Ammonium sulfate

(endorsed 2005)

Ammonium sulfate is used as a source of ammonia to react with chlorine in drinking water treatment, to form chloramines. Chloramination is not as powerful as chlorination but provides a longer lasting residual in the water distribution system.

General description

Ammonium sulfate, $\text{(NH}_{4}\text{)}_{2}\text{ SO}_{4}$, is an off-white crystal which is soluble in water (up to a concentration of 10 g/L). It has a specific gravity of 1.77 (at 20°C), and is available in several grades as 60–100% effective product.

Ammonium sulfate can be stored in rubber-lined vessels or in containers made from stainless steel (type 316), neoprene, monel, fibreglass-reinforced plastic, polyethylene or polyvinyl chloride. If the ammonium sulfate is dry, cast iron can also be used.

Chemistry

Ammonium sulfate is by-product of the manufacture of caprolactam (a nylon-base material), coal gas and coke. It can also be prepared by the reaction of ammonia with sulfuric acid. It dissolves in water to form ammonium hydroxide ($\text{NH}_{4}\text{OH}$ — equation 1), which then releases ammonia gas ($\text{NH}_{3}$ — equation 2):

$$\text{(NH}_{4}\text{)}_{2}+\text{2H}_{2}\text{O}\rightarrow\text{2NH}_{4}\text{OH}+\text{H}_{2}\text{SO}_{4} \newline \text{NH}_{4}\text{OH}\leftrightarrow\text{NH}_{3}+\text{H}_{2}\text{O}$$

Solutions of ammonium salts or aqueous ammonia (ammonia dissolved in water) have an alkaline pH. The actual pH depends on the concentration and the temperature. It is important to vent facilities storing ammonium salt solutions, because of the formation of ammonia gas. Ammonium salt solutions and aqueous ammonia have the same characteristics; therefore, the same care should be taken during handling.

Typical use in Australian drinking water treatment

Ammonium sulfate is used as a source of ammonia for disinfection (see ammonia fact sheet for further details).

The amount of ammonium sulfate to be added can be determined by multiplying the required ammonia level by the molecular ratio of 7.77 (i.e. $\text{(NH}_{4}\text{)}_{2}\text{SO}_{4}$ = 7.77 x $\text{NH}_{3}$). The ammonia fact sheet includes information on levels needed for chloramination.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. Ammonium sulfate may contain moisture and insoluble material as well as the following chemical contaminants (JECFA, NRC 1982):

• aluminium

• arsenic

• chloride

• lead

• pyridine

• selenium

• iron

• nickel

Residual and by-product formation in drinking water

When employed in drinking water treatment, ammonium sulfate 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.

Excessive dosage can lead to biological growth in distribution system (see ammonia fact sheet for further details).

Status

Ammonium sulfate was originally 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

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B302-00. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

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.

JECFA (Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Joint Expert Committee on Food Additives). Compendium of Food Additive Specifications. FAO Food and Nutrition Papers 52 (two volumes). Available at https://www.who.int/foodsafety/publications/jecfa/en/

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

NRC (National Research Council) (1982). Water Chemicals Codex. Committee on Water Treatment Chemicals, Food and Nutrition Board, Assembly of Life Sciences, NRC.

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