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desalination procss osmosis where plant water is used
Have been noted.
Desalination removes dissolved minerals (including but not limited to common salt) from seawater, brackish water or treated waste water. The amount of water in the sea is enormous, but before it can be used for water resources, dissolved salts in the water must be removed or substantially reduced.
Desalination of seawater could produce unlimited supplies of fresh water and could solve many water resources problems if it were possible to do it inexpensively. Unfortunately, desalination is an expensive process, the water produced generally costing more than from other sources, as it has high capital costs and requires a lot of energy, so it is not usually the first choice for a water supply.
Desalination is generally used only where there is no other possible source of water, as all other sources would be cheaper, with the possible exception of long-distance water transfer schemes. Desalination is used in wealthy but arid coastal areas, where it is economic to pay a higher price for water - Saudi Arabia produces 70% of its drinking water by desalination.
The Arabian Peninsula and Iran, for example, have a greater desalination capacity than all the rest of the world, using energy from their abundant oil resources to produce water. The four countries with over 106 m3 per day desalination capacity are Saudi Arabia, the USA, the United Arab Emirates and Kuwait.
Energy costs are a substantial part of the cost of desalinated water. It takes a considerable quantity of energy to desalinate a cubic metre of water by distillation, around 300 MJ m-3 for seawater, so the cost of desalination by this means depends directly on the energy cost. If a country has a cheap source of energy, desalination may be practical.
In California, which has a number of desalination plants, the selling cost of the water is US$1-4 m-3. A desalination plant which began operation in 1992 in Santa Barbara in California had a capital cost of US$36 million for 12 × 106 m3 a year, at a selling price of US$2.4 m-3. However, this plant has now been decommissioned as this water proved to be more expensive than other water sources.
There are various processes for desalinating water. The one most commonly used is distillation, which is similar to the natural evaporation of seawater in the hydrological cycle.
Many arid countries receive large amounts of solar energy and this can be used as the energy source in a solar distillation process. However, solar distillation needs large areas of solar stills and produces only small quantities of water - a maximum of only 5 litres per day for each square metre of still area. It is usually used only in remote villages in arid developing countries.
Larger quantities of water can be produced by distillation plants where the saline water is heated by more concentrated energy sources, and plants producing over 103 m3 per day are common.
Most of the larger plants, such as the 106 m3 per day plant in Jubail, Saudi Arabia, use the distillation method. The efficiency of distillation plants (ratio of usable output water to input water) ranges from 15-50%.
Another important desalination process is reverse osmosis. This uses high pressure to force saline water through a semipermeable plastic membrane, which filters out both suspended and dissolved substances. Reverse osmosis is more suitable for desalinating water with a lower salinity than seawater.
The world’s largest reverse osmosis plant is at Ashkelon, Israel: it can produce 3 × 105 m3 per day. The project is expected to cost US$200 million, and produce water at a production cost of around US$0.5 m-3 .
Unit 6 Rain-making
Attempting to induce an increase in precipitation by artificial means is called rain-making. Clouds consist of minute droplets of water, but not all clouds produce rain, and when it rains, it doesn’t always pour; only a small fraction of the water droplets in each cloud reaches the ground as precipitation.
The idea that human intervention - a rain-dance, perhaps - might encourage a cloud to give up a little extra water has been around since ancient times. More recently, would-be rain-makers have attempted direct intervention, by delivering various chemicals from aero-planes in an effort to wring more rain from the clouds, a practice known as ‘cloud seeding’.
There is no possibility of rain-making in cloudless arid areas - the main condition for rain-making is to have water in the atmosphere as clouds.
For rain to fall, the water droplets in clouds must condense around small particles of solid material, until it forms drops heavy enough to fall as rain. If there are no solid particles to act as nuclei for condensation, there will be no rainfall.
Cloud-seeding supplies nuclei around which condensation can begin. This will only work for clouds where the water content is high enough for the air to be supersaturated - and the warmer the air, the more water droplets it can contain before the conditions for precipitation are reached.
The substances used to seed clouds are commonly silver iodide, common salt or dry ice (solid carbon dioxide). The substance is released into the cloud from the ground, aircraft (a more expensive method) or rockets.
So why isn’t rain-making used more often?
The main reason is that if the clouds are not there, you cannot seed them, so this cuts out most of the arid areas of the world where it would be of most use.
Another reason is that it may not work for summer rainfall. Another concern is that artificially removing water from the atmosphere in one area may reduce the precipitation elsewhere - rain-making may simply redistribute the precipitation. However, research has found that there seems to be no evidence of a decrease in precipitation downwind of rain-making projects.
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