Brine rejection is an unavoidable consequence of purifying saline water, primarily through desalination processes. This term describes the creation of a highly concentrated salt solution, or brine, that remains after freshwater is extracted from sources like seawater or brackish groundwater. The resulting hypersaline effluent is a concentrated byproduct of clean water production. Its management presents a significant challenge as the world increasingly relies on desalination to meet rising water demand.
The Mechanism of Brine Rejection
The creation of rejected brine is an inherent physical requirement of the dominant desalination method, reverse osmosis (RO). Reverse osmosis works by applying high pressure to the source water, forcing the water molecules through a semi-permeable membrane. This membrane is engineered with fine pores that allow small water molecules to pass through to the low-pressure side, yielding purified water.
The dissolved salts, minerals, and other impurities, known collectively as solutes, are physically too large or possess an electrical charge that prevents them from passing through the membrane pores. As water is continuously pushed across the membrane, the concentration of these rejected solutes steadily increases on the high-pressure side. The system must continuously discharge a portion of this concentrated solution to prevent the salts from accumulating and damaging the membrane or halting the process.
The pressure required to drive the process must overcome the natural osmotic pressure, which is the force that would otherwise cause freshwater to flow into the saltier solution. As the salt concentration of the remaining water increases, the osmotic pressure also rises, demanding higher operating pressures to maintain the separation. For typical seawater RO systems, the concentrated brine stream represents between 50% and 60% of the original intake volume.
Characteristics of Rejected Brine
The specific nature of the rejected brine is determined by the source water quality and the pre-treatment chemicals used at the plant. Its defining feature is an extremely high concentration of Total Dissolved Solids (TDS), often containing twice the salinity of the original source water. For instance, if seawater has a salinity of around 35,000 milligrams per liter (mg/L), the rejected brine can easily exceed 70,000 mg/L.
Beyond naturally occurring salts like sodium chloride, magnesium, and calcium, the brine also contains chemical residues added during purification. Pre-treatment involves substances such as anti-scalants to prevent mineral buildup, coagulants to remove suspended particles, and sodium hypochlorite for disinfection. Since these chemicals do not pass through the RO membrane, they are concentrated and discharged along with the salts.
The brine is often discharged at a higher temperature than the receiving water body due to the mechanical energy input from the high-pressure pumps. This thermal energy, combined with the high salinity, can lead to a reduction in the dissolved oxygen levels within the brine. Trace amounts of heavy metals, such as nickel, iron, and chromium, may also be present due to corrosion within the plant’s piping and equipment.
Environmental Management and Disposal
The disposal of rejected brine presents a significant environmental challenge due to its density, temperature, and chemical load. When discharged into the ocean, the hypersaline, dense brine tends to sink rapidly and spread along the seabed, creating a layer of highly concentrated salt water. This can alter the local environment, negatively impacting benthic communities—organisms living on the seafloor—that are not adapted to high salinity levels and low oxygen conditions.
To mitigate this impact, the most common disposal strategy for coastal plants is sub-surface ocean discharge using diffusers. Diffusers are specialized systems that rapidly mix the brine with a large volume of ambient seawater at a high velocity, promoting quick dilution before the effluent can settle. Proper placement of these outfalls, guided by environmental studies, is necessary to protect sensitive marine habitats.
For inland desalination facilities, direct discharge into surface water is often not feasible, necessitating alternative management methods. Deep well injection involves pumping the brine into deep, porous geological formations far below usable aquifers. Another technique is the use of evaporation ponds, which rely on solar energy to evaporate the water, leaving behind solid salts that can be landfilled or repurposed.
Some facilities are also exploring Zero Liquid Discharge (ZLD) technologies. ZLD uses specialized concentrators and crystallizers to recover virtually all the water and produce a solid salt product, eliminating the liquid waste stream entirely.