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1.7: Disinfection

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    Learning Objectives

    After reading this chapter you should be able to identify and explain the following:

    • Disinfection terminology
    • Regulations
    • Types of disinfectants used during water treatment

    The final step in the water treatment process before finished or treated water enters a clearwell for storage is the disinfection process. Disinfection is the process where chemical agents are added to a water source to kill or inactivate pathogenic microorganisms. Pathogenic microorganisms are disease-causing and must be eliminated from treated water. As population sizes increase and freshwater sources become scarcer, the ability to remove and deactivate microorganisms becomes increasingly important. Another factor to consider, especially in California, is stricter regulations due to advancements in technology and water quality testing.

    Water treatment process diagram
    Figure \(\PageIndex{1}\): Image of water treatment by the EPA is in the public domain

    Disinfection Basics

    Treatment plant operators use the two-part process of removal and deactivation of microbiological constituents in water. Most of the pathogens water treatment professionals remove and deactivate from drinking water have adapted to living in the bodies of warm-blooded animals. Pathogens thrive and survive in those environments. Outside those environments, these pathogens can stay dormant until they are consumed. Even more frightening, some of the illnesses can cause death. In the United States, our water is generally safe to drink and we often take for granted that turning on a tap will produce a flow of potable water.

    Because of limitations in testing, it is difficult to indicate the presence of specific waterborne illnesses caused by virus, bacteria, and Giardia. Water professionals use tests such as the total coliform test to look for the likely presence of waterborne disease. The Surface Water Treatment Rule sets specific guidelines for removal and treatment to ensure the removal and inactivation of pathogenic organisms.

    Strict regulations set forth by the Safe Drinking Water Act were created to ensure the public’s drinking water was safe to consume. To ensure drinking water is safe for human consumption, 3 log removal and deactivation or 99.9% of Giardia lamblia is required. For viruses, 4 log or 99.99% removal and deactivation is required. Bacteria fall in the middle of viruses and Giardia so the government determined it was not necessary to have regulations specifically regulating their inactivation and removal.

    Below is a list of waterborne diseases and illnesses.

    Table 7.1: Waterborne Diseases and Illnesses


    Internal Parasite from Protozoa

    Virus caused

    • Anthrax
    • Dysentery
    • Cholera
    • Gastroenteritis (Stomach Flu)
    • Leptospirosis
    • Paratyphoid
    • Salmonella
    • Shigellosis (Shigella)
    • Typhoid fever
    • Dysentery
    • Ascariasis (round worm)
    • Cryptosporidiosis from Cryptosporidium
    • Giardiasis from Giardia
    • Gastroenteritis
    • Heart anomalies
    • Hepatitis A
    • Meningitis
    • Poliomyelitis

    Purpose of Disinfection

    Operators disinfect water to destroy the harmful organisms listed in the above chart. Filtration is used to remove the organisms while disinfection kills them or deactivates them. Operators do not sterilize water because sterilization would kill everything in the water. The process of disinfection relies heavily on everything that occurs downstream in the treatment process. As water enters the treatment plant in the form of raw water, the chemistry of that water affects how well the specific disinfectant will work at each stage of the treatment process.

    What Affects Disinfection?

    There are several characteristics of water that can affect treatment. Water is more easily disinfected with higher temperatures. In lower temperatures, longer contact times may be required and larger amounts of chemical must be used. Higher turbidity rates will decrease disinfection as well. The chapters before have covered how critical it is to remove suspended material early and efficiently. Excess turbidity will require greater amounts of chemical to properly disinfect the water supply. Chemicals such as chlorine can interact with organic and inorganic matter. Chlorine's ability to interact with these constituents may reduce or eliminate the effectiveness of the disinfectants.

    Types of Disinfectants Physical

    Physical disinfection is not widely used to treat potable water at this time. Ultraviolet rays are starting to be used more consistently but chemical means of disinfection must still be used as ultraviolet disinfection does not carry a disinfectant residual. The process is very expensive and thus is not used by large scale treatment operations in the United States. Other means of physical disinfection include boiling and ultrasonic wave production. Agencies will call for boil water notifications during emergencies and when there is a waterborne illness outbreak but it is not used as a primary means to disinfect drinking water.

    UV light as a stage in the process to disinfect water
    Figure \(\PageIndex{2}\): Image by the EPA is in the public domain

    Types of Disinfectants Chemical

    There are several chemical disinfectants available in drinking water applications. The most commonly used in the United States is Chlorine. The topic of Chlorine will be discussed in greater detail in the following chapter. The basics will be covered below as well as a background on the other chemicals available. The chemicals will be broken down into subcategories based on their practical usage in the United States.


    • Iodine⁠—It is commonly used for emergency treatment in the form of droplets or tablets. It is not used by the water treatment industry because of its cost and the potential health hazards to pregnant women and possible thyroid issues, which can develop with frequent use.
    • Bromine⁠—It is not used by water treatment facilities as it is very corrosive and can cause severe skin burns. It is used more commonly as a disinfectant in swimming pools. When it reacts with choline (a common nutrient from plants and animals) in water, it can create disinfectant byproducts. It was used by the United States Navy for a time, but most systems have been removed because of bromine’s corrosiveness.
    • Sodium Hydroxide and Lime⁠—More frequently used to sterilize pipes. They are not used as an everyday disinfectant because of the bitter taste that is left behind after application. Sodium hydroxide and lime are more often used to increase the ph of the water in the distribution system after treatment with gas chlorine.

    More Common

    • Chlorine Dioxide⁠—Used as a water treatment disinfectant and oxidizer. It does not react with ammonia which is an issue with chlorine. Chlorine dioxide is used as a disinfectant but is also very effective at removing iron, manganese, taste, odor, and color from treated water. Cryptosporidium is resistant to chlorine but is not resistant to chlorine dioxide. Up to 70 percent of chlorine dioxide is converted to chlorite, which is a regulated disinfectant by-product so the dosage rates when using it as a disinfectant must be lower than 1.4 mg/L. Chlorine dioxide must also be made on site which necessitates higher operational and maintenance costs.
    • Ozone⁠—Ozone was first used in Europe in the early 1900’s. It is a strong disinfectant that also reduces taste and odor issues. The drawback of ozone is that it is very expensive to produce, has high electrical costs, has limited solubility, and does not leave a residual in the treated water because it is so reactive. If bromide is present in the water, ozone can react with it to form bromate, an undesirable DBP (this is an issue for SCV Water). Ozone is very efficient at disinfecting Cryptosporidium so it is generally used as a secondary disinfectant along with chlorine or chloramines.

    Most Common

    • Chlorine⁠—The most widely used disinfectant in the United States is free chlorine. Chlorine can be added as a gas in the form of chlorine gas, as a solid in the form of calcium hypochlorite, or as a liquid in the form of sodium hypochlorite. Most likely, you have a bottle of sodium hypochlorite in your house. We call it bleach. The use of free available chlorine has declined over the years because of the discovery of disinfectant by-products (DBPs). This topic will be discussed in further detail in the next chapter.
    • Chloramines⁠—The use of chloramines has become more common in recent years in order to reduce DBPs mainly trihalomethane (THM). Chloramines are also referred to as combined chlorine as it is the combination of chlorine and ammonia. Chloramines are also effective at eliminating taste and odor problems and the residual lasts longer in the distribution system. However, chloramine disinfection is not as strong as chlorine and the improper addition of ammonia can lead to excessive amounts of ammonia in the treated water which results in nitrification.
    Ozone Generator
    Figure \(\PageIndex{3}\): Image by olegschedrin0 is licensed under CC0


    The Safe Drinking Water Act (SDWA) is the basis of all drinking water regulations in the United States. It is the umbrella in which all new regulations and rules have subsequently been created and enacted. The SDWA regulates drinking water standards in the United States along with its territories. We take for granted all of the research and technology we have available that allows us to never really be concerned about the quality of our drinking water. The SDWA was passed in 1974 and set fourth standards to regulate public water sources. It was amended in 1986 to include some basic principle definitions:

    • Defined regulated contaminants and approved treatment techniques
    • Defined criteria for filtration of drinking water
    • Defined criteria for disinfecting surface and groundwater
    • Outlawed the use of lead material in drinking water facilities

    After a large public health crisis in Milwaukee, Wisconsin, provisions were made to support drinking water programs through operator training and certification programs. All entities serving water to the public were required to meet program standards with regards to training and certification. In 1999, the government allowed states to hold primacy over drinking water certification programs as long as federal minimums were met. States such as California often have stricter standards than the federal standards.

    Surface Water Treatment Rule

    The Surface Water Treatment Rule (SWTR) was enacted in 1990 and sought to prevent waterborne illness from surface water sources. Water systems with supplies from surface sources which are susceptible to carrying viruses, legionella, and Giardia lamblia, have to follow new requirements with regards to filtration and disinfection known as the multiple barrier approach. The rule also required that systems that used groundwater as a source for drinking water had to adhere to SWTR standards if their water source could come into contact with surface water sources.

    Water treatment plants would have to achieve removal and deactivation requirements through the combined efforts of filtration and disinfection. The removal and deactivation of 99.9% of Giardia or 3 Logs and the removal and deactivation of 99.99% of viruses or 4 Logs was the new standard set forth. This requirement is measured by monitoring combined effluent turbidities in the combined filters and meeting disinfection requirements through the CT calculation. The CT calculation will be covered in greater detail in the following chapter.

    Groundwater Rule

    The Groundwater Rule (GWR) was established in 2009 in response to the frequency of groundwater contamination from surface water runoff sources. The rule requires monitoring for systems that do not disinfect to make sure microbiological contamination is not occurring. If a groundwater supplier did use disinfection, they are to meet 99.99% virus inactivation much like groundwater sources.

    Total Coliform Rule

    The final rule which indirectly relates to water disinfection is the Total Coliform Rule (TCR) which was established in 1990. As stated several times within the text, it is nearly impossible and certainly too costly to test for every type of microbiological contaminant that could lead to a public health risk. Instead, the TCR uses a risk based process which tests for the “worst case scenario”. Coliforms grow in warm blooded animals just like viruses, bacteria, and Cryptosporidium. They pose no health risk to humans and they grow more abundantly than forms of microbiological agents that will do us harm. If coliforms are present in the water supply, there is a chance for a public health concern.

    In the event of a positive coliform test, the downstream and upstream sampling sites as well as the site where the positive sampling occurred will be retested. System maps and sampling plans are a requirement of the TCR. The amount of samples the water supplier takes is based on the population served. Systems which collect less than 40 samples a month can only have one positive sample before notifying the public of a Maximum Contaminant Level (MCL) violation. Systems that collect more than 40 samples a month must not have positives in more than 5% of their coliform samples. If you work at a water supplier that takes 50 samples a month and had 3 positive samples are you in compliance with the TCR?

    \[\begin{align*} 47 ÷ 50 &= 0.94 \times 100 \\[4pt] &= 94\% \end{align*}\]

    So 6% of the samples were positive so you would not be in compliance) or...

    \[\begin{align*} \dfrac{3}{50} &= 0.06 \times 100 \\[4pt] &= 6\% \end{align*}\]

    Therefore, 3 positives are 6% of the total samples.

    Disinfectant By-Product Rule

    The disinfectant by-product rule (DBPR) was put in place to protect the public from cancer causing risks associated with disinfectants reacting with organic and inorganic matter in treated drinking water. Disinfectant by-products such as trihalomethanes are classified as volatile organic compounds. The Stage 1 DBPR established maximum contaminant levels for several DBPs including:

    • Trihalomethane (TTHMs) - 80 ug/L or 80 ppb
    • Haloacitic Acid (HAA5) - 60 ug/L or 60 ppb
    • Bromate - 10 ug/L or 10 ppb
    • Chlorite - 1.0 ppm

    While it is all together possible to remove DBPs from treated water with activated carbon, it is a very expensive treatment process and not cost effective for large treatment operations. Other forms of disinfection are possible but will also cause DBP formation. Chlorite formation is associated with chlorine dioxide treatment, while bromate is associated with ozone treatment. The issue of DBP formation becomes even more problematic as chlorite and bromate are often found naturally in source water.

    The stage II DBP Rule enhanced regulations on DBP formation by targeting water sources that are more vulnerable to DBP formation and the rule also requires monitoring for HAA5s and THMs. The number of samples taken and the number of sampling sites is based on the size of the population served by the water agency. The use of chloraminated water is being used more commonly to combat DBP formation but using chloramines instead of other disinfectants has other risks associated with its use which will be covered in greater detail in the next chapter.

    This is the link to the quick reference guide from the EPA with information on the Stage I and Stage II DBP rule.

    Chapter Review

    1. What is residual chlorine?
      1. Chlorine used to disinfect
      2. The amount of chlorine after the demand has been satisfied
      3. The amount of chlorine added before disinfection
      4. Film left on DPD kit to measure residual
    2. When Chlorine reacts with natural organic matter in water it can create ___________.
      1. Disinfectant by-products
      2. Coliform bacteria
      3. Chloroform
      4. Calcium
    3. What are trihalomenthanes classified as?
      1. Salts
      2. Inorganic compounds
      3. Volatile organic compounds
      4. Radio
    4. What disinfectant is used for emergency purposes and not utilized in the water treatment industry?
      1. Chlorine
      2. Iodine
      3. Ozone
      4. Chlorine dioxide
    5. What is the disinfectant with the least killing power but that has the longest lasting residual?
      1. Chlorine
      2. Ozone
      3. Chlorine dioxide
      4. Chloramines
    6. The active ingredient in household bleach is ___________.
      1. Calcium hypochlorite
      2. Calcium hydroxide
      3. Sodium hypochlorite
      4. Sodium hydroxide
    7. Cryptosporidium is not resistant to this chemical:
      1. Ozone
      2. Chlorine dioxide
      3. Chlorine
      4. Both 1 and 2
    8. The Removal and inactivation requirement for Giardia is?
      1. 99.9%
      2. 99.99%
      3. 99.00%
      4. 90%
    9. If a coliform test is positive, how many repeat samples are required at a minimum?
      1. None
      2. 1
      3. 3
      4. Depends on the severity of the positive sample
    10. Your water system takes 75 coliform tests per month. This month there were 6 positive samples. What is the percentage of samples which tested positive? Did your system violate regulations?
      1. 3% Yes
      2. 5 % No
      3. 8 % Yes
      4. 10 % No

    1.7: Disinfection is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.

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