Water desalination process is a system that removes dissolved salt and other contaminant from water in order to obtain fresh water. This water may be used as drinking, industrial or irrigation purposes. This process is used those areas where fresh water is insufficient but unlimited supply of saline water like Saudi Arabia, UAE, USA, Israel, Kuwait, Bahrain, Qatar, Oman, Australia, North Africa. More than 100 countries use the desalination process. Without sodium chloride this water may also contains some other contaminants such as organic matters, biological and mineral contents, algae, calcium carbonate etc. These substances should be removed from saline water. Generally the TDS value of the produce water is less than 500 ppm which is suitable for most of the domestic, industrial and agricultural uses.
The saline water can be classified as seawater, brackish water, low salinity water and very salinity water. Brackish water has more salinity than fresh water but less than seawater. It can be produced in different ways such as mixing of seawater and fresh water, occur as in brackish fossil aquifers, certain human activities etc.
| Type of water | TDS value |
|---|---|
| Fresh water | Less than 1000 ppm |
| brackish water | 1000-10000 ppm |
| Highly saline water | 10000-35000 ppm |
| Seawater | Above 35000 ppm |
Types of desalination process
There are so many types of water desalination process. It can be achieved by different water treatment process such as membrane process, distillation process, ion exchange, freezing, geothermal process etc. The distillation process can be further classified into multi stage flash distillation, multiple effect distillation and vapor compression. On the other hand the membrane process can be classified into reverse osmosis, electrodialysis, nanofiltration, membrane distillation and forward osmosis. Among them the most of the plants use reverse osmosis processes whereas the multi-stage flash distillation system is in second position.
Here we describe the reverse osmosis desalination process. Mainly the process contains three steps: pretreatment, reverse osmosis, and post-treatment.
Pretreatment
Pretreatment is very significant step for reverse osmosis. This step is used to remove algae, sediment, suspended solids and other particles and adjusting the pH to increase efficiency of the next reverse osmosis step. It may contain multimedia filter tanks such as sand filter, diatomaceous earth filter or cartridge filters.
Reverse Osmosis
In RO, the pretreated water pumped with high pressure through semi-permeable membranes, therefore the freshwater separated from saltwater by keeping salt and other minerals behind. A Small percentage of salt may remain in freshwater; it may up to 500 ppm. This fresh water is suitable for consumption. On the other hand the water behind the membranes is too much salty which is known as brine. This brine needs to be discharge into the sea by diluation with sea water to reduce the impact on sea. Normally the size of the RO membrane pore is about 0.001 microns.
Post-treatment
The fresh water, after the reverse osmosis is almost ready for uses. It needs to be treated as per consumption. In post-treatment step, it may require pH adjustment, disinfection or degasification before use as potable water. For industrial uses the water needs more pure water.
Desalination pros and cons
Generally the water desalination process requires more energy compared to other processes like groundwater or rainwater treatment. Although the membranes are sensitive but the system is simple and requires low maintenance. The brine disposes very carefully so it has a negligible environment impact.