Desalination is the process of taking salty or “brackish water” and turning it into drinking water. Countries, municipalities, armed services and ships have to produce freshwater by desalination because it is limited or unavailable. Desalination technology produces freshwater and along with it industrial advancements to areas that otherwise might have remained undeveloped and inhabitable. More importantly, the health and welfare of many people have been improved by a reliable source of purified drinking water. Turning seawater into potable water has several advantages. Seawater is always available since 97% of the water on Earth is brackish. It is a reliable resource with seemingly unlimited supply capabilities. Desalination operations can be established in coastal zones and send the freshwater to distant areas. Desalination is a complex multi-stage process that involves a series of specialized filtration techniques.
Sea water desalination plant
Turning seawater into drinking water is much more complicated when compared to processing raw freshwater into potable water fit for consumption. Besides being "salty," brackish water and seawater are teeming with living, dead, organic and inorganic particulate matter. Every gallon of seawater contains millions of living and dead algae cells, bacteria, and zooplankton. Microscopic analysis also reveals fragments of marine plants, mucus from fish and invertebrates along with sand, carbonate particles and ground up crustacean shells. The latest threat to desalination plants are transparent exoploymers (TEPs). A major contributor to fouling, TEPs are gelatinous colloids (0.4-200 microns). The gels come from the surface of algae, bacteria and other aquatic organisms. The surface of the ocean may contain 3,000 to 40,000 TEPs particles per milliliter of water. TEPs carry bacteria and provide a ready nutritional supply to stimulate biofouling throughout the desalination plant. All of this suspended matter is held in suspension by water movement, swimming motion of plankton and algae, and electrostatic forces (zeta potential). The first stage of the desalination process focuses on removing the suspended solids and colloids from the water. Raw brackish water is agitated with a coagulation agent like ferric sulfate or more advanced polyelectrolyte flocculation chemicals. The idea is to collapse the electrostatic charges that keep the particulate matter in suspension so it falls to the bottom of a settling chamber or raceway. Some coagulants also cause the particles to clump together, forming heavier agglomerations that settle more quickly.
The water is now filtered through a multi-media chamber containing layers of granular media like course gravel, fine sand, garnet and anthracite. The flow rate is slow to allow capture of the particles. Suspended particles are trapped within the media, much like a large sand filter. The flow rate is specified in gallons per minute in relation to the square feet of filter media. The multi-media filter is backwashed periodically to flush away captured particulates and debris. The brackish water may also go on to a diatomaceous earth process to strain out more particles. Sediment cartridge filters act as the final pretreatment stage. The five-micron filter cartridges capture particles that slip through the coagulation and flocculation process. The number of suspended particles leaving this stage is monitored and must be kept within the system’s operating specifications to prevent clogging of the RO membrane.
MP-70R Water Pump
Brackish Water Membranes
Seawater ranges between 27,000 to 35,000 ppm total dissolved solids (TDS). The Red Sea is even higher at 40,000 to 45,000 ppm TDS. The reverse osmosis membrane has a pore size of about .001 microns, which is about 1/100,000th the diameter of a human hair. Under pressure, freshwater passes through the membrane while concentrated saltwater containing minerals and pollutants are flushed to waste or saved for further recycling and processing.
The osmotic membrane allows water to pass through it at much higher rates than dissolved salts. Some desalination designs use electrostatically charged RO membranes to improve the separation of minerals and salts from the water. Desalination systems use multiple reverse osmosis membrane filters to increase the amount of purified product water produced per day. Each RO filter housing contains a spiral-wound semi-permeable membrane. The desalination process concentrates the mineral and salt ions around the membrane film during the desalination process. However, if the salts are concentrated beyond the allowable saturation level, solid mineral scale can form on the membrane. Anti-scalant is added as a requirement of RO process design to prevent damaging mineral scale from forming inside the RO system.
Polaris UVA-24B UV Reactor