Water is disinfected but never completely sterilized in the water treatment process. This disinfection is a two part process that includes:
Courtesy of Eric Karch and David Loftis
In "water treatment terms" 1 log inactivation is referred to as 1 credit inactivation. Different types of filtration are assigned certain removal credits. For example, conventional filtration is worth 2.5 credits for Giardia cysts. Since the EPA requires 3 log (credit) removal, an additional 0.5 credit inactivation from disinfection must be attained.
Varying degrees of disinfection can be attained by altering the type and concentration of disinfectant, as well as the time water is in contact with the disinfectant. The decision to use one type of disinfectant versus another will set the precedence for the remainder of the values needed to attain the proper disinfection. The time 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. [Notice that the units of contact time are (mg/l)(min).]
Figure 2: Graphical Representation of Chick's Law Edit Heading Taken from R.C. Hoen's CE 4104 Spring Notes.
A relationship between kill efficiency and contact time, was developed by Harriet Chick while she was a Fellow in the Pasteur institute in Paris, France. The research yielded data supporting her relationship that is shown in Figure 2. (No) represents the initial number of organisms and N is the number of organisms at time t. As contact time between water and disinfectant increases, the ratio of No/N decreases as Chick's Law predicts.
|Table 1: C*t for Removal of Giardia Cysts in Relation to Log Removal and pH|
|Log Removal||pH <6||pH 6.5||pH 7.0||pH 7.5|
Information from the Virginia Department of Health Waterworks Regulations
|Table 2: C*t Values for the 99% Inactivation at 5 Degrees Celsius of Organisms Using Various Disinfectants|
|Organism||Free Chlorine (pH 6-7)||Chlorine Dioxide (pH 6-7)||Ozone (pH 6-7)|
|Giardia lamblia cysts||47-150||-||0.5-0.6|
* 99% inactivation at 25 degrees C
Hoff, J.C., Inactivation of Microbial Agents by Chemical Disinfectants, EPA/600/2-86/067, 1986
Last Updated: April 4, 2012