CT is a measure of the strength of the disinfectant for the time that the water and disinfectant are in contact. CT is determined by multiplying the residual disinfectant concentration (C) by contact time (T). In order to calculate log inactivation for compliance with the surface water treatment rule the CT Achieved and CT Required must be calculated for each disinfection segment in your system.
Calculating CT Achieved For A Segment
EPA rules state - “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.”
To meet EPA compliance you must calculate the CT Achieved for a day. There are 2 methods you can choose:
1. calc the CT Achieved for each hour and report the minimum,
- calc the CT Achieved for the hour that the Peak Hourly Flow rate (for the segment) occurred.
V1: Hourly Flow Rate in MGD
V100: Clearwell Water Depth (Ft.) Hourly Minimum
V101: Clearwell Volume in MG
Method 1: Calculate CT Achieved for each hour:
- Calculate segment volume in MG.
Based on current conditions (i.e. level in tank, number of filters on, etc…)
V101 = V100 * 28.29 * 7.48 / 1000000
Where: 28.29 is Cross-Sectional Area of a 3 foot radius (πr^{2})
7.48 is number of gallons in 1 cubic foot
1000000 is used to convert gallons to MG
- Determine Baffling Factor (T10/T)
Baffling Condition |
T10/T |
Baffling Description |
Unbaffled (mixed flow) |
0.1 |
None, agitated basin, very low length to width ratio, high inlet and outlet flow velocities. Can be approximately achieved in flash mix tank |
Poor |
0.3 |
Single or multiple unbaffled inlets and outlets, no intra-basin
Baffles |
Average |
0.5 |
Baffled inlet or outlet with some intra-basin baffles |
Superior |
0.7 |
Perforated inlet baffle, serpentine or perforated intra-basin
baffles, outlet weir or perforated launders |
Perfect (plug flow) |
1.0 |
Very high length to width ratio (pipeline flow), perforated inlet, outlet, and intra-basin baffles |
- Calculate T10 time in minutes
T10 = Detention Time (Mins) of Segment * Baffling Factor
= ((Segment Vol (MG) / Flow (MGD) ) *24 * 60) * Baffling Factor
V111 = (V101 / V1 * 24 * 60) * 0.3
- Calculate CT Achieved
CT Achieved (mg/L-mins) = CL2 Residual (mg/L) * T10 (Mins)
V115 = V141 * V111
Method 2: Calculate CT Achieved for Peak Hourly Flow Rate hour:
- Calculate segment volume in MG.
Based on current conditions (i.e. level in tank, number of filters on, etc…)
V101 = DDWHEN(V100,V1,”MAX",V1,"",V1,"") * 28.29 * 7.48 / 1000000
Where: DDWHEN(V100,V1,”MAX”,V1,"",V1,"") returns V100 for the hour when the Max Flow (V1) occurred.
28.29 is Cross-Sectional Area of a 3 foot radius (πr^{2})
7.48 is number of gallons in 1 cubic foot
1000000 is used to convert gallons to MG
- Determine Baffling Factor (T10/T)
Baffling Condition |
T10/T |
Baffling Description |
Unbaffled (mixed flow) |
0.1 |
None, agitated basin, very low length to width ratio, high inlet and outlet flow velocities. Can be approximately achieved in flash mix tank |
Poor |
0.3 |
Single or multiple unbaffled inlets and outlets, no intra-basin
Baffles |
Average |
0.5 |
Baffled inlet or outlet with some intra-basin baffles |
Superior |
0.7 |
Perforated inlet baffle, serpentine or perforated intra-basin
baffles, outlet weir or perforated launders |
Perfect
(plug flow) |
1.0 |
Very high length to width ratio (pipeline flow), perforated inlet, outlet, and intra-basin baffles |
*For a clearwell with no baffling the Baffling Factor is 0.1
- Calculate T10 time in minutes
T10 = Detention Time (Mins) of Segment * Baffling Factor
= ((Segment Vol (MG) / Flow (MGD) ) *24 * 60) * Baffling Factor
V111 = (V101 / DDMAX(V1,0,23) * 24 * 60) * 0.1
- Calculate CT Achieved
CT Achieved (mg/L-mins) = CL2 Residual (mg/L) * T10 (Mins)
V115 = V141 * V111
Calculating CT Required for Segment
Determine which CT Required you need to calculate. Depending on type of disinfectant used and what you are trying to remove (Giardi or Viruses) different equations or lookup tables are used. In this example we will calculate the 3 log (AKA CT99.9) inactivation of Giardi and the 4 log (CT99.99) inactivation of Viruses.
Three (3) Log Inactivation of Giarda by Free Chlorine
There are three methods for calculating the CT Required but they all derive from the CT Tables provided in the EPA Guidance Manual Disinfection Profiling and Benchmarking. All three methods use the Water Temperature, pH, and CL2 Residual:
V121: Water Temp in Deg C (Deg C = (Deg F – 32) * 5/9)
V131: pH
V141: Cl2 Residual at the end of the segment
Method 1: Use WIMS Lookup Tables
You can download the lookup tables here. Unzip files to your \HachWIMS\Client\LookupTables folder.
Go to Lookup Table Setup and load the following tables:
0.5 C, 5C, 10C, 15C, 20C, and 25C
CT_Giardia_FC_pt5C_3LOG
CT_Giardia_FC_5C_3LOG
CT_Giardia_FC_10C_3LOG
CT_Giardia_FC_15C_3LOG
CT_Giardia_FC_20C_3LOG
CT_Giardia_FC_25C_3LOG.
Then enter the equation:
V151: CT Required - 3 Log IA of Giardi
IF(V121<5,TABLE("CT_Giardia_FC_pt5C_3LOG",V131,V141,3,0),
IF(V121<10,TABLE("CT_Giardia_FC_5C_3LOG",V131,V141,3,0), IF(V121<15,TABLE("CT_Giardia_FC_10C_3LOG",V131,V141,3,0), IF(V121<20,TABLE("CT_Giardia_FC_15C_3LOG",V131,V141,3,0), IF(V121<25,TABLE("CT_Giardia_FC_20C_3LOG",V131,V141,3,0), TABLE("CT_Giardia_FC_25C_3LOG",V131,V141,3,0))))))
Method 2: Use EPA Regression Equation
This method is calculated using regression equations developed by Smith et al. (1995). The equations can be found in Appendix E of USEPA Disinfection Profiling and Benchmarking Guidance Manual (EPA 815-R-99-013), August 1999.
V151: CT Required - 3 Log IA of Giardi
CT = (0.353*I)(12.006+e^{ }^{(2.46-0.073*temp+0.125*C+0.389*pH)}) (for temperature < 12.5 °C)
CT = (0.361*I)(-2.261+e ^{(2.69-0.065*temp+0.111*C+0.361*pH)}) (for temperature >= 12.5 °C)
Where:
I = 3, the number of logs inactivation required
Temp= temperature in degrees Celsius
C = residual chlorine concentration in mg/L
pH = the negative log concentration of hydrogen ion
e = 2.7183, the base for the natural logarithm
Therefore the Math Toolbox equation would be:
IF(V121<12.5,0.353*3*(12.006+EXP(2.46 -.073*V121+0.125*V141+0.389*V131)), 0.361*3*(-2.261+EXP(2.69-0.065*V121+0.111*V141+0.361*V131)))
Method 3: Use CTR3LGC Math Toolbox and/or Spread Function
This is a method for approximating the CT Tables for 3 Log Inactivation of Giardia Cysts. The CT99.9 calculations for free chlorine use a complex interpolating polynomial which correctly reproduces virtually all of the CT table values for chlorine achieving a coefficient of correlation of .99999. Calculated values vary less than the instruments used to gather the data used in the calculations. The interpolating polynomial for calculating CT99.9 were developed by Russell J. Topham with the Utah Division of Drinking Water as part of his work with water treatment processes under the Surface Water Treatment Rule.
V151: CT Required for 3 Log IA of Giardi
=CTR3LGC (Disinfectant ID, Water Temperature, pH, Cl2 Residual)
=CTR3LGC(1, V121, V131, V141)
Disinfectant ID has one of the following settings:
1 - Free Chlorine
2 - Chlorine Dioxide
3 - Chloramines
4 - Ozone
Four (4) Log Inactivation of Viruses by Free Chlorine
Use the CT_InactivationofViruses_FC lookup table (table C-7 EPA Guidance - Disinfection Profiling and Benchmarking).
V121: Water Temp in Deg C (Deg C = (Deg F – 32) * 5/9)
Method 1: Use WIMS Lookup Tables
You can download the lookup tables here. Unzip files to your \HachWIMS\Client\LookupTables folder.
Go to Lookup Table Setup and load:
CT_InactivationofViruses_FC.
V155: CT Required for 4 Log IA of Viruses
TABLE("CT_InactivationofViruses_FC ",V121,4,3,0)
Calculate Segment Inactivation
- Calculate the CT Ratio for 3 Log of Giardia and 4 log of Viruses.
CT Ratio = CT Achieved / CT Required
V161 = V115/V151
V165 = V115/V155
- Calculate the CT Log Inactivation
Giardia Log Inactivation = 3 * CT Ratio
V171 = 3 * V161
Viruses Log Inactivation = 4 * CT Ratio
V175 = 4 * V165
Calculate the total Inactivation
We now must calculate the Segment Inactivation for all disinfection segments (steps). Then total the Inactivation for all segments to get the total Inactivation:
V901 = C171 + C271 +…
V905 = C175 + C275 + …