By ADDING BLEACHING POWDER-

Chlorinated Lime or Bleaching Powder (CaO, 3CaOCl2, 3H2O)

In general the powder is readily available and inexpensive. It is stored in corrosion resistant cans. When fresh, it contains 35% active chlorine. Exposed to air, it is quickly loses it effectiveness. It is usually appliec in solution from which is prepared by adding the powder to a small amount of water to form a soft icream. Stirring prevents lumping when more water is added. When the desired volume of the solution had been prepared, it is allowed to settle before decanting. Solutions should have concentrations between 5 and 1% free chlorine, the settled sludge.

High Test Hypochlorite (HTH) is a stabilized version of calcium hypochlorite $(CaOCl_{2})$ containing between 60% and 70% available chlorine . These chemicals must be handled with great caution. They are caustic, corrosive and sensitive to light. They should be stored in tightly closed containers and in darkened spaces, accessible only to authorized personnel. When handling the material, contacts with skin, eyes and other body tissues must be avoided. Chlorine corrodes metal and to a less extent, wood and some synthetic materials. Metal parts which come in contact with the chemicals should be resistant.

About 1 kg of bleaching powder is added to about 1000 liters of water. Bleaching powder reacts with water to form HOCI and nascent oxygen which are powerful germicides i.e.

$CaOCI_{2}+H_{2}O\rightarrow Ca(OH)_{2}+CI_{2}$

$CI_{2}+H_{2}O \rightarrow HOCI+HCI$ HOCI $\rightarrow$ HCI+[O] (Nascent Oxygen)

Germs +[O] $\rightarrow$ Germs are oxidised.

Disadvantages of adding bleaching powder:

1. Bleaching powder introduces calcium in water therefore making it harder.

2. Bleaching powder undergoes decomposition continuously; therefore it should be analyzed for its effective chlorine contents.

2. CHLORINAION (No breakpoint chlorination)

Chlorination is the most widely used method for drinking water disinfection. It is effective and economical. Its use requires some knowledge about the complex processes that take place during chlorination. Those processes will be briefly summarized in the following paragraphs.

Chlorination is known as the addition of chlorine gas or some other oxidizing chlorine compound (sodium or calcium hypochlorite, chlorinated lime, chlorine dioxide) to the water to be treated. The actual agent is hypochlorous acid (HOCI) which forms when chlorine is added to wate:

$Cl_{2}+H_{2}O \implies HOCl _H^{+} + Cl^{-}$

Hypochlorous acid also forms subsequent to dissociation, when chlorinated lime or hypochlorites are added:

$NaOCl \implies Ca^{2+} + 20Cl^{-}$

$Ca(OCl)_{2} \implies Ca^{2+} + 20Cl^{-}$

$HOCl \implies H^{+} + OCl^{-}$ This chemical equilbrium depends on pH and temperature. At pH levels between 3 and 6 , hypochlorous acid dissociates poorly. Chlorination is most effective in that range of pH. At pH levels greater than 8, hypochlorite ions predominate or exist almost exclusively. Hence the disinfecting effect drops off rapidly as the pH level increases.

In the field, the chlorine demand of water of a given quality can be determined as follow: One lifer samples of the water are taken. Chlorine solution of a known concentration is added and mixed with water. After 30 minutes of contact time, the residual chlorine content is measure, The difference to the amount added then yields the chlorine consumption.

Chlorine demand=chlorine consumption + desired residual

Usually 1% chlorine solutions are applied. The chlorine flow is set such that a chlorine residual level of between 0.1 and 0.3 mg/l is obtained. Higher levels are recommended if rapid re-contamination is likely.

Characteristics of ideal disinfections:

. Should not be toxic to humans

. Should be economic

. Should acts at room temperature killing all microorganisms

Should retain water quality during storage and transportation

Advantages:

. Chlorine is powerful oxidizing agent thereby getting rid of a lot of bacteria in water

. Chlorine is commercially available

. Chlorine is very economical

Disadvantages:

. Chlorine does not exterminate all bacteria. complex microorganisms are known to become dormant in the presence of chlorine but not exactly killed

. Chlorine is not green. It is harmful to the environment.

. Commercially available chlorine (Hypo) disintegrates into Chlorine gas rapidly when exposed.

Factors affecting efficiency of chlorine:

1. Temperature of water: Efficiency increases with rise in temperature
2. Time of contact: With time. efficiency goes on decreasing
3. PH of water: At low PH values (between 5-6.5)the efficiency is higher

3. DISINFECTION BY OZONE-

O3 $\rightarrow$ O2 +[O] Nascent oxygen

Ozone (O3) is one of the most effective disinfectants. As a powerful oxidant, it reduces the contents of iron, manganese, and lead, and eliminates most of the objectionable taste odor present in water. Its effectiveness does not depend on the pH value, temperature or ammonia content of the water. Since ozone is relatively by passing a current of dried and filtered air (or oxygen) through between two electrodes (plates or tubes) subjected to an alternating current potential difference. A portion of oxygen is converted into ozone.

This principle of ozone production had been used in Europe for a long time, since it has the advantage of being applicable under a wide range of conditions. It leaves no chemical residuals behind in the treated water. On the other hand, no lasting protection against re contamination is provided either. Capital costs for the instrumentation of ozone production and feeding, as well as operating costs dues to the electrical energy requirements are very high. Moreover, operation of ozonizers requires continuous and skilled monitoring. The operational requirements therefore exceed the resources available in rural areas of most developing countries.

Factors affecting ozonation:

. The effectiveness of ozonation

. Susceptibility of bacteria, viruses, & microorganisms to ozone

. Extent of solubility of ozone

. Concentration of ozone

Advantage of Ozone Disinfection:

. $O_{3}$ is also highly efficient at killing bacteria, viruses and protozoa and is always generated on-site. It does not require transportation or storage of dangerous materials

. As it disinfects, it oxidises inorganic, and organic impurities such as iron and manganese. It will also oxidize sulphides which can assist filtration

. As it is a 50 percent stronger oxidizer than chlorine, it requires significantly less contact time to remove inorganic/organic compounds than conventional methods.

. Ozone produce less THM disinfection by-products than result from chlorine disinfection(but bromate may be formed). Waste waters treated with this disinfection do not contain chlorine or chlorinated disinfection by products, thereby making dechlorination unnecessary.

Disadvantages of Ozone Deisinfection:

. Because of its toxicity, this chemical need to be consistently monitored.

. Within the ozone system there are higher than fatal concentrations of ozone and the release of this gas is would clearly be highly dangerous. Ozone detectors and other safety steps are essential.

. A related disadvantage is the need to ensure conformance with building regulations and fire codes concerning the storage of liquid, high- purity oxygen. Although it forms less THM's than chlorine based disinfectants if bromine is present in the water bromate will be formed. This is equally as dangerous.

. Ozone has a half-life of 20 minutes in air and water depending therefore leaving no residual kill of pathogens in the water after this time and another disinfectant must be added

. It is also the most expensive technology in this review, the systems need to highly automated and very reliable and this comes at a cost

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