Ecotoxicological assessment of distillate product from a pilot-scale brine concentrator
Internal Report 599
Supervising Scientist Division
Department of Sustainability, Environment, Water, Population and Communities
Steadily increasing process water inventory at the Ranger uranium mine has become a major operational issue for Energy Resources of Australia Ltd (ERA). Following an assessment of potential technology options ERA decided that brine concentration was the most viable technology to reduce the process water inventory. A brine concentrator produces large volumes of a purified water product (distillate) and a waste stream containing the salts present in the process water (brine concentrate). The distillate will be released into the environment via a yet to be determined method, while the brine concentrate will be returned to the tailings storage facility (TSF). Rio Tinto – Technology and Innovation (RT-TI, Bundoora, Victoria) were engaged by ERA to conduct trials on a pilot-scale brine concentrator plant. Two key aims of RT-TI trial were to (i) demonstrate that the distillate does not pose risks to operator health or the environment, and (ii) provide data to assist with designing water management and disposal systems. To assist with addressing the aquatic environment protection aspect, eriss undertook a comprehensive toxicity testing program of the pilot plant distillate. The aims of the toxicity test work were to: (i) detect and quantify any residual toxicity of the distillate and, (ii) in the event effects were observed, to identify the toxic constituent(s) of the distillate.
Initial toxicity screening of the distillate was conducted with a limited range of dilutions of the distillate using three aquatic species which had previously displayed sensitivity to treated process water permeate from the Ranger Treatment Water Plant. Specifically, Chlorella sp. (72-h cell division rate), Hydra viridissima (96-h population growth rate) and Moinadaphnia macleayi (3-brood reproduction) were exposed to Magela Creek water (MCW) control and three dilutions of the distillate (ie 0, 25, 50 and 100% distillate). Further testing was conducted on a second batch of distillate using the same concentration range and two additional species, Lemna aequinoctialis (96-h growth rate)and Mogurnda mogurnda (96-h larval survival). The toxicity of the second batch of distillate was also assessed using Chlorella sp., H. viridissima and M. macleayi, although only at 0 (MCW) and 100% distillate, in order to assess the inter-batch reproducibility of the test methods.
In order to identify the toxic constituents of the distillate, a range of Toxicity Identification Evaluation (TIE) tests were conducted using the sole sensitive species, H. viridissima. The TIE tests involved assessing the relative toxicity of distillate samples produced by specific physical and chemical manipulations to change its composition or the speciation of specific constituents of potential concern. The results enable conclusions about potential primary toxicants. Six TIE tests were conducted to identify the cause of adverse effects on H. viridissima.
The distillation process reduced all major ions, ammonia and metals to near detection limits. Some organic compounds that were not detected in the feed water were detected at low µg L-1 concentrations in the distillate. The toxicity tests results showed that the distillate was of low toxicity to four of the five organisms tested. However, the population growth rate of H. viridissima was reduced by ~50% and 100% following exposure to undiluted (ie 100%) distillate samples from the first and second batch, respectively.
Initial chemical analysis of the distillate indicated that ammonia, manganese (Mn) and an organic component were potential candidate constituents for causing a toxic response. However, initial TIE results suggested none of these constituents were causing or contributing to the observed negative effect on H. viridissima. Specifically, pH manipulation (raising pH) and stripping to remove ammonia that was present indicated that ammonia was not causing the effect. Whilst the pH manipulation suggested Mn may be contributing to the effect, the effect of addition of Ethylenediamine tetraacetic acid (EDTA, a chelating agent) indicated that this was unlikely. Removal of the organic component did not change the toxicity of the distillate, discounting organics as a cause of toxicity.
In light of the above negative findings, the issue of major ion deficiency was specifically investigated as a potential cause of the effect on H. viridissima. Firstly, Ca addition was investigated due to its importance for nematocyst function and other physiological processes in H. viridissima. The addition of 0.2 and 0.5 mg L-1 Ca to the distillate resulted in a 61% and 66% recovery relative to the Synthetic Soft Water (SSW) control, suggesting Ca deficiency as a reason for the effect of distillate on H. viridissima. An additional test was conducted that involved the addition of sodium (Na), potassium (K) and Ca at concentrations that were 0, 50 and 100% that of SSW (SSW contains 0.5, 1.0 and 0.4 mg L-1 of calcium, sodium and potassium, respectively). The results showed a 100% and 96% recovery of H. viridissima population growth rates with the addition of 50 and 100% major ions, respectively. This strongly indicates that the majority of the adverse effect from the distillate on H. viridissima was due to major ion deficiency issue rather than a chemical toxicity.
Despite the substantive removal of toxic effect by replacement of major cations, the concentrations of Mn in the distillate (130-230 µg L-1) remained a concern as they were higher than the IC10 of 60 µg L-1 previously reported for H. viridissima in circumneutral pH (6.0–7.0) soft water. Additionally, the lack of major ions in the distillate had the potential to exacerbate Mn toxicity. Therefore, the effects of Mn in the presence of reduced concentrations of major ions were examined using modifed SSW (ie pH ~6.0 with 0, 50 and 100% Na, K and Ca concentrations). Manganese concentrations of 250 µg L-1 caused a 10–20% reduction in growth rate, independent of the major ion concentrations. Consequently, in addition to the recognised issue with deficiencies of major ions in the distillate, a potential for Mn toxicity was also identified.
- Supplementation of the distillate with major ions (Ca, Na and K) may be required prior to its discharge to the off-site aquatic environment. This could be achieved actively by direct addition of relevant salts or passively by passing the distillate through a wetland system/watercourse and/or blending with mine site waters prior to discharge;
- While the conditioning of the distillate through a wetland/watercourse is likely to improve water quality (by increasing major ion concentrations and, potentially, reducing dissolved Mn concentrations), the risk of exhausting of the system’s capacity to sustainably contribute the required loading of salt may need to be considered if large volumes are to be flushed through the system;
- Further site-specific data are needed to adequately assess the environmental risk of Mn in the distillate.
- A baseline monitoring program for organic compounds in the TSF is needed to establish the likelihood of significant concentrations of sVOCs and VOCs entering the feed water, hence indicating the potential for transfer to the distillate. The distillate should also be monitored for organic compounds following the commissioning of the full-scale plant.
- The effect of anti-scalant and anti-foaming agents that may be added to the concentrator’s feed water needs to be assessed. This may be achieved with laboratory toxicity tests prior to the commissioning of the full-scale plant.
- The distillate product from the full-scale plant will need to be assessed for toxicity and, if necessary, a TIE conducted to determine the cause(s) of any measured effects.