A desalination process essentially separates saline (salty) water into two parts - one that has a low concentration of salt (treated water or product water), and the other with a much higher concentration than the original feed water, usually referred to as ‘concentrate’.
Thermal and membrane technologies are the two major types of technologies that are used around the world for desalination. Both technologies need energy to operate and produce potable water. Within those two broad types, there are sub-categories (processes) using different techniques.
Thermal technologies have rarely been used for brackish water desalination, because of the high costs involved. They have however been used for seawater desalination and can be sub-divided into three groups: Multi-Stage Flash Distillation (MSF), Multi-Effect Distillation (MED), and Vapor Compression Distillation (VCD). The majority of plants that use thermal technology have been built overseas, primarily in the Middle East, where energy resources have been plentiful and inexpensive.
Membrane technologies can be subdivided into two broad categories: Electrodialyis/Electrodialysis Reversal (ED/EDR), and Reverse Osmosis (RO). ED/EDR is a voltage driven membrane process that was commercially introduced in the 1960s, about 10 years before reverse osmosis (RO). Although ED was originally conceived as a seawater desalination process, the electrical process works better for lower salinity water. The few ED units that are located in Texas are those that are used in low-salinity applications such as surface water desalination.
Reverse Osmosis (RO) was commercialized in the 1970s and currently is the most widely used method for desalination in the United States. The RO process uses pressure as the driving force to push saline water through a semi-permeable membrane (a type of filter) into a product water stream and a concentrated salty water stream. Nanofiltration (NF) is a membrane process that is used for removal of salt ions such as Calcium, Magnesium, and Sulphate. RO, on the other hand, is used for removal of only Sodium and Chloride. RO processes are used for desalinating both brackish water and seawater.
The process is explained below:
Osmosis is a natural phenomenon by which water from a low salt concentration passes into a more concentrated solution through a semi-permeable membrane. The Reverse Osmosis process occurs when pressure is applied to the solution with the higher salt concentration solution, the water will flow in a reverse direction through the semi-permeable membrane, leaving the salt behind. An RO desalination plant essentially consists of four major systems:
Pretreatment is very important in RO because the membrane surfaces must remain clean. Therefore, all suspended solids must be first removed, and the water pre-treated so that salt precipitation or microbial growth does not occur on the membranes. Pretreatment may involve conventional methods such as a chemical feed followed by coagulation/flocculation/sedimentation, and sand filtration. Conventional filtration methods begin with coagulation chemicals that are used to attract particles together that will not readily settle or filter out of the water. Flocculation is a gentle mixing of coagulated raw water that allows particles now "sticky" from the addition of coagulant, to gather to form larger, heavier particles called "floc." The sedimentation process settles out larger suspended particles and the floc created through the coagulation/flocculation process and the heavy particles fall to the floor.
Pretreatment may also involve membrane filtration processes such as microfiltration (MF) and ultrafiltration (UF). MF and UF processes are fundamentally the same as RO membrane filtration except in terms of the size of the molecules they retain. The choice of a particular pretreatment process is based on a number of factors such as feed water quality characteristics, space availability, and RO membrane requirements.
High pressure pumps supply the pressure needed to enable the water to pass through the membrane and have the salt rejected. The pressures range from about 150 pound-force per square inch (psi) for slightly brackish water to 800 - 1,000 psi for seawater.
Reverse Osmosis Process consists of a pressure vessel and a semi-permeable membrane inside that permits the feed water to pass through. RO membranes for desalination are spiral wound elements that are actually constructed from flat sheet membranes. Membrane materials may be made of cellulose acetate or of other composite polymers.
In the spiral wound design, the membrane envelope is wrapped around a central collecting tube. The feed water under pressure flows in a spiral path within the membrane envelope, and pure (desalinated) water is collected in the central tube. As a portion of the water passes through the membrane, the remaining feed water increases in salt content.
Concentrated Salty Water Disposal
A portion of the feed water is discharged without passing through the membrane. Without this discharge, the pressurized feed water would continue to increase in salinity content, causing super-saturation of salts. The amount of feed water that is discharged as concentrate ranges from about 20 percent for brackish water to about 50 percent for seawater.
Stabilization consists of stabilizing the water and preparing it for distribution. The post-treatment might consist of adjusting the pH and disinfection. If the desalinated water is being combined with other sources of water supply, it is very important to ensure similar water quality characteristics in both water sources.
Two developments have helped to reduce the operating cost of RO plants during the past decade: the development of more efficient membranes and the use of energy recovery devices. The newer membranes have higher rate of water flow per unit area, improved rejection of salts, lower prices and longer service life.
It is now common to use energy recovery devices connected to the concentrate stream as it leaves the pressure vessel at about 20-50 psi less than the applied pressure from the high-pressure pump. The energy recovery devices are mechanical and consist of turbines, pressure exchangers or other devices that rotate and produce energy, thus assisting the RO process in reducing the overall energy needs. The energy recovered can be as high as 25-35 percent of the input energy for seawater RO.