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Contact time calculator, United States Environmental Protection Agency
Contact time (Ct) calculations
Identification of suitable site-specific Ct values need to take the following into account:
- Raw water quality, i.e. organic matter, alkalinity, pH, etc.
- Expected variation in temperature, pH and organic matter-seasonal effects, weather events, etc.
- Pathogen profile in raw water
- Extent and performance of treatment before disinfection-organic matter removal
- Design and size of the contact tank
What is log inactivation?
Log inactivation is the number or per cent of microorganisms inactivated through the disinfection process and is a measure of the effectiveness of the disinfection process. It is influenced by variables including disinfectant concentrations, temperature, pH and disinfectant type. For example: a 4 log inactivation values means that 99.99% of microorganisms in raw water have been inactivated by disinfection.
Per cent reduction calculation
Per cent reduction = (A − B) [(A − B) × 100] ÷ A
Where:
A is the number of viable microorganisms before treatment
B is the number of viable microorganisms after treatment
Log reduction calculation
Log reduction (log_{10}) = log_{10} (A ÷ B)
Where:
A is the number of viable microorganisms before treatment
B is the number of viable microorganisms after treatment
Convert log reduction to per cent reduction
P = (1 − 10^{-L}) × 100
Where:
P is the per cent reduction
L is the log reduction
See Table 42 for tabulated values.
Table 42. Relationship of log reduction to percentage reduction
Log reduction | Reduction |
---|---|
1 | 90% |
2 | 99% |
3 | 99.9% |
4 | 99.99% |
5 | 99.999% |
6 | 99.9999% |
What is Ct?
Ct (minutes × mg/L) is a measure of the reaction time of the disinfection process. It is defined as the product of: C, the residual disinfectant concentration in mg/L (at the end drinker) and T, the disinfectant contact time in minutes. See Table 43 (below) for a description of the data required to calculate Ct.
Contact time
Ct = concentration × time
Where:
Concentration is disinfection residual
Time water is in contact with the disinfectant
Table 43. Data required to calculate Ct
Parameter | Unit |
---|---|
Peak hourly flow, Q | litres per minute (Lpm) |
Residual disinfectant concentration, C | mg/L |
Disinfectant type | N/A |
Temperature | °C |
pH | standard units (s.u.) |
Ct and log inactivation calculation overview
Step 1—Calculate detention time
Step 1A—Calculate theoretical detention time (TDT)
TDT = V ÷ Q
Where:
TDT = theoretical detention time (minutes)
V = volume, based on low water level
Q = peak hourly flow
Step 1B—Calculate actual detention time (T)
T = TDT × BF
Where:
T = actual detention time (minutes)
TDT = theoretical detention time (minutes)
BF = baffling factor (a measure of short-circuiting). See Table 44 for typical baffling values.
BF = T_{10} ÷ TDT
Where:
TDT = theoretical detention time (minutes)
T_{10} = the time taken for 10% of the incoming water to exit the tank
Table 44. Typical baffling conditions (The Standards and Guidelines for Municipal Water, Wastewater and Storm Drainage Systems, AENV 1997)
Baffling condition | T_{10}/T ratio | Baffling description |
---|---|---|
Unbaffled (mixed flow) | 0.1 | None, agitated basin, very low length to width ratio, high inlet and outlet flow velocities |
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 |
Step 2—Calculate CtCALC
Ct_{CALC} = C × T
Where:
Ct_{CALC} = concentration time, calculated value (minutes × mg/L)
C = residual disinfectant concentration measured during peak flow (mg/L)
T = actual detention time (minutes)
Step 3—Calculate Ct_{99.9}
Step 3A—Determine Ct required for specific microbe 3 log reduction (Ct_{99.9}) using look-up tables and water treatment plant information.
The Ct required for 3 log inactivation of a specific microbe (designated as Ct_{99.9}) is available in tables for different disinfectants, as shown in Watson et al. (2019). The Ct_{99.9} depends on the species of microorganisms targeted, type of disinfectant, residual disinfectant concentration (C), temperature, and pH.
For example, using Table 45 to look up the Ct for a 3-log reduction (Ct_{99.9}) for Giardia lamblia for water with a 0.8mg/L chlorine residual and a pH of 7.5 at 0.5°C would be 246.
Ct_{CALC} > Ct_{99.9} indicates that the water system is meeting at least 3 log microbe inactivation.
Table 45. Example of Ct table for Giardia lamblia 3 log reduction (Ct_{99.9}) for free chlorine at 0.5°C
Temperature = 0.5°C | |||||||
---|---|---|---|---|---|---|---|
Disinfectant concentration (mg/L) | pH | ||||||
≤6.0 | 6.5 | 7 | 7.5 | 8 | 8.5 | 9 | |
≤0.4 | 137 | 163 | 195 | 237 | 277 | 329 | 390 |
0.6 | 141 | 168 | 200 | 239 | 286 | 342 | 407 |
0.8 | 145 | 172 | 205 | 246 | 295 | 354 | 422 |
1.0 | 148 | 176 | 210 | 253 | 304 | 365 | 437 |
Step 3B—Calculate log inactivation ratio
Log inactivation ratio = Ct_{CALC }÷ Ct_{99.9}
Where:
Ct_{CALC} = concentration time, calculated value (minutes · mg/L)
Ct_{99.9} = concentration time to inactivate 3-log of specific microbe (minutes · mg/L) from table
Example log inactivation calculation
Measured at peak flow:
Peak flow, Q = 250L/min
Free chlorine residual, C = 0.8mg/L
pH = 6s.u.
Temperature = 0.5°C
Cylindrical basin information:
Inner tank radius, r = 6m
Minimum tank water level, d = 9m
No baffling, BF = 0.1
Background – calculate basin volume, V
V = π × d × r^{2}, cylindrical basin volume equation
V = 3.1416 × 9m × (6m)^{2} = 1017.9m^{3}
V = 1017.9m^{3} × 1000L/m^{3}
V = 1,017,870L
Step 1—Calculate detention time
Step 1A—Calculate TDT
TDT = V ÷ Q
TDT = 1,017,870 L ÷ 250L/min
TDT = 4,071.48 minutes
Step 1B—Calculate actual detention time
T = TDT × BF
T = 4,071.48 minutes × 0.1
T = 407.148 minutes
Step 2—Calculate Ct_{CALC}
Ct_{CALC} = C × T
Ct_{CALC} = 0.8mg/L × 407.148 minutes
Ct_{CALC} = 325.72 minutes·mg/L
Step 3—Calculate specific microorganism log inactivation (e.g. Giardia lamblia)
Step 3A—Determine the Ct required for Giardia lamblia 3 log reduction
Determine Giardia lamblia Ct_{99.9} from Ct Table 45 at temperature = 0.5°C, pH = 6s.u., free chlorine residual = 0.8mg/L.
Ct_{99.9} = 145 minutes·mg/L
Step 3B—Calculate Giardia lamblia log inactivation ratio
Giardia lamblia log inactivation ratio = Ct_{CALC} ÷ Ct_{99.9}
Giardia lamblia log inactivation ratio = 325.72 minutes·mg/L ÷145 minutes·mg/L
Giardia lamblia log inactivation ratio = 2.24 ratio
The Giardia lamblia log inactivation ratio is >1. Therefore the water system is meeting at least 3 log virus inactivation for Giardia lamblia.