Polyaluminium silica sulfates

(endorsed 2005)

Polyaluminium silica sulfates are a relatively new group of coagulants in the treatment of drinking water. They are effective for removal of metals, colour and turbidity, and readily forms floc even in clean water.

General description

Polyaluminium silicate sulfate, $\text{Al}_{A}\text{(OH)}_{B}\text{(SO}_{4}\text{)}_{C}\text{(SiO}_{X}\text{)}_{D}\cdot\text{E(H}_{2}\text{O)}$ (also known as aluminium hydroxide silicate sulfate) are pale yellow in colour and appears slightly cloudy to clear. It is usually supplied with a minimum of 9.8% $\text{Al}_{2}\text{O}_{3}$, a basicity of about 54% and a specific gravity of 1.32–1.36 (at 25°C). It has a pH of 2.8–3.6. It can be stored in fibreglass, plastics and stainless steel.

Chemistry

Polyaluminium silicate sulfate is manufactured from alum, soda ash, sodium silicate and sodium aluminate.

Polyaluminium silicate sulfate solution is a polymerised coagulant solution containing aluminium in short chains. The high basicity of polyaluminium silicate sulfate assists in flocculation, because the coagulant does not require alkalinity to form the initial floc. The charge on colloidal particles and dissolved organics is neutralised by adsorption onto the very small flocs that form initially. Silicate compounds in polyaluminium silica sulfate help to form larger flocs faster than with many other coagulants.

Typical use in Australian drinking water treatment

Polyaluminium silicate sulfate is used as a coagulant in the treatment of water and wastewater and to assist sludge blanket formation at start up. Polyaluminium silicate sulfate forms floc rapidly, even in cold water. It tends to form floc even with clean dilution water; therefore, it should be added as supplied (i.e. undiluted).

Typical concentrations of polyaluminium silicate sulfate used in drinking water treatment depend on the quality of the water to be treated and the purpose of the treatment. Polyaluminium silicate sulfate doses are typically 5–100 mg/L, but may be higher if the water is particularly dirty. The appropriate concentration should be determined by laboratory trials. Polyaluminium silicate sulfate must be used undiluted in jar tests.

Contaminants

The following contaminants may be present depending on the manufacturing process:

• 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, polyaluminium silicate 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.

Status

Polyaluminium silicate sulfate was endorsed by the NHMRC for use as a drinking water treatment chemical in 2005.

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.

Clifford DA (1999). Ion Exchange and Inorganic Adsorption. 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, 9.1–9.91.

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

McGregor S (2002) Pass for P.A.S.S. on OHS and treatment. WaterWorks, December pp 12-15.