Ammonia

(endorsed 2005)

Ammonia, $\text{NH}_{3}$, is added to drinking water to react with chlorine to form chloramine disinfectants. Chloramination is not as powerful as chlorination but provides a longer lasting residual in the water distribution system.

General description

Ammonia, $\text{NH}_{3}$, is a colourless gas or liquid, with a sharp, intensely irritating odour. It is lighter than air and easily liquefied by pressure. Ammonia has a boiling point of –33.5°C, a freezing point of –77.7°C, and a specific gravity of 0.8 as a liquid. Ammonia gas is combustible and is very soluble in water. When hydrated, ammonia can attack copper, zinc and alloys containing these metals. Ammonia can be supplied as a compressed liquid (anhydrous ammonia), dissolved in water (aqueous ammonia) or as solutions of ammonium salts (e.g. ammonium sulfate).

Gaseous ammonia is compatible with some steels, stainless steel (type 316), neoprene and monel. Aqueous ammonia can be stored in iron, steel, stainless steel, fibreglass-reinforced plastic or rubber-lined vessels.

Chemistry

Ammonia is prepared commercially in vast quantities, mostly using the Haber process to combine nitrogen directly with hydrogen. It can also be made using the cyanamide process, and is produced as a by-product of the destructive distillation of coal. Most of the ammonia produced is used to make fertilisers.

The reactions between ammonia and chlorine are complex, but the simplified equations shown below are often used. The chloramines produced are monochloramine ($\text{NH}_{2}\text{Cl}$ — equation 1), dichloramine ($\text{NH}\text{Cl}_2$ — equation 2) and trichloramine or nitrogen trichloride ($\text{N}\text{Cl}_3$ — equation 3).

$$\text{NH}_{3}^{+}+\text{HOCl}\rightarrow\text{NH}_{2}\text{Cl}+\text{H}_{2}\text{O}\hspace{2cm}\text{(1)} \newline \text{NH}_{2}\text{Cl}+\text{HOCl}\rightarrow\text{NHCl}_{2}+\text{H}_{2}\text{O}\hspace{2cm}\text{(2)} \newline \text{NHCl}_{2}+\text{HOCl}\rightarrow\text{NCl}_{3}+\text{H}_{2}\text{O}\hspace{2cm}\text{(3)}$$

Other products are also formed, such as nitrogen ($\text{N}_{2}$) and nitrate ($\text{NO}_{3}^{-}$).

The sum of the concentrations of the three chloramine species is referred to as ‘combined chlorine’ and is often expressed as $\text{Cl}_{2}$, in the units of mg/L. The sum of the combined chlorine concentration and the free chlorine concentration (i.e. hypochlorous acid and hypochlorite ion) is referred to as ‘total chlorine’. The relative amounts of the three species of chloramine formed depend on the ratio of chlorine to ammonia, the pH and the temperature. Monochloramines are preferred because they do not cause the taste and odour problems that can arise with dichloramines and trichloramines. Users should refer to available data on how pH and the ratio of chlorine to ammonia affect the distribution of chloramines (see discussion in the following section), and should be aware of the breakpoint phenomenon (whereby chlorine applied in sufficient doses will oxidise ammonia and eliminate chloramines, forming a free chlorine residual).

Typical use in Australian drinking water treatment

In drinking-water treatment, ammonia is added with chlorine (at a fixed ratio of ammonia to chlorine) to produce chloramine disinfectants. Chloramines react with bacteria and oxidisable material more slowly than free chlorine, but last longer than free chlorine. Depending on the order and process used trihalomethanes (THMs) may form. Chloramines thus tend to be used as a secondary disinfectant to provide a disinfectant residual in the distribution system, but may also be used as a primary disinfectant if an appropriate contact time is allowed. Chloramines are particularly suited to providing disinfectant residuals in long distribution systems, where it is difficult to maintain a residual using chlorine.

To produce monochloramine, the pH should be between 8 and 9, and the chlorine to ammonia ratio should be between 3:1 and 4:1. A ratio above 4:1 may produce chlorinous odours. Ammonia may be added before or after chlorine. In primary disinfection, chlorine is usually added first, because it kills bacteria, viruses and spores much more efficiently than does monochloramine, provided that sufficient contact time is allowed for disinfection before the ammonia is added. Ammonia and chlorine can be added together, provided that contact time is sufficient to ensure disinfection.

Chloramines present in water are harmful to people on kidney dialysis and to animal species in aquaria; therefore, it is important for water utilities using chloramination to inform consumers at risk.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. Ammonia is generally supplied at 99.9 % purity or better, but the product may include a very small amount of oil (hydrocarbons), heavy metals and water.

Residual and by-product formation in drinking water

When employed in drinking water treatment, ammonia should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines. Free ammonia liberated in the distribution system may contribute to nitrification problems or biological growth.

Chloramines may form some halogenated organic by-products. THMs may also be produced, but to a much lesser extent than with chlorination. More information on chloramines can be obtained from the Chloramine Fact Sheet in the Australian Drinking Water Guidelines.

Status

Aqueous ammonia was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th Edition. American Public Health Association, Washington, DC.

Connell GF (1996). The Chlorination/Chloramination Handbook. Water Disinfection Series, American Water Works Association. Denver, Colorado.

Hass CN (1999). Disinfection. In Water Quality and Treatment, A Handbook of Community Water Supplies, Letterman RD (ed). American Water Works Association, 5th edition. McGraw-Hill Professional, New York, 14.1–14.60.

Lewis RJ (1993). Hawley’s Condensed Chemical Dictionary, 12th edition. Van Nostrand Reinhold, New York.

White GC (1992). Handbook of chlorination and alternative disinfectants, 3rd edition. Van Nostrand Reinhold, New York.