What is Disinfection of Chlorine in Drinking water?
In this article, we are going to discuss the disinfection of chlorine in drinking water. The term ‘disinfection’ is used to mean the destruction of infective organisms in water to such low levels that no infection of disease results when the water is used for domestic purposes including drinking.
The organisms in water which it may be necessary to kill by disinfection include bacteria, bacterial spores, viruses, protozoa and protozoa cysts, worms and larvae. The efficiency of disinfection depends on numerous factors- the type of disinfectant used, the amount applied and the time for which it is applied; the type and numbers of organisms present; and the physical and chemical characteristics of the water.
Factors affecting the disinfection of chlorine in drinking water
The following factors have to be taken into account when treating water with chlorine.
1.The stage at which chlorine is applied.
Chlorine is often applied at more than one stage in the treatment of water. ‘Pre-chlorination’ comprises the application of chlorine to water (often raw water) before it is processed through treatment works, e.g. before clarification and filtration. ‘Intermediate-stage chlorination’ is chlorine added between stages of treatment. ‘Final chlorination’ means the final disinfection of water before it is put into supply.
The purposes of pre-chlorination and intermediate-stage chlorination are often partly biological such as to reduce bacterial content, prevent bacterial multiplication, and restrain algal growth; and partly chemical, such as to assist in the precipitation of iron and manganese and achieve other oxidation benefits. Final chlorination is always for the purpose of disinfecting the water and to maintain a residual in the distribution system so that it is safe for drinking.
2.Effect of turbidity.
The effect of turbidity in water is to make it difficult for the penetration of chlorine and therefore the destruction of bacteria in particles of suspended matter. It is always necessary, therefore, that final disinfection by chlorine is applied as a final stage in water which contains low turbidity. For effective disinfection, the World Health Organization 3 (WHO) suggests a guide level value for turbidity of less than 1 NTU.
3.Consumption of chlorine by metallic compounds.
A substantial amount of chlorine in drinking water may be used to convert iron and manganese in solution in the water into products which are insoluble in water. Reduction of these parameters by upstream processes is therefore desirable. Typically iron and manganese should be less than 0.1 mg/1 as Fe and 0.05 mg/1 Mn, respectively. If at the point of chlorine application their levels are too low to justify removal, the dose must take their demand into account.
4.Reaction of chlorine with ammonia compounds and organic matter.
The ammonia compounds may exist in organic matter or, alternatively, ammonia may exist separately from organic matter (see Section 6.6); in either case, they will form combined chlorine which, as mentioned in the previous section, is not so effective a bactericide as free chlorine.
Chlorine may be used in the oxidation of some organic matter. Ammonia should not exceed 0.015 mg/1 as N. When this value is exceeded or when organic matter is present, an allowance should be made both in the chlorine dose and contact time to satisfy the chlorine demand prior to disinfection.
5. Low temperature causes a delay in disinfection.
A very substantial decrease in killing power takes place with lowering of temperature. The difference in kill rate of bacteria between the temperatures of 20 and 2~ is noticeable both with free and combined chlorine. This must be borne in mind when fixing the contact period (see below).
6. Increasing pH reduces the effectiveness of chlorine in drinking water.
In free chlorine and in combined chlorine the more effective disinfectants in each case, i.e. hypochlorous acid and dichloramine respectively, are formed in greater quantities at low pH values than at high values. Thus disinfection is more effective at low pH values; the guide level value suggested by WHO is less than 8.
These are the factors affecting the disinfection of drinking water.