Desalination

Many areas in Western Australia have access to brackish groundwater that requires treatment to remove salts before use. This process typically produces two streams, the reduced-salinity water for use and a high-salinity waste stream known as brine. Brine requires appropriate management, which is a key limitation of the desalination process. The two seawater desalination plants operating in Western Australia manage brine by discharging it back to the sea. Inland brackish water desalination systems, however, are usually smaller and have limited brine management options.

Desalination technologies

Most commercial desalination plants in Australia employ membrane-based desalination, typically reverse osmosis (RO), for brackish water (1,000 to 15,000 mg/L total dissolved solids (TDS) as a measure of mineral content). These plants are often smaller than seawater desalination plants, require pre-treatment based on the characteristics of feed water, and are generally limited to 70% recovery of reduced-salinity usable water.

Electrodialysis (ED)/electrodialysis reversal (EDR), and high-temperature thermal distillation are other well-established desalination techniques. Although EDR requires pretreatment to remove solids, microorganisms, and organic contaminants that cause deposits and clogging, it can achieve higher water recovery rates for feed water salinities below 5,000 mg/L TDS. It must be operated below current density limits to avoid water dissociation and unacceptable system efficiencies.

Conventional thermal desalination can reduce brine volumes, but it is not widely used in Australia due to its high energy use and cost.

Emerging technologies could address some of the challenges faced by conventional desalination technologies. However, their performance, reliability, operation and maintenance requirements remain to be proven at scale and under site specific conditions.

Brine management options

Brine management strategies include minimising brine production by maximising water recovery, disposing of residual brine, and finding beneficial uses for it. A review of brine management methods has been developed. Practical implementation of a brine management method requires careful site-specific consideration of environmental impacts, discharge regulatory compliance, and economic feasibility (all based on the chemical composition and quantity of the brine). In Australia, the most common options for inland brine management are evaporation ponds and land application, with few test cases of disposal in aquifers (subsurface water bodies in geological formations).

Brine minimisation options

Depending on feedwater characteristics and the desalination process, pre-treatments can increase water recovery and minimise brine waste. Some pre-treatment methods include pH and temperature control, clarification and solids separation, disinfection, oxidation, media filtration, and softening. Brine minimisation is particularly expensive, however, it is likely required because brine disposal options are limited for inland desalination.

Brine disposal options

Up to 50% of the costs for inland desalination are associated with brine disposal, depending on the disposal volume, brine characteristics, disposal method, location, distance to discharge point, and local climate.

BRINE DISPOSAL METHOD	ADVANTAGE	DISADVANTAGE	SUITABILITY FOR INLAND BRACKISH DESALINATION
Evaporation ponds	Easy to construct and operate	Limited to small brine flows, Climate dependant, Require high maintenance to mitigate environmental impacts, Potential regulatory restrictions, Cost	Generally suitable
Surface water discharge	Can be applied for medium to large brine flows depending on receiving water conditions, Less infrastructure, Low cost	May negatively impact aquatic ecosystems, Requires regular monitoring, Involves difficult and complex permit procedures	May be suitable if salt lakes are present
Sewer discharge	Easy to implement, Less infrastructure. Low cost	Limited to small size brine flows, May impact wastewater treatment plant capacity and operation	Only suitable if desalination plant is located near an urban area
Land application	Easy to implement, Less infrastructure, Low cost	Requires large land area, Limited to small plants, May require some pre-treatment, Potential impact to soil, waterways and groundwater with seepage and contaminant runoff issues	May be suitable in some areas, site specific long term environmental impact assessment required
Groundwater well injection	Minimal surface footprint	Potential groundwater contamination and other environmental impacts, Specific brine borefield infrastructure required, Cost, Difficult and complex permit procedures	May be viable in areas with suitable hydrogeology (e.g. if a saline aquifer is available), site specific long term environmental impact assessment required
Encapsulation	Long-term containment of contaminants/brine, Lower environmental impact if contained safely	Limited to small size brine flows, Lined capsules with chemically durable and heat-resistant liner material required, High capital and operating costs	Regional encapsulation facilities near brine pond or use within ponds not in operation

Beneficial use of brine

Beneficial reuse of brine is gaining more attention, because it can reduce the cost of disposal and potentially recover valuable resources like metals, non-metals, and other valuable elements and raw materials from brine wastes. However, beneficial reuse of brine may not eliminate disposal requirements entirely. The advantages and challenges of some of the brine use options are shown in the table below.

BRINE USE METHOD	ADVANTAGES	LIMITATIONS
Aquaculture	Economic benefits, May reduce salt load in brine, Potential to scale up using large quantities of brine, Co-cultivation with different species	Brine may require pre-treatment or augmentation, Requires environmental consideration and disease outbreak management, Requires residual brine management
Algae cultivation	Potential for reuse with salt and nutrient circulation, Potential carbon co-utilisation (carbon dioxide (CO2) capture and utilisation), Combined cultivation -algae, aquaculture and plants	May require pre- and post- management of brine
Salt-tolerant plant	Reuse potential for lowering brine concentration, Harvested plants can be used for fodder, food, medicine, biomass, biofuel, land stabilisation, etc.	Require experience in plant species selection, May require pre- and post- management of brine
Solar salt gradient ponds	Potential to use heat for low-temperature desalination with high incident solar radiation	Requires regular maintenance, Requires environmental consideration, Dependant on climate
Energy storage and production	Potential use as brine or as raw material in different applications (low-cost sodium-based electrode material, additive and electrolyte in sodium ion battery as alternative to lithium-ion battery, pressure restarted osmosis for electricity production)	Currently use in small quantities/scale, Requires further development and improvement in efficiencies
Recovery of value-add resources from brine	Potential targeted high-value product recovery	Purity, quantities, economic feasibilities require further investigation and developments

Regulatory considerations

Numerous legislative policies and procedures about establishing and operating desalination plants exist

These include:

• the environmental impact assessment (EIA) process covered by both federal and state regulations
• individual landholders and small-scale desalination plants (water production capacity less than 100 kL/d) require site-specific assessment of proposal and environmental approvals
• brine discharge guidelines focus on minimising the impacts of disposal to surface and groundwater systems, marine and land environments through monitoring, compliance and reporting
• policies more specifically related to groundwater management, including those related to managed aquifer recharge (MAR).

Brine management policies in Western Australia focus on compliance with environmental standards, waste management strategies, and the protection of marine and groundwater ecosystems. The actual policies may vary depending on the brine management option adopted.