How much ozone do I need to destroy pathogens? The question is similar to asking “how much heat do I need to cook an egg?” This question is more easily answered when put in terms of time and temperature. Five minutes in boiling water can produce a softboiled egg. Ten minutes in boiling water will produce a hardboiled egg. The ozone question can be answered in a similar way: About three seconds of exposure in 0.5 ppm ozonated water can destroy 99% of E.coli bacteria. Six seconds of exposure in 0.5 ppm ozonated water can destroy 99.99%. Time and ozone concentration are the two main factors needed to how much ozone is needed.
If the ozone concentration is lower, it takes longer to destroy the bacteria. In a similar way, it takes longer to cook meat when the temperature is lower. A higher temperature cooks faster, but can also have undesirable side effects. Higher concentrations of ozone destroy pathogens more quickly, but also can have undesirable side effects. When cooking a piece of meat, the goal is to reach a particular internal temperature. In the disinfection industry, the goal is a particular Contact Time or CT value. The CT value is often given in units of mg/min -1 which is equivalent to ppm x time in minutes.
The CT disinfection value is a number that tells you when a particular type of pathogen has been “cooked” or inactivated to the desired level. The numbers come from a CT value chart. For example, the chart here gives a set of CT values for inactivating cryptosporidium. The CT value needed to inactivate 99% (2 Log) of the cryptosporidium at 15 degrees Celsius is 12. If my ozone concentration in the water is 2ppm, then I need to maintain that level of ozone in the water for 6 minutes. Ozone concentration (2ppm) x Time (6 min) = 12.
Another chart gives the CT values for inactivating 99% of a variety of different pathogens at 5 degrees Celsius with four different kinds of disinfectants. E.coli bacteria have a very low CT value of 0.02 with ozone. A 0.5 ppm concentration of ozone requires only 0.04 minutes (2.4 seconds) of contact time to inactivate 99% of E.coli. Chlorine is also an oxidant, but it is not as strong an oxidant as ozone. The chart shows the CT values of three different forms of chlorine. All of them have a higher CT value and therefore require a higher concentration or a longer contact time for the same level of disinfection.
When you start looking at CT charts, you will notice that water temperature has a significant impact on CT values. In cold water, ozone does not react as quickly as it does in warmer water. Keep in mind, however, that the ozone level in warmer water declines more quickly as it oxidizes things. As the ozonated water moves through a pipe or reaction chamber, it may begin at 4 ppm, and end at 2 ppm. (see charts at end of post)
Temperature is not the only factor to consider. Minerals or other organic compounds in the water will be oxidized by the ozone and reduce the concentration. Contact time may also vary depending on water demand. A CT value table provides a solid starting point, but all the other factors that affect ozone and limit contact of ozone with a particular organism must be considered when determining how much ozone will be needed.
A five gallon bucket and a stopwatch will give a fairly good measurement of your water flow in gallons per minute. Ozonated water flowing at 5 gallons per minute through a 10 gallon tank will provide about 2 minutes of contact time. Dissolved ozone test kits are a low cost method of measuring the ozone levels in water. Dissolved ozone sensors that provide a continuous digital reading of dissolved ozone levels are much more expensive. Measuring the Oxidation Reduction Potential (ORP) is a cheaper option, but does not give a direct ppm measurement. However, some sampling with a test kit can provide a fairly accurate correlation chart (see blog post) of ORP and dissolved ozone levels in your water.
Related blog posts and links to products.
Dissolved ozone test kits