Disifection

DISINFECTION

The filtered water from the Slow or Rapid sand filters normally contains some harmful pathogenic (disease causing) bacteria. These bacteria must be killed in order to make the water safe for drinking. The process of killing these harmful bacteria is called disinfection and the chemicals used in this process are called disinfectants. Disinfection not only kills the bacteria during treatment but also prevents any recontamination during the distribution of water to the consumer. The disinfectant chemicals used should therefore be able to give residual sterilizing effect for a long period.

Minor Methods of Disinfection

1. Boiling of water (Sterilization): Most effective method since boiling of water kills all the bacteria and micro-organisms. However, this method is not feasible for large scale public water supplies. During epidemics, the consumers are advised to boil the water before drinking.

2. Treatment with excess lime: Excess lime added increases the pH of water to greater than 9.5, when E-coli present in water will die. The bacterial removal efficiency is upto 99 to 100%. However, the excess lime added has to be removed by re-carbonation or other suitable methods, before supplying it to the consumers. The dosage of lime is between 10 to 20 ppm of Calcium Oxide.

3. Treatment with ozone: Ozone gas is faintly blue gas of pungent odour and is  

Because of its instability, ozone readily breaks down into oxygen and nascent oxygen (O). The nascent oxygen is a powerful oxidizing agent and removes the bacteria as well as organic matter from water. The dosage is 2 to 3 ppm and contact period is 10 minutes. In addition, Ozone removes the colour, taste and odour from water. However, it is very costly.

4. Treatment with iodine and bromine: Addition of iodine and bromine to water kills the pathogenic bacteria. The dosage is 8 ppm and contact period is 5 minutes. They are not used for treating large scale public supplies, but may be used for small water supplies for army troops, private plants, swimming pools Is etc.

4. Treatment with ultra-violet rays: UV rays are effective in killing both the active bacteria as well as spores. Though Sun is a powerful source of UV rays, it requires large exposure area and long time. Hence UV rays are generated by mercury vapour lamps enclosed in a quartz globe. Water should be made to flow in thin films over the lamp and it should be colourless with turbidity less than 15 ppm. This method is costly and hence not commonly used except in private buildings, office buildings, institutions and swimming pools.

6. Treatment with potassium permanganate: Common method in rural areas. Potassium permanganate (KMnO,) is dissolved in a bucket of water and is mixed with the well water. The dosage is 1 to 2 mg/L with contact period of 4 to 6 hours. Addition of potassium permanganate imparts pink colour to water and the water should not be used during the first 48 hours.

7. Treatment with silver, called Electro-Katadyn process: Silver when immersed in water exerts an inhibiting action on bacterial life. Silver ions, with or without activators (palladium or gold) are deposited on particles of granular activated carbon. Bacteria laden water contacting the silver impregnated carbon release minute quantities of silver - 25 to 40 ppb which acts as a disinfectant. The contact time varies from 10 to 60 minutes. Since silver is costly, this method is suitable for small installations or for private individual houses.

Chlorination

Chlorine is universally adopted for disinfecting public water supplies. It is cheap, reliable, easy to handle, easily measurable and capable of providing residual disinfecting effects for long periods, thus affording complete protection against future recontamination of water in the distribution system. The only disadvantage is that when used in excess, it When chlorine is added to water, it forms hypochlorous acid or hypochlorite ions, which have immediate disastrous effect on microorganisms.

Cl₂+ H₂O → HOCI (hypochlorous acid) + HCI

HOCI→ H⁺ (hydrogen ion) + OCH (Hypochlorite ions)

The hypochlorous acid is unstable and may break into hydrogen ions and hypochlorite ions. The above reaction is reversible and depends on pH of water. The sum of hypochlorous acid, hypochlorite ions, and molecular chlorine existing in water is called free available chlorine. Out of these, hypochlorous acid (HOCI) is more destructive and the pH of water is maintained slightly less than 7, so as to control the dissociation of HOCI.

Moreover, the chlorine will immediately react with ammonia present in water to form chloramines,

The chloramines formed are stable and are found to possess disinfecting properties. When the added chlorine has consumed all the ammonia available in water, then it will persist as free chlorine. The combination of chlorine with ammonia in the form of chloramines is called the Combined Chlorine, and is less effective in causing disinfection compared to free chlorine.

The free chlorine as well as the combined chlorine (chloramines) will cause germicidal action on bacteria and pathogens. The free chlorine will instantaneously kill the pathogens, while the combined chlorine will provide long term germicidal effect.

In general, most of the waters are satisfactorily disinfected if the free chlorine residual is about 0.2 mg/l, 10 minutes after the chlorine is applied.

Various forms in which chlorine can be applied:

•  Liquid chlorine or chlorine gas

•  Hypochlorites or bleaching powder

•  Chlorine tablets

• Chloramines ie., mixture of ammonia and chlorine

• Chlorine dioxide

(i) Bleaching powder: Bleaching powder or calcium hypochlorite Ca(OCI), is a chlorinated lime containing about 33% of available chlorine. It loses its strength during storage or exposure of air. It is therefore used only on small installations or under emergency conditions.

The process of chlorination with hypochlorites is known as hypochlorination. ovito Commerical compounds such as HTH (high test hypochlorites), Pittclor, Pittcide, no Hoodchlor etc are used instead of bleaching powder. The HTH has chlorine content of 65 to 70%. Hypochlorites are applied to water as a solution by means of hypochlorite feeding apparatus. It consists of a solution tank connected to a constant level feeding tank. The dosage of the solution is adjusted by means of an adjustable pinch clamp.

(ii) Chloramines: Chloramines, the combination of ammonia and chlorine are widely used as they produce a more stable disinfecting residual than produced by chlorine alone. In this treatment, ammonia is added to water just before the chlorine is applied. The usual proportions are 1 part of ammonia to 4.5 parts of chlorine by weight. Ammonia may be used in the form of gas or liquid or ammonium sulphate or ammonium chloride. Since the disinfecting action of chloramines is slower than chlorine alone, a contact period of 2 hours should be provided before the water is used.

(iii) Free Chlorine: Chlorine is generally applied in gaseous form or in liquid form. Gaseous chlorine is a greenish-yellow poisonous substance, with a typical odour and is 2.48 times heavier than air. Liquid chlorine is amber coloured oily liquid and about 1.44 times heavier than water. Unconfined liquid chlorine rapidly onino vapourises to gas, 1 volume of liquid yields 462 volumes of gas. Chlorine is stored and supplied in liquid form in metal containers under pressure. Since liquid chlorine is highly corrosive, the cylinders containing liquid chlorine are provided with special fittings. Chlorine gas is a respiratory irritant and it can cause varying degrees of irritation to skin, mucous membranes and respiratory system. Chlorine cylinders should be stored in a cool well-ventilated room. The chlorine dose depends upon : Organic matter present in water, pH of water, amount of carbon dioxide present in water, temperature and time of contact.

(iv) Chlorine Dioxide: Bactericidal properties of chlorine dioxide is greater than chlorine. The chlorine dioxide gas is unstable, and is therefore produced at the

It is harmless in aqueous solution. It does not react with organic materials to produce any harmful substances. It has greater oxidizing ability than chlorine. The dosage varies from 0.5 to 1.5 ppm and is not affected by variations in pH. However due to its high cost of production, it is rarely used.

Forms of Chlorination

Depending upon the stage at which chlorine is applied to water, chlorination may be of the following forms:

(i) Plain chlorination: The application of chlorine to raw or untreated water supply as it enters the distribution system. It also includes the chlorination of raw waters in tanks or reservoirs to check the growth of weeds, organic matter, algae and bacteria. It also removes colour and odour from water. This is done when water is relatively clear with turbidities less than 20 to 30 ppm. The normal dose is between 0.5 to 1 ppm.

(ii) Pre-chlorination: It is the application of chlorine to water before its treatment yllups (filtration or sedimentation). This helps in reducing the amount of coagulants required because of the oxidation of organic matter by chlorine. The dosage of chlorine is adjusted such that the chlorine residual is 0.1 to 0.5 ppm.

Advantages of pre-chlorination:

• It reduces the quantity of coagulants required

• It reduces the bacterial load on filters

• It controls algae and plankton growth in basins and filters

• It eliminates tastes and odours

(iii) Post-chlorination: It is the application of chlorine in water after its treatment. This is the standard procedure followed in which chlorine is added to water as it leaves the rapid sand filters and before it enters the distribution system. The dose of the chlorine should be so adjusted that the residual chlorine is about 0.1 to 0.2 ppm. It is useful for protection against recontamination during distribution.

(iv) Double or multiple chlorination: It refers to the application of chlorine at two or more points in the purification process. Generally, double chlorination is resorted to, in which chlorine is applied just before water enters the sedimentation tanks, and after it leaves the filter plants. This is done specially when raw water is highly The advantages of double chlorination are similar to those of pre-chlorination with greater factor of safety against pathogenic microorganisms.

(v) Break Point Chlorination: When chlorine is applied to water, two actions take place one after the other:

• Chlorine acts as disinfectant and kills bacteria.

• Chlorine acts as oxidizing agent and it oxidizes the organic matter.

Chlorine Demand: Chlorine compounds are good oxidizing agents and hence they react with organic and inorganic impurities present in water, before the disinfection is achieved. The amount of chlorine consumed in the oxidation of these impurities is known as chlorine demand of water. Only after the chlorine demand of water is fulfilled, the chlorine will appear as free available chlorine to kill the pathogenic microorganisms. Chlorine demand is therefore, the difference between the amount of chlorine added to water and the quantity of free available chlorine remaining at the end of the specified contact period.

Break point chlorination is a term which gives us an idea of the extent of chlorine to be added to the water. In fact, it represents, that much dose of chlorination, beyond which any further addition of chlorine will equally appear as free residual chlorine.

When chlorine is added to pure water which has no organic impurities But, water generally has some impurities or chlorine demand and as such results in curve ABC, shown in figure 2.21. When chlorine is added to the water, it first reacts with the ammonia present in the water, so as to form chloramines. The residual chlorine curve (AB) therefore increases with the applied chlorine dosage but shall be slightly less than the applied chlorine as some chlorine is consumed for killing the bacteria. If the addition of chlorine is continued abizorby beyond the point B, chlorine oxidizes the organic impurities in water and therefore, the residual chlorine curve falls down, (BC). This point "C" is called the break point, as any chlorine that is added to water beyond this point, breaks through the water and totally appears as residual chlorine. The addition of chlorine beyond break point is called break point chlorination.

General practice is to add chlorine beyond the break point and ensure a residual of 0.2 to 0.3 mg/l of free chlorine.

(vi) Super chlorination: Super-chlorination is the application of excessive amount of chlorine (5 to 15 mg/l) to the water to give about 1 to 2 mg/l of residue beyond the break point. This may be required in some special cases of highly polluted waters containing high concentration of organic impurities, or during pinsgro di epidemics of water borne diseases or when water contains cysts of E. histolytica (organism causing amoebic dysentery). Sometimes, even higher doses may be used and the resultant water is dechlorinated before distribution.