Continuing the postings related to the application of ozone in water treatment, we will cover the use of ozone for disinfection. In general, disinfection with ozone, and other oxidizing disinfectants, is measured by CT. This is a product of the disinfection residual by the time of exposure to the residual. If one imagines a curve of disinfection residual versus time, the area under the curve would represent CT. For ozone people refer to the law of fours, 0.4 mg/l for 4 minutes will reduce viruses by 4 orders of magnitude, i.e. a 99.99% reduction.
In drinking water treatment the EPA has established regulations for calculating CT and the CT value associated with the inactivation of specific pathogens at a particular temperature. Spartan has a presentation which discusses these regulations (EPA CT Regulations). Depending on the particular situation of a water treatment plant, a certain number of disinfection credits are required for different pathogens.
The EPA is promoting multiple pathogen barriers to accumulate the total number of credits. Thus one combines the benefits of filtration, ozone and say chlorine to reach the degree of inactivation that is needed. Besides the obvious benefit of wearing “belts and suspenders†for such an important application, synergies can develop between the methods that actually enhances disinfection. In addition, ozone provides other benefits, which we have discussed recently. Thus adding ozone to provide some additional disinfection credits while gaining benefits for color removal or taste and odor is may be a good idea.
Ozone in particular works by damaging the membranes of micro organisms. For a more detailed discussion of how ozone disinfects follows this link. Compared to other oxidizing disinfectants, ozone works very quickly. For example, for giardia lamblia, ozone requires a CT that is 1,000 times less than chlorine.
In order to use ozone effectively, one needs to know two things about the system, the hydraulic detention time (HDT), the amount of time the water is exposed to the ozone, and the ozone residual. In a multi chamber contact vessel, ozone residual may be measured in several of the chambers, especially at the exit. The HDT is more difficult to measure since the flow through a contact vessel or tank and have some short circuiting. Thus one cannot simply take the volume for the tank and divide by the flow rate to know the HDT.
Dissolved ozone monitors are available for determining the ozone concentration accurately. The HDT is often measured by tracer studies. A tracer chemical is added to the inlet of the contact vessel and the tracer measured at the outlet of the tank. In a multi chamber contact vessel, the actual HDT verses the apparent HDT typically is 60-70%.
Ozone is a multi functional water treatment chemical that is an extremely effective disinfectant either alone or in combination with other compounds and methods. Spartan supplies ozone disinfection systems for water treatment.