Disinfection of Water: 8 Methods | Water Treatment | Water Engineering

The following are the minor methods of disinfection:

Method # 1. Boiling:

The bacteria present in water can be killed by boiling it for a long time. This is the most effective method of disinfection of water. However, this method cannot be used to disinfect water on a large scale for public water supplies, for economical reasons.

Moreover, it can only kill the existing bacteria and cannot take care of the future possible contaminations. Boiling of water may, however, be adopted individually by the consumers for disinfecting water before use for drinking and domestic purposes in emergencies like accidental contamination of public water supply or during epidemic breakout.

Method # 2. Excess Lime Treatment:

It has been found that when pH value of water is greater than 9.5, E-coli and other bacteria present in water cannot survive. Thus when enough lime is added to water to raise its pH value to 9.5 or more most of the bacteria will be killed.

The removal of bacteria in this case is to the extent of 99.93 or even 100%. The necessary dose of lime to be given to water for disinfection is between 10 and 20 p.p.m. However, after disinfection the excess lime is to be removed from the water by a suitable method of re-carbonation, before supplying it to the consumers.

It has also been found that most of the bacteria present in water cannot survive if the water becomes highly acidic with pH value below 3. Thus disinfection of water can also be achieved by the addition of acid to the water.

Method # 3. Iodine Treatment:

In this method iodine (I₂) is used for disinfection of water. Iodine is a bluish black solid which when added to water forms hypoiodous acid (HIO) and the dissociation of the hypoiodous acid resulting in the formation of hypoiodite ion (IO).

At pH value of 7 the percentage of iodine (I₂), hypoiodous acid (HIO) and hypoiodite ion (IO) have been reported to be 52, 48 and 0 for a total iodine residual of 0.5 mg/l. However, with the increase in the pH value of water the percentage of iodine as I₂ decreases while that of iodine as hypoiodous acid (HIO) increases. Both iodine and hypoiodous acid are equally good disinfectants.

Iodine reacts less with organic matter as compared to chlorine and it is relatively stable in water. Further iodine does not react with ammonia to form iodamines but it oxidizes ammonia. It also oxidizes phenols. Because of these reasons less iodine is required to obtain free iodine residual. The usual dose of iodine is about 8 p.p.m and contact time with water is 5 minutes. The iodine is also available in the form of pellets or small pills for disinfection purposes.

Iodine is less dependent on pH value and temperature of water, time of contact and nitrogenous impurities than chlorine. It can also kill amoebic cysts which chlorine does not. Further as compared to chlorine iodine provides longer lasting protection against pathogens and reduced offensive tastes and odours.

Because of these advantages over chlorine, iodine is better for post disinfection than chlorine. However, iodine is more costly than chlorine, on account of which the use of iodine for disinfection has been limited to only small water supplies such as for swimming pools, army troops in the field etc.

Method # 4. Bromine Treatment:

In this method bromine (Br) is used for disinfection of water. Bromine is a heavy dark reddish-brown liquid which when added to water forms hypobromous acid (HOBr) and the dissociation of the hypobromous acid resulting in the formation of hypobromite ion (OBr). The bactericidal effect of bromine is almost similar to that of chlorine.

Further bromine also reacts with ammonia to form monobromanine and dibromanine. Monobromanine is a strong bactericide almost as strong as free bromine in contrast to monochloramines. However, because of higher cost and less effectiveness, bromine has not been used for disinfection of water in large public water supplies. It has been used for disinfection of swimming pool waters on a limited scale.

Method # 5. Ozone Treatment:

In this method ozone (O₃) is used for disinfection of water. Ozone is a faintly blue gas of pungent odour. It is an unstable gas which tends to break down to normal oxygen (O₂) and nascent oxygen (O) (containing single atom of oxygen).

The nascent oxygen is very effective in oxidizing the organic matter and it is very powerful in killing bacteria. Ozone being unstable it cannot be stored, but it has to be produced at the point of use. Ozone is produced by passing dry air or oxygen between two high potential electrodes.

The mixture of air or oxygen and ozone produced by the ozone generator is introduced into the water to be treated by injecting or diffusing it into a mixing chamber. The dosage of ozone is about 2 to 3 p.p.m. to obtain residual ozone of 10 p.p.m and the contact period is about 10 minutes.

When ozone is added to the water to be treated it first reacts with the chemical impurities present in the water. Once this ozone demand of water has been satisfied, the free residual ozone available in water kills the micro­organisms present in it and the necessary disinfection of water is accomplished.

The advantages and disadvantages of ozone treatment for disinfection of water are as indicated below:

Advantages of Ozone Treatment:

(i) Ozone is a more powerful disinfectant than chlorine. Ozone is effective in killing some pathogens like cysts and certain viruses which cannot be killed by chlorine.

(ii) The efficiency of disinfection by ozone remains unaffected by temperature and pH value of water over a wide range.

(iii) The bactericidal action is more rapid with ozone than with chlorine and hence short contact time is required for ozone.

(iv) Ozone is highly effective in removal of tastes, odours, colour, iron and manganese from water.

(v) Ozone, unlike chlorine, does not impart offensive tastes and odours to water, nor does it usually produce toxic substances such as chlorinated hydrocarbons.

(vi) There is no danger of over treatment because ozone decomposes to oxygen.

(vii) Ozone rapidly oxidizes organic impurities present in water.

(viii) Since ozone is generated at the point of use, there are no transport and storage problems. Also it does not involve storage of dangerous chemicals.

Disadvantages of Ozone Treatment:

(i) Ozone is more costly than chlorine.

(ii) Since ozone is quite unstable it does not provide residual protection against recontamination.

(iii) Ozone is required to be produced as it is needed and for its production electricity is required, and hence it may be difficult to adjust treatment to variations in load, or to changes in water quality with regard to ozone demand.

(iv) Since ozone cannot be stored and supplied in cylinders, complicated ozone manufacturing equipment called ozonizer is required to be installed at the treatment plant.

Although ozone treatment has several advantages but because of its high cost it has not been extensively used for disinfection of water in large public water supplies in our country. However, in European countries ozone has been extensively used for disinfection of water for large public water supplies. Further ozone treatment has been used for disinfection of water on a small scale such as for swimming pool waters.

Method # 6. Potassium Permanganate Treatment:

In this method potassium permanganate (KMnO₄) is used for disinfection of water. Potassium permanganate works as a powerful oxidising agent and it is found to be effective in killing cholera bacteria. However, it is less effective in killing bacteria of other water-borne diseases.

This method of disinfection is not used for large public water supplies but it is commonly used in rural areas where the water supplies are mostly from wells which usually contain lesser amount of bacteria. Potassium permanganate is dissolved in a bucket of water and it is thoroughly mixed with well water. The normal dosage of potassium permanganate varies from 1 to 2 mg/l with a contact time of 4 to 6 hours. Further the well water should not be used for atleast 48 hours of the addition of potassium permanganate to it.

Addition of potassium permanganate to water imparts it some pink colour. Further the water treated with potassium permanganate, with the passage of time, produces a dark brown coating on porcelain vessels and this coating is difficult to remove except with scratching or rubbing.

Method # 7. Silver Treatment:

It has been found that silver has the property of disinfecting water as it is effective in destroying the bacterial spores and algae present in comparatively clear water. Silver can be introduced in water either in the form of a silver salt or by immersing silver or silver coated electrodes in water and applying an electrical potential of about 100 volts. The later process is called Electro-Katadyn Process. The dosage of silver varies from 0.05 to 1 p.p.m with a contact time of 15 minutes to 3 hours.

The silver treatment does not develop any taste and odour in water and it does not create any harmful effect on human body. However, silver is costly and hence the field of silver treatment is limited to private individual houses only.

Method # 8. Ultra-Violet Ray Treatment:

The ultra-violet rays offer an effective method for the disinfection of water. The ultra-violet rays are the invisible light rays. These rays are basically found in sunlight. As such the exposure of water to sunlight leads to destruction of micro-organisms which is primarily due to ultra-violet rays.

However, the treatment with sunlight requires large exposure area and long time. Hence ultra-violet rays are artificially generated by a machine which consists of a mercury-vapour lamp enclosed in a quartz globe. These lamps commonly use 220-volts D.C., supply.

The ultra-violet rays kill both active bacteria and spores, which are difficult to destroy by other means. The destructive power of these rays begins in the blue-green region of the spectrum with a wavelength of 0.490 µ and increases in effectiveness to 0.149 µ.

For efficient disinfection by ultra-violet rays the following conditions are necessary:

(i) Water should be free from suspended and colloidal substances causing turbidity. The turbidity of water should not be more than 15 to 20 p.p.m.

(ii) Water should not contain light absorbing substances such as phenols, alkyl benzene-sulphonate (ABS) and other atomic compounds.

(iii) Water should flow in thin films or sheets with depth of water not exceeding 10 cm, and it should be close to the ultra-violet rays.

(iv) Water should be well agitated but not mixed with air.

(v) Adequate intensity and time of exposure of ultra-violet rays should be applied.

The advantages of ultra-violet ray treatment are that exposure is for short periods, no foreign matter is actually introduced and no taste and odour is produced. Further over exposure does not result in any harmful effects. The disadvantages of this treatment are that no residual effect is available for protection against recontamination, and there is need for a rapid field test for assessing the treatment efficiency.

Moreover, the ultra-violet treatment is costly and it is unsuitable for disinfection of water on a large scale for public water supplies. It can, however, be adopted for water supply installations of private buildings, private institutions, office buildings, and particularly of swimming pools where if chemicals are used for disinfection, there are chances of developing harmful effects to the skins of people taking bath.