Methods of Water Treatment

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Methods of Water Treatment

Methods of water treatment are :

  1. Mechanical treatment
  2. Thermal treatment
  3. Chemical treatment

1. Mechanical Treatment :

  1. The suspended materials in water can be removed by sedimentation, coagulation and filteration In the process of sedimentation the water is allowed to stand still in big tanks for some time.
  2. It allows the solid matters to settle down due to gravity and the clean water is taken out. In the process of coagulation, a coagulant like aluminium sulphate or sodium aluminate is added.
  3. It reacts with salts and forms a flock which makes the small particles adhere to each other and they form larger particles which settle down easily.
  4. In the filtration process the water is passed through gravity or pressure feed filter.
  5. It removes the suspended materials which adhere to filter material at the top and the clear water drains from the bottom.
2. Thermal Treatment :
  1. The dissolved gases like O2, CO2, H2S, air and other gases in water are responsible for corrosion since these react with impurities and form acids.
  2. Therefore, these gases are removed from water before supplied to the boiler by the method of thermal treatment.
  3. It includes distillation and deacrative heating of water.
1. Deaeration :
  1. It is also called as degasification process. As mentioned above, it is the most important process of feed water treatment for removal of O2  and CO2  Because these gases make the water corrosive as they react with metal to form iron oxide.
  2. The presence of these dissolved gases in water decreases with the increase in temperature and their magnitude becomes almost negligible at about 100°C, the saturation temperature of steam at atmosphere pressure.
  3. A thermal deacrator is shown in Figure A. It is a direct contact type feed water heater, It consist of a horizontal deaerator storage tank and a vertical column of deaerator. 
  4. The feed water from intermediate with pressure (1.P.) feed water heater is sprayed from top in the deaerator vertical column and the bled steam from H.P turbine from bottom of deaerator. 
  5. These two come in direct contact, as a result the steam condenses and the feed water is heated. The dissolved gases alongwith vapours are released during heating and these are vented from the deaerator through the connection at the top. 
  6. The deaerator works at about 1 – 1.25 bar pressure, for medium pressure boilers of about (40 – 50) bar and deaerator works at about (5 – 8) bar pressure for high pressure boilers (100 bar).
Thermal Deaerator
Figure A
2 . Evaporation : 
  1. This method is used when makeup water requirement for boiler are small. In this the pretreated raw water is evaporated with the help of bled steam from boiler. 
  2. Both evaporated water and condensed steam from evaporator are fed back as make up water into the steam power plant. 
  3. A single effect submerged evaporator is shown in Figure B. In this evaporator, the pre-treated raw water at the rate of my is supplied from above and the bled steam at the rate of m, is fed into the steam chest. This steam flows into the tubes and gets condensed by transferring heat to raw water. 
  4. The water gets evaporated from the top surface into vapour which are withdrawn and supplied to plant. 
  5. The condensate alongwith pure saturated water is returned to boiler as make up water.
Single Effect Submurged Evaporator
Figure B

3. Chemical Treatment :
There are two methods :
(a) Internal treatment : 
It is suitable for treatment of water as feed to the boiler working upto a pressure of 20 bar. In this method, the soluble salts responsible for scale formation are converted in soluble compounds which are then removed from the boiler by blow down process. 
Various reagents like trisodium phosphate and disodium phosphate are added to keep the sludge in fluid state. It prevents the sludge from adhering to boiler surface. 
(b) External treatment : 
The treatment of water for high pressure boilers is carried out externally in tank where the raw water is received and reagents are mixed. In order to accelerate the process of water treatment, the water is heated before addition of reagents. Some of the methods are being discussed below : 
(i) Lime soda treatment : In this method calcium hydroxide and soda ash is added to remove carbonate and sulphate hardness of calcium and magnesium salts respectively and to precipitate them. The reactions are :

Ca (OH)2 + (HCO3)2  → 2 Ca CO3  ↓
2H2O

Ca (OH)2 + Mg (HCO3)2 →
2 Mg CO3  ↓ 2H2O

Na2
CO3 (soda ash) + Ca SO4 →
Ca CO3 ↓ Na2 SO4

Na2
CO3 + Mg SO4 → 
Mg CO3 ↓ Na2 SO4

(ii) Zeolite treatment :
Zeolites are natural or artificial compounds which are capable of exchanging their base. A sodium zeolite Na2 Z (Na2 Al2 Si2 O3) process uses natural Zeolite like green sand which reacts with calcium and magnesium salts to form Zeolites. These Zeolites can be regenerated by addition of sodium chloride through the bed.
The chemical reactions can be written as :
Process with Zeolite :

Ca (HCO3)2 Na2 Z → Ca
Z + 2Na HCO3

Ca SO4 Na2 Z → Ca  Z + Na2 SO4

Regeneration process with chloride solution :

Ca  Z + 2 NaCl → CaCl2 Na2 Z

It can be seen that the method of removing hardness is simple and inexpensive. Only problem is that this system cannot work with turbid water above the certain turbidity. In such cases, the coagulation and filteration of water will be necessary before subjected to zeolite treatment of water.

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