In dealing with brine that has gone through a desalination process, there are several options for disposal, each with both advantages and disadvantages. This is an important topic to consider, since the explosion of desalination plants across the world has resulted in massive increases in reject brine that needs to be safely treated or disposed of. Desalination brine can be particularly difficult to manage compared to other sources since it is often laden with residual chemicals from treatment. Another point to be considered is that different desalination processes vary in their efficiency. RO facilities may produce more fresh water from seawater compared to thermal processes, for example, but the resulting brine is also significantly more concentrated and difficult to manage.
In selecting an ultimate disposal method, several factors are generally considered, including the volume and concentration of salts, the presence of chemicals and other contaminants, the location of the facility where it is produced as well as where it is discharged, the capacity of suitable disposal options, overall costs and expansion possibilities, and public acceptance.
Ultimately, the key to a sustainable desalination industry is implementation of an effective, low-chemical consumption, low-energy consumption, and low-cost strategy.
Minimization
Waste minimization is an important first step in handling almost any waste brine. Brines that are still mostly liquid take up a considerable amount of space, and disposal can represent a significant loss of the target product. After a typical desalination process, more fresh water can be extracted by employing additional membrane-based cycles, which in turn concentrates the brine further, including other minerals and processing chemicals. This process is energy intensive and requires special handling or additional equipment, which drives up costs. Ultimately, the brine reaches a point where further concentration is impractical, and the final waste product must still be disposed of.
Surface Water Discharge
Discharging waste brine into surface waters like rivers, lakes, and wetlands can be accommodated, but not as a reliable, long-term solution since the volume of produced brine is increasing rapidly and available surface waters and their related ecosystems are highly vulnerable to damage. In relatively small amounts, brine is diluted and the increase in mineral and salt concentrations can be insignificant if the receiving body of water is large enough. Healthy ecosystems can remove a certain number of contaminants, particularly in wetlands that include salt-tolerant plants.
When counting on natural or even constructed ecosystems, however, the receiving water’s capacity to process brine must be carefully monitored. If the plants in wetlands die from overexposure to salts, for example, it is unlikely to recover and that represents a permanent loss of purification options, as well as extremely long-term detrimental effects on the local environment.
Wastewater Treatment Plants
Some small desalination plants dispose of reject brine by simply dumping it into municipal sewers. This requires permits and the brine may not be accepted if it will bring the treated sewage outside of acceptable discharge parameters.
Injecting a certain amount of reject brine into wastewater can benefit the treatment process by lowering BOD (biochemical oxygen demand) of the mix, which helps support a healthy load of beneficial bacteria critical in wastewater treatment. On the other hand, the brine also increases TDS (total dissolved solids) in the treated water which can make it unsuitable for some re-use options. These limitations make wastewater treatment incompatible with managing a growing stream of brine waste.
Deep Well Injection
Wells intended for permanent storage of hazardous liquids like concentrated brine are designed to store those liquids thousands of feet below the ground. The wells are situated in porous subsurface rock formations that are not vulnerable to earthquakes and are not near mineral resources. These wells must be designed to eliminate any movement of wastes into or around aquifers or other underground water sources. Stringent requirements and the potential for negative environmental effects generally make the permitting, site evaluation and monitoring requirements quite expensive. There have also been increasing reports of seismic activity related to the use of deep disposal wells in places as unlikely as Oklahoma.
Since wells have a limited, predetermined capacity and they’re designed to hold, not disperse the waste brine, a steady or increasing production of brine inevitably requires additional wells. They can also be quite expensive to drill and operate. Since not all locations even have suitable geology for injection wells, reliance on this type of disposal is ultimately unsustainable.
