WATER IMPURITIES AND THEIR CLASSIFICATION
In this article we discussed about the different type of water impurities. when the rain water is free falling from the cloud after that it comes with the different type of gases and get polluted , after that rain water falling on the ground and then it mixed with the may impurities like mud, dirt , silica, clay and many more ground impurities . according to the water treatment these impurities classified in different categories like non ionic impurities or undissolved impurities ,Ionic impurities or dissolved impurities, Gaseous impurities. These impurities are discussed below in brief way.
The impurities present in water can be broadly classified as:-
1) Non ionic impurities or undissolved impurities
2) Ionic impurities or dissolved impurities.
3) Gaseous impurities.
Non ionic impurities
Major non ionic impurities are:
1) Turbidity or suspended solids.
3) Taste and odour
4) Organic matter
5) Colloidal silica
Turbidity is caused by the presence of suspended solids in water. The latter is a measure of total weight of dry solids present whereas turbidity is an optical effect. The suspended impurities include clay, sand, algae and precipitated iron.
Colour is normally expressed in Hazen units and is caused by the presence of colloidal suspension and aquatic growth. It is also caused by dissolved organic substance due to decomposition of vegetation. Some water may have colour due to presence of iron.
Taste and odor
Many water have bad taste and odor. Most organics and some inorganic chemicals contribute taste and odor. These chemicals can originate from municipal or industrial waste or from natural resources such as decomposition of vegetable matter.
Organic impurities can be due to vegetable decomposition. It is also due to organic matter which comes from animal and human fecal matter or it’s degradation products, industrial waste and agriculture pesticides and herbicides.
Micro Organism are normally not present in deep well water but are quite common in surface water. Microorganisms are objectionable in water because they are sources of disease and slime formation.
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Ionic or dissolved impurities
Major ionic impurities can be subdivided as cationic and anionic impurities.
Major cationic impurities
Major anionic impurities
1) Bi carbonates
Calcium and Magnesium
Calcium and magnesium are the most common dissolved impurities found in water. When calcium and magnesium salts are present in considerable amount it imparts hardness to water i.e. it does not allow lather to form with soap. The salts of calcium and magnesium that causes hardness are divided into two types.
- a) Carbonate or temporary hardness.
- b) Non carbonate or permanent hardness.
Sodium and potassium
Sodium compounds are very soluble and the element is present in most natural waters. Levels can range from less than 1 ppm to several thousand ppm as in Brine or sea water. They are harmless in most water supplies as long as they are not present in very high amount. Being alkaline in nature they can accelerate corrosion.
Iron and Manganese
Iron can be found in water supplies as:
- a) Dissolved iron.
- b) Precipitated iron – oxidised iron both filterable and colloidal.
- c) Organic bound iron.
- d) Components of living organism – Iron bacteria.
In ground and surface water supplies iron generally exists as divalent ferrous salts or as organic complexs. with few exception exists as divalent manganous. Iron can induce corrosion in pipelines. It can also cause staining. Bacterial iron can cause clogging of pipelines.
Usually found in water which has undergone coagulation. Can cause deposits in cooling system and contribute to complex boiler scales.
Alkalinity in water is due to presence of Bicarbonates, carbonates and hydronyl ions. In raw water alkalinity is mainly due to bicarbonates. Some times carbonates ions may also be present. Carbonates and particularly hydronyl ions are rarely encountered in untreated waters. Hydronyl ions normally get introduced during treatment of water. Alkalinity is determined by using standard acid solution using methyl or
phenolphthatein indicator. Alkalinity determined by using methyl orange indicator is termed as M-Alkalinity or Total Alkalinity. P-Alkalinity is determined by using phenolphthalein as indicator. The different type of alkalinity present in water supplies can be calculated from M and P-Alkalinity value determined by titration.
Chlorides are present in nearly all waters. The chloride irons may be in combination with one or more cations, calcium, magnesium, iron and sodium. Chlorides of these salts are present in water because of their high solubility in water. Excessive chloride in water causes corrosion. It also impart taste to water. Chlorides are prominent in crevice corrosion and pitting.
Most raw water contains sulphates due to leaching and erosion of sulphate minerals and oxidation of sulphides. They can also be due to industrial waste discharge and farm drainage. Sulphate also occur due to aerobic oxidation of organic matter. Sulphate in water causes corrosion and scaling in boiler. Sulphate are normally found as calcium, magnesium and sodium salt. Sulphate are also aggressive to concretes.
Fluorides are industrially not important but it’s absence as well as presence in excess is harmful in Drinking water. Fluoridation and Defluoridation is very important in drinking water technology.
Most natural water contain silica ranging from 1 ppm to 100 ppm. Silica is an oxide of silicon which is a major constituents of igneous and meta morphic rocks, of clay minerals such as kaolin and of fieldspars and quartz. Silica can exist in various form as simple silicates or as a complex polymeric material. Colloidal silica is rarely present in Bore well water but is commonly present in surface water. Silica if not removed can cause deposits on boiler, cooling tower, turbines and other industrial equipments which are difficult to remove.
The major gaseous impurities are carbon di-oxide, oxygen, hydrogen sulphide, ammonia and chlorine.
Carbon-di-oxide is found in waters coming in contact with decaying organic matter or carbonaceous material. Surface water contains small amount of free CO2 but well water may contain more than 100 ppm. Organic decomposition is the major cause for CO2 in well water. Depending on the pH of water carbon-dioxide may be present either as free carbonic acid (dissolved CO2 gas) or in a semi carbide form (as bicarbonates) or in a combined form (as carbonates), free CO2 depresses the pH and thus accelerates corrosion.
Oxygen is practically absent in artesian water, but is usually present in surface waters rather in high concentration. Oxygen is corrosive to metals but its absence in natural waters could produce other obnoxious gases such as methane H2S etc. Oxygen is also vital to all living things in natural waters. Dissolved oxygen is highly corrosive and should be removed either chemically or mechanically especially in Boiler feed water.
Hydrogen Sulphide (H2S)
Hydrogen sulphide is found in some well waters in areas where the soil contains certain types of organic matter, decomposing under anaerobic conditions. The usual amount of sulphide is small (less than 10 ppm) although as much as 100 ppm has occasionally encountered. H2S imparts unpleasant odor to water, promoters metal corrosion, and can be cause of clogging of pipes owing to growth of sulfur bacteria.
Nitrogen may get into water from atmospheric air, on decomposition of organic rests and reduction of nitrogen compounds by denitrifying bacteria.
Ammonia is introduced into surface water due to industrial and agricultural pollution. It is corrosive to copper and brass at pH 9.0. Ammonia can be removed by various methods-Deaeration, chlorination or by hydrogen cation exchange if in ionic form.
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