Ecotoxicological assessment of a polyelectrolyte flocculant

2010

Internal Report 575
Harford AJ, Hogan AC & van Dam RA
Supervising Scientist Division
Department of the Environment, Water, Heritage and the Arts

About the report

Flocculant blocks are commonly used in water treatment processes to reduce suspended sediment loads of the water column. The mining industry has been increasingly interested in the application of flocculant blocks, with the aim of improving the quality of water released into the environment. Energy Resources of Australia Ltd (ERA) implemented the use of flocculant blocks during the 2006-07 wet season to reduce suspended sediment (and associated adsorbed metal) concentrations in a number of its on-site water bodies. To ensure appropriate on-site water management, ERA required information on whether the use of the flocculant blocks would introduce unacceptable toxicity to the Pond Waters at Ranger.

This study investigated the biological impacts of a flocculant block that contained an anionic polyacrylamide (PAM) active ingredient and a polyethylene glycol (PEG) based carrier. The toxicity of the whole flocculent block was assessed and the individual components of the block were also tested separately. Previous studies using acute tests with Northern hemisphere species indicated that the toxicity of the flocculant block was relatively low. However, this study used primarily chronic, sub-lethal tests to assess toxicity. Five Northern Australian tropical freshwater species (ie Chlorella sp, Lemna aequinoctialis, Hydra viridissima Moinodaphnia macleayi and Mogurnda mogurnda) were exposed to various concentrations of the flocculent block, PAM and PEG.The test solutions were measured for total organic carbon (TOC) as indicators of the total amount of PAM and PEG present, while total nitrogen (N) was measured as an indicator of the concentration of PAM alone. Viscosity was measured to provide a metric for assessing the physical effects of the medium itself on the organisms.

The results showed an extremely wide range of 'toxicity', with the flocculant blocks being essentially non-toxic to the duckweed, fish and algae (IC50>3600 mg L-1, IC10>780 mg L-1, nominal concentrations), slightly 'toxic' to the hydra (IC50=1180-4250 mg L-1, IC10=120-160 mg L-1, nominal concentrations) and highly 'toxic' to the cladoceran (IC50=10 mg L-1, IC10 = 5 mg L-1, nominal concentrations). Investigation of the individual components indicated that the PAM was the primary 'toxicant' in the flocculant blocks. Increased viscosity at higher concentrations of the product was one of the possible contributing factors to the adverse effects observed in the cladocerans.

Water quality trigger values were calculated using species sensitivity distributions. In the event that 95 or 99% species protection levels (equating to TVs of 0.05 and 2 mg L-1 TOC) were to be applied then it may not be possible to use measurements of TOC or N as surrogates for the flocculant block constituents. The reason for this is that such low concentrations are essentially at or below the effective detection limits for these methods of analysis. In the event that ERA wishes to monitor the presence of flocculant block constituents, a TV protection level of 80%, (ie 30 mg L-1 TOC) for mine site water bodies would make monitoring of TOC levels as a signal of flocculant block contamination achievable.