THE NEED FOR DESALINATION AND TYPES OF TECHNOLOGIES FOR DESALINATION
As the world population is growing, the need for freshwater is increasing. Water desalination is a means for producing fresh water from saline water abundant in seas and oceans. Various technologies have been used to desalinate saline water with different performance characteristics. This article outlines current desalination technologies and compares their performance in terms of input and output water quality, amount of energy required, environmental impact and cost. It was found that adsorption desalination technology is a promising method for desalinating seawater due to its low running cost and low environmental impact as it uses waste energy resources.
Desalination technologies are divided into three major groups, namely: (i) thermally-activated systems in which evaporation and condensation are the main processes used to separate salts from water, (ii) pressure-activated systems where a pressure is applied on the salty water that forces it through a membrane, leaving salts behind and (iii) chemically-activated desalination methods. Thermally activated systems include, multi-stage flash distillation (MSF), multiple-effect distillation (MED), vapor compression distillation (MVC), humidification – dehumidification desalination (HDH), solar distillation (SD) and freezing (Frz). In these systems, heat transfer is used either to boil or freeze the seawater or brackish water to convert it to vapor or ice, so the salts are separated from the water. Pressure- activated systems use permeable membranes to create two zones where water can pass through leaving salt behind.
These technologies consist of reverse osmosis (RO), forward osmosis (FO), electro-dyalysis (ED) and nanofiltration (NF). Chemically-activated desalination systems include ion-exchange desalination (I.Ex), liquid–liquid extraction (LLE) and gas hydrate (G.Hyd) or other precipitation schemes Recently, adsorption technology (Ads) has been investigated for desalination application. In this technology an adsorbent material with high affinity to water like silica gel can be used to separate the water from the salts.
Figure 1 shows a flow chart of the various desalination technologies.
PROCESS DESCRIPTION FOR MOST COMMON DESALINATION TECHNOLOGIES
Some of the most common types of technologies implemented in Desalination process are as follows:
- Reverse osmosis (RO):This is the most extensive and advanced desalination system in the world, used in over 60% of facilities worldwide. The reverse osmosis process consists of applying pressure to a salt-water solution and forcing it through a semipermeable membrane whose function is to allow the passage of the solvent (water) but not the solute (dissolved salts). The solvent (water) passes through the membrane, from the side where the salt concentration is highest, toward the side where the salt concentration is lower. The result is that the concentrated solution part is minimized in favor of the freshwater, which increases in quantity. This water treatment process can be performed thanks to the external application of energy in the form of pressure, which overcomes the natural osmotic pressure of the solution.
- Multiple effect distillation (MED):The process consists of heating water to the point at which it evaporates and then condenses to obtain fresh water. This desalination procedure is performed in various stages, with the temperature and pressure decreasing in each stage until the desired result is achieved. The heat obtained from the condensation also serves to distill the water again.
- Multi-stage flash evaporation (MSF):
In this process, the water is introduced in the form of fine drops into a chamber at below saturation pressure. Some of these water drops immediately convert into vapor, which later condenses to form desalinated water. The remaining water enters another chamber at lower pressure than the former and repeats the process until the desired outcome is achieved.
As the world’s population continues to grow, existing water supplies will become increasingly insufficient. Water demand and usage increase with increasing population, desalination of sea water will become an increasingly important source of usable water. Any comprehensive plan addressing mankind’s energy usage or ecologic impact must account for the effect of desalination; responsible development requires attention to the most energy-efficient methods of purifying water.
- Youssef, P. G., Al-Dadah, R. K., & Mahmoud, S. M. (2014). Comparative analysis of desalination technologies. Energy Procedia, 61, 2604-2607.