The protection of public health from the transmission of waterborne diseases by disinfecting water has been recognized since the early 1900’s. Of the existing processes used to produce safe drinking water, chlorination of public water supplies ranks as one of the most important.
Chick’s Law was later modified by H.E. Watson (1908) to include the coefficient of specific lethality . The Chick-Watson Equation is shown below.
initial number of organisms
number of organisms at time t
concentration of disinfectant (mg/L)
contact time (min.)
coefficient of specific lethality
coefficient depending on
disinfectant type and pH