Water Treatment

WATER TREATMENT

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• First source is surface water such as rivers, lakes and deep wells.

• Properties:

Low in dissolved solids High in suspended solids Significant quality changes caused

by seasons and weather High in permanent hardness

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SOURCES OF WATER

• Second source comes from wells.• Properties: High in dissolved solids Low in suspended solids High iron and magnesium content Low in oxygen but may contain sulfide

gas Relatively constant quality and

temperature High in temporary hardness

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• Typical Impurities in Natural Water.

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• Dissolved chemicals are corrosive to metals and form scale deposits on heated surfaces in process equipment.

• Scale impedes heat transfer and corrosion weakens equipment.

• Removal or reduction of the impurities in natural water is called treatment.

• The objective of water treatment is to reduce the level of selected impurities to an economical concentration that is acceptable for end use.

• Softening=process to remove hard metal ions such Mg++ and Ca++

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• Water treatment

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CRUDE FILTRATIONRemove suspended solidsSimplest form of solids removal is a settling basin or pond where hold up time is provided for large particle s to settle out by gravity.Settling basin has little effect on small suspended particles and no effect on colloidal-sized particles.A sand or gravel filter is common way to remove intermediate –sized particle.An activated carbon bed remove small particles and has ability to modify odor and taste.

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CHEMICAL AND PHYSICAL TREATMENT OF NATURAL WATER

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Coagulation and flocculationCoagulation involves neutralizing the negative charges on small particles and provide means for the suspended particles to adhere to each other.Flocculation is bridging together of the coagulated particles to produce larger group of particles called flocs.Most common coagulants are iron and aluminium salts.Ferric and alumina irons have 3 positive charges and effective to react with negatively charge colloidal particles.

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SODA LIME PROCESSCa and Mg is the major source of fouling and scale formation on equipment surface .Precipitation process= the chemical added react with dissolved minerals(total hardness) in water to produce precipitate.Lime treatment is used to remove temporary hardness. Hydrated lime reacts with calcium and magnesium bicarbonates to form insoluble carbonates precipitate.

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LIME TREATMENT TO REMOVE TEMPORARY HARDNESS

• The precipitants- calcium carbonate(CaCO3) and magnesium hydroxide (Mg(OH)2) are sludge like solids that can be removed by settling and blowdown in a clarifier.

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WATER TREATMENT CLARIFIER

• The precipitant, calcium carbonate and other solids is removed using coagulation and flocculation to aid settling followed by blowdown and purge to the sludge.

• The chemical reactions are slow and take place at very low concentration.

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SODA ASH TREATMENT TO REMOVE PERMANENT HARDNESS

• Process in which an impurity is removed from a process stream through adherence to the surfaces of a solid to which it attracted or bonded.

• Cation: an ion with a positive charge• Anion: a negatively charged ion such as OH or Cl.• In an ion exchange version of an adsorption

process to soften water, the process stream containing the impurities is passed through a bed of ion exchange resin that is preloaded with ion of the same charge.

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ADSORPTION

• Water treatment to remove cations is a good example. Water contains salt of metals such as calcium, magnesium and iron.

• An iron exchange resin is loaded with sodium ions, the sodium ion is exchanged with metal ion which remains attached to the surface of resin bead.

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CONTINUE

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THE SODIUM CYCLE

• Ion exchange softening dependent on the sodium(Na+) cycle. Resins called zeolytes have the physical property to attract and hold positive ions that cause hardness.

• Water containing hard metal ions(Mg++ and Ca++) are passed through the bed and displace the soft Na+ ions that were previously loaded on the surface of the zeolyte.

Cation removal process

•Remove cations(Na+ and Fe++) and replace with H+

•Done by passing water through an iron exchange bed saturated with H+ ions.•The cations are adsorbed and displace H+ from negatively charged resin. The H+ irons go out with the water.

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DEMINERALIZATION

Anion removal process

•Exchange anions like SO4

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(sulfate) with hydroxyl (OH-) ions.

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