Disinfection with ozone
Disinfection with ozone / Kinetics of the contact time with ozone Water is disinfected but never fully sterilized in the water treatment process. This disinfection is a two-part process that includes: Particle removal by filtration. A general rule of thumb is that high turbidity in the effluent is a potential health risk because viruses and bacteria can hide within the rough texture of the particles. Therefore, the removal of the particles reduces the possibility of pathogenic microorganisms in the effluent. (see Particles figure) Inactivation of pathogenic microorganisms by chlorine, chlorine dioxide, ozone or other disinfectants: contact time and kinetics are simply a measure of inactivation due to the time and concentration of the disinfectant. USEPA has developed regulations for the minimum death rates (inactivation) necessary for public water to be considered potable. These regulations include minimal disinfection of: • Three records (99.9%) for Giardia Lamblia cysts • Four records (99.99%) for enteric viruses In "water treatment terms", 1 record deactivation is called 1 credit inactivation. Different types of filtration are assigned certain elimination credits. For example, conventional filtration is worth 2.5 credits for Giardia cysts. Since EPA requires the deletion of 3 records (credit), an additional inactivation of 0.5 credits must be achieved per disinfection. Various degrees of disinfection can be achieved by altering the type and concentration of disinfectant, as well as the time the water is in contact with the disinfectant. The decision to use one type of disinfectant versus another will establish the precedence for the rest of the values necessary to achieve adequate disinfection. The time that the untreated water is exposed to the disinfectant and the concentration of that disinfectant are the main factors in the equation that will be discussed in the next section [Note that the contact time units are (mg / l) (min)] . Relationship between death efficiency and contact time
Harriet Chick developed a relationship between death efficiency and contact time while a member of the Pasteur Institute in Paris, France. The research yielded data supporting their relationship as shown in the graph. 'No' represents the initial number of organisms and 'N' is the number or organisms in 'Time'. As the contact time between the water and the disinfectant increases, the N / No ratio decreases as Chick's Law predicts.
Factors Affecting C * t Values
As the pH increases, the C * t value also needs to be increased. This can be explained by examining the effects of pH on free chlorine. As pH increases, there is more weak disinfectant (OCI-) than strong disinfectant (HOCI-), which increases the value of C * t. See Table 1 below. The higher the registry deletion, the higher the C * t, as can be seen in the table below.
Table 1: C*t for Removal of Giardia Cysts in Relation to Log Removal and pH
Information from the Virginia Department of Sanitary Regulations The strength of a disinfectant directly affects the C * t. For a weak disinfectant, the C * t will have to be higher than for a strong disinfectant. As Table 2 below shows, ozone is the strongest disinfectant, therefore the required C * t value is lower compared to chlorine and chlorine dioxide. Different organisms have different resistance to disinfectants. If an organism has a strong resistance to a certain disinfectant, the C * t will be higher than for an organism with a weaker resistance. See Table 2 below.
Table 2: C*t Values for the 99% Inactivation at 5° C of Organisms Using Various Disinfectants
Hoff, J.C., Inactivation of Microbial Agents by Chemical Disinfectants, EPA/600/2-86/067, 1986