# Ozone disinfection Kinetics

Kinetics of ozone disinfection

Chemical disinfection by ozone can be achieved by bringing water in contact with gaseous ozone for a certain period of time. The kinetics of the deactivation of pathogenic microorganisms (disinfection) is comparable to a chemical reaction. The most commonly used model to describe water disinfection by ozone is the Chick-Watson law. This law can be mathematically represented as follows [6,22]:

k = Cn . t

k = reaction-constant, dependent on the type of microorganism and the disinfectant
C = disinfectant concentration
t = contact time, period of time that the disinfectant is in contact with water
n = constant

In most cases n equals 1, causing the deactivation of bacteria to become a first-order reaction. When the n constant (nearly) equals 1, Watson’s law can be approached as [22]:

k = C . t

During disinfection, this Ct-value is used. This value is a multiplication of the disinfectant concentration (C) in mg/L and contact time (t) in miutes, which is needed to deactivate a microorganism. Various levels of deactivation can be achieved. This is often expressed as a log reduction:

1 log reduction = 90% deactivation
2 log reduction = 99% deactivation
3 log reduction = 99,9% deactivation
4 log reduction = 99,99% deactivation

Much research has been conducted on Ct-values for various types of microorganisms and for various disinfectants. Data on Ct-values in literary sources may differ. While comparing disinfectants, the CT-value must always be associated with the log reduction. Apart from concentration and time there are other factors that influence this Ct-value. Examples are pH value, sunlight, water temperature, mixture of water and the disinfectant, and contact chamber design [6].

In the table below, Ct-values of the main types of microorganisms are represented, when ozone is used as a disinfectant. A lower concentration (C) in a larger time interval (t) is preferred [30]. Contact time is not ment to exceed 5 minutes, because this causes a decrease in residual concentration of ozone in the system [28].

 Microorganism kO3 (mg min l-1) T (oC) pH (-) log reduction Reference E.coli 0,009 12 * 4 log 9 Legionella pneumophila 1,05 12 * 2 log 9 Rotavirus 0,006-0,06 5 6-7 2 log 5 Giardia lamblia (cysts) 0,17 25 7,2 2 log 16 Giardia muris (cysts) 0,27 25 7 2 log 16 Cryptosporidium parvum (oocysts) 5,39** 20 * 2 log 11

* no availbale data
** based on a more resistant chain