Chick’s law and Watson’s law - law for disinfection

Basis of disinfection

Chick’s law

Inactivation of micro-organisms in a disinfection plant depends on the time that the micro-organism are exposed to the disinfectant. This is described by Chick’s law:

dn / dt = kN

where:

dn/dt= necessary time to inactivate n micro-organisms

k =time constant depending on disinfectant,type of micro-organism and water quality

N =number of live micro-organisms t =time.

This differential equation can be integrated within limits to give the following equation:

N1= N0e−kt

where:

N0=number of micro-organisms at the start

N1=number of micro-organisms after time t.

Watson’s law

Based on the results of Chick’s law, Watson’s law can be developed:

where:

Λ = coefficient of specific toxicity

C =concentration of disinfectant

n =exponent (normally around 1)

t =time after start-up.

This means that the relation between the number of active and inactive micro-organisms is a product of the concentration of the disinfectant and the exposure time.