Water Technology 5

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EXTERNAL TREATMENT (SOFTENING METHODS)

WATER

Dr. E. LaxminarayanaMail: [email protected]

Water used for industrial purposessufficiently pure.

The process of removing hardness producingsubstances from water is called softening ofwater.

1 . Lim e-Sod a pr ocess:

In this method soluble Ca & Mg salts in waterare converted in to insoluble compounds, byadding calculated amount of Lime and soda.

CaCO3 and Mg(OH)2, so precipitated, arefiltered off.


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Cold l im e soda p r ocess:

In this process calculated amount of chemicals(lime and soda) are mixed with water at RT

At RT the precipitates formed are finely divided,will not be settle down easily.

So, it is essential to add small amount of

coagulants (Alum, Sodium aluminate, Aluminiumsulphate).

These coagulants help in the fine precipitation.

Sodium aluminate also helps the removal of

silica and oil. This cold L-S process provides water-containing

50-60ppm.


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NaAlO2 + H2O NaOH + Al(OH)3

Al2(SO4)-3+Ca(HCO3)2 Al(OH)3+ CaSO4+ CO2

HOT LI ME SODA PROSESS:It involves treating water with softening chemicals

at a temperature of 80-150oC

The reactions proceed faster

Softening capacity increased

Precipitates and sludge formed settle down rapidly

No coagulants are needed

Much of the dissolved gases driven out of the waterFiltration of softened water is fast


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Advan t ages o f Lim e Soda p r ocess:

Very economical. Lesser amount of coagulants is needed.

This process increases the pH of water andprotects from corrosion.

Mineral quantity is reduced.

Disadv ant ages o f L im e Soda p r ocess:

Careful operation and skilled supervision is

required. Disposal of large amount of sludge is difficult.

Removes hardness up to 15ppm.


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2 .ZEOLI TE ( PERMUTI T) PROCESS:

Zeolite is hydrated sodium alumino silicate. It is also called as Permutits.

It capable of exchanging its sodium ions

Structure of sodium Zeolite is Na2O.Al2O3.xSiO2.yH2O.

Where x = 2-10, y = 2-6


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Zeolites are two types

1.Natural Zeolites: These arenon-porous.

Ex.: Na2O.Al2O3.4SiO2.2H2O

Synthetic Zeolites: These are porousand possess gel structure.

These have higher exchange capacity

Prepared by heating together chinaclay, faldspar and soda.


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Process:Na2Ze + Ca(HCO3)2 NaHCO3 + CaZeRegenera t ion : After some time the zeolite is converted in to Ca and

Mg Zeolites and it gets exhausted. At this stage the supply of this hard water is stopped

and the exhausted Zeolite is reclaimed by treating

with 10% NaClCaZe + NaCl CaCl2 + Na2ZeL im i ta t i ons : If the water is turbid, the suspended matter is to be

removed. Large quantity of iron and manganese should be

removed; otherwise they form Mn and Fe Zeolites. if mineral acids are present, they may destroy the

Zeolite bed.


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Advantages:It removes the hardness up to 10ppm.

The equipment used is very compact.No impurities are precipitated, so there is no formation

of sludge or scale.It requires less time.

It requires less skill.

Disadvantages: Treated water contains more sodium.

It removes only positive ions. High turbidity water cannot be treated.


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I on Ex chan ge Pr ocess:

It is also called de-ionization or de-mineralisation.

Ion exchange resins are in soluble, long chain,cross-linked polymers with a micro porousstructure.

Resins containing acid groups (COOH, SO3H) arecapable of exchanging their H+ ions with othercations.

Resins containing acid groups (OH-, NH2-) arecapable of exchanging their OH- ions with otheranions.

Cation exchange resin

Anion exchange resin


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Dem ine ra l i za t ion o f B rack ish W ate r The process of removing common salt from the

water is known as desalination or

demineralization. The water containing dissolved salts with a

peculiar salty taste is called Brackish Water.Brackish water is demineralized by two methods:


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Electro dialysis:In this method ions are pulled out of the salt water by

passing direct currentIn this process electrodes and plastic membrane pair

are used.When direct current is passed through saline water,

Sodium ions start moving towards negative poleThe chloride ions start moving towards the positive pole

through the membrane.During this process the conc. of NaCl decreases in the

central component while it increases in two sidecomponents

Desalinated water is removed from the centralcomponent from time to time


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For efficient separation ions-selectivemembranes are employed.

Ex. RCOO-, RSO3- and R4N+Cl-Elect r o d ia ly s is cel l consists of a large

number of paired sets of plastic membranes.

Advan tages : Most compact unit

Most economical

If electricity is easily available, it is bestsuited


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Rever se Osm osis : When two solutions of unequal concentrations

are separated by a *SPM, flow of solvent takes

place from dilute to concentrated sides, due toosmosis. If a hydrostatic pressure in excess of osmotic

pressure is applied on the concentrated side thesolvent flow reverses.

The membrane filtration is also called as Super-filtration or Hyper-filtration.

In this method 15-40 Kg/Cm2 is applied.


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Advan tages :

It removes ionic, non-ionic, colloidal substances

etc. Cost is on the replacement of SPM (life time

2years)

Converts sea water into drinking water and

obtaining water for very high pressure boilers.

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Water Technology 5