Common Water Disinfectants and How to Use Them

Disinfection of your source water is important to remove bacteria, viruses and protozoa that can cause human illness. There are several options for disinfectants, but your choice will depend a great deal on what pathogens are present in your source water and what other barriers you’ve provided to ensure your drinking water is safe. Disinfecting the water from your drinking water source is one of the barriers in the multi-barrier approach to safe drinking water. Other barriers include source protection, water quality monitoring, system management, etc. 

Common drinking water disinfectants include the following:

  • Sodium hypochlorite
  • Calcium hypochlorite
  • Chlorine gas
  • Chloramines
  • Chlorine dioxide
  • Ozone
  • Ultraviolet light radiation

The Inactivation and Destruction of Pathogens (bacteria, viruses and protozoa):

The disinfectants listed above are effective against bacteria such as: Escherichia coli (E. coli), Campylobacter, Cholera, Legionella, Salmonella and Shigella. Chemical disinfectants tend to be more effective as the water temperature increases and the pH decreases. However, these disinfectants are not equal in their effectiveness against all pathogens. 

As discussed under Physical Removal of Pathogens (Filtration), enteric (intestinal) viruses are much smaller than bacteria and protozoa so filtration alone is not an effective method of treatment against them. If your water source is vulnerable to enteric viruses, disinfection technologies need to be employed (either individually or in combination) to meet the minimum pathogen log reduction requirements. 

Enteric viruses can be inactivated by chemical disinfectants such as chlorine, ozone and chlorine dioxide. Some disinfectants are more effective against  protozoans (Cryptosporidium oocysts and Giardia lamblia cysts) compared to other disinfectants.

A pathogen log reduction credit is a value assigned to a specific drinking water treatment process, expressed in log units, for the removal or inactivation of specific microorganism. Chemical and ultraviolet light disinfectants (discussed below) will have different pathogen log reduction credit values assigned to them. Similarly, different types of filtration are also assigned different pathogen log reduction credit values. You can read more in the BC Guidelines for Pathogen Log Reduction Credit Assignment.

When designing a water treatment system, you should consult with your regional health authority’s Drinking Water Officer (DWO) and Public Health Engineer (PHE). 

Turbidity and How it Impacts Disinfection

Turbidity is caused by particles suspended or dissolved in water that scatter light; making the water appear cloudy or murky. Particulate matter can include sediment (especially clay and silt), fine organic and inorganic matter, soluble coloured organic compounds, algae, and other microscopic organisms.

Bacteria, viruses and protozoans can attach themselves to the suspended particles in turbid water. These particles then interfere with disinfection by shielding contaminants from the disinfectant (e.g. chlorine or UV light radiation). For more information about turbidity filtration processes, see Physical Removal of Pathogens (Filtration).

Disinfection By-Products (DBPs)

Most disinfectants form by-products (DBPs). There are health concerns associated with each DBP. For example, DBPs associated with chlorination are:  trihalomethanes (THMs) and haloacetic acids (HAAs). For more information about DBPs, refer to Health Canada’s Guidelines for Canadian Drinking Water Quality.

Advantages and Disadvantages of the Different Types of Disinfectants

Each disinfectant’s efficacy varies significantly depending on:

  • the typical target pathogen(s) found in the source water
  • the quality of the source water (e.g., pH, temperature, colour, hardness, total organic carbon, mineral content, etc.) 
  • the disinfectant contact time.

Each disinfectant has advantages and disadvantages. Under Resources at the bottom of this web page, Table A (“Advantages and Disadvantages of Drinking Water Disinfectants”) provides a quick reference guide to the advantages and disadvantages of the different disinfectants. 

The advantages and disadvantages of each disinfectant relates to its:

  • effectiveness against the different waterborne pathogens (bacteria, viruses and protozoa)
  • formation of disinfection by-products (DBPs)
  • ability to provide a stable disinfectant residual in the distribution system
  • ability to maintain effectiveness when presented with elevated turbidity or total dissolved solids
  • ability to resist interference from elevated hardness, iron and manganese
  • technical complexity (operator training requirements, safety concerns, etc.)
  • creation of taste and odour problems (aesthetics)
  • cost (i.e. costs regarding construction/installation, onsite production, operation, maintenance, etc.)

The Pros and Cons of Each Disinfectant

Chlorine is very effective against bacteria and enteric viruses but less effective against protozoans (especially Cryptosporidium). It forms a residual and is the most commonly used chemical for secondary disinfection. Chlorine can be used in three forms for disinfection purposes.  

 

Sodium hypochlorite is a liquid, with approximately 12-15% available chlorine.

Pros:

  • Effective against E. coli bacteria and most microorganisms
  • Effective at inactivating enteric viruses
  • Effective against Giardia protozoa when the proper CT is provided 
  • Keeps a residual in the distribution system (secondary disinfection)
  • Can oxidize iron and manganese - allowing them to be filtered out
  • Technology well understood and relatively easy to use

Cons:

  • Not effective against Cryptosporidium protozoa
  • Can cause taste and odour problems
  • Forms DBPs when organic substances are present 
  • Corrosive if exposed to certain metal materials (as with calcium hypochlorite and chlorine gas)
  • Wastewater requires dechlorination after disinfection to reduce chlorine to receiving waters
  • Dissipates over time as undiluted bleach loses half its strength in 6 months to one year from the date of manufacture

 

Calcium hypochlorite is a solid, with approximately 70% available chlorine.

Pros:

  • Effective against E. coli bacteria and most microorganisms
  • Effective at inactivating enteric viruses
  • Effective against Giardia protozoa when the proper CT is provided 
  • Keeps a residual in the distribution system (secondary disinfection)
  • Can oxidize iron and manganese - allowing them to be filtered out
  • Available in easy to use granules or tablets
  • More stable and safer to handle than sodium hypochlorite or chlorine gas

Cons:

  • Not effective against Cryptosporidium protozoa
  • Can cause taste and odour problems
  • Forms DBPs when organic substances are present 
  • Corrosive if exposed to certain metal materials (as with sodium hypochlorite and chlorine gas)
  • Wastewater requires dechlorination after disinfection to reduce chlorine to receiving waters
  • Readily absorbs moisture 
  • Reacts slowly with moisture in the air to form toxic chlorine gas if not stored in air-tight containers
  • While not flammable, it acts as an oxidizer with combustible material and may react explosively with ammonia, amines, or organic sulfides

 

Chlorine gas has 100% available chlorine. It is not recommended as a disinfectant for Small Water Systems.

Pros: 

  • Effective against E. coli bacteria and most microorganisms
  • Effective at inactivating enteric viruses
  • Effective against Giardia protozoa when the proper CT is provided 
  • Keeps a residual in the distribution system (secondary disinfection)
  • Can oxidize iron and manganese - allowing them to be filtered out
  • Strongest form of chlorine regarding disinfecting capability

Cons:

  • Not effective against Cryptosporidium protozoa
  • Can cause taste and odour problems
  • Forms DBPs when organic substances are present 
  • Corrosive if exposed to certain metal materials (as with calcium hypochlorite and sodium hypchlorite)
  • Wastewater requires dechlorination after disinfection to reduce chlorine to receiving waters
  • Chlorine gas is extremely toxic and is classified as a pulmonary irritant
  • Advanced operator training is required before use
  • Chlorine gas is not a recommended disinfectant for small water systems

Your Public Health Engineer (PHE) should provide guidance regarding the required chlorine concentration and contact time (the CT value) prior to your first drinking water service connection. CT tables are available for the minimum treatment requirements for viruses and protozoans (oocysts of Cryptosporidium and cysts of Giardia lamblia).

Chlorine and other chemical disinfectants are discussed in Section 8 of the Guidelines for Pathogen Log Reduction Credit Assignment in the Drinking Water Officers’ Guide (DWOG).

Chloramines are formed by combining chlorine and ammonia. Chloramine species include monochloramine, dichloramine and trichloramine. Monochloramine is the predominant species in drinking water. Monochloramine is preferred for use in disinfecting drinking water because of its relative stability in drinking water and because it rarely causes taste and odour problems when compared with dichloramine and trichloramine. Compared to the other chemical disinfectants, chloramines are weak disinfectants against viruses and protozoa and CT is extremely difficult to establish. Consequently, chloramine is not recommended as a primary disinfectant for protozoa. However, Chloramine forms a more stable residual than chlorine alone and it is effective for secondary disinfection.

Pros:

  • Forms a more stable residual than chlorine (suitable for secondary disinfection)
  • Forms less DBPs than chlorine
  • Forms less taste and odour causing compounds in water
  • Technology well understood

Cons:

  • A weak disinfectant
  • Less effective than chlorine against viruses and protozoa
  • Poorly oxidizes iron and manganese
  • Compared to chlorine, it is more persistent in the environment
  • Usually requires a more powerful disinfectant for primary disinfection

Chlorine dioxide is produced by reacting sodium chlorite with chlorine or hydrochloric acid. It is an effective disinfectant against pathogenic microorganisms including Cryptosporidium and Giardia (protozoan oocysts and cysts, respectively). It is typically more expensive and complicated to implement, particularly in small treatment systems. The pH of the water does not affect the efficiency of the application. Chlorine Dioxide can not be stored as a gas or in liquid form, so it must be generated on site. Chlorine dioxide is not recommended for secondary disinfection because of its relatively rapid decay.

Pros:

  • More effective than chlorine or chloramines against microorganisms
  • Controls taste and odour better than chlorine in some cases
  • Forms less trihalomethanes (THMs) and haloacetic acids (HAAs) than chlorine (i.e., DBPs)

Cons:

  • Must be produced on site
  • Forms additional DBPs such as chlorite and chlorate
  • Requires daily chlorite and chlorine dioxide monitoring
  • Costs more for equipment and chemicals than chlorine
  • Takes more technical skill to use

Ozone is produced by electrical discharge through air or oxygen. Ozone is a molecule made of three oxygen atoms (O3). Because ozone is an unstable compound, a gas, it must be produced on-site using an ozone generator. Ozone is a strong disinfectant and it’s very effective against pathogenic microorganisms including Cryptosporidium and Giardia (protozoan oocysts and cysts, respectively). Ozone is typically expensive and energy intensive to produce and thus it has been predominantly used in municipal wastewater treatment facilities. Ozone decays rapidly after being applied during treatment and cannot be used to provide a secondary disinfectant residual.

Pros:

  • Most powerful disinfectant for treating drinking water
  • More effective than chlorine dioxide
  • Effective against the protozoans (Giardia and Cryptosporidium)

Cons:

  • Must be produced on site
  • Takes more technical skill to use
  • Forms bromate and other DBPs
  • Requires bromate monitoring
  • Does not provide residual (re secondary disinfection)

Ultraviolet light radiation can be used as a non-chemical disinfectant by using ultraviolet radiation at certain wavelengths. Ultraviolet (UV) ‘light’ is part of the light that comes from the sun. UV light cannot be seen as it lies between visible light and X-rays along the electromagnetic spectrum. The shortwave UV-C has disinfection properties. UV-C has a wavelength range of 200 nanometer (nm) to 280 nm. A nm is one billionth of a metre. One of the advantages of using UV disinfection is that the disinfection by-products typically associated with the use of chemical disinfectants are not formed. 

Pros: 

  • Effective against bacteria and the protozoans Giardia and Cryptosporidium
  • Does not form DBPs

Cons:

  • Requires pre-filtration to remove sediment & suspended particles
  • Severely affected by elevated turbidity, total dissolved solids, hardness, iron and manganese
  • Less effective against certain viruses (e.g. Adenovirus serotypes 40 & 41)
  • Takes more technical skill to use (training required)
  • Does not provide residual (re secondary disinfection)

About UV Reactors:

The effectiveness of this process is impacted by the general water quality parameters and the recommended standards are shown below in Table 1. A filter should be installed upstream of the UV disinfection system to ensure proper UV reactor performance as turbidity impacts its effectiveness.

Parameter

Value

Turbidity

< 1.0 NTU

Hardness

< 120 mg/L

Iron

< 0.3 mg/L

Manganese

< 0.05 mg/L

Hydrogen sulphide (if odour present) 

Non-detectable

Total suspended solids (TSS)

< 10 mg/L

pH

6.5 to 9.5

Total coliform

< 1000/100 mL

UV Transmittance (UVT) b

> 75 %

a  Source:  Table 3 of BC Guidelines for Ultraviolet Disinfection of Drinking Water, Drinking Water Officers’ Guide

b  UVT for fair water quality, U.S. EPA Guidance Manual on Alternative Disinfectants and Oxidants (1999). 

UVT is a measure of the percentage of incident light at a specified wavelength transmitted through a material (e.g. water) over a specified distance (pathlength normally 1 cm). UVT is typically measured at 254 nm.

How the UV lamp works:

UV light inactivates pathogens by damaging their nucleic acids (DNA and RNA) so that they cannot replicate and infect humans. The degree of pathogen inactivation depends upon the UV dose that is applied. UV dose is expressed as the product of UV intensity, expressed in milliwatts per square centimeter of exposed area (mW/cm2) and the amount of time that a microorganism is exposed to UV light in a reactor vessel (measured in seconds). 

Pathogen Log Reduction Credits for Protozoans and Viruses (UV):

UV differs from chlorination in that it is highly effective at inactivating both Cryptosporidium and Giardia (protozoan oocysts and cysts, respectively) but it is less effective against certain viruses. The BC Guidelines for Ultraviolet Disinfection of Drinking Water state that UV systems certified to NSF/ANSI Standard 55 Class A, can achieve a 3-log (99.9%) inactivation of protozoa and either a 0.5-log (66.6%) or 2-log (99%) inactivation of viruses. 

An NSF/ANSI Standard 55 Class A certified UV unit is effective at achieving a 4-log (99.99%) inactivation for most enteric (intestinal) viruses, with the exception of Adenovirus. Adenovirus, a double-stranded DNA virus, is the target pathogen for establishing UV virus inactivation requirements and it is more resistant to UV radiation than single stranded RNA viruses such as Hepatitis A. Adenovirus serotypes 40 and 41 cause the majority of Adenovirus-related gastroenteritis.

If a DWO considers a drinking water source to be at risk of human fecal contamination based on a source water assessment, only a 0.5-log virus reduction credit should be assigned because Adenovirus has a high level of resistance to UV treatment. Under such circumstances, two or more forms of treatment (e.g. chemical disinfection and UV disinfection), would be necessary to provide additional virus inactivation. The BC Guidelines for Ultraviolet Disinfection of Drinking Water also state that if the DWO does not consider the source to be at risk of human fecal contamination, a 2-log virus reduction credit should be assigned based on rotavirus inactivation. Rotavirus is the most resistant virus to UV disinfection after Adenovirus.

 

Validation

Protocol or

Certification

Standard a

Minimum UV

Dosage

Maximum Pathogen Log Reduction Credits Assigned

Cryptosporidium

Oocysts

Giardia Cysts

Viruses

NSF/ANSI Standard 55

(Class A Systems only)

40 mJ/cm2 at wavelength of 254 nm

3

3

0.5 or 2

U.S. EPA UVDGM

Validated dose ≥

required dose for

target pathogen log inactivation

Determined on a case

by case basis

Determined on a case

by case basis

Determined on on a case

by case basis

DVGW W294

RED = 40 mJ/cm2 at wavelength of 254 nm

3

3

0.5 or 2

* Source:  Table 4: Pathogen Log Reduction Credit Assignment, BC Guidelines for Ultraviolet Disinfection of Drinking Water, Drinking Water Officers’ Guide

a The Validation Protocol or Certification Standard (the first column of Table 2 above), as examples, shows the Certification Standard NSF/ANSI Standard 55 Class A and two Validation Protocols:  1) U.S. EPA UVDGM and 2) DVGW W294 (RED = Reduction Equivalent Dose)