This article throws light upon the fourteen main factors that influence the disinfecting power of chlorine. Some of the factors are: 1. Active Chlorine 2. Available Chlorine 3. Free Available Chlorine 4. Combined Available Chlorine 5. Combined Available Residual and Free Available Chlorine Residual 6. Chlorine Demand 7. Chlorine Dose 8. Plain Chlorination 9. Pre-chlorination and Others.
Factor # 1. Active Chlorine:
The quantity of molecular chlorine corresponding to the oxidizing power of a given compound with respect to KI in an acid medium is called active chlorine.
ADVERTISEMENTS:
Active Cl2% = nM/M0 × 100.
Where n is the number of hypochlorite ions in a molecule of a chlorine compound. M0 is the molecular mass of chlorine compound and M is the molecular mass of chlorine.
The amount of chlorine liberated from a sample of bleaching powder with excess of dilute acids or CO2 is called available chlorine.
Factor # 2. Available Chlorine:
The strength of chlorine compounds, i.e. their oxidizing capacity is generally expressed as available chlorine. It is called chlorine equivalent. The amount of chlorine librated from a sample of bleaching powder with excess of dilute acids or CO2 is called available chlorine.
Factor # 3. Free Available Chlorine:
The chlorine existing in water as HOCl, OCl–, Cl2 and H2OCl+ is defined as free available chlorine.
Factor # 4. Combined Available Chlorine:
The chlorine in water in chemical combination with ammonia or other nitrogenous compounds such as NH2Cl, NHCl2, and NCl3 is called combined available chlorine.
Factor # 5. Combined Available Residual and Free Available Chlorine Residual:
ADVERTISEMENTS:
If chloramines have been formed, the orthotolidine test will show a residual which is called combined available residual.
If additional chlorine, is added then chloramines get oxidized besides the oxidation of other compounds. The orthotolidine residual will show HOCl, Cl2 and OCl– if any, and it is known as free available chlorine residual.
Factor # 6. Chlorine Demand:
It is defined as the difference between the amount of chlorine added and the amount of chlorine present as a residual, either free or combined, after some designated period.
Factors # 7. Chlorine Dose:
The chlorine dosages can be calculated from the dosage in milligrams per litre by the following equation
ADVERTISEMENTS:
Pound of chlorine = gallon of water × 8.34/1,000000 × dosage in mg/litre
Factor # 8. Plain Chlorination:
It is a term given when cities surface water is used with no other treatment than chlorination. For disinfections, a dose of 0.5 mg/litre or more is required to obtain a combined available residual.
Factor # 9. Pre-chlorination:
It is a term used when chlorine is added in the water before any other treatment is given to it. It improves coagulation and reduces tastes and odours caused by sludge in the sedimentation tank.
Factor # 10. Post Chlorination:
This term is applied to the addition of chlorine to water after all other treatments. Dosage depends upon the type of water and may be between 0.25 to 0.5 mg/litre so as to get a combined available residual of 0.1 to 0.2 mg/litre as water leaves the plant. Sometimes greater residuals are needed to have greater protection against contamination and also checks growth in mains.
Factor # 11. Super Chlorination:
ADVERTISEMENTS:
Sometimes heavy dose of chlorine is given to the water when quick disinfectant action is needed to destroy odours or tastes, but carried to the break point. This is called super chlorination and it can be followed by dechlorination.
Aeration will remove chlorine, hypochlorous acid, dichloramine and trichloramine slowly and slowly from the water. Sometimes NaHSO3 and Na2SO3 are used. Activated carbon can also be used.
Factor # 12. Break Point Chlorination:
When chlorine is added to water it reacts with organic and inorganic compounds. The term break point indicates complete oxidation of chloramines and other chlorine combinations. The residue above the break point is mainly free available chlorine but sometimes NCl3 also occurs. Dosages are likely to be 7 to 10 mg/litre or more may be obtained.
Factor # 13. Free Residual Chlorination vs Combined Residual Chlorination:
The main aim of pre-chlorination and post-chlorination is to obtain a combined residual although a free chlorine residue is also present there. For better safety, it is necessary that sufficient chlorine should be added to destroy ammonia in the free residual chlorination and the resulting residue will be a very efficient disinfectant.
The odorous material can be eliminated and a residue can be maintained in the distribution system once its chlorine demand is satisfied and water is not exposed to sunlight.
Depending on the stages of treatment at which chlorine is added and also the expected results of chlorination various forms of chlorination are in practice.
These are:
(a) Plain chlorination
(b) Pre chlorination
(c) Post chlorination
(d) Double chlorination
(e) Super chlorination
(f) Breakpoint chlorination and
(a) Plain chlorination:
This type of chlorination is used to indicate that only chlorine treatment is given to water from deep wells, lakes, reservoirs etc. This is called plane chlorination.
In other words, if raw water is suspended for consumption after giving only chlorine treatment, this is called plain chlorination.
(b) Pre chlorination:
When chlorine is added to raw water before any treatment, it is called pre-chlorination.
Pre-chlorination reduces the taste and odour of water, improves coagulation and also controls the growth of algae.
(c) Post chlorination:
When chlorine is applied after all the treatments of purification of water it is called post-chlorination.
(d) Double chlorination:
When chlorine is added to raw water at more than one point, the process is called double-chlorination.
(e) Super chlorination:
Super chlorination is generally adopted in cases of emergency like a breakdown or in cases of water which is highly polluted. The application of chlorine beyond the stage of break point is also known as super chlorination.
After super chlorination, the excess of chlorine is necessary to be removed by the process of dechlorination before water is sent for consumption.
(f) Breakpoint chlorination:
When chlorine is added to a sample of water and the available chlorine is estimated after a few minutes. It would be observed that available chlorine is less than the added. This is due to the consumption of chlorine in oxidizing some oxidisable matter present in water.
If we further take some samples, it would be observed that available chlorine left in each sample after a few minutes’ increases with the increase in the quantity of chlorine added.
Such a curve continues in straight line until a definite point, known as breakpoint, is reached and then with additional chlorine, there is a decrease in available chlorine. Hence, after the breakpoint the quantity of available chlorine after each increment of added chlorine is almost equal to the quantity of added chlorine.
Factors # 14. Chloramines:
If an excess of ammonia is present in water then chloramines are formed in larger amount by the addition of chlorine to water. These chloramines have been used as an odour preventive with satisfactory bactericidal effects if contact period is long enough.
They enter the cell walls and destroy the bacteria. For getting chloramines in water, sometimes-anhydrous ammonia is used if insufficient natural ammonia is present in water.
Gaseous ammonia feeding equipment is similar to that used for chlorine. The main advantage of chloramines in water is that they provide and maintain a stable residual throughout a distribution system. According to Kudesia, when ammonia reacts with chlorine water then chloramines are formed at pH values noted against them as follows at 24°C.
