Calculating CT Disinfection

How effective a chemical disinfectant is in eliminating pathogens is based on the:

  • residual concentration of the chemical disinfectant (chlorine, ozone, chlorine dioxide, chloramines, etc.)
  • water temperature
  • pH
  • contact time (the time that the given disinfectant residual is held before the first service connection as discussed below).

This relationship is commonly referred to as the CT concept. CT is the product of the residual concentration of the disinfectant (C) measured in mg/L and the disinfectant contact time (T) measured in minutes.

CT = C x T

= Concentration (mg/L) of the disinfectant x Time (the contact time in minutes)

To account for disinfectant decay, the residual concentration “C” is usually determined at the exit (outlet) of the chemical contact chamber rather than using the initial concentration at the point of injection/addition.

For the purpose of pathogen log reduction credit assignment, worst-case scenario operating conditions should be considered. Worst-case operating conditions reflect the most challenging conditions for a water system. To determine the effective CT value, it is recommended that the following worst-case operating conditions be used: 

  • the peak hourly flow rate
  • the minimum disinfectant residual (e.g., Free Chlorine) in the water measured at the outlet of the chemical contact chamber (not measured at the point of injection/addition)
  • the minimum temperature of the water during disinfection (i.e. worst case)
  • the maximum pH of the water during disinfection (i.e. worst case)
  • minimum water depth in reservoirs (i.e. low-level alarm depth)
  • the baffling factor for the chlorine contact tank

Where appropriate, CT values can be calculated for each process step of the treatment train and the values summed. Calculations should be based on the residual concentration of disinfectant at the end of each process step.

For a water system using surface water or groundwater at risk of containing pathogens (GARP) sources, the CT must provide both:

  • a minimum of 4-log virus reduction
  • a minimum 3-log protozoan (Giardia cysts and Cryptosporidium oocysts) reduction

The BC Guidelines for Pathogen Log Reduction Credit Assignment of the BC Drinking Water Officers’ Guide (DWOG) provides more information on how to calculate CT and it provides tables with CT values for the inactivation of viruses, Giardia cysts and Cryptosporidium oocysts. It provides CT values for systems using free chlorine, chlorine dioxide, ozone, and chloramines for primary disinfection. The majority of surface water systems in BC use free chlorine for primary disinfection and the calculation of CT. This Guideline also provides:

  • CT factor calculations based on examples using different types of source water (surface water, GARP wells, etc.) 
  • information regarding the determination of “CT Calculated” and “CT Required”

Health Canada’s Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Chlorine (section 4.5.2 – Primary Disinfection) provides more information on CT disinfection.

The first step in calculating CT is to determine the volume of water (V) in the unit process, measured in cubic metres (m3). Clear wells, reservoirs and contact pipes are typically used to provide disinfectant contact time and the contact volume depends on the geometry of the contact chamber. 

Volume of a Rectangular contact chamber or reservoir:

The volume (V) of water (m3) is calculated by multiplying the chamber’s internal length (l) by the internal width (w) by the minimum water depth (d).

Volume (V) of a rectangular chamber/reservoir = l x w x d

Volume of a Cylindrical contact chamber or pipe:

The volume (V) of water (m3) is calculated by multiplying pi (π = 3.1416) by the internal chamber/pipe radius squared (r2) by the minimum water depth (d) of the chamber/reservoir. For the volume of a linear pipe, substitute the length (l) for the water depth (d).

Volume (V) of a cylindrical chamber/reservoir:  V = π r2 x d

Volume (V) of a length of pipe:  V = π r 2 x l

The contact time “T” is typically calculated using a T10 value, which is defined as the length of time during which 10% of the influent water passes through the contact chamber. Using T10 in CT calculations ensures that 90% of the treated water meets or exceeds the contact time. The T10 value is usually estimated based on the geometry and flow conditions (baffling) of the contact chamber, basin or reservoir. As ozone reactions occur quickly in water, T10 calculations are not always appropriate to assess contact time when ozone is used as the disinfectant.

The theoretical detention time (T) measured in minutes, is calculated as the volume (V) of water in the contact chamber measured in cubic metres (m3) divided by the flow rate (Q) of water through the chamber measured in m3/minute.

For the purpose of calculating CT under worst-case operating conditions, the peak hourly flow rate should be used.

T = V/Q = Volume (m3)/Flow rate (m3/minute)

T10 is calculated as the theoretical detention time (T) of water in the contact chamber measured in minutes, multiplied by the baffling factor (T10/T). 

T10 = T x BF = Theoretical Detention Time (minutes) x Baffling Factor (T10/T).

The baffling factor is used to adjust the theoretical detention time to a more realistic value of CT. The baffling factor is also known as the short-circuiting factor. The baffling factor for a particular contact chamber can be estimated based on the configuration (geometry) of the chamber and the degree of short-circuiting. Typical baffling factors are set out in Table 1 below.

Table 1:  Baffling Factors1

Baffling Factor


Baffling Condition

Baffling Description



No baffles, agitated basin, very low length-to-width ratio, high inlet and outlet flow velocities, inlet and outlet at the same level



No baffles, agitated basin, very low length-to-width ratio, high inlet and outlet flow velocities, inlet high and outlet low or vice versa



Single or multiple unbaffled inlets and outlets, no intra-basin

baffles, vertical perforated pipe for an inlet and/or outlet



Baffled inlet or outlet, vertical perforated pipe for an inlet or

outlet, with some intra-basin baffles



Perforated inlet baffle, perforated intra-basin baffles, outlet

weirs or perforated launders



Serpentine baffling throughout



Pipeline flow, very high length-to-width ratio (≥160:1) with

turbulent flow

1 Source:  Table 3: Baffling Factors of the Guidelines for Pathogen Log Reduction Credit Assignment of the BC Drinking Water Officers’ Guide (2022)