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“What is a CT Value?”: The (Simple) Math Behind Water Disinfection

Pictured: The top of a WellMate contact tank. Contact tanks are used to ensure proper contact time between water and a disinfectant.
Pictured: The top of a WellMate contact tank. Contact tanks are used to ensure proper contact time between water and a disinfectant.

Disinfection of your water is crucial to ensuring that your water 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.  All of these methods, however, require sufficient contact time between the water and the disinfectant.  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.

CT Calculation Formula

See the formula below for calculating CT when using chlorine:

CT = Concentration of free chlorine (C mg/L) * contact time (T Minutes)

where

Free chlorine = Concentration measured in milligrams per liter (mg/L)
Cutaway of a retention tank, used to increase contact time
Cutaway of a retention tank, used to increase contact time (click to enlarge)

Since different contaminants require different CT values for complete disinfection, it’s 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.

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

U.S. EPA CT VALUES (mg/L x min) for the Inactivation of Giardia Cysts with Ozone at Different Temperatures and pH from 6 to 9. Temperature °C (°F)

Inactivation 0.5(32.1) 5(41) 10(50) 15(59) 20(68) 25(77)
0.5 log 0.48 0.32 0.23 0.16 0.12 0.08
1.0 log 0.97 0.63 0.48 0.32 0.24 0.16
1.5 log 1.50 0.95 0.72 0.48 0.36 0.24
2.0 log 1.90 1.30 0.95 0.63 0.48 0.32
2.5 log 2.40 1.60 1.20 0.79 0.60 0.40
3.0 log 2.90 1.90 1.40 0.95 0.72 0.48

CT Values for Inactivation of Viruses by Ozone Temperature, °C

Inactivation <1

5

10

15

20

25
2‐ log 0.48 0.32 0.23 0.16 0.12 0.08
3‐ log 0.97 0.63 0.48 0.32 0.24 0.16
4‐ log 1.50 0.95 0.72 0.48 0.36 0.24

CT Values for 3‐log (99.9%) Inactivation of Giardia Cysts by Free Chlorine at Water Temperature 10.0 °C (50°F)

Free Residual, Mg/L

pH < 6.0

pH 6.5

pH 7.0

pH 7.5

pH 8.0

pH 8.5

pH < 9.0

< 0.4

73

88

104

125

149

177

209

0.6

75

90

107

128

153

183

218

0.8

78

92

110

131

158

189

226

1.0

79

94

112

134

162

195

234

1.2

80

95

114

137

168

200

240

1.4

82

98

116

140

170

206

247

1.6

83

99

119

144

174

211

253

1.8

88

101

122

147

179

215

259

2.0

87

104

124

150

182

221

265

2.2

89

105

127

153

186

225

271

2.4

90

107

129

157

190

230

276

2.6

92

110

131

160

194

234

281

2.8

93

111

134

163

197

239

287

3.0

95

113

137

166

201

243

292

Now you’re ready to start disinfecting.  Want to read more about CT values and contact tanks?  Download our free guide here below:

 

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