5.3.Factors that influence water disinfection

• CT: A factor for measurement of the contact time between disinfectant and microorganism and the concentration of disinfectant. CT is used for calculating the concentration of disinfectant, which 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

• In case of application of a particular disinfectant to water, it does not only react with pathogenic microorganisms. It also reacts with other impurities, such as soluble metals, particles of organic matter and other microorganisms. The use of a specific disinfectant for reactions with all these substances gives information for disinfection demand of the water. When these basic disinfection demands are satisfied, 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 for 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.
• CT is commonly used to determine the effect of a particular disinfectant against a certain microorganism under specified conditions. There is a difference between the relative effects of chemical disinfectants against different microorganisms. Often a certain level is added to CT, for example 99 %. This means that 99 % of the microorganisms are deactivated by the disinfectant. CT can be used to compare the effect of various disinfectants against microorganisms (Table 1).
• According to Table 1, ozone is the most effective disinfectant; the CT value of ozone is very low. Chloramines are least effective and cannot be used against Giardia lambia. Chlorine is effective against E. coli bacteria and Polio virus. The CT value of chlorine used against Giardia lambia is much higher than that of chlorine used against E. coli bacteria and Polio virus.

Table 1. Comparison of CT values for the 99 % inactivation of microorganisms at 5°C