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Saturday, 13 April 2019

WATER TREATMENT - BASIC WATER CHEMISTRY

          Water occurs in nature "pure" and what ever be the source always contains impurities either in solution or in suspension. The determination of these impurities makes analysis of water necessary and removal and control of these impurities make water treatment essential.

Water sources:
The various sources of water can be broadly classified as:
          a) Rain water
          b) Surface water (River, Streams, Ponds, Lakes and Reservoirs)
          c) Ground water (Springs, Shallow wells and Deep wells)
of the above, logically rain water is the purest but even this collects and dissolves atmospheric gasses and impurities in air. Further, once in contact with the earth's crust, the rain water will gradually dissolve various materials.

Various (Majour) impurities:
The major impurities of water can be classified in three main groups:
          1) Non-ionic and undissolved
          2) Ionic and Dissolved
          3) Gaseous

1) Non-ionic Impurities:  These are mainly, salt, mud, dirt and other suspended matter, micro-organisms, bactiria and other organic matter, oil and corrosion products. It goes without saying that drinking water and most industrial water supplied should be clear and organic-free.

2) Ionic and dissolved Impurities:  Any salt which dissolves in water disassociates into positively charged ions called "Cations" and negetively charged ions called "Anions". Since these permit the water to conduct electricity, these salts are called electrolytes.
          Some of the most common Cations in water are:
      Calcium, Magnesium, Sodium and Iron, and rarely Ammonium, Potassium and Manganese. These cations are associated with Anions like Bicarbonates, Carbonates, Hydroxides (the sum of which is termed as Alkalinity), Sulfates and Chlorides. Presence of Nitrites and Phosphates is normally not very common. In the water treatment field, the preferred method of expression of these dissolved impurities is in terms of Equivalent Calcium Carbonate, abbrevated to as "CaCO3". This is because Calcium Carbonate is a good common denominator as it has a molecular weight of 100, which facilitates calculations.
          Moreover, in this form of analysis, the sum of Cations or total Cation always equals the total Anions. Quantitatively, these are expressed in parts per million or milligram/ltr. One part per million equals one ten thousandth of one percent (0.0001%). One part per million means one part in a million parts, for example, one liter in a million liters of water or one Kg in a million Kgs of water.
          Of all the dissolved impurities, hardness is perhaps the most troublesome. Hardness is due to compounds of Calcium and Magnesium. On heating water caontaining these salts, Carbon Dioxide is released from solution and the Bicarbonates are converted into Carbonates which are insoluble and form scales and deposites. Other salts of Calcium and Magnesium like Sulfates and Chlorides have lower solubility than Sodium salts and participated out at high temperatures. Bicarbonates of Calcium and Magnesium are known as the "Alkaline hardness" or "Temporary hardness" and chlorides, sulfates, nitrates etc., of Calcium and Magnesium are known as "Neutral" or "Permanent hardness". Sodium salts are highly soluble but can be corrosive if present in large quantities such as Sodium Chloride or Sodium Bicarbonate.
          Dissolved Silica is another troublesome impurity, especially in water fed to Boilers of very high temperatures and pressures. Even in lower pressure boilers, it could form a very hard type of scale by acting as a binding agent.
          The natural water contains solid, liquid and gaseous impurities and therefore, this water cannot be used for the generation od steam in the boilers. The impurities present in the water should be removed before it'suse in steam generation. The necessity for reducing the corrosive nature & quantity of dissolved and suspended solids in feed water has become increasingly important with the advent of high pressure, critical & supercritical boilers.

Impurities in water:
The impurities present in the feed water are classified as given below -

-- Undissolved and suspended solid materials.
-- Dissolved salts and minerals.
-- Dissolved gases.
-- Other materials (as Oil, Acid) either in mixed or unmixed forms.


A. Undissolved and suspended materials: Turbidity and sediments:
          Turbidity in the water is suspended insoluble matter including coarse particles (mud, sediment, sand etc.,) that settle rapidly on standing. Amounts range from almost zero in most ground waters and 60,000 ppm in muddy and turbulent river water. The turbidity of feed water should not exceed 5 ppm. These materials can be removed by settling, coagulation and filtration. Their presence is undesirable because heating or evaporation produces hard stony scale deposits on the heating surface and clog the fluid system. Both are objectionable as they cause damage to the boiler system.

B. Dissolved salts and minerals:

a) Calcium and Magnesium salts: The calcium and Magnesium salts present in the water in the form of carbonates, bicarbonates, sulfates and chlorides. The presence of these salts is recognized by the hardness of water (Hardness of water is tested by soap test). The hardness of water is classified as temporary and permanent hardness. The temporary hardness is caused by the bicarbonates of calcium and magnesium and can be removed by boiling. The boiling converts the soluble bicarbonates into less soluble carbonates which can be removed by simple blow down method. The permanent hardness of the water is caused by the presence of chlorides, sulfates and nitraes of calcium and magnesium and they cannot be removed just by boiling because they form a hard scale on heating surfaces.
          A standard amount of measurement of hardness is taken as being the amount of Calcium Carbonate (CaCO3) in th water and is referred to in part per million (ppm) or grains per gallon (grains/gallon*17.1=ppm).

b) Sodium and Potassium salts:  These are extremly soluble in water and do not deposit unless highly concentrated. Their presence is troublesome as they are alkaline in nature and accelerated the corrosion.

c) Chlorides:  Majority of the chlorides cause increased corrosive action of water.

d) Iron:  Most common soluble iron in water is ferrous bicarbonate. The water cantaining ferrous bicarbonate deposits become yellowish and reddish sediments of ferric hydroxide if exposed to air. Majority of ground surface water contains less than 5 ppm but even 0.3 ppm can create trouble in feed water system by soft scale formation and accelerating the corrosion.

e) Manganese:  It also occurs in similar form and it is also equally troublesome.

f) Silica:  Most natural water contains silica from 1 to 100 ppm. Its presence is highly objectionable as it forms very hard scale in Boilers and forms insoluble deposits in turbine blades. In modern high pressure boilers its presence is reduced as low as 10-50 ppb.

g) Microbiological Growth:  Various growth occur in surface water (lake & river). The micro-organisms include diatoms, molds, bacterial slimes, algae, manganese & sulphate reducing bacteria and many others. These can cause coating on Heat Exchanger and clog the flow passages and reduce the heat transfer rates.

h) Colour:  Surface waters from swampy areas become highly colored due to decaying vegetation. Colour of feed water is objectionable as it causes foaming in Boilers and may interfere with treatment processes. It is generally removed by chlorination and absorption by activated carbon.

C. Dissolved Gases:

a) Oxygen:  It presents in surface water in dissolved form with variable percentage depending upon the water temperature and other solid contents in water. Its presence is highly objectionable as it corrosive to iron, zinc, brass and other minerals. It causes corrosion and pitting of water lines, boiler tubes. Its effect is furthur accelerated at high temperatures.

b) Carbon Dioxide:  The river water contains 50 ppm and well water contains 2 to 50 ppm of C02. It also causes the corrosion of stream, water and condensate lines, It also helps to accelerate the corrosive action of oxygen.
          wThe other gases are H2S, CH4, N2, and many others but their percentages are negligible, therefore, their effects are not discussed here.

D. Other minerals:
a) Free Mineral Acid:   Usually present as Sulphuric or hydrochloric acid and causes corrosion. The presence is reduced by neutralization with alkalis.

b) Oil:  Generally, the lubricating oil is carried with steam into the condenser and through the feed system to the Boiler. It causes sludge, scale and foaming in Boilers. It is generally removed by strainers and baffle seperators.
          The effects of all the impurities present in the water are the scale formation on the different parts of the boiler system and corrosion. The scale formation reduces the heat transfer rates and clog the flow passage and endager the life of the equipments by increasing the temperature above safe limit. The corrosion phenomenon reduces the life of the plant rapidity. Therefore, it is absolutely necessary to reduce the impurities below a safe limit for the proper working of the power plant.

C. Dissolved Gases:
The atmosphereic gases found in naturally occuring waters, only two Carbon Dioxide and Oxygen, are the main causes of many corrosion related problems.


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