Purification of Water

Water is one of the essential elements that we need to survive. Our planet is covered by water but unfortunately, that is not fit for consumption. Therefore, to consume it, we have to purify it. Purification of water on a large scale includes different steps of purification of water on a large

Scale. Large-scale water purification is done for the water that is provided in large cities and towns. The Steps of Water Purification are:

Step 1: Storage of Water

Step 2: Filtration of Water: It is The Most Important Step. 

• It can be done with Slow sand (biological) filters.
  • The heart of this filter is the Schmutzdecke layer, which is 2-3 cm in thickness. This layer is the Vital/biological/Zoogleal layer, which is responsible for purifying water. It removes organic matter, holds back bacteria, and oxidizes ammoniacal nitrogen into nitrates. In simple words, it disinfects water. 
  • There is nothing above the sand bed which means there is a mechanical operation in the Rapid Sand filter.
• Other filters that can be used are Rapid sand (mechanical) filters, also known as mechanical filters.
  • There is no vital layer, so mechanical operation is used. It functions by utilizing the concepts of gravity and pressure.
  • These are of two types:
    •  Paterson's Gravity Type: Water gets filtered through a sand bed under its weight.
    • Candy's Pressure Type: Water is passed through the bed under pressure, which is higher than atmospheric pressure.

Step 3: Disinfection of Water 

This includes methods like Chlorination, Ozonation, and Ultraviolet irradiation.

Increasing water pollution and other environmental hazards have made it crucial to use water meticulously to preserve it for future generations. Therefore, we need to purify water for daily activities by Purification of water on a small scale. For example, If we are in a rural area or small-scale place, like a factory, then water can be purified by various means, which include

1. Boiling: 

• Boiling is the best method for the purification of water. It kills everything, but boiling is not preferred as a mode of purification of water because
  • It is costly.
  • It consumes a lot of gas.
  • It has no residual activity. This means that once we boil water, after 4-6 hours, we will need to boil it again, as the Water will not remain disinfected.
  • Retreatment of boiling is required every 4-6 hours.

2. Chlorination is the Second-best method for purification. It is second because it does not kill everything like it cannot kill all spores and ova.

• In case we need to kill the polio virus or cyclops, the dose of chlorine will have to be increased. However, this will alter the taste and smell of water.
• Chlorine has a very important property, which is Residual Activity. This means water, once chlorinated, doesn't have to be re-treated again.
  • This residual activity is also known as Free chlorine.
  • When chlorine is added to water (also known as Combined Chlorine), it will form HOCl (hypochlorous acid) + HCl (Hydrogen chloride). The alkalinity of water neutralizes HCl. 90% of disinfecting properties are due to Hypochlorous acid, and 10% is due to Hypochlorite ions.
•  The major disinfecting property of chlorine is due to Hypochlorous acid.
• The amount of chlorine required to disinfect water is known as Chlorine demand.
• Chlorine is most commonly used as bleaching powder.
  • 100g of bleaching powder releases 33 grams of chlorine.
• Chlorine also oxidizes Iron, Manganese, and Hydrogen sulfide- Controls algae, slime organisms, and AIDS coagulation.
• The amount of chlorine required or demanded to destroy bacteria and to oxidize all the organic matter and ammoniacal substances present in water is known as the chlorine demand of water.
• Estimation of chlorine demand of water (or the dose of Bleaching water required for disinfecting of water) is done by the Horrocks apparatus.

• Horrocks apparatus estimates the chlorine demand of water.
  • It contains 1 Black cup and 6 white cups. 
  • We use an indicator here, which is starch iodide. The color changes to blue during the procedure of estimation.
  • All 6 cups are of the same size and shape.
  • There are markings made on six white cups at increasing levels.
  • In the Black cup, we have to prepare the solution.
  • (nx2) gms of bleaching powder will disinfect 455 liters of water. where n is the first cup that will show a color change.

Also Read: Water: Sources, Diseases and Quality

Test for Chlorination of Water 

The test used for chlorinating water is the Ortho Toluidine (OT) Test or the OTA test (Ortho toluidine arsenite test).

• If we are using this test, again there would be a color change.
• OTA tests can detect both free and combined chlorines separately.
• OT tests can also detect free chlorine separately. It can detect combined chlorine but not separately.

There are some set Levels of residual chlorine required for water to be usable. They are as follows:

• Drinking water: More than 0.5 mg/L for a contact period of 1 hour.
• Swimming pool sanitation: more than 1 mg/L and contact period will be 1 hour.
  • To kill poliovirus or cyclops: more than 2 mg/L for a contact period of 1 hour.
  • Post-disaster phase: more than equal to 0.7 mg/L for a contact period of 1 hour.

1. Oxidization
   • OZONE: Powerful oxidizing and strong virucidal agent.
     • The dose for portable water treatment varies from 0.2 - 1.5 mg/L.
   • UV radiation: This doesn't have any residual activity.
2. Chemical Disinfection
   • For this method of disinfection, Bleaching powder can be used: 33% of chlorine.
   • Chlorine Solution: High test Hypochlorite: Perchloron is a calcium compound that contains 60-70% available chlorine.
   • A single tablet of 0.5 mm of chlorine is sufficient to disinfect 20 liters of water.
   • Iodine is used for emergency water disinfection. Two drops of a 2% ethanol solution of iodine will suffice for one liter of clear water, with a contact time of 20-20 min.
     • Iodine tablets are available as Nesfield's and Bursoline's tablets.
   • Potassium permanganate is no longer recommended.

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