Disinfectants can effectively kill pathogenic microorganisms (bacteria, viruses, and parasites). Some microorganisms can be resistant. E. coli bacteria are more resistant to disinfectants than other bacteria and for this reason they are used as indicator organisms. But several viruses are even more resistant than E. coli which absence does not mean that the water is safe. Protozoan parasites like Cryptosporidium and Giardia are very resistant to chlorine.
The effect of the disinfection activity of a particular disinfectant also depends upon the age of the microorganism. The young microbial population is more vulnerable to the killing effect of the disinfectant. In older populations, different reserve metabolites like polysaccharide shell over their cell wall, makes them more resistant to disinfectants. When 2.0 mg/L chlorine is used, the required contact time to deactivate bacteria that are 10 days old is 30 minutes. For bacteria of the same species and of the age of 1 day, 1 minute of contact time is sufficient. Bacterial spores can be very resistant. Most disinfectants are not effective against bacterial spores.
The nature of the water that requires treatment has its influence on the disinfection. Materials in the water, for example iron, manganese, hydrogen sulphide and nitrates, often react with disinfectants, which disturbs disinfection. Turbidity of the water also reduces the affectivity of disinfection. Microorganisms are protected against disinfection by turbidity.
The temperature also influences the affectivity of disinfection. Increasing temperature usually increases the speed of reactions of disinfection. However, increasing temperature can also decrease disinfection, because the disinfectant falls apart or is volatized.