{"id":1867,"date":"2014-01-21T10:08:02","date_gmt":"2014-01-21T18:08:02","guid":{"rendered":"http:\/\/support.cleanwaterstore.com\/blog\/?p=1867"},"modified":"2025-07-30T13:14:07","modified_gmt":"2025-07-30T20:14:07","slug":"ct-values-explained","status":"publish","type":"post","link":"https:\/\/www.cleanwaterstore.com\/blog\/ct-values-explained\/","title":{"rendered":"“What is a CT Value?”: The (Simple) Math Behind Water Disinfection"},"content":{"rendered":"

\"Smiling<\/h1>\n

Water disinfection is crucial to ensuring that it is safe to drink and free of harmful bacteria and other organisms. The primary methods of disinfection are chlorination, hydrogen peroxide injection, ozone, and UV light, each requiring absolute quantification of contact time and dosage to ensure effectiveness.<\/p>\n

Proper contact time must also be complemented by the correct dosage of a disinfectant; multiplying the concentration of your disinfectant by the time of contact with your water will give you a CT value. This calculation is based on the equation: CT = Concentration x Time. See the formula below for calculating CT when using chlorine:<\/p>\n

CT = Concentration of free chlorine <\/em>(C mg\/L<\/em>) * contact time <\/em>(T Minutes<\/em>)<\/span><\/p><\/blockquote>\n

The formula for calculating CT values is essential for understanding the efficiency of PCR amplification. The exponential phase, where reactants are in excess, is crucial for obtaining reliable quantitative data. Data points obtained from this phase are deemed the most valuable for calculating initial target quantities, ensuring the accuracy of gene quantification.<\/p>\n

Free chlorine = Concentration measured in milligrams per liter <\/em>(mg\/L<\/em>)<\/span><\/p><\/blockquote>\n

\"Cutaway<\/a><\/p>\n

Since different contaminants require different CT values for complete disinfection, it\u2019s important that you find the right amounts of disinfectant and contact time needed to adequately disinfect your water. Note that higher concentrations of disinfectants are required when contact time is shorter; for this reason, longer contact times are preferable. The effectiveness of disinfection can be significantly affected by factors such as water quality and the presence of organic matter.<\/p>\n

Now that you know how to calculate your CT value, consult the charts below to determine precisely what value you\u2019ll need to disinfect your water based on its condition and contaminants. Note that a 3-log removal means a 99.9% removal rate, while a 4-log removal will remove 99.99% of viruses\/bacteria.<\/p>\n

CT Values for Giardia Cyst Inactivation Using Ozone<\/h2>\n

U.S. EPA CT values (mg\/L \u00d7 min) based on water temperature and pH (6 to 9)<\/em><\/p>\n\n\n\n\n\n\n\n\n\n
I<\/b>n<\/b>a<\/b>c<\/b>t<\/b>i<\/b>v<\/b>a<\/b>t<\/b>i<\/b>o<\/b>n<\/b><\/td>\n0.<\/b>5<\/b>(32.1)<\/b><\/td>\n5<\/b>(4<\/b>1<\/b>)<\/b><\/td>\n10(<\/b>5<\/b>0<\/b>)<\/b><\/td>\n1<\/b>5<\/b>(59)<\/b><\/td>\n2<\/b>0<\/b>(<\/b>6<\/b>8<\/b>)<\/b><\/td>\n25(<\/b>7<\/b>7<\/b>)<\/b><\/td>\n<\/tr>\n
0.5 log<\/td>\n0.48<\/td>\n0.32<\/td>\n0.23<\/td>\n0.16<\/td>\n0.12<\/td>\n0.08<\/td>\n<\/tr>\n
1.0 log<\/td>\n0.97<\/td>\n0.63<\/td>\n0.48<\/td>\n0.32<\/td>\n0.24<\/td>\n0.16<\/td>\n<\/tr>\n
1.5 log<\/td>\n1.50<\/td>\n0.95<\/td>\n0.72<\/td>\n0.48<\/td>\n0.36<\/td>\n0.24<\/td>\n<\/tr>\n
2.0 log<\/td>\n1.90<\/td>\n1.30<\/td>\n0.95<\/td>\n0.63<\/td>\n0.48<\/td>\n0.32<\/td>\n<\/tr>\n
2.5 log<\/td>\n2.40<\/td>\n1.60<\/td>\n1.20<\/td>\n0.79<\/td>\n0.60<\/td>\n0.40<\/td>\n<\/tr>\n
3.0 log<\/td>\n2.90<\/td>\n1.90<\/td>\n1.40<\/td>\n0.95<\/td>\n0.72<\/td>\n0.48<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

CT Values for Virus Inactivation Using Ozone<\/h2>\n

Required CT values (mg\/L \u00d7 min) for 2-log to 4-log virus removal across various water temperatures<\/em><\/p>\n\n\n\n\n\n\n
I<\/b>n<\/b>a<\/b>c<\/b>t<\/b>i<\/b>v<\/b>a<\/b>t<\/b>i<\/b>o<\/b>n<\/b><\/td>\n<<\/span><\/b>1<\/b><\/td>\n\n

5<\/b><\/p>\n<\/td>\n

\n

10<\/b><\/p>\n<\/td>\n

\n

15<\/b><\/p>\n<\/td>\n

\n

20<\/b><\/p>\n<\/td>\n

25<\/b><\/td>\n<\/tr>\n
2\u2010 log<\/td>\n0.48<\/td>\n0.32<\/td>\n0.23<\/td>\n0.16<\/td>\n0.12<\/td>\n0.08<\/td>\n<\/tr>\n
3\u2010 log<\/td>\n0.97<\/td>\n0.63<\/td>\n0.48<\/td>\n0.32<\/td>\n0.24<\/td>\n0.16<\/td>\n<\/tr>\n
4\u2010 log<\/td>\n1.50<\/td>\n0.95<\/td>\n0.72<\/td>\n0.48<\/td>\n0.36<\/td>\n0.24<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

CT Values for 3-log Giardia Cyst Inactivation Using Free Chlorine<\/h2>\n

CT values (mg\/L \u00d7 min) at 10\u00b0C (50\u00b0F), across various pH levels and chlorine concentrations<\/em><\/b><\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Free Residual, Mg\/L<\/b><\/td>\n\n

pH < <\/span>6.0<\/b><\/p>\n<\/td>\n

\n

pH 6.5<\/b><\/p>\n<\/td>\n

\n

pH 7.0<\/b><\/p>\n<\/td>\n

\n

pH 7.5<\/b><\/p>\n<\/td>\n

\n

pH 8<\/b>.<\/b>0<\/b><\/p>\n<\/td>\n

\n

pH 8<\/b>.<\/b>5<\/b><\/p>\n<\/td>\n

\n

pH < <\/span>9.0<\/b><\/p>\n<\/td>\n<\/tr>\n

< <\/span>0.4<\/td>\n\n

73<\/p>\n<\/td>\n

\n

88<\/p>\n<\/td>\n

\n

104<\/p>\n<\/td>\n

\n

125<\/p>\n<\/td>\n

\n

149<\/p>\n<\/td>\n

\n

177<\/p>\n<\/td>\n

\n

209<\/p>\n<\/td>\n<\/tr>\n

0.6<\/td>\n\n

75<\/p>\n<\/td>\n

\n

90<\/p>\n<\/td>\n

\n

107<\/p>\n<\/td>\n

\n

128<\/p>\n<\/td>\n

\n

153<\/p>\n<\/td>\n

\n

183<\/p>\n<\/td>\n

\n

218<\/p>\n<\/td>\n<\/tr>\n

0.8<\/td>\n\n

78<\/p>\n<\/td>\n

\n

92<\/p>\n<\/td>\n

\n

110<\/p>\n<\/td>\n

\n

131<\/p>\n<\/td>\n

\n

158<\/p>\n<\/td>\n

\n

189<\/p>\n<\/td>\n

\n

226<\/p>\n<\/td>\n<\/tr>\n

1.0<\/td>\n\n

79<\/p>\n<\/td>\n

\n

94<\/p>\n<\/td>\n

\n

112<\/p>\n<\/td>\n

\n

134<\/p>\n<\/td>\n

\n

162<\/p>\n<\/td>\n

\n

195<\/p>\n<\/td>\n

\n

234<\/p>\n<\/td>\n<\/tr>\n

1.2<\/td>\n\n

80<\/p>\n<\/td>\n

\n

95<\/p>\n<\/td>\n

\n

114<\/p>\n<\/td>\n

\n

137<\/p>\n<\/td>\n

\n

168<\/p>\n<\/td>\n

\n

200<\/p>\n<\/td>\n

\n

240<\/p>\n<\/td>\n<\/tr>\n

1.4<\/td>\n\n

82<\/p>\n<\/td>\n

\n

98<\/p>\n<\/td>\n

\n

116<\/p>\n<\/td>\n

\n

140<\/p>\n<\/td>\n

\n

170<\/p>\n<\/td>\n

\n

206<\/p>\n<\/td>\n

\n

247<\/p>\n<\/td>\n<\/tr>\n

1.6<\/td>\n\n

83<\/p>\n<\/td>\n

\n

99<\/p>\n<\/td>\n

\n

119<\/p>\n<\/td>\n

\n

144<\/p>\n<\/td>\n

\n

174<\/p>\n<\/td>\n

\n

211<\/p>\n<\/td>\n

\n

253<\/p>\n<\/td>\n<\/tr>\n

1.8<\/td>\n\n

88<\/p>\n<\/td>\n

\n

101<\/p>\n<\/td>\n

\n

122<\/p>\n<\/td>\n

\n

147<\/p>\n<\/td>\n

\n

179<\/p>\n<\/td>\n

\n

215<\/p>\n<\/td>\n

\n

259<\/p>\n<\/td>\n<\/tr>\n

2.0<\/td>\n\n

87<\/p>\n<\/td>\n

\n

104<\/p>\n<\/td>\n

\n

124<\/p>\n<\/td>\n

\n

150<\/p>\n<\/td>\n

\n

182<\/p>\n<\/td>\n

\n

221<\/p>\n<\/td>\n

\n

265<\/p>\n<\/td>\n<\/tr>\n

2.2<\/td>\n\n

89<\/p>\n<\/td>\n

\n

105<\/p>\n<\/td>\n

\n

127<\/p>\n<\/td>\n

\n

153<\/p>\n<\/td>\n

\n

186<\/p>\n<\/td>\n

\n

225<\/p>\n<\/td>\n

\n

271<\/p>\n<\/td>\n<\/tr>\n

2.4<\/td>\n\n

90<\/p>\n<\/td>\n

\n

107<\/p>\n<\/td>\n

\n

129<\/p>\n<\/td>\n

\n

157<\/p>\n<\/td>\n

\n

190<\/p>\n<\/td>\n

\n

230<\/p>\n<\/td>\n

\n

276<\/p>\n<\/td>\n<\/tr>\n

2.6<\/td>\n\n

92<\/p>\n<\/td>\n

\n

110<\/p>\n<\/td>\n

\n

131<\/p>\n<\/td>\n

\n

160<\/p>\n<\/td>\n

\n

194<\/p>\n<\/td>\n

\n

234<\/p>\n<\/td>\n

\n

281<\/p>\n<\/td>\n<\/tr>\n

2.8<\/td>\n\n

93<\/p>\n<\/td>\n

\n

111<\/p>\n<\/td>\n

\n

134<\/p>\n<\/td>\n

\n

163<\/p>\n<\/td>\n

\n

197<\/p>\n<\/td>\n

\n

239<\/p>\n<\/td>\n

\n

287<\/p>\n<\/td>\n<\/tr>\n

3.0<\/td>\n\n

95<\/p>\n<\/td>\n

\n

113<\/p>\n<\/td>\n

\n

137<\/p>\n<\/td>\n

\n

166<\/p>\n<\/td>\n

\n

201<\/p>\n<\/td>\n

\n

243<\/p>\n<\/td>\n

\n

292<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Introduction to CT Values<\/h2>\n

CT values, also known as cycle threshold values, are crucial to quantitative PCR (qPCR) and real-time PCR (polymerase chain reaction) tests. CT values are derived from real-time PCR assays designed to detect specific genetic sequences. They represent the number of amplification cycles required to detect a specific genetic sequence, such as viral RNA, in a patient sample.<\/p>\n

CT values help quantify the amount of target nucleic acid present in a sample, which is vital for determining whether a test is positive or negative. In the context of COVID-19, CT values are instrumental in detecting the presence of the SARS-CoV-2 virus in patient samples, providing critical information for diagnosis and treatment. In clinical settings, CT values provide insights into viral load and help guide treatment decisions.<\/p>\n

Calculating CT Values<\/h2>\n

CT values are calculated using real-time PCR software, which meticulously analyzes the fluorescence signal emitted during amplification. The software ensures consistent calculation of CT values by using standardized algorithms. The software sets a fluorescence threshold, and the cycle number at which the reaction crosses this threshold is defined as the CT value.<\/p>\n

The amplification process is often visualized through a graph, showing the relationship between fluorescence units and PCR cycle numbers. This value is inversely proportional to the amount of target nucleic acid present in the sample.<\/p>\n

In simpler terms, lower CT values indicate a higher concentration of viral RNA in the original sample, while high CT values suggest only trace amounts of viral RNA. This relationship helps clinicians and researchers gauge the viral load in a patient sample accurately.<\/p>\n

Factors That Influence CT Measurements in Water Disinfection<\/h2>\n

Several factors impact the accuracy and reliability of CT measurements in water disinfection. Key considerations include:<\/p>\n