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Another common way to look at disinfection effectiveness is to employ the CT concept. This is typically used in the
design drinking water disinfection systems. This is expressed as a product of the average concentration of ozone
(residual) multiplied by the time over which an organism is exposed to the residual. This product is referred to as CT and
typically has the units mg-min/liter. This approach is conceptually the same as the lethality coefficient, but calculated in a
somewhat different manner.

For a given organism and temperature, the CT will correlate to a certain log reduction of that organism. A four log
reduction would mean that 99.99% of the organism has been removed. Each organism will have a different response for
a given biocide. Below are some CT values for 3 log reduction (99.9%) of giardia lamblia.

Comparing ozone to chlorine for this organism, for the same exposure time using the CT approach shows that ozone is
60 times more effective than chlorine.

Biocide | CT |
---|---|

Chlorine | 122 |

Chloramines | 2200 |

Chlorine dioxide | 26 |

Ozone | 2 |

Organism | CT | Log Reduction |
---|---|---|

Virus | 1.3 | 4 |

E. Coli | 0.02 - 0.03 | 2 |

Streptococcus F. | 0.01 - 0.03 | 2 |

Legionella | 0.3 - 1.1 | 2 |

Total Coliform | 0.19 | 6 |

There is normally a regulatory requirement for a CT, CT (Req). For example, Ground Water Rule requires a four long reduction of viruses. The calculated value CT (Cal) for a drinking water system must meet or exceed CT (Req). CT (Cal) is usually based on the value for water reaching the first customer. This is more relevant for chlorine than ozone since chlorine has a persistent residual that lasts for many hours. The ozone residual has a half-life measured in minutes.

The calculation for CT begins with measuring the residual of the disinfectant. For ozone there is normally an instantaneous demand and then a decay in ozone concentration as a function of time. As noted above, this decay occurs over a matter of minutes. In pure water at 20 degrees C the half-life of ozone is about 20 minutes. So, in one hour the concentration has been reduced by a factor or eight after the instantaneous demand has been satisfied. There are instruments that allow for the continuous monitoring of ozone concentration. Multiple monitoring points throughout the contact tank provide a profile of the residual concentration.

The apparent time, T, is the volume of the tank divided by the flow. In reality, most tanks allow for some of the water to short circuit the tank and have a shorter contact time than predicted by the simple division of volume by flow rate. To arrive at the proper number, one can use tracer studies where tracer chemicals are added to the water and are measured as they exit the tank. the amount of time for 10% of the amount to exit is the allowed contact time T(10). Alternatively, regulatory authorities such as the USEPA provide baffling factors that can be used to estimate the actual contact time. For a tank with multiple baffles this baffling factor will be 0.5 to 0.6. So, if the simple T=Volume/Flow Rate equation where to indicate a contact time of 20 minutes, application of a baffling factor of 0.5 would suggest a true contact time of 10 minutes.

The average concentration multiplied by the corrected contact time would provide the actual CT value. If the concentration can be measured through the contact tank and plotted with time, the area under the curve is also a measure of the true CT value.