyphosphates aredecomposed for energy supply, causing phosphates to bereleased into the aerobic zone.

When the Acinetobacter enters the aerobic zone it takes upphosphates and stores them as polyphosphates in the celltissues. This causes the phosphate content of the wastewaterto decrease.


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Organic matter
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Organic matter Organic matter present in surface water is

mostly of vegetable and animal origin andconsists essentially of large molecular weightcarboxylic acids collectively termed as humicand fulvic acids.

carry negative charge and therefore areadsorbed by a strong base resin in DM plant.

Organic matter is harmful if present in boiler feedwater as it breaks down in the boiler drum,

depresses the pH and causes corrosion. A lower pH increases the risk of silica carry-over

in steam.


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Removal of Organic matter

Chlorination- added as liq Chlorine or

Bleaching powder .destroys the bacteria or

any other micro organism present.

Ozonization-

UV Light

Removal of organic matter


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Removal of organic matter Biological water purification is performed to lower the

organic load of dissolved organic compounds.

Microrganisms, mainly bacteria, do the decomposition ofthese compounds.

There are two main categories of biological treatment:

aerobic treatment and anaerobic treatment.

Aerobic water treatment means decomposition of organicmatter by bacteria that need oxygen during thedecomposition process.

Anaerobic water treatmentmeans decomposition of organicmatter by microrganisms that do not use oxygen.

In aerobic systems the water is aerated with compressed air

whereas anaerobic systems run under oxygen freeconditions.

Chlorination of water


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Chlorination of waterCl2 + H2O -> HOCl + HCl

If the pH of the wastewater is greater than 8, the hypochlorus acid will

dissociate to yield hypochlorite ion HOCl H+ + OCl If ammonia is present in the wastewater effluent, then the hypochlorus acid

will react to form one three types of chloramines depending on the pH,

temperature, and reaction time.

Monochloramine and dichloramine are formed in the pH range of 4.5 to 8.5,

however, monochloramine is most common when the pH is above 8. When

the pH of the wastewater is below 4.5, the most common form of chloramine istrichloramine which produces a very foul odor.

Monochloramine: NH3 + HOCl -> NH2Cl + H2O

Dichloramine: NH2Cl + 2HOCl -> NHCl2 + 2H2O

Trichloramine: NHCl2 + 3HOCl -> NHCl3 + 3H2O

Chloramines are an effective disinfectant against bacteria but not againstviruses. As a result, it is necessary to add more chlorine to the wastewater to

prevent the formation of chloramines and form other stronger forms of

disinfectants.

2NH2Cl + HOCl -> N2 + 6HCl + H2O


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Oxidizing Biocides

Chlorine -diffuses through the cell walls of micro-organism reaching the cytoplasm to produce achemically stable nitrogen-chlorine bond with the cellproteins.

The optimum pH values of cooling water in whichchlorine dosing is best effective, is 6.5 to 7.5

Certain micro-organisms sometime become immune tothe regular dose of chlorine so Shock Chlorinationemploying heavy doses of chlorine for few hours is done

to kill the micro-organisms


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Chlorine dioxide

Preparation

2NaClO2 + Cl2 2ClO2 + 2 NaCl

Structure-

Chl i i t d ith 25% di hl it


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Chlorine gas is reacted with a 25% sodium chlorite

solution to produce chlorine dioxide.

in water treatment is used as a pre-oxidant prior to


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in water treatment is used as a pre oxidant prior tochlorination of drinking water to destroy natural waterimpurities that produce tri-halomethanes on exposure to

free chlorineChlorine dioxide is also superior to chlorine whenoperating above pH 7 due to presence of ammonia

Chlorine dioxide is used in many industrial water

treatment applications as a biocide including coolingtowers

Chlorine dioxide is less corrosive than chlorine andsuperior for the control of legionella bacteria.

It is more effective as a disinfectant than chlorine inmost circumstances against water borne pathogenicmicrobes such as viruses , bacteria and protozoa

O


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Ozone treatment

An oxygen molecule (O2) in the stratosphere isbroken into 2 oxygen atoms (O + O) by

absorbing ultraviolet light energy from the sun.

The oxygen atom (O) is now free to react with anoxygen molecule (O2) to create an ozone

molecule (O3).

O2 + UV => O + O

O + O2 => O3

O ti


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Ozone generation

Ozone generation by corona-discharge ozone is produced from oxygen as a direct result

of electrical discharge.

This corona-discharge ruptures the stableoxygen molecule and forms two oxygen radicals.

These radicals can combine with oxygenmolecules to form ozone.

Colloidal silica


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Colloidal silica Escapes clarifier system and resin

treatment. Silica polymerize into colloids which

behave differently than silica in solutions

Colloids undetectable by colorimetrictesting for reactive silica

Colloids also dont carry any charge so

escape resin treatment. Colloids break down at high temp andpressure

Colloidal silica


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Colloidal silica Colloidal silica is non-ionic, and is found in

surface waters. It creates problems in water treatment because

of its stability as an un-ionized compound, whichmakes it difficult to remove using ion exchange

processes. It can cause some resin fouling where colloidal

silica levels are very high.

Colloidal silica slips through the demineralisation

(DM) plant to get converted into reactive silicaat high temperature and pressure leading tosevere problems in boilers.

H f l i t


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Harmful impacts When reverse osmosis (RO) plants are used in

water treatment, colloidal silica and associated

impurities can foul RO membranes leading to

drop in productivity.

Colloidal particles are small particles,intermediate in size between true solutions and

suspended matter. They can be assumed to be

any particle larger than 10 A units and < 1micronin diameter

f ti f ll id l ili


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formation of colloidal silica

As water passes through and over various soilsthe formation of carbon dioxide and organicacids resulting from microbial activity results inacid degradation of the silicate minerals

(particularly clay particles).This acid attack on minerals dissolves the iron,

aluminum etc. and interaction of thesecomponents with silica results in the formation of

colloidal silica that is stabilised with a coating of

organic matter.

M t f C ll id l ili


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Measurement of Colloidal silica

Estimation of non-reactive silica in water is oftendifficult as it is present in very small quantities(generally < 1.0 mg/l).

Non-reactive silica is measured indirectly by the

difference between total silica (reactive silica +colloidal silica) and reactive silica.

Total silica may be measured by solubilising

non-reactive silica by reacting it with hydrofluoricacid (HF) and then employing usual calorimetricprocedures.


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removing/reducing colloidal silica


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removing/reducing colloidal silica Coagulation/Flocculation: Difficulties in removing

non-reactive silica arise from the fact that it is notpresent as a simple colloidal particle and is notamenable to coagulation under normalconditions.

It is mostly present as hydrated silica associated

with organic matter naturally present in soil andhydrated oxides of

iron & aluminum.

Coagulation, at best, is a physical process and,

under ideal conditions, one can therefore expectto remove up to 80 to 90% of non-reactive silica

present in water.

Retention time in clarifiers 1 to 4 hrs depending on water


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Retention time in clarifiers 1 to 4 hrs depending on waterrequirements and water quality.

Proper operation of clarifiers can reduce Ca to 35-40 ppm

as CaCO3 and turbidity < 10 NTU. Filtration can reduce turbidity to 10 psi from normal.

ACF- good breeding ground for bacteria which escaped

oxidising biocide, so regular checking for organics andmicrobiological content.

ACF produced from different minerals and one type of ACFwill have different properties from other.

If high iron and Mn and dissolved gases like CO2,NH3,H2S

than aeration recommended to remove these impurities.Forhigh levels of these impurities Mn greensand required


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Sodium phosphate and iron oxide which act as

insulators leading to increase in temp.

In coagulation- charge neutralisation- flocformation due to Van der walls force of attraction

Silica is removed as Mg Silicate in lime Ca(OH)2

, Soda (Na2CO3) softening process. Removal of silica depends upon the conc of

Magnesium in water.

Supplemental silica removal chemicals like MgO

or MgCo3 are added to enhance silica removal.

Deionization and softening


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Deionization and softening Deionisation is commonly processed through

ion exchange. Ion exchange systems consistof a tank with small beds of synthetic resin,which is treated to selectively absorb certaincations or anions and replace them by

counter-ions. The process of ion exchange lasts, until all

available spaces are filled up with ions.

The ion-exchanging device than has to be

regenerated by suitable chemicals.

DM PLANT


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From filter

water pumps

DM

water

storage

tank

ACFWAC SAC WBA

SBAMB

DEGASSER

Air

To main plant forboiler make up

For circuit rinse

ion exchange reactions


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ion exchange reactionsthe ion exchange reactions are reversible.

The degree the reaction proceeds to theright will depend on the resins preference. or

selectivity, for cations compared with its

preference for hydrogen ions.

strong acid resins have a preference for

cations(Ca++ or Mg++) over hydrogen.

Despite this preference, the resin can be

converted back to the hydrogen form bycontact with a concentrated solution of HCl

or H2SO4


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Attachment of Functional Group


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Attachment of Functional Group.

Following the formation of the styrene/divinyl

benzene copolymer, functionalization of the

polymer structure is done to convert the polymer

into an ion exchange resin.Functionalities added to the copolymer include

sulfonate, quaternary amine, and tertiary amine

groups.

Types of resins


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Types of resins

There are four main types differing in their functional groups:

strongly acidic (typically, sulfonic acid groups, eg.

sodium polystyrene sulfonate)

strongly basic, (quaternary amino groups, for example,

trimethylammonium groups)

weakly acidic (carboxylic acid groups)

weakly basic (primary, secondary, and/or ternary amino

groups, eg. polyethylene amine)

resins


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resins
http://upload.wikimedia.org/wikipedia/commons/0/08/Ion_exchange_resin_beads.jpg


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Weak Acid Cation Resin


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Weak Acid Cation Resin

Weak acid cation resins are utilized to

remove cations (primarily calcium andmagnesium) which are bound to alkalinity.

However, they also remove monovalent

cations (sodium and potassium) when totalalkalinity exceeds total hardness.

When sulfuric acid is employed, it firstmust be diluted to 0.5-1.0% (prior to its

introduction to the weak acid resin) in orderto limit calcium sulfate fouling potential.

CATION EXCHANGER AND ANION EXCHANGERSERVICE AND REGENERATION


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DF - Down Flow

NF - Nozzle flushin

Regeneration line to weak

exchanger

DrainDrain

Weak Strong

SI

SOSO

Acid/Alkali injection

DF

Air

VentAir

Vent

NF

SI

BO

BO

BIBI

RO

RO

MIXED BEDSERVICE AND REGENERATION


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SERVICE AND REGENERATION

Air

Vent

SI

SODrain

Alkali injection

Acid injection

NF

Air

MIXED BEDRESIN SEPARATION


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RESIN SEPARATION

CATION

EXCHANGE

RESIN

ANIONEXCHANGE

RESIN

THE MANUFACTURING


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PROCESS

The cation ion exchange manufacturingprocess consists of the following steps:

Polymerization (styrene and divinylbenzene )

Sulfonation (Introduction ofSO3H group)

Neutralization(treatment with NaOH)

rinse. (To remove extra NaOH)

THE SOFTENING PROCESS


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THE SOFTENING PROCESS When soap comes in contact with calcium

and magnesium, a curd forms (precipitate). Industrially, hardness forms scale on

pipes, boilers, heat exchangers and

cooling towers. deposits will reduce the heat transfer

capabilities of a system as well asincrease the cost of operating the system .

as hardness increases so does thepotential for scaling.

Softening


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Softening

Sometimes chemicals are included toreduce the hardness or mineral content

of drinking water. This usually involves the

exchange of sodium for calcium and

magnesium and, sometimes, the removalof iron and manganese.

features


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features Gel-type softening resins are translucent (light

will pass through bead) while macroporoussoftening resins are opaque.

Softening resin works on the principle of

selectivity Monovalent ions, like sodium have one positive

charge, and are held onto the resin less tightly

than divalent molecules like calcium and

magnesium (which have two positive charges).

calcium ion will displace a sodium ion .

Cont


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the greater the molecular weight, the

greater the affinity of the resin for this ion.

Calcium will displace magnesium since

calcium has a molecular weight of 40.1

and magnesium has a molecular weight of24.3.

In the softening process, two sodium ions

are released for one calcium ion or onemagnesium ion

Cont


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Cont

The waste regenerant will contain theexcess sodium chloride (NaCl), calcium

chloride (CaCl2) and magnesium chloride .

no matter how much salt is used in theregeneration process; the resin will never

fully regenerate to the sodium form.

DEMINERALISATION


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In ACF Bed removes the residual Cl2and fine

turbidity present in filtered water After ACF, water enters the SAC Bed, where

the Cations in water are replaced with H+ as

follows:

RH + Na Cl RNa + HCl

2 RH + CaSO4 R2Ca + H2SO4

2 RH + Ca(HCO3)2 R2Ca + 2H2CO3

DEMINERALISATION


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All Cations are exchanged in SAC Bed to

form acids like HCl, H2SO4, HNO3 etc.Bicarbonate is exchanged in SAC formcarbonic acid (H2CO3).

In the Degasser the water is sprayed fromtop and air blow (which has very less CO2content) is given in counter direction.

H2

CO3

H2

O + CO2 The process is similar to removal of

dissolved O2in Deaerator using steam.


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MIXED BED


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Mixed Bed (MB) contains a mixture of SAC

and SBA resins and act as polishing unitfor water from SBA. The output of MB is

Demineralised Water and is practically

free from all ions. DM water is having a conductivity


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degasification


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g Alkalinity on passing through cation beds

turns into CO2, so economical to removebefore anion removal

Forced draft or vacuum degasification. In

former water in droplets falls and airpasses through removing CO2 and

concentrating with O2 while in other

removal of gases by vacuum .

regeneration


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regeneration Cocurrent- most strongly held ions to pass

through whole resin attaching anddetaching several times which ultimatelyneed more quantity of acid.

Silica removal enhanced by heating theregenerant water to 120 F for type I and105 for type II resins

SAC & SBA require 3 times the H+ or OH-

of ionic loading whereas WAC & WBArequire stoichiometric concentrations

Alkalinity


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Alkalinity

Alkalinity is based upon the bicarbonateion (HCO3), the carbonate ion (CO3), and

the hydroxide ion (OH).

The bicarbonate ion will be prevalent inwater when the pH ranges from 4.3 to 8.3.

When the pH exceeds 8.3, CO3 becomes

prevalent at pH above of 10 OH will exist.

chemical oxygen demand (COD)


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Definition

Standard method for indirect measurement of the amount of

pollution (that cannot be oxidized biologically) in a sample of

water. COD test procedure is based on the chemical-

decomposition of organic and inorganic contaminants,

dissolved or suspended in water. The result of a COD test

indicates the amount of water dissolved oxygen (expressedas parts per million or milligrams per liter of water)

consumed by the contaminants, during two hours of

decomposition from a solution of boiling potassium

dichromate. Higher the COD, higher the amount of pollution

in the test sample. For the contaminants that can be oxidizedbiologically, biological oxygen demand (BOD) method is

used

Pore sizes


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Pore sizes Membrane pore sizes can vary from .1 to 5,000

nanometers (nm) depending on filter type. "Particle filtration" removes particles of 1,000 nm or

larger.

Microfiltration removes particles of 50 nm or larger.

"Ultrafiltration" removes particles of roughly 3 nm orlarger.

"Nanofiltration" removes particles of 1 nm or larger.Reverse osmosis is in the final category of membranefiltration,

"Hyperfiltration", and removes particles larger than .1 nm. FRP-Fibre Glass reinforced plastic

Ultra Filtration


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Ultra Filtration

The best way of ensuring maximum removal ofnon-reactive silica is to remove the bulk of it in

the pretreatment plant

polish it with an ultra filtration (UF) system

installed at the outlet of the mixed bed (MB) unit.

UF is a pressure activated process employing

a semi-permeable membrane with asymmetric

structure and can be effectively used for

removal of non-reactive silica

UF membrane


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UF membrane


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RO process


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RO process In the RO process, water from a pressurized saline solution is

separated from the dissolved salts by passing through a

water-permeable membrane. The permeate (the liquidflowing through the membrane) is directed to flow through themembrane by the pressure differential created between thepressurized feedwater and the product water, which is atnear-atmospheric pressure. The remaining feedwatercontinues through the pressurized side of the reactor as

brine. No heating or phase change takes place. The majorenergy requirement is for the initial pressurization of thefeedwater. For brackish water desalination the operatingpressures range from 250 to 400 psi, and for seawaterdesalination from 800 to 1 000 psi.

To reduce the concentration of dissolved salts remaining, aportion of this concentrated feedwater-brine solution iswithdrawn from the container.

Different stages of RO Process


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Different stages of RO Process

A reverse osmosis system consists of four majorcomponents/processes: (1) pretreatment, (2)pressurization, (3) membrane separation, and (4) post-treatment stabilization .

Pretreatment:The incoming feedwater is pretreated to

be compatible with the membranes by removingsuspended solids, adjusting the pH, and adding athreshold inhibitor to control scaling caused byconstituents such as calcium sulphate.

Pressurizat ion:The pump raises the pressure of thepretreated feedwater to an operating pressureappropriate for the membrane and the salinity of thefeedwater .

Separat ion :The permeable membranes inhibitthe passage of dissolved salts while permitting thedesalinated product water to pass through


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desalinated product water to pass through.Applying feedwater to the membrane assembly

results in a freshwater product stream and aconcentrated brine reject stream.

Two of the most popular membranes are spiralwound and hollow fine fiber. They are generallymade of cellulose acetate, aromatic polyamides,or, nowadays, thin film polymer composites.

Stabil izat ion :The product water from themembrane assembly usually requires pHadjustmentand degasification . The product

passes through an aeration column in which thepH is elevated from a value of approximately 5 toa value close to 7


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New membranes are being designed to operate at higherpressures (7 to 8.5 atm) and with greater efficiencies(removing 60% to 75% of the salt plus nearly all organics,viruses, bacteria, and other chemical pollutants).

The main operational concern related to the use of reverseosmosis units is fouling. Fouling is caused when membranepores are clogged by salts or obstructed by suspendedparticulates. It limits the amount of water that can betreated before cleaning is required. Membrane fouling canbe corrected by backwashing or cleaning (about every 4months), and by replacement of the cartridge filter elements

(about every 8 weeks). The lifetime of a membrane hasbeen reported to be 2 to 3 years,

Effectiveness of the Technology


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Earlier, it was tough to separate product waters from 90% ofthe salt in feedwater at total dissolved solids (TDS) levels of 1

500 mg/1, using pressures of 600 psi and a flux through themembrane of 18 l/m2/day.

Today, typical brackish installations can separate 98% of thesalt from feedwater at TDS levels of 2 500 to 3 000 mg/1,using pressures of 13.6 to 17 atm and a flux of 24 l/m2/day -and guaranteeing to do it for 5 years without having to replace

the membrane. Today's state-of-the-art technology uses thin film composite

membranes in place of the older cellulose acetate andpolyamide membranes. The composite membranes work overa wider range of pH, at higher temperatures, and withinbroader chemical limits.

In general, the recovery efficiency of RO desalination plantsincreases with time as long as there is no fouling of themembrane.

Advantages of RO


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g The processing system is simple; the only complicating factor

is finding or producing a clean supply of feedwater tominimize the need for frequent cleaning of the membrane.

Low maintenance, nonmetallic materials are used inconstruction.

Energy use to process brackish water ranges from 1 to 3 kWhper 1 0001 of product water.

RO technologies can make use of an almost unlimited andreliable water source, the sea.

RO technologies can be used to remove organic andinorganic contaminants.

Aside from the need to dispose of the brine, RO has a

negligible environmental impact. The technology makes minimal use of chemicals.

Disadvantages


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The membranes are sensitive to abuse.

The feedwater usually needs to be pretreated to remove particulates(in order to prolong membrane life).

There may be interruptions of service during stormy weather (whichmay increase particulate resuspension and the amount ofsuspended solids in the feedwater) for plants that use seawater. .

Brine must be carefully disposed of to avoid deleteriousenvironmental impacts.

There is a risk of bacterial contamination of the membranes; whilebacteria are retained in the brine stream, bacterial growth on themembrane itself can introduce tastes and odors into the productwater

RO technologies require a reliable energy source.

Desalination technologies have a high cost when compared to othermethods, such as groundwater extraction or rainwater harvesting.

Suspended solids and TDS


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p

Suspended solids represent the undissolved matter inwater, including dirt, silt, biological growth, vegetation,and insoluble organic matter.

When minerals dissolve in water, ions are formed. Thesum of all minerals or ions in the water in the total

dissolved solids or the TDS. Iron can be soluble orinsoluble. Insoluble iron can clog valves and strainersand can cause excessive sludge build up in low lyingareas of a water system. It also leads to boiler depositsthat can cause tube failure. Soluble iron can interfere inmany processes, such as printing or the dying of cloth. Indomestic water systems, porcelain fixtures can bestained by as little as 0.25 ppm of iron.


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Acid Rain


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As rain passes through the atmosphere it picks

up gasses such as carbon dioxide, sulfur dioxideand oxygenetc. Many of the gasses assimilatedby a drop of rain are acidic in nature or react withother compounds to form acids, and this is whyrain water will generally have a pH below 7.

When the drop of water reaches the surface ofthe earth it starts to pick-up certain substancesthat it contacts. For example, as water percolatesthrough the soil it may come in contact with alime stone (calcium carbonate) deposit. The

calcium carbonate will react with CO2, formingcalcium bicarbonate which is a primaryconstituent of hardness in water.

Resin specs


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The total capacity is the resins theoretical

capacity. Unit = meq/ml

Required 1.9 - 2.0

Water Retention, % 45 - 48The water retention is the amount of water that is

found inside the bead. standard softening resin

is made up of approximately 44 - 48% water. If

the resin is allowed to dry out a 40% reduction inresin volume will result.

Cont..


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The bead count measures the percentage of wholeperfect beads.

Bead Count, % Whole 95

Excessive amounts of broken beads will cause pressuredrop and channeling in the bed.

The screen distribution or particle size distribution is a

parameter that is run to show bead size Standard softening resin have a distribution of 16 U.S.

standard mesh to 50 U.S. standard mesh. This equatesout to be 0.3 mm to 1.2 mm in diameter. The percentage

of -50 mesh resin is important, since the finer the resinthe greater the pressure drop.


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pH is important since it is an indication of whether or notthe resin was properly neutralized and rinsed. Resin issoaked in a neutral brine solution. If the pH drops below7 a portion of the softening resin is still in the hydrogenform. This occurs since the sodium is picked up by the

resin and hydrogen is released in its place. Thehydrogen combines with the chloride from the saltsolution to form hydrochloric acid. If a high pH isdetected, the NaOH (caustic) was not completely rinsedfrom the resin. The pH range for this test should fall

between 6.5 and 9.5.


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Gel-type softening resins are translucent (lightwill pass through bead) while macroporoussoftening resins are opaque. Any type of resincan be manufactured with different percentages

of crosslinking. However, macroporous resinsare usually manufactured to be stronger,requiring higher crosslinking. The DVBconcentration will generally run from 10 to 20%.

With this greater crosslinking the resin becomesmore resistant to oxidation from substances likechlorine


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TDS is expressed as CaCO3. To check thewater analysis for accuracy, the total cations(Ca, Mg, K and Na) as CaCO3 should beequivalent to the TDS.

why when caustic (NaOH) is added to acidicwater and the pH is elevated to 8 that no OHalkalinity exist?

The answer goes back to pH. No OH alkalinity

can exist at this pH. The OH will react with CO2to form HCO3, at this pH.


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Strong Acid Cation-The hydrogen and sodium forms of strongacid resins are highly dissociated and the exchangeable Na+and H+ are readily available for exchange over the entire pHrange. The exchange capacity of strong acid resins isindependent of solution pH.

Weak Acid Cation Basins- Weak acid resins exhibit a much

higher affinity for hydrogen ions than do strong acid resins.This characteristic allows for regeneration to the hydrogenform with significantly less acid than is required for strong acidresins. Almost complete regeneration can be accomplishedwith stoichiometric amounts of acid. The degree ofdissociation of a weak acid resin is strongly influenced by the

solution pH. Consequently, resin capacity depends in part onsolution pH. a typical weak acid resin has limited capacitybelow a pH of 6.0. making it unsuitable for deionizing acidicmetal finishing wastewater.

Exchange capacity


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g p y

Dissolved Oxygen in Fresh Water


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Alkalinity


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Alkalinity is based upon the bicarbonateion (HCO3), the carbonate ion (CO3), and

the hydroxide ion (OH).

The bicarbonate ion will be prevalent inwater when the pH ranges from 4.3 to 8.3.

When the pH exceeds 8.3, CO3 becomes

prevalent at pH above of 10 OH will exist.

CaCO3 equivalent, the industry

standard


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standard

permits the comparison of one ion toanother

performed by multiplying the ion by a

conversion factor. These conversion factors are derived fromthe equivalent weight of the ion which isdivided into the equivalent weight of

CaCO3 Ca has a conversion factor of 2 5

Documents

DEMINERALISATION OF WATER.ppt