Desalination using Single Phase Processes
Single phase desalination processes always keep the source water in a liquid state.
Reverse Osmosis (RO)
Reverse osmosis is a technique where seawater is forced through thousands of tightly wrapped membranes at very high pressure. The membranes allow water molecules to pass through while dissolved compounds and chemicals are left behind.
Electrodialysis Reversal Desalination (EDR)
EDR is a process where electricity is applied to electrodes to pull dissolved salts through an ion exchange membrane to separate water from the salts. Periodically, the electric current is reversed, which drives the salts and other contaminants off the membrane surface to avoid clogging or fouling. One of EDR’s major advantages is that this ability to prevent the membranes from clogging or fouling means that added chemicals like descalers are not necessary to maintain operation.
New Technologies in Desalination:
TSSE
Temperature Swing Solvent Extraction is an emerging desalination technology that is both non-evaporative and membrane-less, making it uniquely suitable for challenging issues including high salinity reject brine from sources like hydraulic fracturing operations and other desalination plants. TSSE uses a low-polarity, recyclable solvent, with temperature-dependent solubility, to extract pure water from saline feeds using low-temperature swings compatible with sustainable energy sources like solar collectors, geothermal energy, and low-grade waste heat. A study in June 2020 demonstrated that this technology is able to produce zero-liquid discharge even from ultra-high salinity brines
Nanofiltration
In reverse osmosis, clear water is separated from salt and other dissolved compounds employing membranes which allow water molecules to pass through but prevent the passage of larger molecules. In the past few years, advanced materials scientists have developed a thin film composite nano filter membrane with ultrahigh permeance and high rejection. These qualities greatly improve the efficiency of desalination processes by rejecting a higher percentage of salt molecules while improving the passage of water molecules.
How Are Brines Different?
Brine is a solution of water and potentially high concentrations of several kinds of salts, although we usually think of NaCL (Sodium Chloride, or table salt) when we discuss brines treated to make drinking water. These brines can range from merely unsuitable for agriculture and drinking water to highly concentrated and caustic liquids that can corrode metal and even deplete oxygen in the marine environment, leading to huge mid-ocean dead zones.
Brine can come from salt lakes and oceans, from naturally occurring underground aquifers, from industrial processes like mining, hydraulic fracturing, and even agriculture. Naturally produced brines can include organic wastes, different types of salts, and a variety of minerals. Reject brines created through desalination processes are more highly concentrated and are likely to include a variety of treatment and cleaning chemicals, and potentially even some heavy metals due to corrosion of pipes and equipment.
While high quality brines are preferred for producing drinking water, the concentrated wastewater from any desalination process invariably presents significant challenges for disposal. Poor quality feed water ultimately intended for human consumption, on the other hand, usually requires multiple courses of pretreatment, depending on the technology, including mechanical filtration, coagulation, and flocculation. Chemicals such as anti-scalants and chlorine are also commonly used in pretreatment. These chemicals must then be removed again during the treatment process before the water is drinkable. Once all the impurities are removed, they remain in the reject brine at the end of the process. Strategies for the management and disposal of the very diverse natures of reject brine are an increasingly urgent focus for scientists around the world.
