This article throws light upon the top eight methods used for disinfection of drinking water. The methods are: 1. Disinfection by Light 2. Disinfections by Heat 3. Metal Ions 4. Alkalies and Acids 5. Surface Active Chemicals 6. Disinfection by Ozone 7. Halogens as Disinfectants 8. Other Disinfectants.
Method # 1. Disinfection by Light:
As we know sunlight is a natural disinfectant. The ultraviolet light is a good source for the disinfection of drinking water. The common surface of ultraviolet light is a mercury lamp constructed of quartz.
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For proper disinfection, it is necessary that water must be free from light absorbing substances like aromatic and phenolic compounds.
Radiant energy occurs in discrete units or quanta,
E = hcλ.
Where E = energy of single quantum in erg,
H = Planck’s constt (6.62 × 10-27 erg – sec)
C = Velocity of light (3 × 1010 cm per sec) and
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λ = Wavelength of radiation in centimeter.
By definition, a germicidal unit is an intensity of 100 mw per sq. cm for radiation of wave length 2537 Å. Exposures of Escherichia coli to 3000, 1500 and 750 mw-sec per cm2 produce 99, 99, 99 and 90 % kills or decontamination factors of 104, 102 and 10 respectively.
Water disinfections by solar heat:
Solar water disinfection, also known as SODIS is a method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a free and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage.
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Exposure of water to sunlight has been shown to deactive diarrhoea causing organisms in polluted drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms.
i. Ultra Violet rays interferences directly with the metabolism and destroys cell structures of bacteria.
ii. UV rays (wavelength 320-400 nm) reacts with oxygen dissolved in water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens.
iii. Cumulative solar energy (including the infrared radiation component) heats the water. If the water temperature rises above 50°C, the disinfection process is three times faster.
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Process of solar disinfection:
i. Colourless, transparent plastic bottles (2 litre or smaller size) with a few surface scratches are chosen for use. The labels are removed and the bottles are washed before first use.
ii. Water from contaminated sources is filled into the bottles. To improve oxygen saturation, bottles can be filled three quarters, shaken for 20 second (with the cap on), then filled completely and recapped. Water in very cloudy with a turbidity higher than 30 NTU must be filtered prior to exposure to the sunlight.
iii. Filled bottles are then exposed to the sun. Bottles will heat faster and to higher temperatures if they are placed on a sloped sun-facing corrugated metal roof as compared to thatched roofs.
iv. The treated water can be consumed directly from the bottle or poured into clean drinking cups.
Method # 2. Disinfections by Heat:
Water will be disinfected if it is raised to its boiling point.
Method # 3. Metal Ions:
(i) Silver:
Silver ions are bactericidal although they are neither viricidal nor cystidal. Disinfection occurs even at the low concentration of silver (16/µg per litre, where µg = microgram per litre). Researches have shown that one part of silver can destroy all the bacteria in one hundred million parts of water. A minute amount of silver can kill the germs immediately in a glass of water.
(ii) Copper:
Copper ions have proved good agicidal although they are weakly bactericidal. Copper is the main element of our health and our life style. The wealth and power of many great kings of the past could be attributed largely to the possession of copper. Today we are no less dependent upon it because copper plays a vital role in all branches of our engineering science and architecture.
Furthermore, copper is one of the trace elements essential to the healthy life of many plants and animals usually, occurring as part of the prosthetic group of oxidizing enzymes. On the other hand, copper can be toxic in larger quantities, especially to the lower members of the human food chain. Obviously, a full coverage of all aspects of copper in the environment would fill a large number of volumes.
Copper has been established as a component of a number of different plant enzymes and is necessary for the activity of several specific enzymes. Essentially all copper containing metalloenzymes are concerned with the catalysis of oxidation – reduction type reactions in which O2 is the electron acceptor.
(iii) Zinc:
Zinc has also been proved very helpful if present in small quantity in water. Under specific conditions, it kills a number of viruses and other microorganisms.
(iv) Bromine Chloride (BrCl):
This oxidant has been suggested as an alternative to chlorine for disinfecting drinking water. Bromine chloride produces trihalomethanes at a much faster rate than chlorine. As disinfectant BrCl behaves similarly to chlorine and chlorine dioxide.
Plants:
(i) Tulsi leaves:
Tulsi leaves have been proved very helpful in killing viruses and other micro- organisms – both in water and milk.
(ii) Amla:
The wood of Amla (Planthus emblica) in used to clear small rain ponds in Indian peninsula.
(iii) Drumstick tree (Moringa oliefera) which is Sudan is called the clarifier tree. It produces seeds which are used for water purification.
(iv) Seeds of honge (Pongamia glabra) and nuts of Nirmali tree (Strychnos potatorium) are used as water clarifiers.
Method # 4. Alkalies and Acids:
It has been found that pathogenic bacteria die in a short time in highly acidic or alkaline water, e.g. at very low (< 3) and very high (> 11) pH value.
Method # 5. Surface Active Chemicals:
Researches have shown that the anionic detergents are weakly destructive while cationic detergents are strongly destructive. The neutral detergents have only intermediate position. Detergents have been used as disinfectants in rinse water and waste water of eating utensils.
Method # 6. Disinfection by Ozone:
The ozone which is prepared from, (a) silent electri-discharge in air oxygen, (b) electrolysis of water, is used in drinking water treatment. The common uses of ozone etc.
(a) Disinfection
(b) Destruction of sulphites
(c) Removal of turbidity
(d) Oxidation of taste, odour and colour producing compounds.
(e) Oxidation of other organic compounds
(f) Precipitation of iron and manganese and
(g) Destruction of many surfactants.
Ozone is used for the disinfection of drinking water in U.S.A., Switzerland, West Germany, Prance and in some African countries.
Ozone produces no disinfection below a critical concentration but above that concentration it gives complete disinfection. For example, the decontamination factor of ozone is zero and 104 at concentration 0.42 ppm and 0.53 ppm for E. Coli in water while in case of chlorine the decontamination factor is 10 at concentration of 0.2 ppm and becomes 103 at 0.48 ppm.
The capital and running cost of ozonation equipment is costlier than chlorine and moreover ozone is a toxic substance and hence great precaution is to be taken in using it. The danger limit in treatment plants is at 0.2 mg of O3 per m3 of air. It leaves no measurable residual.
Method # 7. Halogens as Disinfectants:
Chlorine, bromine and iodine can be used for the disinfection of drinking water.
Method # 8. Other Disinfectants:
(i) Coloured glass:
Recent researches of V.P. Kudesia have shown that certain colours (yellow, orange) can be used as disinfectants, e.g., if water is kept in an orange coloured glass (through which rays may pass), then 30-40 percent micro-organisms die out within a few minutes. On the other hand, blue and green colours invite some germs (non — pathogenic) which are helpful to the human beings in flu’, dysentery, heat etc.
(ii) Cowdung:
The cowdung if pasted on the walls of a house can protect it from the radioactive rays. Perhaps for this reason, it has been used since antiquity in the houses. It also serves as a disinfectant for many reptile animals and insects. Cowdung & fallen leaves get rotten after some-times which acts as natural fertilizer and increase soil fertility.
(iii) Kudesia and co-workers have discovered that with the help of bark of babul and neem, we can remove impurities of Cd, Mn and Cr (iii) and (vi) in water. Just take a can of water and place same pieces of bark of babul for about 10 minutes then 80-90% of these toxic metals will be absorbed by the bark pieces and the water can safely be used for other purposes.
This technique can safely be applied in industries to isolate the toxic metals from effluents.
Disinfection by Potassium Permanganate:
Reaction with organic compounds — Potassium permanganate is also used as a disinfectant. It is a good oxidizing agent and oxidizes alcohols, aldehydes, amines, acids including amino acids.
The possible mechanism of acid catalysed oxidation of amino acids by potassium permanganate in acidic medium can be given below:
(i) Effect on micro – organisms:
KMnO4 is a powerful germicidal agent. It kills E. coli, yeasts, mould, spores and bacterial spores but its effectiveness is slower than chlorine. The effect of KMnO4 on viruses has shown that polioviruses I, II, and III were completely inactivated by exposure to 0.89 ppm at pH 7 at 25°C.
(ii) Effect of chlorine dioxide on micro-organism:
The toxicity of chlorine dioxide in yeasts, moulds, bacteria and bacterial spores has been studied by Kudesia.
The vegetative bacteria —
(1) Escherichia coli,
(2) Salmonella typhosa and
(3) Staphylococci are sensitive to 0.05 to 0.2 ppm chlorine dioxide.
Some yeast has also been found sensitive between 0.08 to 0.35 ppm chlorine dioxides.
It has been discovered that chlorine dioxide is more powerful and effective than chlorine, bromine or iodine for removal of algae from contaminated cooling towers. The results have shown that 0.10 ppm chlorine dioxide killed all the E. coli present in tap water within 5 minutes at 25°C. The results exhibit that it is stronger than bromine in killing E. coli.
The effect of ClO2 on viruses has also been investigated. The polioviruses I, II and III were completely inactivated by exposure to 0.3 ppm of chlorine dioxide at pH 7 at 25° C. It is noticed that in cases of some polioviruses and enter viruses, ClO2 is more effective than chlorine, bromine or iodine.