Interestingly, the treated water coming out of this process is often too clean to discharge as it can cause harm to aquatic ecology. To overcome this, treated water is blended with partially treated water (water treated through all other stages except for RO) to make it more suitable for it’s intended use.
While disposal of treated waste water to rivers and streams has been permitted in the Surat and Bowen Basin, it is generally preferable to find more beneficial uses and more responsible long term solutions for the water.
DERM guidelines prioritise the disposal options that should be favoured. The most preferable option is that produced water be either provided as a substitution for a licensed groundwater allocation (ie. providing the water to local landholders to reduce their own groundwater requirement) or to inject it back into a local aquifer. Given the large volumes of produced water, injection will ultimately be required in addition to providing a supply to the local community.
The main benefit of injection for landholders and the environment is that it works towards balancing the net loss of groundwater and can be directed to mitigate the drawdown effects in shallower aquifers. Injection can be targeted to specific aquifers and areas that groundwater models have predicted drawdown effects may occur making this a useful tool to mitigate landholder impacts. Currently, the target aquifers have not been defined by any of the major operators but they would most likely include the Springbok and Hutton Formation aquifers, and potentially the Condamine River Alluvial aquifer. Various studies are under way to further define the viability of different injection methods and locations.
While this all sounds good so far, there are problems with the process. The quality of water being injected must be carefully monitored and matched to the quality of the formation it is being injected to. Differences in pH, salinity or mineral composition can result in range of chemical reactions that often result in minerals of the aquifer being dissolved, or new minerals being precipitated out effectively clogging the aquifer around the injection well.
However, the most pressing concern is the disposal options for the highly saline brine produced by the RO treatment process. This brine has a salinity similar to that of sea water. Several disposal options are being considered. Evaporation is not likely due to the risk of leakage from evaporation ponds impacting on shallow aquifers and DERMs avoidance of evaporative disposal. Shipping or piping the brine to the coast does not appear to be economically viable.
Currently, the most likely disposal option is the injection of the brine to a very deep, isolated formation that has no perceived value. The hard rock basement beneath the sedimentary aquifers is the most likely target. The brine injection wells would need to be fracked to improve the injection rates in the otherwise low permeability basement.
The potential for the brine to move into fresh aquifers is the key concern. This is generally considered unlikely as the brine is much denser than the other water and will sink to the bottom. Combined with the hydraulic isolation of the deep layers, this appears to be the most likely scenario.
Injection trials conducted by both DERM (focussing on the Condamine Alluvium) and the various operators are underway. I’ll offer a review of these reports when they’re released.