Polyacrylamide

(endorsed 2005)

Polyacrylamide is used in water treatment as an aid to coagulation, flocculation, clarification, filtration or handling of sludge.

General description

Polyacrylamide, $\text{(CH}_{2}\text{CHCONH}_{2}\text{)}_{n}$, is a white crystalline solid. It is hydrophilic, with molecular weights of 1–30 million daltons, and chain lengths of 1.4 × $\text{10}^{4}$ to 4.2 × $\text{10}^{5}$ monomer units. Polyacrylamide is available in anionic, cationic or non-ionic forms, and in a variety of molecular weights and charge densities, to suit the particular characteristics of the water to be treated. It may be supplied as a powder, as an aqueous solution, dispersed in a light mineral oil or bound up in a solid cake that slowly dissolves when immersed.

Appropriate handling materials for polyacrylamide include fibreglass-reinforced plastic, polyethylene, polypropylene, polyvinyl chloride, stainless steel and coated steel.

Chemistry

Polyacrylamide is usually manufactured by the polymerisation of the acrylamide monomer (AM) to form a non-ionic polymer, polymerisation of AM with acrylic acid salts to form an anionic polymer, or polymerisation of AM with cationic monomer to form a cationic polymer.

Typical use in Australian drinking water treatment

In drinking water treatment, polyacrylamide may be added:

• as a coagulation aid, immediately after coagulation, to strengthen the precipitate formed

• as a flocculation aid, at the start of flocculation, to increase the agglomeration of the floc

• as a clarification aid, before clarification, to help settle floc, bind dissolved air bubbles to floc (in dissolved air flotation, DAF) or bind floc to microsand

• as a filtration aid, before filtration, to minimise floc shearing and to improve adsorption of floc onto the filter medium

• to backwash water, to minimise filter ripening periods

• to sludge for thickening or dewatering, to improve performance.

As a coagulation, flocculation or clarification aid, polyacrylamide is typically used at concentrations of 0.05–0.3 mg/L. As a filter aid, it is usually applied in lower doses (0.01–0.1 mg/L). For sludge handling, typical doses of polyacrylamide are 0.5–2 kg per tonne of dry solids for thickening, and 1–4 kg for dewatering.

High doses of polyacrylamide can cause clogging and blockages, particularly in filter beds. Therefore, where high doses of polymers are used in water treatment, it is best to clean filters by both air scouring and water washing. Even with relatively low doses of polyacrylamide, filter beds should be inspected regularly for signs of polymer build-up. Regular measurement of the headloss accumulation rate in a filter is also useful.

Care should be taken in making up polymer solutions to minimise the formation of lumps of undissolved polymer (referred to as ‘fish eyes’). The polymer should be mixed with the water using a well-designed eductor, so that each grain of polymer is separately introduced to the water.

The polymer solution should also be suitably aged before dosing to obtain best performance. Aging requires gentle mixing of the polymer solution for 1–4 hours (refer to manufacturer for specific polymer aging times).

While most polymers are at least chlorine resistant, making up polymer solutions with chlorinated water can reduce their effectiveness.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in this product, depending on the raw materials used:

• acetamide

• acetone

• acrylamide

• acrylic acid

• acrylonitrile

• copper

• hydroquinone

• methacrylamide

• methyl ether hydroquinone

• peroxide

• propanamide

• sulfate

Residual and by-product formation in drinking water

Polyacrylamides contain varying residual amounts of unreacted acrylamide monomer.

When employed in drinking water treatment, polyacrylamide should be used in such a way that any contaminants or by-products formed by the use of the chemical do not exceed guideline values in the Australian Drinking Water Guidelines.

Status

Polyacrylamide, acrylic acid polymers and copolymers were endorsed by the NHMRC as drinking water treatment chemicals in 1977 and 1979. The revision undertaken in 2003 did not result in any change to 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 B453-01. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Brown L and Rhead MM (1979). Liquid chromatographic determination of acrylamide monomer in natural and polluted aqueous environments. Analyst 104:391–399.

Letterman RD and Pero (1990). Contaminants in Polyelectrolyets used in Water Treatment. American Water Works Association 82(11): 87–97.

NICNAS (National Industrial Chemicals Notification and Assessment Scheme) (2002). Priority Existing Chemical Assessment No 23: Acrylamide, NICNAS, Canberra.