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1.2: Contact Time Calculations

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    Learning Objectives

    • Outline disinfection process and system requirements
    • Define CT concept
    • Calculate flow rates and detention time
    • Calculate CT value

    CT Concept

    Disinfection is a key component of the multi-barrier approach to providing safe drinking water.

    Water treatment can be broken into two phases:

    • The water treatment process
    • Disinfection

    CT is defined as the disinfectant residual concentration (i.e. C) multiplied by the effective contact time (i.e. T). The corresponding unit is mg/L-min. The effective contact time T is also known as T10. T10 refers to the effective contact time, which is the time it takes 10% of the volume of a unit to pass through that unit and T is referred to as theoretical detection time. In completing CT calculations, the following operating or design conditions must be applied to determine the effective contact time provided at a water treatment plant:

    • The peak hourly flow rate (typically the pump peak flow)
    • Minimum normal operating level of the storage reservoir, clear well or tank
    • The baffling factor for the chlorine contact tank
    • Minimum disinfectant residual measured at the end of each disinfection segment, or the minimum disinfectant residual allowed in the Permit to Operate
    • Minimum temperature of the water undergoing disinfection
    • Maximum pH of the water undergoing

    The baffling factor (BF) of a contact tank is used to adjust the theoretical detention time to a more realistic value of the T and reduces the effective storage volume to account for potential short-circuiting. It is expressed as T10/T, where T10 refers to the effective contact time, which is the time it takes 10% of the volume of a unit to pass through that unit and T is referred to as theoretical detection time. A reliable and accurate method to determine the BF (T10/T) of a disinfection system is through the use of a tracer study or computational fluid dynamics modeling.

    For daily operation at facilities where CT calculation is an on-going operating requirement, CT may be calculated using actual values for reservoir volume, flow, temperature, pH, chlorine residual, and other required factors. However, for design purposes, conservative CT calculation must be used to determine if the system will meet CT requirements at all times. For a seasonal operation system, CT calculation for winter and summer conditions may be required to cover the worst-case scenario.

    In a case where more than one disinfectant is used, or where there are multiple disinfectant injection points, disinfection segments should be identified. In the case of multiple disinfection segments, the CT calculation is performed for each individual disinfection segment and then summed to get for the total CT value for the entire system.

    CT Values are an important part of calculating the disinfectant dosage for the chlorination of drinking water. The goal of disinfection is the inactivation of microorganisms. Inactivation depends on:

    • The microorganism
    • The disinfectant being used
    • The concentration of the disinfectant
    • The contact time
    • The temperature
    • The pH of the water

    Disinfection Process

    Disinfection destroys harmful organisms. Disinfection can be accomplished physically or chemically. Physical methods include:

    • Physically removing the organisms from the water
    • Introducing motion that will disrupt the cells’ biological activity and kill or inactivate them

    Chemical methods alter the cell chemistry causing the microorganism to die. The most widely used disinfectant chemical is chlorine. Chlorine is easily obtained, relatively inexpensive, and most importantly, leaves residual chlorine that can be measured. Other disinfectants are also used. Presently, an interest in disinfectants other than chlorine exists because of the carcinogenic compounds that chlorine can form (THMs).

    Physical Means of Disinfection

    • Ultraviolet rays can be used to destroy pathogenic microorganisms. To be effective, the rays must come in contact with each microorganism. The ultraviolet energy disrupts various organic components of the cell causing a biological change that is fatal to the microorganism. This system has not had widespread acceptance because of the lack of a measurable residual and the cost of operation. Currently, the use of ultraviolet rays is limited to small or local systems and industrial applications. Oceangoing ships use these systems for their water supply. Advances in UV technology and concern about disinfection byproducts produced by other disinfectants have prompted a renewed interest in UV disinfection.
    • Heat has been used for centuries to disinfect water. Boiling water for about 5 minutes will destroy essentially all microorganisms. This method is energy-intensive and thus expensive. However, it is the only practical treatment process for disinfection in the event of a disaster when individual local users are required to boil their water.
    • Ultrasonic waves have been used to disinfect water on a limited scale. Sonic waves destroy microorganisms by vibration. This procedure is not yet practical and is expensive.

    Chemical Disinfectants Other Than Chlorine

    • Iodine has been used as a disinfectant in water, but its use has been limited to emergency treatment of water supplies. Although it has long been recognized as a good disinfectant, iodine’s high cost and potential physiological effects on pregnant women has prevented widespread acceptance. The recommended dosage is two drops of iodine (7% available iodine) in a liter of water.
    • Bromine has been used only on a very limited scale for water treatment because of its handling difficulties. Bromine causes skin burns on contact. Because bromine is a very reactive chemical, residuals are hard to obtain. This lack of a measurable residual also limits its use. Bromine can be purchased at swimming pool supply stores.
    • Bases, such as sodium hydroxide and lime, can be effective disinfectants but the high pH leaves a bitter taste in the finished water. Bases can also cause skin burns when left too long in contact with the skin. Bases effectively kill all microorganisms (sterilize). Although this method has not been used on a large scale, bases have been used to sterilize water pipes.
    • Ozone, in the United States, has been used for taste and odor control. The limited use has been due to its high costs, lack of residual, difficulty in storing, and maintenance requirements. Although ozone is effective in disinfecting water, its use is limited by its solubility. The temperature and pressure of the water being treated regulate the amount of ozone that can be dissolved in the water. These factors tend to limit the disinfectant strength that can be made available to treat water. Many scientists claim that ozone destroys all microorganisms. Unfortunately, significant residual ozone does not guarantee that treated water is safe to drink. Organic solids may protect organisms from the disinfecting action and increase the amount of ozone needed for the disinfection process. In addition, ozone residuals cannot be maintained in metallic conduits for any period of time because of ozone’s reactive nature. The inability of ozone to provide a residual in the distribution system is a major drawback to its use. However, recent information concerning the formation of THMs by chlorine compounds has resulted in a renewed interest in ozone as an alternative means of disinfection.


    Chlorine is a greenish-yellow gas with a penetrating and distinctive odor. The gas is two-and-a-half times heavier than air. Chlorine has a high coefficient of expansion. If the temperature increases by 50 F, the volume will increase 84 to 89 percent. This expansion could easily rupture a cylinder or a line full of liquid chlorine. For this reason, no chlorine containers should be filled to more than 85 percent of their capacity. One liter of liquid chlorine can evaporate and produce 450 liters of chlorine gas.

    Chlorine is nonflammable and nonexplosive, but it will support combustion. When the temperature rises, so does the vapor pressure of chlorine. When the temperature increase, the chlorine gas inside a chlorine container will increase. This property of chlorine must be considered when:

    • Feeding chlorine gas from a container
    • Dealing with a leaking chlorine cylinder

    Chlorine Disinfection Action

    The exact mechanism of chlorine disinfection action is not fully known. One theory holds that chlorine exerts a direct action against the bacterial cell; thus destroying it. Another theory is that the toxic character of chlorine inactivates the cell’s enzymes, which enable living microorganisms to use their food supply. As a result, the organisms die of starvation. From the point of view of water treatment, the exact mechanism of chlorine disinfection is less important than its demonstrated effects as a disinfectant.

    When chlorine is added to water, several chemical reactions take place. Some involve the molecules of water, and some involve organic and inorganic substances suspended in the water. Water combines with inorganic and organic materials to form chlorine compounds. If chlorine is continued to be added to the water, eventually a point exists where the reactions with organic and inorganic materials stop. At this point, the chlorine demand has been satisfied.

    When the amount of chlorine needed to satisfy the chlorine demand and the amount of chlorine residual needed for disinfection is added, the chlorine dose is computed. The chlorine dose is the amount of chlorine that has to be added to the water to disinfect it.

    • Cldose , mg/L = Cldemand , mg/L + Clresidual , mg/L where...
      • Cldemand , mg/L = Cldose , mg/L - Clresidual , mg/L and...
      • Clresidual , mg/L = Cldose , mg/L - Cldemand , mg/L
      • Clcombined = Combined Chlorine forms
      • Clfree = Free Chlorine

    Chlorine Reactions with Water

    Free chlorine combines with water to form hypochlorous and hydrochloric acid. In solutions that are dilute and have a pH above 4, the formation of HOCl (hypochlorous acid) is almost complete and leaves little free chlorine.

    • Chlorine + Water Hypochlorous Acid + Hydrochloric Acid

    Depending on the pH, some hypochlorous acid will disassociate and produce a hydrogen ion and a hypochlorite ion. Hypochlorous acid is a weak acid and is poorly dissociated at pH levels below 6. Below pH 6, the free chlorine is almost all in the hypochlorous form. Above pH 9, almost all of the free chlorine is in the hypochlorite form and almost none is in the hypochlorous form.

    • Hypochlorous Acid Hydrogen ion + Hypochlorite

    Normally, in water with a pH of 7.5, approximately 50 percent of the chlorine present will be in the hypochlorous form and 50 percent will be in the form of hypochlorite. This phenomenon is important since hypochlorous and hypochlorite differ in their respective disinfection ability, hypochlorous has a much greater disinfection potential than hypochlorite.

    Chlorine Reactions with Impurities in Water

    Most water contains some impurities. The more common impurities that react with chorine that effects the disinfection ability of chlorine are:

    • Hydrogen sulfide and ammonia are two inorganic substances that may be found in water when it reaches the disinfection stage of treatment. Their presence can complicate the use of chlorine for disinfection purposes. Hydrogen sulfide and ammonia are reducing agents, and they give up electrons easily. Chlorine reacts rapidly with these particular reducing agents producing some undesirable results. Hydrogen sulfide produces an odor that smells like rotten eggs. It reacts with chlorine to form sulfuric acid and elemental sulfur (depending on temperature, pH, and hydrogen sulfide concentration). Elemental sulfur is objectionable because it can cause odor problems and will precipitate as finely divided white particles that are sometimes colloidal in nature. The chemical reaction between hydrogen sulfide and chlorine is:
      • Hydrogen Sulfide + Chlorine +Oxygen Ion Elemental Sulfur + Water + Chloride Ions
      • The chlorine required to oxidize hydrogen sulfide to sulfur and water is 2.08 mg/L chlorine to 1 mg/L hydrogen sulfide. The complete oxidation of hydrogen sulfide to the sulfate form is:
        • Hydrogen Sulfide + Chlorine + Water Sulfuric Acid + Hydrochloric Acid
      • When chlorine is added to water containing ammonia, it reacts rapidly with the ammonia and forms chloramines. Therefore, less chlorine is available to act as a disinfectant. As the concentration of ammonia increases, the disinfectant power of the chorine drops off at a rapid rate.
    • When organic materials are present in water, being disinfected with chlorine, the chemical reaction that takes place may produce suspected carcinogenic compounds. The formation of these compounds can be prevented by limiting the amount of prechlorination and by removing the organic materials before chlorination of the water.


    The use of hypochlorite to treat potable water achieves the same result as chlorine gas. Hypochlorite may be applied in the form of calcium hypochlorite or sodium hypochlorite. The form of calcium hypochlorite most frequently used to disinfect water is known as High Test Hypochlorite.

    • Calcium Hypochlorite + Water Hydrochlorous Acid + Calcium Hydroxide
    • Sodium Hypochlorite + Water Hydrochlorous Acid = Sodium Hydroxide

    In systems, where calcium hypochlorite is used a problem occurs when sodium fluoride is injected at the same point as the hypochlorite. A server crust forms when the calcium and fluoride ions combine.

    Differences between Chlorine Gas and Hypochlorite Compound Reactions

    The only difference between the reactions of hypochlorite compounds and chlorine gas is the side reactions of the end products. The reaction of chlorine gas tends to lower the pH by the formation of hydrochloric acid, which favors the formation of hypochlorous acid. The hypochlorite tends to raise the pH with the formation of the hydroxyl ions from the calcium or sodium hydroxide. At a high pH of around 8.5 or higher, the hypochlorous acid is almost completely dissociated to the ineffective hypochlorite ion. This reaction also depends on the buffer capacity of the water.

    Onsite Chlorine Generation

    Small water systems are generating chlorine on site for their water treatment processes. Onsite generation of chlorine is attractive due to the lower safety hazards and costs involved. Onsite generated chlorine systems produce 0.8 percent sodium hypochlorite. This solution strength is below the lower limit deemed a hazardous liquid, with obvious economic and safety advantages.

    The operator’s only duties with onsite generation systems are to observe the control panel daily for proper operating guidelines and to dump bags of salt every few weeks. Since the assemblies include an ion exchange water softener, mineral deposits forming with the electrolytic cell are minimal, with an acid cleaning being necessary only every few months. Cell voltage is controlled at a low value to maximize electrode life, which is about 3 years. Process brine strength and cell current determine chlorine production at the anode, while hydrogen gas is continually vented from the cathode. The units include provisions for storing the chlorine solution to deliver chlorine for several days in the event of a power failure or other problems causing equipment failure.

    Breakpoint Chlorination

    In determining how much chlorine the operator will need for disinfection, operators must remember a certain chlorine residual in the form of a free available chlorine residual is the goal. Chlorine in this form has the highest disinfecting ability. Breakpoint chlorination is the name of this process of adding chlorine to water until the chlorine demand has been satisfied. Further additions of chlorine will result in a chlorine residual that is directly proportional to the amount of chlorine added beyond the breakpoint. Public water supplies are normally chlorinated past the breakpoint.

    Assume the water being chlorinated contains some manganese, iron, nitrite, organic matter, and ammonia. When a small amount of chlorine is added, the chlorine reacts with (oxidizes) the manganese, iron, and nitrite. No disinfection and no chlorine residual occurs. When additional chlorine is added, enough to react with the organics and ammonia, chlororganics and chloramines form. The chloramines produce a combined available chlorine residual which is chlorine combined with other substances, mainly ammonia. Combined residuals have poor disinfecting power and may cause tastes and odors.

    By adding more chlorine, the chloramines and some of the chlororganics are destroyed, which results in a drop in combined chlorine residual. When all of the chloramines are gone, adding more chlorine produces free available residual chlorine which is free in the sense that it has not reacted with anything and is available in that it can and will react if needed. Free available residual chlorine is the best residual for disinfection. It disinfects faster and without the swimming pool odor of combined residual chlorine. The point at which the chlorine residual curve bottoms out is called the breakpoint, and chlorination beyond this point is called breakpoint chlorination. In water treatment plants today it is common practice to go past the breakpoint. This process means that the treated water will have a very effective disinfectant because it is in the form of free available residual chlorine.

    Graph of breakpoint chlorination - text description follows image.
    Figure \(\PageIndex{1}\): Breakpoint Chlorination Curve – Image by State of New South Wales NSW Ministry of Health is licensed under CC BY

    CT Values

    The purpose of the SWTR is to ensure that pathogenic organisms are removed or inactivated by the treatment process. To meet this goal, all systems are required to disinfect their water supplies. For some water systems using very clean source water and meeting the other criteria to avoid filtration, disinfection alone can achieve the 3-log (99.9-percent) Giardia and 4-log (99.99-percent) virus inactivation levels required by the Surface Water Treatment Rule.

    Several methods of disinfection are in common use, including free chlorination, chloramination, use of chlorine dioxide, and application of ozone. The concentration of chemical needed and the length of contact time needed to ensure disinfection are different for each disinfectant. Therefore, the effectiveness of the disinfectant is measured by the time (t) in minutes of the disinfectant’s contact in the water and the concentration (C) of the disinfectant residual in mg/L measured at the end of the contact time. The product of these two factors (C × t) provides a measure of the degree of pathogenic inactivation.

    The required CT value to achieve pathogenic inactivation is dependent upon the organism in question, type of disinfectant, pH, and temperature of the water supply. Time or T is measured from point of application to the point where C is determined. T must be based on peak hour flow rate conditions. In pipelines, T is calculated by dividing the volume of the pipeline in gallons by the flow rate in gallons per minute (GPM). In reservoirs and basins, dye tracer tests must be used to determine T. In this case, T is the time it takes for 10 percent of the tracer to pass the measuring point.

    A properly operated filtration system can achieve limited removal or inactivation of microorganisms. For this reason, systems that are required to filter their water are permitted to apply a factor that represents the microorganism removal value of filtration when calculating CT values to meet the disinfection requirements.

    The factor (removal credit) varies with the type of filtration system. Its purpose is to take into account the combined effect of disinfection and filtration in meeting the SWTR microbial standards.

    The effectiveness of disinfection is demonstrated through the concept of contact time (CT), which is defined as a product of a disinfectant residual concentration(C), in mg/L and the effective disinfectant contact time (T), in minutes.

    • CT Disinfection demonstrates that the required disinfection is being achieved.
    • CT Disinfection is a straightforward three-step process. These steps include:
      • Determine how much CT is need
      • Determine how much CT is achieved
      • Ensure CT achieved is more than CT required

    The CT value is developed to relate the levels of inactivation under different operational conditions. For true groundwater systems, a CT value must be achieved that provides a minimum of a 4-log virus reduction/inactivation; while all surface water or Groundwater Under Direct Influence (GUDI) systems, a CT value must be achieved that provides a minimum of a 0.5 log Giardia and 2-log virus reduction/inactivation. Depending on the treatment process, additional Crypto and Giardia removal/inactivation may be required for a surface water source. Significant deterioration of water quality may require further removal or inactivation of viruses, Cryptosporidium, and Giardia.

    CT is simply the concentration of chlorine in water times the time of contact that the chlorine has with the water before the first customer tap.


    If a water system is providing water to a community, and if the well capacity is 100 gpm, the residual chlorine concentration is 0.1 mg/L free chlorine measured at the tank, the pump capacity is 250 gpm, and no baffling is provided except that the inlet to the storage tank is located at the top and the outlet is located at the bottom on the opposite wall, the pH is 7.5, the tank capacity is 50,000 gallons and the lowest operating volume is 25,000 gallons, and the water temperature is 20oC, then what is the CT value?

    Equations: Total Detention Time = Lowest Operating Volume/Peak Flow

    The operating volume should be taken during peak hour demand.


    The well capacity is irrelevant in the example because it has no bearing on the peak flow of the water system. However, the pump's peak flow of 250 gpm does have an effect on the CT value, and the rule describes that the lowest operating volume must be used in CT calculation.

    • Peak Flow = 250 gpm
    • Baffling Factor = 0.3 (from Baffling Factor Table)
    • Lowest operating tank volume = 25,000 gallons
    • CT required from table = 3 mg/L-min
    • Total Detention Time = Lowest Operating Volume/Peak Flow = 25,000 gallons/250 gpm
    • Total Detention Time = 100 minutes
    • Contact Time = Total Detention Time x Baffling Factor (from Baffling factor table) = 100 minutes x 0.3
    • Contact Time = 30 minutes
    • CT calculation = Residual Chlorine Concentration x Contact Time = 0.1 mg/L x 30 minutes
    • CT calculation = 3 mg/L / minute
    • Inactivation Ratio = Inactivation Ratio = CT cal /CT req = 3 mg/L-min / 3 mg/L -min
    • Inactivation Ratio = 1

    All surface and groundwater under the influence of surface water systems should be using CT.

    Disinfection to demonstrate that you are achieving sufficient water treatment to inactivate protozoa and viruses. Disinfection with free chlorine that inactivates protozoa will usually provide enough CT to kill viruses as well.

    Practicing CT Disinfection is the safe thing to do. CT Disinfection is the disinfection standard. CT Disinfection is also the water treatment industry standard for disinfection. It is the best method to ensure that the water you are providing your customers is safe. If your water has been exposed to the surface, it has also been exposed to surface contamination, whether from livestock, wild animals or other human activity.

    In water treatment practice, tables of the product C × t are used to calculate disinfection dosages. These tables express the required CT values to achieve the desired removal of microorganisms of interest in drinking water, such as Giardia lamblia cysts, for a given disinfectant under constant temperature and pH conditions.

    The disinfection of water is crucial to ensuring that the water is safe to drink and free of harmful bacteria and other organisms. The primary methods of disinfection are chlorination, hydrogen peroxide injection, ozone, and UV light. These methods, however, require sufficient contact time between the water and the disinfectant. Proper contact time must also be complemented by the correct dosage of a disinfectant; multiplying the concentration of the disinfectant by the time of contact with the water will provide a CT value.

    Review Questions

    1. Describe the disinfection process using chlorine.
    2. Define CT concept.
    3. What is the detention time in hours for a sedimentation basin that contains 240,000 gallons with flow into the basin being 1,700 gpm?
    4. What is the flow rate if the detention time is 2 hours and the basin contains 100,000 gallons of water?
    5. If a water system is providing water to a community, and if the well capacity is 200 gpm, the residual chlorine concentration is 0.5 mg/L free chlorine measured at the tank, the pump capacity is 300 gpm, and no baffling is provided, the pH is 7.5, the tank capacity is 25,000 gallons and the lowest operating volume is 20,000 gallons, and the water temperature is 20oC, then what is the Contact Time value (CT value)?

    Test Questions

    1. ________is the product of the concentration of a disinfectant and the contact time with the water being disinfected. It is typically expressed in units of mg-min/L. The goal of disinfection is the inactivation of microorganisms.
      1. Free chlorine residual
      2. Chlorine demand
      3. CT value
      4. Breakpoint chlorination
    2. ____________ is the name of this process of adding chlorine to water until the chlorine demand has been satisfied. Further additions of chlorine will result in a chlorine residual that is directly proportional to the amount of chorine.
      1. Free chlorine residual
      2. Chlorine demand
      3. CT value
      4. Breakpoint chlorination
    3. _________is a key component to the multi-barrier approach to providing safe drinking water.
      1. Free chlorine residual
      2. Chlorine demand
      3. CT value
      4. Chlorination
    4. Which of the following is not a physical means of disinfection?
      1. Ultraviolet rays
      2. Heat
      3. Bases
      4. Ultrasonic waves
    5. Which of the following is not a chemical disinfectant?
      1. Iodine
      2. UV light
      3. Bases
      4. Ozone
    6. _________is also the water treatment industry standard for disinfection. It is the best method to ensure that the water you are providing your customers is safe.
      1. Free chlorine residual
      2. Chlorine demand
      3. CT Disinfection
      4. Breakpoint chlorination

    This page titled 1.2: Contact Time Calculations is shared under a CC BY license and was authored, remixed, and/or curated by John Rowe (ZTC Textbooks) .