Factors that influence water disinfection
This stands for the contact time between disinfectant and microorganism and the concentration of disinfectant. CT is used to calculate how much disinfectant is required to adequately disinfect water. C refers to the final residual concentration of a particular chemical disinfectant in mg/L. T refers to the minimum contact time (minutes) of material that is disinfected with the disinfectant. Therefore, the units of CT are expressed in mg-min/L.
CT = disinfectant concentration x contact time = C mg/L x T minutes
When a particular disinfectant is added to water, it does not only react with pathogenic microorganisms, but also with other impurities, such as soluble metals, particles of organic matter and other microorganisms. The utilization of a disinfectant for reactions with these substances make up the disinfection demand of the water. The disinfection demand must first be satisfied, before a residual disinfectant concentration can be established. The disinfectant concentration that has to be added to water is made up by the sum of the disinfection demand and the residual disinfectant concentration. Once there is a residual disinfectant concentration, this residual concentration has to be maintained during the required contact time to kill pathogenic microorganisms. To adequately disinfect the water it is therefore required to supply the water with a higher disinfectant concentration than the concentration required to kill pathogenic microorganisms.
Usually a dose of 12-20 mg/L chlorine is required to result in a free chlorine residual concentration of 6-8 mg/L. The time required to deactivate a particular microorganism decreases when the applied disinfectant concentration (mg/L) is increased. Laboratory tests are conducted, to find out which contact time is most effective.
The CT is commonly used to determine the affectivity of a particular disinfectant against a certain microorganism under specified conditions. There is a difference between the relative affectivity of chemical disinfectants against different microorganisms. Often a certain level is added to the CT, for example 99%. This means that 99% of the microorganisms are deactivated by the disinfectant. CT can be used to compare the affectivity of various disinfectants against microorganisms (table 1).
Table 1. Comparison of CT values for the 99% inactivaton of microorganisms at 5 °C
The type of microorganism
The age of the microorganism
Water that requires treatment