Types of chemical disinfectants containing chlorine are hypochlorite, chloramines, and chlorine-dioxide.
Instead of using chlorine gas, some plants apply chlorine to water as a hypochlorite, also known as bleaching powder. Hypochlorites are less pure than chlorine gas and are less dangerous. Temperature, light, and physical energy can all break down hypochlorites before they are able to react with pathogens in water. Hypochlorites work in the same manner as chlorine gas. They react with water and form the disinfectant hypochlorous acid. In general, disinfection using chlorine gas and hypochlorites occurs in the same manner. The differences lie in how the chlorine is fed into the water and on handling and storage of the chlorine compounds. In addition, the amount of each type of chlorine added to water will vary since each compound has a different concentration of chlorine.
There are three types of hypochlorites viz., sodium hypochlorite, calcium hypochlorite and commercial bleach.
Sodium hypochlorite (NaOCl) comes in a liquid form which contains up to 12% chlorine. The reaction of sodium hypochlorite with water is shown below:
Sodium hypochlorite + Water -> Hypochlorous Acid + Sodium Hydroxide
NaOCI + H20 –> HOCl + NaOH
Sodium hypochlorite may be prepared by absorbing chlorine gas in cold sodium hydroxide solution: .
2NaOH + Cl2 –> NaCl + NaOCI + H2O
Sodium hydroxide and chlorine are commercially produced by the chloralkali process, and there is no need to isolate them to prepare sodium hypochlorite. Hence NaOCl is prepared industrially by the electrolysis of sodium chloride solution without any separation between the anode and the cathode. The solution must be kept below 40°C (by cooling coils) to prevent the formation of sodium chlorate. The commercial solutions always contain significant amounts of sodium chloride (common salt) as the main byproduct, as seen in the equation above.
A solution of sodium hypochlorite is frequently used as a disinfectant and as a bleaching agent. A 12% solution is widely used in waterworks for the chlorination of water. High-test hypochlorite (HTH) is sold for chlorination of swimming pools and contains approximately 30% sodium hypochlorite.
Sodium hypochlorite as a disinfectant has the following advantages:
It can easily be stored and transported.
Disadvantages of sodium hypochlorite as a disinfectant are:
Sodium hypochlorite is a dangerous and corrosive substance
Calcium hypochlorite (Ca(OCl)2), also known as bleaching powder or chlorinated lime is a white amorphous powder with pungent smell of chlorine. When freshly made it contains about 30 – 50% of available chlorine. It is however, unstable compound, and on exposure to air, light, moisture, it rapidly looses its chlorine content. Its strength should be first ascertained in the laboratory before using it for chlorination. The reactions of calcium hypochlorite with water are shown below:
Calcium hypochlorite + Water —> Hypochlorous Acid + Calcium Hydroxide
Ca(OCI)2 + 2 H2O —> 2 HOCl + Ca(OH)2
The usual method is to make up a suspension of powder in water, lime in it is allowed to settle, and the supernatant chlorine-water is drawn off for dosage purposes. Assuming the bleaching powder releases 33.3 % of its weight as chlorine, some typical solutions used might be as shown below.
Chlorine strength and amount of bleaching powder required
|Chlorine solution strength,||Chlorine content||Amount of bleaching powder required|
|%(mg/L)||(g/L)||at 33.3% w/w, (g/L)|
|0.01 % (1000 mg/L)||1||3|
|0.003% (300 mg/L)||0.3||1|
Considerable problems arise in the use of bleaching powder. There is a heavy sediment of lime when bleaching powder is mixed with water; this sediment must be disposed of, but the supernatant still contains a large amount of suspended lime which tends to clog up any kind of drip-feeding device that can be used. The WHO recommends first making up a 1 % solution comprising 40g of bleaching powder to one litre of water (approximately assuming 15% available chlorine). It is then recommended that three drops of this solution should be added to one litre of water for drinking. The strong solutions of chlorine are very unstable and will rapidly lose their chlorine content if exposed to the air or sunlight. Hence, they must be stored in closed dark-walled containers. Even when so stored a 0.1 % solution will lose a substantial amount of its chlorine in three to seven days.
In general bleaching powder is an unsatisfactory medium for the application of controlled doses of chlorine on a daily basis. Its use should be avoided wherever possible because it so often results in overdosing of a water with chlorine; consumers’ objection to the strongly chlorinous taste.
Disadvantages of the calcium hypochlorite or bleaching powder are:
Commercial bleach is the bleach which is available in a grocery store. The concentration of commercial bleach varies depending on the brand.
Some plants use chloramines rather than hypochlorous acid to disinfect the water. To produce chloramines, first chlorine gas or hypochlorite is add
inally, the dichloramine may react with hypochlorous acid to form a trichloramine:
Dichloramine + Hypochlorous Acid —> Trichloramine + Water
NHCl2 + HOCl —> NCl3 + H2O
The number of these reactions depends on the pH of the water. In most cases, both monochloramines and dichloramines are formed. Monochloramines and dichloramines, can be used as a disinfecting agent and are called combined chlorine residual because the chlorine is combined with nitrogen. This is in contrast to the free chlorine residual of hypochlorous acid which is used in other types of chlorination. Chloramines are weaker than chlorine, but are more stable, so they are often used as the disinfectant in water treatment systems. Chloramines are effective at killing bacteria and some protozoans, but they are very ineffective at killing viruses.
Chlorine dioxide, (ClO2), is a very effective form of chlorine since it will kill protozoans, Cryptosporidium, Giardia, and viruses that other systems may not kill. In addition, chlorine dioxide oxidizes all metals and organic matter, converting the organic matter to carbon dioxide and water.Chlorine dioxide can be used to remove sulfide compounds and phenolic tastes and odors. When chlorine dioxide is used trihalomethanes are not formed, and hence, chlorination process is unaffected by ammonia. Moreover, chlorine dioxide is effective at a higher pH than other forms of chlorination.
However, chlorine dioxide is not used in all systems because chlorine dioxide must be generated on site, which is a very costly process requiring a great technical expertise. Unlike chlorine gas, chlorine dioxide is highly combustible and care must be taken when handling the chlorine dioxide.