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1.3: Disinfection

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    Student Learning Outcomes

    After reading this chapter, you should be able to:

    • List the various types of disinfectants
    • Explain the disinfection process
    • Describe the concepts of breakpoint chlorination and nitrification

    ​​​​​Disinfection is often thought of as drinking water treatment. However, it is more of a conditioning and preventative process. The process of disinfection is designed to inactivate pathogenic organisms in water with chemical oxidants or equivalent agents. This simply means the destruction (killing) of microorganisms, which poses a threat to public health. In contrast, drinking water treatment usually involves the removal of contaminants. Another term that sometimes is confused with disinfection is sterilization. Sterilization is the destruction of all organisms and not just pathogens. While sterilization may result in providing safe drinking water, it is not a necessary process and it would result in higher costs.

    The following chemicals can be used for the chemical disinfection of water:

    • Chlorine—Cl2
    • Chlorine dioxide—ClO2
    • Hypochlorites—OCl-
      • Sodium hypochlorite—NaClO
      • Calcium hypochlorite—Ca(ClO2)
    • Trichloroisocyanuric acid—C3Cl3N3O3
    • Ozone—O3
    • Bromine—Br2
    • Iodene—I
    • Various bases

    There are other means of disinfection besides the use of chemicals. We will refer to these as physical disinfectants. The following list can be used as a physical mean to disinfect water:

    • Ultraviolet light—The UV rays must come in contact with each organism in order to provide disinfection
    • Heat—Boiling water for about five (5) minutes is usually sufficient to destroy essentially all microorganisms in the water
    • Ultrasonic waves—Sonic waves destroy microorganisms by vibration

    Chemical Disinfectants other than Chlorine Based Compounds

    Four (4) non-chlorine based disinfectants were listed above and while the can provide a means to disinfect and kill microorganisms in drinking water, there are drawbacks to each of them

    • Iodine—Using iodine as a disinfectant in drinking water can be effective, but it has a relatively high cost. In addition, iodine has potential serious physiological side effects, especially to pregnant women
    • Bromine—There are safety concerns and difficulties handling bromine. Direct contact with the skin can cause burns and residuals are hard to maintain. Bromine is commonly used is spas and swimming pools.
    • Ozone—Ozone is commonly used at drinking water treatment plants and reduces bad tastes and odors. Ozone is not usually used in distribution systems is because of the lack of a residual, high costs, and maintenance requirements.
    • Bases—Two of the more common bases include lime and sodium hydroxide. They leave a bitter taste and can also burn skin.

    Chlorine and Chlorinated Disinfectants

    Disinfection with chlorine and chlorinated compounds result in a disinfectant residual of free chlorine or total chlorine. Free chlorine is the amount of “free” and available chlorine to perform the disinfection process. As chlorine combines with nitrogen related compounds (as we will describe in more detail with chloramination) a combined chlorine residual occurs. This combined chlorine, along with any available free chlorine makes up a total chlorine residual.

    Chlorine Gas

    Chlorine is the most common disinfectant used in drinking water. It is in the form of a gas, provides a relatively long-lasting residual, and tends to lower the pH of the water. Chlorine gas is greenish-yellow in color and has a high coefficient of expansion. Therefore, chlorine cylinders should not be filled more than eighty-five (85) percent. Chlorine cylinders come in one hundred (100), one hundred fifty (150), and one (1) ton sizes. Only forty (40) pounds per day can be withdrawn from the smaller sized cylinders unless they are equipped with an evaporator. Cylinders are equipped with a fusible plug, which is designed to melt at temperatures between 158F and 156F to prevent the cylinder from combusting. One-ton cylinders have six (6) fusible plugs and two (2) valves to withdraw the chlorine. Chlorine gas is 2.5 times heavier than air and ventilation in chlorine rooms needs to close to the floor. Commonly, vents are twelve (12) inches above the floor. Leaks can be detected by waving an ammonia-soaked rag, which creates a white cloud. Chlorine gas has an immediately dangerous to life and health (IDLH) level of 10 ppm. Chlorine can combine with organics in drinking water created halogenated disinfection by-products.

    Hypochlorites of Sodium and Calcium

    Sodium hypochlorite comes in the form of a liquid and the chlorine concentration is commonly twelve and one half (12.5) percent. Calcium hypochlorite comes in the form of a solid as granules or tablets and is typically sixty-five (65) percent. Calcium hypochlorite is also known as high test hypochlorite (HTH). When dissolved in water hypochlorites tend to raise the pH. Hypochlorites can combine with organics in drinking water created halogenated disinfection by-products.

    Chlorine Dioxide

    Chlorine dioxide is highly effective in controlling waterborne pathogens while minimizing disinfection by-products. It is an effective means of controlling taste and odor issues. It can be expensive to use, especially in smaller quantities and can be difficult to handle.


    Chloramine is a disinfection process using chlorine and ammonia together. This disinfection process is referred to as chloramination and it results in a combined or total chlorine residual. There are several reasons chloramination is used instead of chlorine alone. Chloramines are produced under three different processes; pre-ammoniation with post-chlorination, pre-chlorination with post-ammoniation, or concurrent addition of chlorine and ammonia. Concurrent addition produces the lowest disinfection by-products and pre-chlorination produces the highest disinfection by-product levels.

    Physical Disinfection

    While chemical disinfectants and chlorine specifically provide the majority of disinfection in drinking water systems, there are several other means of disinfecting water without using chemicals. These are referred to as physical means of disinfection and they include:

    • Ultraviolet rays—Ultraviolet (UV) light rays and inactivate microorganisms. However, the light must come in direct contact with each organism in order to inactivate. Therefore, it is not a sufficient process in drinking water systems. UV systems are commonly used with fish aquariums.
    • Heat—Boiling water is an efficient process to destroy microorganisms in water. “Boil Water” orders are often implemented whenever there is an issue with a drinking water system. Boiling water requires heat and time. Typically is takes approximately five (5) of boiling to destroy all microorganisms.
    • Ultrasonic waves—Sound waves generate cavitation bubbles in liquids resulting in intense shear forces and high stress

    Factors Influencing Disinfection

    Depending on the disinfection process, whether physical or chemical, there are things affecting the effectiveness. For example, UV light must come in direct contact with the organisms to work. We will look at six (6) variables influencing the disinfection process.

    The pH of the water plays a pivotal role in the effectiveness of disinfection, especially when using chlorine or chlorine related compounds. When using free chlorine with water, hypochlorus and hydrochloric acids are formed. In dilute solutions with a pH above 4, the formation of hypochlorus acid is most complete and leaves little chlorine in the solution. However, hypochlorus acid is a weak acid and poorly dissociated at pH levels below 6. The higher the pH, the greater percent of hypochlorite ion exists. Hypochlorus acid has a greater disinfection potential than hypochlorite ion. Therefore, pH plays an important role with disinfection. At a pH of approximately 7.2, 60% of dissolved chlorine exists as hypocholrus acid. At a pH of 8.5, approximately 90% of the dissolved chlorine exists as hypochlorite ions. Therefore, chlorine as a disinfectant is more efficient at pH levels around 7.

    Temperature also influences disinfection. The higher the temperature the more efficiently water can be disinfected. At lower temperatures, longer contact times are required. Adding larger quantities of chlorine can speed up the disinfection process. One major disadvantage of warmer waters exposed to the atmosphere is the increased dissipation rate of chlorine into the atmosphere.

    Excessive turbidity in water supplies will greatly reduce the efficiency of the disinfection process. Any suspended solids present in the water supply can shield microorganisms from the disinfectant. In addition, some types of suspended solids can create an increase in chlorine demand, this results in less available chlorine to react with pathogens.

    If some of the turbidity or if other substances in the water are in the form of organic compounds, chlorine disinfectants are greatly reduced. In addition, unwanted by-products can be formed, including trihalomethanes and halo acetic acids. The overall effect is a reduction in the overall chemical available for disinfection.

    Various other non-organic reducing agents can also impact the disinfection process. The demand for chlorine for all reducing agents must be satisfied before chlorine becomes available for disinfection. Inorganic reducing agents impacting chlorine disinfection include, but are not limited to hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions.

    Chloramination vs. Chlorination

    Both chemicals are widely used to disinfect drinking water. Each has its benefits and drawbacks. As previously described, Chloramination results in total chlorine residual and chlorine creates a free chlorine residual.

    Both free and total are efficient with inactivating/killing microorganisms, including heterotrophic plate count bacteria and pathogenic organisms. They both can penetrate biofilm and reduce coliform regrowth. While free chlorine is a stronger oxidizer, chloramines provide a longer-lasting residual.

    At the correct ratio between chlorine and ammonia, taste and odor problems can be controlled. If water contains organic compounds, free chlorine can be combined with these compounds creating disinfection by-products, such as trihalomethane and halo acetic acid compounds. Chloramines reduce this disinfection by-product formation.

    Breakpoint Chlorination

    As chlorine is initially added to water, reducing compounds are destroyed. Both organic and inorganic reducing agents contribute to this first stage of disinfection. As a result, no chlorine residual is present. Understanding this is critical, but it is also counter-intuitive. As you add chlorine no chlorine residual is detected. More chlorine must be continually added. The next stage of breakpoint chlorination is the formation of chloroganics and chloramines. At this point, a residual begins to be detected. As the chloroganics and chloramines start to be destroyed, the residual starts to decrease. Once all the chlororganics and chloramines are completely destroyed, breakpoint is hit and all the chlorine demand is satisfied. At this point, any chlorine added is directly proportional to the chlorine residual measured.

    Chloramination Breakpoint Curve
    Figure \(\PageIndex{1}\): Image by Aliciacdiehl is licensed under CC BY-SA 3.0

    If disinfecting with chloramines, an ideal chlorine to ammonia ratio is 5:1. This means for every part of ammonia added, there should be five parts of chlorine. At this point, the highest total chlorine residual is realized and taste and odor issues are minimized. Lower chlorine to ammonia ratios result in free available ammonia. This creates a potential food source for microorganisms and results in a decreased disinfectant residual. This results in a condition referred to as nitrification. Therefore, it is important to monitor for nitrogen related components to control this condition. If the chlorine to ammonia ratio increases, the disinfectant residual also decreases and unwanted taste and odor compounds increase. If a free chlorine residual is desired and ammonia is not added along with chlorine, then chlorine needs to be continually added until breakpoint is reached.


    Nitrification is an aerobic process in which bacteria reduce ammonia and organic nitrogen into nitrite and then nitrate. Nitrite rapidly reduces free chlorine and can also interfere with the measurement of a free chlorine residual. This results in a loss of total chlorine and ammonia and an increase in heterotrophic plate count bacteria. Higher temperatures and longer detention times in storage facilities increase the potential for nitrification. Water utilities using chloramination as a disinfection practice, usually have a nitrification monitoring plan. This plan would specify and describe steps the utility will take to monitor, prevent, and reduce the affects associated with nitrification. For example, the plan would specify when increased monitoring would be required. It would specify the constituents, which would need to be monitored. It would also describe maintenance activities within the distribution system such as a proactive flushing program to help distribute the chlorine residual and also remove stagnant water. Another example would be to properly cycle the water within storage tanks to prevent or reduce stratification, higher temperatures, and stagnant water. Constituents routinely monitored in systems using chloramines include ammonia, total and free chlorine, nitrite, and heterotrophic plate count bacteria. A reactive approach, which is commonly used by water utilities, is a process referred to as “batch” chlorination. Batch chlorination is the process of adding chlorine to water storage facilities when residuals become too low and/or when nitrification compounds are present.


    There are a variety of methods and chemicals, which can be used to disinfect drinking water, with chlorine and chlorine related compounds being the most common. The disinfection process is critical in making sure drinking water is safe to drink by eliminating pathogenic microorganisms from the water supply. It is important to disinfect source water supplies and it is important to maintain detectable chlorine residuals within the distribution system. There are side effects related to the disinfection process including taste and odor issues and the potential formation of unwanted disinfection by-products. Therefore it is important to have adequate monitoring programs and to make sure the appropriate disinfectant is used.

    Sample Questions

    1. The IDLH of chlorine is ___________.
      1. 5 ppm
      2. 10 ppm
      3. 20 ppm
      4. 100 ppm
    2. A 1,000 ton gas chlorine cylinder has ___________.
      1. 6 valves and 2 fusible plugs
      2. 2 valves and 6 fusible plugs
      3. 4 valves and 2 fusible plugs
      4. 4 fusible plugs and 2 valves
    3. Gas chlorine ___________.
      1. Lowers the pH
      2. Raises the pH
      3. Keeps the pH neutral
      4. None of the above
    4. HTH stands for ___________.
      1. High Tolerant Hypochlorite
      2. Hypochlorite Total Hypochlorate
      3. High Test Hypochlorite
      4. High Total Hypochlorite
    5. After “breakpoint,” ___________.
      1. All chlorine added is free
      2. All chlorine added is combined
      3. Chlorine is not detectable
      4. Chlorine is a weak disinfectant
    6. Chloramines are ___________.
      1. A combination of Clorox and ammonia
      2. A combination of amino acids and chlorine
      3. A combination of chlorine and ammonia
      4. Bad because they create high levels of disinfection byproducts
    7. Chlorine leaks are best detected with ___________.
      1. A chlorine gas detector
      2. A rag soaked with DPD
      3. A rag soaked with ammonia
      4. Your nose
    8. Fusible plugs are designed to melt between ___________.
      1. 155°F and 165°F
      2. 168°F and 175°F
      3. 158°F and 165°F
      4. <150°F
    9. A 150 lb chlorine cylinder is designed to provide 40 ppd of chlorine unless it has a(n) ___________.
      1. Evaporator
      2. Chiller
      3. Extra fusible plug
      4. They can never deliver more than 40 ppd
    10. Which of the following has no effect on the disinfection process?
      1. Turbidity
      2. Temperature
      3. pH
      4. They all affect disinfection

    1.3: Disinfection is shared under a CC BY license and was authored, remixed, and/or curated by Mike Alvord.