▶ Desalination is a process that removes dissolved minerals(including but not limited to salt) from feedwater sources such as
seawater, brackish water or treated wastewater brackish water (TDS1,500 mg/l), and seawater.
The process is explained below:
▶ A desalination process essentially separates saline 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
brine concentrate or simply as ‘concentrate’.
▶ The two major types of technologies that are used around the world for desalination can be broadly classified as either
thermal or membrane.? Both technologies need energy to operate and produce fresh water. Within those two broad types,
there are sub-categories (processes) using different techniques. The major desalination processes are identified in Table
(Desalination Technologies and processes)
- Reverse Osmosis(RO)
- Multi-Stage Flash Distillation (MSF)
- Multi-Effect Distillation (MED)
- Membrane Distillation
- Electrodialysis reversal(EDR)
- Vapor Compression Distillation (VCD)
- Solar Humidfication
▶ Thermal and membrane capacity on a worldwide basis was about 7 billion gallons per day (bgd) in early 2000, with about 50%
in thermal processes and 50% in membrane technologies.This is total installed capacity since the early 1950s, and not all of that
capacity may be in operation. On a global basis, desalination capacity increased at almost 12 percent per year, from 1972
through 1999. There have been over 8,600 desalination plants installed worldwide, with approximately 20 percent of them in the
U.S., the largest number of any countryin the world. In terms of capacity however, the U.S. ranks second globally
(U.S. Department of the Interior, 2003).
▶ Membrane technologies can be subdivided into two broad categories: Reverse Osmosis (RO), and Electrodialyis(ED) or
Reverse Osmosis (RO)
▶ In relation to thermal processes, Reverse Osmosis (RO) is a relatively new process that was commercialized in the 1970s
(Buros, 2000). Currently, RO 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 into a product
water stream and a concentrated brine stream. Nanofiltration (NF) is also a membrane process that is used for removal of divalent
salt ions such as Calcium, Magnesium, and Sulphate.? RO, on the other hand, is used for removal of Sodium and Chloride. RO processes are used for desalinating brackish water (TDS>1,500 mg/l), 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. 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.
This is known as the Reverse Osmosis process or RO process.
▶ An RO desalination plant essentially consists of four major systems:
• Pretreatment system
• High-pressure pumps
• Membrane systems
▶ Pre-treatment 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.
Pre-treatment may involve conventional methods such as a chemical feed followed by coagulation/flocculation/sedimentation,
and sand filtration, or pre-treatment may involve membrane processes such as microfiltration (MF) and ultrafiltration (UF).
The choice of a particular pre-treatment process is based on a number of factors such as feed water quality characteristics,
space availability, RO membrane requirements, etc.
▶ 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 psi for slightly brackish water to 800 - 1,000 psi for seawater.
▶ The membrane assembly consists of a pressure vessel and a semi-permeable membrane inside that permits the feed water to
pass through it. RO membranes for desalination generally come in two types:
Spiral wound and Hollow fiber. Spiral wound elements 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.
A portion of the feed water is discharged without passing through themembrane.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.
▶ Another type of membrane is the hollow fiber design which places a large number of hollow fiber membranes in a pressure vessel.
The pressurized saline water is introduced into the vessel along the outside of the hollow fibers. Under pressure, desalinated
water passes through the fiber walls, and flows in the hollow fibers for collection.This type of design is not as widely used now
as the spiral wound membranes for desalination.
▶ Post-Treatment 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.
(RO Desalination Plant)
▶ 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 flux (rate of water flow per
unit area), improved rejection of salts, lower prices and longer service life (Buros, 2000).
▶ It is now common to use energy recovery devices connected to the concentrate stream as it leaves the pressure vessel at
about 20-50psi 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 (Oklejas, et al., 1996).