The effectiveness of disinfection is calculated by determining the length of time water is in contact with a specific dosage of disinfectant. The required CT to provide target inactivation varies somewhat due to ambient pH or temperature conditions, as well as the strength of the disinfectant, i.e. free chlorine has greater pathogen inactivation properties than chloramines in the same concentration. The actual CT of the disinfection system is then compared to the required values necessary to achieve the desired level of inactivation of key pathogens such as bacteria, viruses, and protozoa. In this classification of pathogens, bacteria are the most prevalent and are the first focus of disinfection. Fortunately, harmful bacteria are relatively easlily inactivated by chlorination.
Viruses are more resistant to chlorination.
Giardia and cryptosporidium are examples of pathogentic protozoa that are particularly difficult to inactivate using normal dosages of chlorine but are almost never found in source waters. The reduction of residual disinfectant levels within a pipeline system is affected by a variety of factors including temperature, presence of organic matter in the water or on the pipe surface, and corrosion of the pipe surface. The level of the residual disinfectant is measured throughout the distribution system using a colorimetric test by which a color change in the sample is compared to a color chart in order to estimate the disinfectant concentration within a reasonable degree of accuracy.