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## C . T Calculation

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### T =contact time= Detention Time

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.”

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.

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

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

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

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

The C.T calculation must be calculated

- Once a year
- Once a month
- Once a week
- Once a day

The peak flow must be measured as total flow through the plant?

- True
- False

The final C.T calculation has what units?

- Mg/L
- Lbs/day
- mg-min/L
- time

Four log removal is?

- 90.00 %
- 99.00 %
- 99.90 %
- 99.99 %

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

Disinfection CTs have been established for?

- Free chlorine
- Chloramine
- Chlorine dioxide
- Ozone
- All of the above

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

C.T Calculation

“the length of time water is retained in a vessel or basin”

The units of T in the final CT calculation are ?

- Seconds
- Minutes
- Hours
- Days

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

Baffle Factor?

Poor circulation

Baffling efficiency = 5-10%

out

in

Poor circulation

Baffling efficiency = 5-10%

out

in

No circulation

Baffling efficiency = 0%

out

in

out

Baffle Factor?No circulation

Baffling efficiency = 0%

in

Good circulation

Baffling efficiency = 30-50%

out

in

Superior circulation

Baffling efficiency = 70%

Baffle Factor?

Perfect circulation

Baffling efficiency = 100%

in

out

Poor circulation/hydropneumatic tank

Baffling efficiency = 10%

in

out

No circulation/Bladder type pressure tank

Baffling efficiency = 0%

Baffle Factor?

“How well the chlorine is in contact with the water”

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

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

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.

20

ft

15 ft

in

700 gpm

out

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:

- 96.17 min
- 67.32 min
- 12.85 min
- 9 min

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)= 67320 gal

DT=VT/FR X BF

Time = 67,320 gal/700gpm X 0.7

Time= 96.17 min X 0.7

Time= 67.32 min

Disinfection C•T Requirements

Chlorine residual concentrations (mg/L)

Contact Time (min)

Water Temperature

Water pH

Disinfection power= [residual chlorine] • time of its contact

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

CTrequiredis # established by EPA to provide log inactivation. Based on Giardia cysts. Look up in charts

Actual C•T

CTactualis # established by multiplying actual chlorine residual by hydraulic detention time and baffle factor.

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”

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.

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.

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