1. Math for Water Technology MTH 082 (pg. 468)
C.T Calculation
“Required by Law”
Math for Water Technology
MTH 082
(pg. 468)

2. C.T calculation “C” is the concentration of chlorine,
“T” is the amount of time the water has been in contact with chlorine before the first user.
Calculated daily and compared against the EPA CT “Required” Table
EPA rules states - “Measurements must be taken on the same day of the week, every week, for one year (52 measurements), during peak hourly flow for that day. Data can be measured manually or with on-line instrumentation.”

3. C.T calculation
Surface Water Treatment Rule: all surface water and GWUDI systems require 99.9% (3-log removal) and/or inactivation of Giardia and 99.99% (4-log) removal of viruses through disinfection and treatment
Ground Water Treatment Rule: If a system is fecal positive may be required to achieve inactivation of 99.99% (4-log) removal of viruses through disinfection and treatment

4. C.T Calculation Problems
To calculate the contact time T, the results of a recent tracer study must be used!!
The peak flow must be measured from the effluent side of the clearwell or contact chamber, NOT the flow through the plant.
The chlorine residual, pH, and temperature values used to calculate CT must be measured at the effluent side of the clearwell or contact chamber, or before the first user.

5. CT Calculation Problems
When using the EPA CT tables, make sure you round down for temperature, and round up for pH.
log-reduction must equal 3.0 for Giardia

6. For surface water systems without filtration, the Surface Water Treatment Rule requires public water systems to?
Provide coagulation and filtration
Maintain a C X T value above the minimum value
Continuously sample for total coliforms
Maintain public notification

7. In the application of chlorine for disinfection, which of the following is not normally an operational consideration?
Mixing
Contact time
Dissolved oxygen pH
None of these answers are correct

8. The chlorine residual, pH, and temperature values used to calculate CT must be measured at?
effluent side of the clearwell
effluent side of the contact chamber
before the first user
Any of the above would be ok

9. The C.T calculation must be calculated
Once a year
Once a month
Once a week
Once a day

10. The peak flow must be measured as total flow through the plant?
True
False

11. The final C.T calculation has what units?
Mg/L
Lbs/day
mg-min/L
time

12. What is the acceptable log reduction for Giardia?1 2 3 4

13. What is the acceptable log reduction for viruses?1 2 3 4

14. Four log removal is?
90.00 %
99.00 %
99.90 %
99.99 %

15. If your temperature is 8 degrees with a pH of 7
If your temperature is 8 degrees with a pH of 7.2, you should use the ____ degree page and the _____ pH table (please look at your tables).
5, 19
10, 8.5
5, 7.5
10, 7

16. Disinfection CTs have been established for?
Free chlorine
Chloramine
Chlorine dioxide
Ozone
All of the above

17. Disinfection C.T are impacted by which of the following?
Chlorine residual concentrations (mg/L)
Contact Time (min)
Water Temperature
Water pH
All of the above

18. T =contact time= Detention Time
C.T Calculation
T =contact time= Detention Time
“the length of time water is retained in a vessel or basin ”

19. The units of T in the final CT calculation are ?
Seconds
Minutes
Hours
Days

20. Want T value to be in minutes
Determining T in CT?
Detention time (DT) = volume of tank = MG (1440min)
flow rate MGD (1day)
Want T value to be in minutes
T = volume X Baffle Factor (table)
flow rate

21. Baffle Factor? Poor circulation Baffling efficiency = 5-10%
out
Poor circulation
Baffling efficiency = 5-10% in
out
No circulation
Baffling efficiency = 0% in
out

22. Baffle Factor? No circulation Baffling efficiency = 0%
Good circulation
Baffling efficiency = 30-50% in
out in
out
Superior circulation
Baffling efficiency = 70%

23. Baffle Factor? Perfect circulation Baffling efficiency = 100%
out
Poor circulation/hydropneumatic tank
Baffling efficiency = 10% in
out
No circulation/Bladder type pressure tank
Baffling efficiency = 0%

24. Baffle Factor? Unbaffled Tank 0.1 Poor Baffling 0.3 Average Baffling
0.5
Superior baffling
0.7
Pipe Flow
1.0
“How well the chlorine is in contact with the water”

25. What is the average detention time in a basin that is 1240 ft3 when the flow rate is 4.1 MGD?
3.3 min
.03 min
303 min
435,512 min

26. What is the average detention time in a basin given the following: diameter = 80' depth = 12.2' flow = 5 MGD
132 min
101 min
144 min
105 min

27. Determine T at the effluent side of a superior baffled contact chamber basin that has the following: diameter = 30' depth = 15' flow = 700 gpm
65.8 min.
79.1 min
75 min.
86.1 min.

28. 96.17 min 67.32 min 12.85 min 9 min DRAW: Given: Formula: Solve:
Determine T at the effluent side of a superior baffled contact chamber basin that has the following: Length of = 30ft, width of 20 ft, depth = 15' flow = 700 gpm?
DRAW:
Given:
Formula:
Solve:
tank= L X W X D , Flow rate 700 gpm, BF=0.7
T= DT X BF
T= volume of tank/flow rate X BF
Vt= L X W X D
Vt=30 ft X 20 ft X 15 ft= 9000 ft3
9000 ft3 (7.48 gal/1ft3)= gal
DT=VT/FR X BF
Time = 67,320 gal/700gpm X 0.7
Time= min X 0.7
Time= min in
700 gpm
out
30 ft 20 ft
15 ft
96.17 min
67.32 min
min
9 min

29. Disinfection C•T Requirements
Chlorine residual concentrations (mg/L)
Contact Time (min)
Water Temperature
Water pH
Disinfection power= [residual chlorine] • time of its contact

30. Disinfection C•T Calculation
C•T= [disinfection concentration mg/L] • contact time (min)
C•T units= mg/L• min or mg •min/L
Required C•T
CTrequired is # established by EPA to provide log inactivation. Based on Giardia cysts. Look up in charts
Actual C•T
CTactual is # established by multiplying actual chlorine residual by hydraulic detention time and baffle factor.

31. Disinfection C•T Calculation
C•T actual =[Residual chlorine]•hydraulic detention time•baffle factor (table)
Hydraulic detention time = volume/flow rate
C•Tact/ CTreq= ratio must be > or = to 1.
C•Tact/ CTreq > 1.0
“You want to make sure that you are actually meeting the necessary requirements”

32. Disinfection C•T Rules
Chlorine residuals used for C•T calculations are measured after contact, but before first customer.
2. Contact times are determined by calculating the hydraulic detention time (HDT) as water flows through pipes and tanks. Based on highest flow of day.

33. Disinfection C•T Rules
3. Water flowing through pipes provides contact times that are equal to the calculated HDT. Circular or rectangular tanks used for contact time are given only partial credit due to short circuiting.

34. Removal Credits Requirements Viruses
Table 7.2: Typical Removal Credits and Inactivation Requirements for Various Treatment Technologies
Process
Typical Log
Removal Credits
Resulting
Disinfection Log
Inactivation
Requirements
Giardia
Viruses
Conventional
Treatment
2.5
2.0
0.5
Direct
Filtration
1.0
3.0
Slow Sand
Diatomaceous
Earth Filtration
Unfiltered
4.0
* Systems must demonstrate to the State by pilot study or other means
that the alternative filtration technology provides the required log
removal and inactivation shown in Table 7-1.