Challenge: Reject brine from seawater desalination plants

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Worldwide, membrane- and thermal-based seawater desalination plants supply a significant portion of drinking, industrial, and agricultural water needs, whilst currently discharging the bulk of reject brine back to the sea at the expense of adverse environmental impacts, high carbon footprint, and the loss of a resource rich in valuable metals and a source of additional water. Despite the growing reliance on desalinated water by jurisdictions around the world to balance the forecasted reduction in groundwater resources, along with growing global population, increased environmental awareness, and a growing realisation of the values embedded in reject brine, there remains a number of fundamental challenges with sustainable use of reject brines from seawater desalination plants, namely:

 

• Nearly all proposed so-called brine mining solutions generate their own residual waste, which in the absence of ZLD (zero liquid discharge) outcomes, are considered unsustainable for broad-based large-scale application. This is a critical requirement, considering that a large proportion of seawater desalination plants are located in highly environmentally sensitive regions such as the Gulf Region, wherein the bulk of reject brine is discharged to closed seas. 

• To date, no sizeable production trials, at large pilot or commercial scale, have been undertaken to confirm the validity and robustness of process flowsheets and product quality for commercial sale from proposed brine mining solutions. Projections of reduced carbon emissions, recovery of critical metals/minerals, and generation of a significant non-water revenue stream from treatment of reject of seawater desalination plants should therefore wait for confirmation of the above. 

• Considering that more than 50% of the volume of seawater introduced to membrane desalination plants returns back to the sea in a concentration twice that of seawater, there remains a major challenge for sustainable recovery of additional water in the proposed brine mining methods, considering that the transition from thermal distillation to membrane desalination using renewable energy is a key factor in achieving circularity through green water production.