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How turbidity and suspended solids are removed in water treatment? |
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Suspended Solids refers to all matter suspended in water. Turbidity indicates the presence of colloidal matter in water.
While suspended solids do make a water sample turbid, there is no quantifiable relationship between turbidity and suspended solids. The coagulation-flocculation
processes facilitate the removal of suspended solids, turbidity and colloids.
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Suspended solids settle rapidly in water. Clay-like material of the size of a few microns take time to settle; while
colloids which refer to particle size in the sub-micron range cannot settle naturally and so the process of
coagulation-flocculation brings about the settling of these substances to effect their removal.
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In raw water, colloids invariably carry a negative charge (ionization of peripheral chemical groups such as hydroxyl,
carboxyl, phosphates, sulphates, etc.). The neutralization of this surface charge is termed as destabilization of the
colloid. The destabilization is achieved through a covalent reaction between these groups and the polyvalent metallic ions
of the coagulant.
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Coagulation is therefore the destabilization of colloidal particles brought about by the addition of a chemical reagent
known as coagulant. Flocculation is the agglomeration of destabilized particles into microfloc and later into bulky floc
which can be settled. The introduction of another reagent called a flocculant
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or a flocculent aid may promote the formation of the floc.
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What are the objectives in water conditioning? |
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The main objective in water conditioning is the removal of reduction of undesirable impurities present in the water and
introduction of certain desirable properties Which render the water non-scaling, non-corrosive and non-fouling.
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Use of untreated water in boilers results in scale and deposit formation, corrosion and carryover. In cooling water
systems, use of untreated water results in problems like scale and deposits, corrosion and fouling.
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The net result is loss in system efficiency, damage to equipment and plant outages resulting in loss of production.
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Similarly in process applications, presence of iron in water causes discoloration if used in the textile industry. Use of hard water for bleaching and dyeing results in appearance
of 'patches' in the fabric. Use of 'ultra pure' water is essential in the semi-conductor industry to
ensure production of quality components.
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The above examples show the importance of water conditioning for any industry.
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How do surface waters differ from ground waters in terms of the impurities present in them? |
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A. Surface waters contain varying amounts of suspended solids consisting mainly of clay, silt-high during
monsoon and low during summer.
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Most surface waters contain organic compounds which are the products of decay of natural vegetable matter such as leaves,
grass etc. These compounds are high molecular weight carboxylic acids and are collectively called 'organic matter'.
The elimination of organic matter is important particularly in demineralization as they affect the performance of the
anion exchangers used in the DM plant. Surface waters also contain floating micro-organisms such as algae.
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These waters are saturated with dissolved oxygen but contain comparatively lower concentrations of carbon dioxide. Surface
waters however do not contain significant quantities of dissolved heavy metals like iron and manganese as they are often
saturated with oxygen.
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Surface waters contain relatively lower concentration of dissolved solids.
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B. Ground waters are practically free from suspended solids as they get filtered when the water percolates
through the strata.
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These waters do not as a general rule contain organic matter or floating microorganisms.
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Ground waters may contain very low amounts of oxygen but very high amounts of carbon dioxide.
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They may also include dissolved heavy metals like iron and manganese. Presence of iron causes the typical 'red'
water and presence of manganese gives rise to the typical 'black' water.
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Ground waters have relatively higher concentration of dissolved solids and most tube well waters are high 1n dissolved solids often exceeding 1000 ppm.
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External treatment involves the passage of water through equipment such
as clarifiers, filters and ion exchange equipment like softeners and demineralizers. The treatment is external to the point of
use.
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Internal treatment on the other hand is the addition of chemicals like scale and corrosion inhibitors to the water at the
point of use to make it more suitable for the proposed application.
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External treatment is sometimes referred to as primary purification or preliminary treatment while internal treatment is
called post treatment or secondary treatment.
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In external treatment coagulation, clarification and filtration are same of the processes employed to remove suspended
solids. Ion exchange processes are employed for removal of dissolved impurities and equipment like degassers and
deaerators are employed to remove dissolved.
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In internal treatment, scale and corrosion control are achieved by dosing suitable chemicals to inhibit and control
formation of scales. In cooling water treatment biocides are employed to minimize bacterial growth. In boiler feed water
treatment chemicals like sodium sulfite and hydrazine are dosed to prevent corrosion due to dissolved oxygen.
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