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New method for water supply
Global water scarcity has risen worldwide concerned. It is reported that, The world's water consumption rate is doubling every 20 years, outpacing by two times the rate of population growth. It is projected that by the year 2025 water demand will exceed supply by 56%, due to persistent regional droughts, shifting of the population to urban coastal cities, and water needed for industrial growth. The portable fresh water which the human race needed is declining. However, water requirements for agriculture, industry and so on are on the rise.
In this condition, advanced scientist think about a new way to solve this question. Some try to turn salt water into potable water which is generally known as desalination.
Desalination describes procedures which are applied to cut down the amount of dissolved impurities in water. As a practical technique of producing potable water, desalination itself is often a costly choice which often has association with electricity generation foliages, where both electricity and extra heat are wasted.
Natural water is normally assorted on the basis of their TDS values.(Dissolved solids are often referred to as total dissolved solids (TDS), and are measured in mg/l ) There are several kinds of water desalination which is chosen depended on the TDS value of the unprocessed water.
The two main available methods of desalination are reverse osmosis and distillation followed by condensation. The main propose of desalination is removal of dissolved species from water either by thermal or membrane processes. In either procedure, the saline solution becomes concentrated, and once the concentration of salts exceeds its saturation limit, they form an unwanted deposit referred to as a fouled or scale layer. Even though precipitating salts coexist in industrial water systems, owning to the complexity of the fouling process, research has primarily been concentrated on a single salt precipitation.
Reverse osmosis performs like a very fine percolator where diluted and more concentrated solutions are parted by a selectively-permeable membrane which allowing passage of certain, especially small, molecules or ions but acting as a barrier to others. Used of biological and synthetic membranes. In this case, the selectively-permeable membrane allows big water molecules to go through, but prevents the pass of salt or other dissolved chemical impurities. (Figure 1)
Distillation is a cheaper one compared to reverse osmosis. An uncomplicated, low embodied way to desalinate impure water by distillation is under using in a lot of areas where coal resources are available. Large-scale distillation units use a process known as Multistage flash distillation, which saves energy, but the facility used is costly and sophisticated. Multi-stage flash distillation links performing on the rule that by raising the pressure in a sealed container, the boiling point of a liquid can be raised; and that by lowering the pressure in a sealed container the boiling point of a liquid can be reduced.
Considering the amount of sea water on the earth, it seems that desalination can be helpful. However, the debate raises that whether desalination can solve the current situation of water scarcity in depth and how it can help. Turning salt water into drinking water is not a solution to tackle global water scarcity, the WWF has said. the objectors hold the opinions that, firstly, desalination itself is rather expensive, high-tech equipments are always needed for desalination which may cost huge amount of money. Secondly, the procedure may release lots of greenhouse gas which, of course, is a sort of game not worth the candle to the environment.
Furthermore, "Desalinating the sea is an expensive, energy intensive and greenhouse gas emitting way to get water," said Jamie Pittock, director of WWF's global freshwater programme. So far as the most advanced reverse-osmosis plants require 3.7 kilowatt hours (kWh) of energy to produce 1,000 litres of drinking water. It can be a huge waste of heat and electricity during the process of osmosis. It is also limited that recently-built desalination plants are at large lied in coastal areas which is indicated the consumption of the desalination itself.
As being previously mentioned, desalination has a lot of limits
However, desalination plants have already played a key role in offering fresh water for drinking and agriculture in areas such as the Middle East, where freshwater supplies are at a shortage. "Water supply, on a global basis, is a problem," commented Richard Bowen, a fellow of the Royal Academy of Engineering. "Desalination is set to become more important because the demand for water is going to increase, and a large percentage of the world's population is situated in coastal areas." Most of desalination plants in Libya use the Mediterranean Sea as the major source for producing drinking and industrial waters. The discharge of the concentrates from desalination plants to the sea may carry different pollutants, which can render the water harmful for human and marine life. These pollutants include halo volatile organic compounds, heavy metals and other materials. The Libyan Authority of water and environment has established limits on pollutants, which may be dangerous to the environment and may be discharged from desalination plants. All seawater desalination plants discharge the concentrate back to the sea, for this reason simultaneous analysis of raw seawater that feeling the desalination plants and concentrates that discharging from different desalination plants were carried out.
Depended on the current circumstance, many countries supports the building of desalination plants. It is reported that Poseidon Resources has gained support from Orange County cities and water districts for its $350 million project that would convert seawater into drinking water to combat the drought plaguing the county and he US Trade and Development Agency (USTDA) will grant US$369,325 to finance a study to determine the feasibility of building a water desalination. All the signs show the population and economic benefits of desalination.
To sum up, we cannot judge desalination in a simple way. As an advanced technique, it is still expected that desalination would more or less solve the problem of global water scarcity. In another way, we can neither depend on the water desalination. Not only because the limits of technique itself, but also the fact that the sense of saving water is fundamental.