Aluminium sulfate (alum)

(endorsed 2005)

Aluminium sulfate (alum) is a general purpose coagulant that is used in water treatment to remove turbidity, natural organic matter (NOM) (including colour), microorganisms and many inorganic chemicals. Removal of NOM reduces the formation of disinfection by-products, because it removes the organic precursors of the by-products.

General description

For use in water treatment, aluminium sulfate (alum) is generally supplied as a bulk liquid, but it can also be supplied in granular form. The concentration of the supplied liquid solution varies, and users should establish the concentration with the supplier. Typically, alum solutions contain 7.5–8.4% $\text{Al}_{2}\text{O}_{3}$ w/w (i.e. 43–50% w/w $\text{Al}_{2}\text{(SO}_{4}\text{)}_{3}\cdot\text{14H}_{2}\text{O}$, and have a specific gravity of 1.28–1.34 at 20°C. Solutions at the upper end of the available strengths may become unstable at low temperatures.

Alum is also available as a crystalline solid with varying degrees of hydration (14–18 $\text{H}_2\text{O}$). It has a pH of 1.2–3.0 and can be stored in rubber-lined containers or in fibreglass, stainless steel (type 316) or plastic.

Chemistry

Alum is produced by the reaction of sulfuric acid with an aluminium-rich ore such as refined bauxite.

In water, the aluminium ion reacts with natural alkalinity (hydroxyl or bicarbonate) or added alkalinity (lime, caustic soda or soda ash) to form aluminium hydroxide species. The hydrolysis proceeds as follows:

$$\text{Al}_{2}\text{(SO}_{4}\text{)}_{3}\cdot\text{6H}_{2}\text{O}\leftrightarrow\text{2Al}\text{(OH)}_{3}+\text{6H}^{+}+\text{3SO}_{4}^{2-}$$

As the hydrolysis reactions proceed, mononuclear products can form polynuclear species. The reactions are complex and the species formed are quite variable. Examples of the species formed are:

• mononuclear: $\text{Al OH}^{2+}$, $\text{Al}(\text{OH})_2^+$, $\text{Al}(\text{OH})_3$ (solid precipitate), $\text{Al}(\text{OH})_4^-$

• polynuclear: $\text{Al}_8(\text{OH})_{20}^{4+}$, $\text{Al}_{13}\text{O}_4(\text{OH})_{24}^{7+}$

The generally positively charged Al species are available to interact with negatively charged colloidal matter in water. Such matter includes inorganic turbidity particles and the high molecular weight fraction of organic compounds present in NOM. The interaction destabilises the repulsive forces between the negatively charged particles, allowing them to collide and agglomerate to form microfloc (a process referred to as adsorption–destabilisation).

At higher concentrations of alum, metal hydroxides precipitate and can enmesh any colloidal particles in a process known as ‘sweep coagulation’, which renders water suitable for clarification. Alum has an optimum pH for coagulation of 5.5–7.5, with the lower end of the range (pH 5.5–6.2) being used for organics removal and enhanced coagulation (see below), and the higher end (pH 6.5–7.5) being used for sweep coagulation. Adsorption–destabilisation to form small floc, which can be removed by contact and direct filtration, typically occurs in the pH range 6–7.

‘Enhanced coagulation’ refers to coagulation at low pH with high doses of alum, and is used to remove NOM. The pH and alum dose need to be optimised, to maximise the removal of dissolved organic carbon (DOC).

Typical use in Australian drinking water treatment

The dose of alum used depends on the properties of the raw water, including (but not limited to) the turbidity, DOC, temperature and alkalinity. Waters of low turbidity often need higher doses of alum to bring about coagulation than more turbid waters. Indeed, waters of low turbidity and high colour are the most difficult to treat.

Typical alum doses (expressed as mg/L $\text{Al}_{2}\text{(SO}_{4}\text{)}_{3}\cdot\text{14H}_{2}\text{O}$) range from 5 to 200 mg/L and may even be as high as 500 mg/L if the water is particularly dirty.

The dose rate for alum is expressed in different units throughout Australia, and it is important to take this into account when comparing rates.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. In Australia, alum is produced by reacting aluminium trihydroxide or refined bauxite with sulfuric acid, and most of the impurities in the alum are derived from these raw materials.

The following chemical contaminants may be present in alum (NRC 1982):

• antimony

• arsenic

• barium

• beryllium

• cadmium

• chromium

• copper

• fluoride

• iron

• lead

• magnesium

• manganese

• mercury

• nickel

• phosphorus

• selenium

• silver

• thallium

• zinc

Residual and by-product formation in drinking water

When employed in drinking water treatment, alum 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.

Aluminium residuals after filtration can cause floc to form in the distribution system, which can cause customer complaints. To minimise residual levels of aluminium, alum should be used at pH and dosage conditions that exceed the solubility of aluminium. At 25°C, aluminium is least soluble at a pH near 6. At colder temperatures, the pH of minimum solubility increases. For example, at 4°C, aluminium is least soluble at pH 6.5–7. Hence, if water is treated at pH 6 throughout the year, levels of residual dissolved aluminium will be higher in winter. Poor dosage selection or inadequate mixing also leads to elevated aluminium residuals.

Status

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

References

Amirtharajah A and Mills KM (1982). Rapid Mix Design for Mechanisms of Alum Coagulation. Journal of the American Water and Wastewater Association 74(4):210–216.

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B403-98. 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.

Letterman RD, Amirtharajah A and O’Melia CR (1999). Coagulation and Flocculation. 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, 6.1–6.66.

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