Polydiallyldimethylammonium chloride

(endorsed 2005)

Polydiallyldimethylammonium chloride (polyDADMAC) is used in the treatment of drinking water as a primary coagulant or, together with an inorganic coagulant, as a coagulation aid. PolyDADMAC reduces the quantities of floc and sludge produced.

General description

Polydiallyldimethylammonium chloride $\text{(C}_{8}\text{H}_{16}\text{N}\cdot\text{Cl)}_{n}$, (also known as polyDADMAC), is a cationic polyelectrolyte with a medium molecular weight range of 105–106 and a high charge density (50–100%). The chemical is available as a powder or aqueous solution (10–60%). PolyDADMAC is not pH sensitive and is chlorine resistant.

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

Chemistry

PolyDADMAC is produced from the diallyldimethylammonium chloride (DADMAC) monomer, which is made from allyl chloride and dimethylamine.

Typical use in Australian drinking water treatment

PolyDADMAC can be used in a conventional treatment process as a primary coagulant for neutralisation and precipitation, in place of metal salts. The positively charged polyDADMAC reacts with turbidity particles and humic substances, which are generally negatively charged. The reaction eliminates the charge, allowing the particles to agglomerate. PolyDADMAC is usually most effective with particulate material; it may be less useful than aluminium and iron salts for treating dilute inorganic suspensions and water with significant amounts of colour.

PolyDADMAC can also be used as a secondary coagulant, to partially replace inorganic salts. A small dose of polyDADMAC may significantly reduce the amount of inorganic salt required (thus reducing floc volume and improving filter run times); often, it also improves treated water quality. PolyDADMAC is used particularly in direct and contact filtration processes, where the objective of coagulation is to produce small, high-density aggregates.

In treatment of drinking water, typical concentrations of polyDADMAC are 0.2–6 mg/L (as 100% polyDADMAC). When polyDADMAC is used, together with an inorganic salt, as a secondary coagulant, concentrations are usually lower (0.2–1 mg/L). The amount of polyDADMAC required should be determined through jar testing. The chemical can be added at concentrations of up to 10 mg/L, provided that the residual concentration of the monomer (DADMAC) does not exceed 2% of the polymer, and that the concentration of the residual monomer does not exceed 0.2 mg/L in the clarified water.

At concentrations above 40%, polyDADMAC is difficult to pump, because of its relatively high viscosity. Excessive polymer concentrations can adversely affect coagulation and filtration by re-dispersing the impurities.

Being highly charged, polyDADMAC should be diluted before it is added to the main water stream, so that it mixes more easily.

PolyDADMAC is usually supplied as a liquid. If supplied as a solid, individuals should seek advice from the supplier of the polymer as how to best prepare it.

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:

• 5-hexenal

• allyl chloride

• DAD monomers

• beryllium

• cadmium

• chromium

• copper

• fluoride

• iron

• lead

• diallyl ether

• dimethylamine

• mercury

• nickel

• phosphorus

• selenium

• silver

• thallium

• zinc

Residual and by-product formation in drinking water

When employed in drinking water treatment, polyDADMAC 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.

Diallyldimethylammonium chloride residues are present in polyDADMAC.

Status

PolyDADMAC was endorsed by the NHMRC for use as a drinking water treatment chemical in 1982. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

NSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B451-98. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Bolto, B. (August, 1994). Polymeric Flocculants in Water Purification. Water Chemistry Supplement in Water Journal 21(4): 431-433.

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