With The Middle East suffering from sever water shortages various countries in the region have resorted to relying heavily on desalination, collectively holding 33% of global planned desalination plants. The desalination process characteristically demands large quantities of energy, rendering the costs associated with desalination greater than its alternatives, which include groundwater excavation and other methods of water recycling. However, the geographic location as well as the water poor climate of the Middle East solidifies desalination as an important method to obtain consumable water.
Desalination processes traditionally are only associated with the transformation of saltwater into water fit for human consumption or irrigation purposes. However, technological advances in desalination methods is quickly allowing for a broader range of inputs into the desalination process, allowing the processing of industrial and agricultural wastewater (brackish water), all the while resulting in a more cost effective practice.
Furthermore, while desalination methods vary depending on the application, they may be categorized into two technologies:
1. Thermal Heating Technology
Thermal technology requires energy in the form of heat to obtain pure water from the distillation process of saline water vapor. The energy requirements for thermal heating are substantial; therefore thermal desalination technology is more popular for salt-water desalination rather than brackish water.
Thermal technology includes three different processes:
a. Multiple Stage Flash Process (MSF)
This process, which is shown here, uses evaporating chambers, called stages, at decreasing pressures to conduct the distillation process. The initial seawater is heated under high pressure and then enters the first chamber, which is at a lower pressure level. The pressure drop causes the water to boil rapidly causing it to evaporate. This process, known as flashing, is further repeated throughout the stages due to the declining pressures across each stage.
The vapor created from each stage is then condensed through heat exchanger tubes, which are kept at low temperatures by cold seawater water, in each stage forming freshwater. A major characteristic of the heat exchanger is that only a small percentage of the feed water is converted into vapor and condensed thus producing a small amount of freshwater.
b. Multi-Effect Distillation
Multi-effect distillation (MED), shown here, contains a series of chambers, called effects, where evaporation and condensation occurs at reduced ambient pressures. In MED, a series of evaporator effects produce water at progressively lower pressures. Due to this decrease in pressure, water is boiled at lower temperatures and subsequently the water vapor produced in the first effect serves as the heating medium for the second and so on.
c. Vapor Compression distillation
Vapor Compression distillation (VCD) may be operated as either a stand-alone process or as an add-on to an existing process, such as MED. As the name implies, vapor compression provides the heat required for water evaporation. The most commonly available configuration contains a mechanical compressor, compressing the vapor and providing heat. VCDs are most commonly found in small-scale applications such as desalination plants for individual neighborhoods, hotels, and hospitals.
2. Membrane Technology
Electrodialyis/Electrodialysis Reversal (ED/EDR)
Although ED and EDR were originally conceived as a seawater desalination process, the electrical process works better for lower salinity water (brackish water). Therefore, membrane technology has been mainly used for treating of brackish wastewater.
Electrodialysis uses an electrical potential to move salts through a membrane, leaving fresh water behind as product water, where as ED relies on the fact that most salts dissolved in water are either positively charged ions called cations or negatively charged ions called anions. Therefore ions are attracted to electrodes at an opposite electric charge. This allows for the construction of selective membranes that only allow passage for either anions or cations.
Inside the plants these membranes are placed in alternate order: Anion-permeable membrane followed by a cation-permeable membrane. As saline solution flows through the system, salt is reduced in one channel, while concentrated solutions are gathered at the electrodes in the spaces between the alternating membranes, which are called cells. One ED unit consists of several hundred cells bound together with electrodes, and is referred to as a stack. Once saline water passes through both membranes, fresh water is produced.
Reverse Osmosis (RO).
Osmosis is a naturally occurring phenomenon in which water containing a low salt concentration passes into a more concentrated solution through a semi-permeable membrane. With reverse osmosis, pressure is applied to the solution with the higher salt concentration solution allowing a reversal in the water flow through the membrane causing the salt to be blocked by the membrane, thus creating fresh water.
The RO desalination process may be subdivided into 4 different stages; the pretreatment stage, high-pressure pump stage, membrane system stage, and the post treatment stage.
The pre-treatment stage includes removing any solid material that may be contained in the water, which could cause harm to the semi-permeable membranes used later in the process. It also constitutes of water pretreatment, to ensure the membranes are free from salt precipitation or microbial growth. This pre-treatment entails methods such as using chemical feed followed by coagulation/flocculation/sedimentation, and sand filtration. Different considerations may affect the type of pre-treatment chosen, which include the quality of the feed water, space considerations, and RO membrane requirements.
The high-pressure pump stage provides the pressure needed to enable the untreated water to pass through the membrane. The pressures vary depending on the salt content of the feed water, ranging from about 150 pounds per square inch (psi) for slightly brackish water to 800 - 1,000 psi for seawater. This allows for a more effective and efficient treatment of saline water.
In the membrane system stage, RO membranes are usually either spiral wound and Hollow fiber. Spiral wound membranes, the most popular membranes, constitute of materials such as of cellulose acetate or of other composite polymers. In the spiral wound design, the membrane is wrapped around a central water collection tube. Under pressure, the feed water then flows within the spiral membrane, allowing for desalinated water to be collected within the central collecting tube (This process is shown here).
After the feed water passes through the membrane and is processed, the remaining water increases in salt content. in the post treatment stage It is necessary that a portion of the feed water is discharged without passing through the membrane, as without this, the pressurized feed water would continue to increase in salinity content, resulting in salt super saturation. The percentage of feed water which is discharged without passing through the membrane depends on the original salinity of the feed water, with an average figure ranging from 20 percent for brackish water to about 50 percent for seawater.
Finally once these 4 processes have been completed, fresh water is produced and can be used for municipal and agricultural purposes.
It is evident that with a plethora of treatment processes, creating new freshwater sources should not be difficult. Furthermore, in an attempt to make these processes less energy intensive and environmentally friendly, countries across the Middle East have been attempting to incorporate renewable energy use with desalination processes to allow for a more efficient and environmentally friendly way of producing fresh water. However in order for this to be achieved heavy investment in the technology and knowledge required is key. Only then can one look ahead for a water rich Middle East.