Water Treatment Technology

Water Treatment TechnologybyNilay Patel

Introduction What is the goal of Water Treatment Technologies? To remove 1) Turbidity 2) Chemicals 3) Pathogenic Contaminants from water sources in the most affordable and expedient manner possible.

Sources of Water1) Rain Water2) Surface Water (Rivers and Lakes) 3) Ground Water (Wells and Springs) 4) Sea Water

Uses of Water Paper Industries Textiles Industries Cooling Industries Boiler Feed Laundries Chemical Industries

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Impurities in Water1) Dissolved Impurities :- carbonates and bicarbonates, chlorides, magnesium, iron, dissolved gases2) Suspended Impurities :- Organic :- oil globules, vegetable and animal matter Inorganic :- clay and sand3) Colloidal Impurities :- finely divided silica and clay, organic waste products, amino acids 4) Micro organisms :- fungi and bacteria

Methods for removal of impurities Screening Sedimentation Coagulation Filtration Sterilization :- 1) by Chlorine or Bleaching Powder 2) by UV Radiation 3) by Ozone

1) Screening : Screening is a process of removing floating materials like wood pieces and leaves from water.

Raw material is allow to pass through a screen having a large number of holes which removes the small and large floating matter.

2) Sedimentation : Sedimentation is the process of removing suspended impurities by allowing the water to stand undisturbed for 2-6 hours in big tanks.

Due to force of gravity, most of the suspended particles settled down at the bottom and they are removed.

Sedimentation removes only 70-75% of the suspended water.

3) Coagulation : Finely-divided silica, clay etc. do not settle down easily and hence can not be removed by sedimentation. Most of these are in colloidal form and are negatively charged and hence do not coalesce because of mutual repulsion . Such impurities are removed by coagulation method When Al2(SO4)3 is added in water, it hydrolyzes to form a gelatinous precipitate of Al(OH)3. It entraps impurities and settle down at the bottom.

4) Filtration : For removing bacteria, colour, taste, odour, fine suspended particles, etc and to produce clear water, filtration is used.

In this process, water is passed through beds of fine sand, coarse sand and other granular material.

The porous material used is the filtration medium and the equipment used for filtration is known as filter. e.g. slow sand filter

5) Sterilization :By Chlorine or Bleaching Powder :- When bleaching powder is added in the water, HOCl is produced and it kills the bacteria.

By Ultraviolet Radiation :- UV Radiation can kill the bacteria. It is useful for sterilizing swimming pool water.

By Ozone :- Ozone is readily absorbed by water. It is highly unstable and decomposes to give nascent oxygen. It kills the bacteria.

Boiler Feed Water One of the chief use of water is generation of steam by boilers.Essential requirements of Boiler Feed Water :-It should be free from - Turbidity, oil, dissolved salts- Hardness & scale forming constituents- Dissolved O2 & CO2- Caustic alkali

If hard Water is directly fed into boiler there arise many problems such as - Priming - Foaming - Sludge & Scale Formation - Caustic Embrittlement

Priming : The process of formation of Wet Steam is called Priming.

Because of priming efficiency and life of machine parts decrease. It is caused by - Very high water level - Presence of foam on the surface - High steam velocity

Foaming : Formation of stable bubbles above the surface of water is called foaming. These bubbles are carried over by steam leading to excessive priming. It is caused by presence of soluble impurities like alkali metal salts and oil. Foaming can be controlled by action of anti foaming agents like synthetic polyamides

Sludge and Scale formation :

Sludge ScaleSludges are soft and non- adherent deposits.Scales are hard deposits which stick very firmly to the inner surface of boiler. 2. Sludges can be removed easily.2. Scales are very difficult to remove.3. Sludges can transfer heat to some extent and is less dangerous.3. Scales are bad conductors of heat and are more dangerous. 4. Sludges are formed by substances like MgCl2 and CaCl2.4. Scales are formed by substances like CaSO4 and Mg(OH)2.

Caustic Embrittlement : It is the phenomenon during which the boiler material becomes brittle. Na2CO3 present in water breaks up and give NaOH and water become alkaline.

This alkali boiler water flows into cracks by capillary action. There the water evaporates and concentration of caustic soda increases. This leads to failure of boiler.

It can be avoided by using sodium phosphate as a softening reagent instead of Na2CO3 and adding tannin or lignin to boiler water which blocks the cracks.

Softening of Water : Softening can be done in two ways

Zeolite Process : Zeolite are naturally occurring hydrated sodium aluminosilicate minerals. Chemical Formula :- Na2OAl2O3xSiO2yH2O It is represented by Na2Ze

Sodium ions are replaced by Ca2+ and Mg2+ ions.

When zeolite comes in contact with hard water, it forms calcium and magnesium zeolites.

Advantages :

1) Water of nearly zero hardness can be obtained.

2) Method is cheap.

3) No sludge formation takes place.

4) Equipments are compact and require small space.

5) Easy Operation.

6) Process can be made automatic and continues.

Disadvantages :

1) This method can not be used for acidic and turbid water.

2) This treatment replaces only cations leaving all anions like HCO3 and Co32- in the soft water.

3) This method can not be used for softening of brackish water because it contains Na+ ions.So, ion exchange reaction will not take place.

Ion Exchange Process : Cations like Ca2+ and Mg2+ and anions like Cl- and SO42- can be removed by cation and anion exchange resins. Resins containing -COOH, -SO3H are capable for exchanging their H+ ions to cationic portion of minerals. It is called cation exchanger. Resins containing NH2 , NHCH3 are capable for exchanging the anionic portion of the minerals. It is called anionic exchanger.

Process : Water is passed through cation and anion exchanger respectively. Water coming out side from anion exchanger is completely free from cations and anions. It is known as deionised water or demineralised water and it is pure as distilled water.

Regeneration : Cation exchange resins are regenerated by passing a dilute solution of HCl through them. Rca + 2HCl RH2 + CaCl2 Anion exchange resins are generated by passing a dilute solution of NaOH. RCl2 + 2NaOH R(OH)2 + 2NaCl

Advantages :

Highly acidic or alkaline water can be treated.Water of nearly zero hardness can be obtained.

Disadvantages :

Equipment is costly and expensive chemicals are needed. If water contains turbidity then the output of process is reduced.

Internal Treatment:1) Colloidal Conditioning : This colloidal Substances get coated over the scale forming particles and thus the coalescence of this particles to a compact scale is prevented. Thus the scale formed remains loose and non- sticky and can be easily removable. starch and glue are colloidal conditioning agents.

Internal Treatment:2) Phosphate Conditioning : Scale formation can be avoided by adding sodium phosphate to boiler water. Phosphate reacts with Ca and Mg salts and formed easily removable sludge. 3CaSO4 + 2Na3PO4 Ca3(PO4)2 + 3Na2SO4 If water is - acidic then trisodium phosphate is used - alkaline then sodium dihydrogen phosphate is used - neutral then disodium hydrogen phosphate is used

Internal Treatment:3) Carbonate Conditioning : Scale formation can be avoided by adding sodium carbonate to boiler water.

CaSO4 + Na2CO3 CaCO3 + Na2SO4

Internal Treatment:4) Calgon Conditioning : It involves the addition of calgon or sodium hexametaphosphate to boiler water. This substance reacts with calcium ions and forms highly soluble complex and thus prevents the precipitation of scale forming salt.

2CaSO4 Na2[Ca2(PO3) 6] + + Na2[Na4(PO3) 6] 2Na2SO4

Internal Treatment:4) Treatment with sodium aluminate : When sodium aluminate is treated with boiler water, it gets hydrolyzed give sodium hydroxide and a gelatinous precipitate of aluminum hydroxide. NaAlO2 + 2H2O NaOH + Al(OH)3 2NaOH + MgCl2 Mg(OH)2 + 2NaCl

Al(OH)3 and Mg(OH)2 entrap finely suspended and colloidal impurities like sand, oil drops and settle to the bottom and can be removed easily.

1) Electrodialysis :

Ions present in saline water migrate towards the oppositely charged electrodes because of applied E.M.F.

Movements of ions takes place through ion selective membranes.

Compartments with even number are filled with pure water and compartments with odd numbers filled with concentrated brine water when the process finish.

Thus the salinity from the water can be removed.

2) Reverse Osmosis :

When two solutions of different concentration are separated by a semi permeable membrane, flow of solvent takes from a region of low concentration to high concentration, until the concentration is equal both sides. This process is called osmosis and reverse process is called reverse osmosis. The driving force in this phenomenon is called osmotic pressure. By this method pure water is separated from sea water. Membranes used are cellulose, acetate, cellulose butyrate etc.

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