Chlorine Contact Time Calculations
Steps to solving a math problem:
a. Read the problem and write down what is “given”.
b. Determine what is being requested with the correct units.
c. Cross out anything “given” that has nothing to do with solving the problem.
d. Select the appropriate formulas & factors.
e. Write down the formula in vertical format.
f. Plug in the “given” numbers into the formula.
g. Make sure units on both sides of the equation match.
h. Convert all units to the same form using conversion factors in vertical format to cancel out like
units in the numerator & denominator.
i. Solve the problem.
Equations:
The operating volume should be taken during peak hour demand but the lowest operating volume
may be substituted with Safe Drinking Water Branch’s approval.
4 : 4 4 !
4 : 4 4
Table 3-2 – Baffling Factors
Factor Description
0.1 None, agitated basis, very low length to width ratio, high inlet
and outlet flow velocities.
Enclosed circular or rectangular tank with single inlet and
outlet line.
Enclosed circular or rectangular tank with inlet on top and
outlet on the bottom, either directly below or on the same side
as the inlet line.
0.3 Single or multiple unbaffled inlets and outlets, no intra-basin
baffles.
Enclosed circular or rectangular tank with inlet on top and
outlet on the bottom on the opposite wall.
0.5 Baffled inlet or outlet with some intra-basin baffles.
0.7 Perforated inlet baffle, serpentine or perforated intra-basin
baffles, outlet weir or perforated launders.
1 Very high length to width ratio (pipeline flow), perforated inlet,
outlet, and intra-basin baffles.
Page 1 CT Calc 2009
Table B-2 – CTreq values for 4-log virus
inactivation of viruses by free chlorine
in mg/L•min
Temperature (°C) pH
6-9 10
0.5 12 90
5 8 60
10 6 45
15 4 30
20 3 22
25 2 15
1. Is the water system providing 4log virus inactivation using 12.5% sodium hypochlorite?
GIVEN Chlorine
Addition
Residual
Chlorine
Measurement
Tank
Well
Booster
Pump
Well capacity = 100 gpm
Residual chlorine concentration = 0.1 mg/L free chlorine, measured at the outlet of the
tank
Tank’s total capacity = 50,000 gallons
Tank’s lowest operating volume = 25,000 gallons
Booster pump capacity = 250 gpm
Tank has no baffles with a separate inlet and outlet. The inlet is located at the top of the
tank and the outlet is located at the bottom of the tank on the opposite wall.
pH = 7.5
Temperature = 20°C
FIND If the water system is providing 4-log virus inactivation.
NOT RELEVANT - The well capacity has no bearing on the peak flow of the water system.
- Using the tank’s total capacity will increase the results and not cover the most
conservative situation when the tank is at its lowest operating volume.
SOLUTION
From Table 3-2, Baffling factor = 0.3.
From Table B-2, CT req = 3 ·
Peak Flow = 250 gpm
Page 2 CT Calc 2009
25,000
250
100
100 0.3
30
0.1 30
3 ·
3 ·
3 ·
4 4
4 1 4
4 4
4 4, ,4 ! Answer: YES
Page 3 CT Calc 2009
2. Is the water system providing 4log virus inactivation using 12.5% sodium hypochlorite?
GIVEN
Residual
Chlorine Chlorine
Addition Measurement
Tank
Well To Distribution System
Well capacity = 600 gpm
Residual chlorine concentration = 0.1 mg/L free chlorine, measured at the outlet of the
tank
Tank’s total capacity = 600,000 gallons
Tank’s lowest operating volume = 200,000 gallons
Peak hour flow = 1,200 gpm
Tank has no baffles with a separate inlet and outlet with high inlet and outlet flow
velocities.
pH = 6.8
Temperature = 25°C
FIND If the water system is providing 4-log virus inactivation.
NOT RELEVANT - The well capacity has no bearing on the peak flow of the water system.
- Using the tank’s total capacity will increase the results and not cover the most
conservative situation when the tank is at its lowest operating volume.
SOLUTION
From Table 3-2, Baffling factor = 0.1
From Table B-2, CT req = 2 ·
Peak flow = 1,200 gpm
200,000
1,200
166.67
166.67 0.1
16.67
Page 4 CT Calc 2009
0.1 16.67
1.67 ·
1.67 ·
2 ·
0.84
Round to 1 decimal place.
0.8
4 4
4 0.8 4
4 3.6
4 4, ,4 !
Answer: NO
Page 5 CT Calc 2009
3. Is the water system providing 4log virus inactivation using 12.5% sodium hypochlorite?
GIVEN
Residual
Chlorine
Chlorine Measurement
Well
First
Customer
Well capacity = 200 gpm
Residual chlorine concentration = 0.1 mg/L free chlorine, measured before the first
customer
Pipeline has a diameter of 12 inches and a length of 1,000 feet between the chlorine
injection point and the first customer.
Peak hour flow = 200 gpm
pH = 6.6
Temperature = 20°C
FIND If the water system is providing 4-log virus inactivation.
NOT RELEVANT - The well capacity has no bearing on the peak flow of the water system.
SOLUTION
From Table 3-2, Baffling factor = 1
From Table B-2, CT req = 3 ·
Peak flow = 200 gpm
12 1
1 7.48
1
4
0.785
0.785 1,000 7.48
5,871.8
Page 6 CT Calc 2009
5,871.8
200
29.36
29.36 1
29.36
0.1 29.36
2.94 ·
2.94 ·
3 ·
0.98
Round to one decimal place
1.0
4 4
4 1.0 4
4 4.0
4 4, ,4 !
Answer: YES
Page 7 CT Calc 2009
