Chloramines are formed by the reaction of ammonia and aqueous chlorine (i.e., HOCl). At first, chloramines were utilised for taste and also odour control. However, it was quickly recognised that chloramines were much more secure than free chlorine in the circulation system and consequently were found to be reliable for regulating bacterial regrowth. During the past two decades, concern regarding chlorinated organics (e.g., THM and HAA formation) in water treatment and distribution systems raised interest in chloramines since they form few byproducts (DBPs).
Chloramines are formed from the reaction of chlorine and ammonia. The resulting blend may have monochloramine (NH2 Cl), dichloramine (NHCl2 ), or nitrogen trichloride (NCl3 ). When chlorine is distributed in water, rapid hydrolysis occurs according to the following reaction:
Cl2 + H2O ? HOCl + H + + Cl –
The equilibrium constant (K eq) at 25 ° C is 3.94 x 104 M¯¹ for this reaction. In diluted water solutions, at a pH of more than 3, the forward reaction is complete. Hypochlorous acid (HOCl) is a weak acid that dissociates as it adheres to:
HOCl ? OCl – + H+ pKa = 7.6
Relative proportions of HOCl and OCl – are dependent upon pH. Both chlorine species in the above reactions are powerful oxidants, efficient in responding to many substances existing in water. In aqueous solutions with pH 7.0 to 8.5, HOCl reacts swiftly with ammonia to form inorganic chloramines in a series of contending reactions (White, 1992).
The simplified stoichiometry of chlorine-ammonia reactions are as follows:
NH3 + HOCl ? NH2 Cl + H2O (monochloramine)
NH2Cl + HOCl ? NHCl2 + H2O (dichloramine)
5NHCl2 + HOCl ? NCl3 + H2O (nitrogen trichloride)
These competing reactions, and several others, mainly depend on pH and are managed to a considerable degree by the chlorine: ammonia nitrogen (Cl2 : N) proportion. Temperature and contact time also contribute to the reaction process. Figure 1 reveals the standard connections between the chloramine species at different Cl2 : N proportions for pH ranging from 6.5 to 8.5.
Figure 1:Theoretical Breakpoint Curve
This number indicates that monochloramine is predominately developed when the used Cl2 : N proportion is less than 5:1 by weight. As the used Cl2 : N ratio rises from 5:1 to 7.6:1, breakpoint response occurs, lowering the residual chlorine level to a minimum. Breakpoint chlorination causes the development of nitrogen gas, nitrate, as well as nitrogen chloride. At Cl2 : N ratios over 7.6:1, free chlorine and also nitrogen trichloride are present.
Figure 2 reveals the partnership between chloramine types as the pH modifications (Palin, 1950). The number shows that dichloramine becomes a dominant species at low pH.
Figure 2: Distribution Diagram for Chloramine Species with pH
To avoid breakpoint reactions, utilities should keep a Cl2: N ratio between 3 and 5 by weight. A proportion of 6 is optimum for disinfection. However, it is challenging to preserve a stable operation at that point in the breakthrough curve. As a result, a Cl2 : N proportion of 4 is usually accepted as optimal for chloramination.
Additionally, over a day approximately, with no modification of pH or Cl2: N proportion, monochloramine will certainly weaken gradually to dichloramine to a ratio of 43 percent NH2 Cl to 57 percent NHCl2 . Dichloramine is relatively unstable in the presence of HOCl; therefore, pure options of this form of monochloramine are challenging to generate and retain. According to the above formulas, chloramines are developed by the reaction of hypochlorous acid to ammonia. Table 1 below summarises the theoretical doses of chlorine as well as ammonia based upon these solutions.
|Reaction||mg Cl2/mg NH3|
|Nitrogen Trichloride (NCl3)||12.5|
|Free residual reaction||9|
Table 1: Chlorine Dose Required for NH3 – Cl2 Reaction
Monochloramine is the preferred chloramine species for disinfecting drinking water. This is due to preference and smell issues associated with dichloramine and nitrogen trichloride. To guarantee that these compounds are not created, standard practice was to limit the chlorine to ammonia proportion to 3:1. However, because of problems such as nitrification and biofilm formation, which can be brought on by excess ammonia, the present technique uses a Cl2 : N ratio in the range of 3:1 to 5:1, with a typical worth of 4:1.
Table 2 below illustrates the calculated response times for monochloromine development at 25 ° C and at chlorine: ammonia ratio of 3:1 (White, 1992). The rate of reaction of monochloramine formation is sensitive to pH.
Table 2: Time to 99 Percent Conversion of Chlorine to Monochloramine
Monochloramine is utilised in drinking water treatment for disinfection and also organism control. Factors of application are based on treatment goals and also contact time disinfection requirements.
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Primary Use of Monochloramines
The primary use of monochloramine in water systems is as a secondary disinfectant for maintaining a residual in the distribution system. Chloramines are an excellent choice for secondary disinfectants because of the following potential benefits:
- Chloramines are not as reactive with organics as free chlorine in forming THMs.
- The monochloramine residual is more stable and longer-lasting than free chlorine, providing better protection against bacterial regrowth in systems with large storage tanks and dead-end water mains.
- The monochloramine residual is more effective in controlling biofilms because of its superior ability to penetrate the biofilm. Controlling biofilms also tends to reduce coliform concentrations and biofilm-induced corrosion.
- Because chloramines do not react with organic compounds, many systems will experience less taste and odour complaints when using chloramines.
Water systems typically found that conversion from free chlorine to monochloramine as the secondary disinfectant significantly reduced coliform concentrations in the distribution system.
The standard dosage range for monochloramine is in the range of 1.0 to 4.0 mg/L. The minimum residual of monochloramine in the distribution system is typically regulated at 0.5 mg/L. To prevent nitrification in a distribution system, a minimum monochloramine dosage of 2.0 mg/L is recommended.
Advantages and Disadvantages when using Chloramine:
- Chloramines are not as reactive with organics as free chlorine in forming disinfection byproducts (DBPs).
- The monochloramine residual is more stable, lasting longer than free chlorine. This provides better protection against bacterial regrowth in systems with large storage tanks and dead-end water mains. However, excess ammonia in the network has the potential to cause biofilm.
- Chloramines do not tend to react with organic compounds. Several systems will experience fewer taste and odour challenges when using chloramines.
- Chloramines are reasonably priced.
- Chloramines are manufactured on-site and consumed immediately.
- The disinfection properties of chloramines are not as strong as other disinfectants found in the market, such as chlorine, ozone, and chlorine dioxide.
- Chloramines are unable to oxidise iron, manganese, and sulfides.
- When using chloramines for secondary disinfection, it may be necessary to periodically convert to free chlorine for biofilm control in the water distribution system.
- Excess ammonia in the distribution lines may lead to nitrification challenges; this occurs mainly in dead-ends and other areas of low disinfection residual.
- Monochloramines are less effective as disinfectants at high pH than at low pH.
- Dichloramines have treatment and operation challenges.
- Chloramines must be made up on site.
Frequently Asked Questions (FAQ)
How long has monochloramine been used to disinfect potable or drinking water?
- Monochloramine has been utilised as a drinking or potable water disinfectant for longer than 90 years.
- Monochloramine has proved to be an effective anti-bacterial cemented by decades of use in Canada, the U.S., Canada, and Great Britain.
- Typically, monochloramine is used alongside chlorine or chlorine dioxide as part of the drinking water treatment process.
- Monochloramine helps protect individuals from waterborne illness.
How is monochloramine normally used?
- Monochloramine is most often made use of to maintain water quality in the pipes.
- Monochloramine offers a durable defence of water quality.
- Monochloramine works as an anti-bacterial since it does not dissipate rapidly.
- Monochloramine aids lower degrees of potentially unsafe disinfection byproducts compared to chlorine.
How is monochloramine used in the disinfection of drinking water?
- Monochloramine is used as a secondary disinfection, giving longer-lasting water treatment as the water moves with pipes to customers.
- Additional disinfection maintains the water’s high quality by eliminating potentially hazardous organisms that might get into the water as it travels through pipelines.
- Monochloramine may be more beneficial than chlorine in killing specific potentially dangerous organisms in pipes such as those that cause Legionnaire’s disease.
Can monochloramine be an effective primary disinfectant?
- Monochloramine takes a lot longer than chlorine to kill most potentially harmful organisms.
- Monochloramine can be used as a primary disinfectant, yet the time required for treatment makes it impractical for most utilities.
- However, because it is much longer-lasting than chlorine, monochloramine is often used as a secondary disinfectant.
Can combinations of disinfectants be used for primary disinfection?
- Primary disinfection usually includes numerous disinfection steps that may start as the water enters the treatment plant.
- When used as a primary disinfectant, monochloramine performance is boosted by incorporating it with other disinfectants.
- The option of which combination of disinfectants to use varies for water utilities based on their demands and requirements.
Does monochloramine have advantages over chlorine as a secondary disinfectant?
- Monochloramine is a lot more chemically stable than chlorine.
- Monochloramine produces less potentially dangerous regulated disinfection byproducts than chlorine.
- Monochloramine is longer-lasting than chlorine, making it helpful in killing particular harmful organisms located in pipes such as those that cause Legionnaires’ illness.