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地表的70%是覆蓋著水，但是絕大部分是不能飲用的海水；而地球上的水僅有3%是淡水，而其中只有1%是可以被輕易取得的地表水。脫鹽的過程是，自取得略帶鹹味的水或是鹽水中，除去水中的鹽分及其他不可溶解的固體成分，因此可得到淡水。
海水淡化的做法，因著很多的原因而成為一種誘人的水資源，尤其是這種供水來源幾乎是無限也不受乾旱的影響。在沿海國家，淡化海水的工程並不受政治變局影響，沒有那些因數個國家共享河流水源所產生的問題。內陸國家如果想從海邊引水，則需要額外的開銷以及合作。海水淡化技術的發展只需要符合當地的需求，用不著大規模的水利工程計劃。海水淡化計劃也不會造成當地原住民被迫遷移，改變當地居民的生活型態，或是嚴重破壞的當地生態。
脫鹽過程主要是將含鹽的水轉化成可飲用的水。此淡化工程也被利用於：淨化含硝酸鹽、除草劑和有機物質的農業排水及工業廢水；藉此改善含高量礦物質的飲用水水質；都市污水處理；改善用飲用水中的味道、惡臭和顏色。
在很多情況下，海水淡化並不是社區供水的唯一來源-雖然這可能在海水淡化的成本降低後將有所改變（對非常缺水的沿海地區來說尤為如此）。通常它與一些成本較低的供水來源一起作為社區供水的來源。1991年，世界約120個國家的海水淡化處理廠每天能夠生產155.4億升的水。在加勒比海、北非及中東的很多地區，除去鹽份的水是城市的主要供水來源。現在，沙烏地阿拉伯海水淡化生產能力名列世界第一，該國海水淡化總產量占世界總量的24%。
人們對海水淡化作用最關心的是，此過程過於昂貴、消耗的能量過多。在某些地方，海水淡化之後的水價是當地傳統水源價格的很多倍（以納米比亞乾旱的北部沿海地區為例，一個新的脫鹽水處理廠生產的脫鹽水將比當地的地下水貴35%）。然而，技術突破正開始降低淡化海水的價格（雖然還沒有以人為方式降到農業用水所支付的那麼低的水平）。人們往往將海水淡化與現有的供水系統進行成本比較，通常這種比較並沒有進行充分公正的成本-收益分析。為了公正起見，應該與開發其他新的供水來源的成本相比較（而且所有的成本都應該包括在分析之內，如環境成本和社會成本）。採用這種比較方法，人們就會發現從經濟方面和環境方面來說，海水淡化都可以和建造水壩、水渠以及進行其他新的水利建設相媲美。
去除的鹽量大大影響了淡化的成本，因為要使用一定的方法來除去鹽份。要去除的鹽份越多，脫鹽的過程也就越昂貴。海水淡化處理廠的生產能力也會影響成本，一般說來，規模較大的工廠的生產成本也就更低一些。淡化處理過程中最重要的因素是能源。目前最先進的技術所需的能源也占了總成本的30~40%。其他因素包括需要處理的水的體積和類型、所選定的處理過程、去除的鹽份的處理（濃縮）、各種規定條款、土地成本以及將水運抵、運離工廠的運輸成本。
【文章連載】
■海水淡化 (上) (下)
版權歸屬 國際河網IRN，環境資訊協會（陶俊譯，李傑、李瑞玉 審校）
中英對照全文：http://e-info.org.tw/issue/water/2001/is-water-irn01100801.htm |
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Some 70 percent of the earth's surface is water, but most of that is undrinkable seawater. By volume, only 3 percent of all water on earth is fresh water, and only about 1 percent is easily accessible surface freshwater. Water desalination is a process used to remove salt and other dissolved solids from brackish or salt water to create fresh water.
Desalination is an attractive water source for many reasons, especially because the supply is virtually limitless and unaffected by drought. For coastal countries, desalted water is not vulnerable to political changes, unlike water supply from shared rivers. For landlocked countries, piping water from the coast involves additional costs and cooperation. Desalting technologies can be built in stages to meet demand, unlike most large-scale water infrastructure projects. Desalination projects also do not lead to the displacement of indigenous peoples, changed agricultural lifestyles or serious ecological impacts.
Desalting processes are mainly used to convert salty water into drinkable water. It is also used to clean up agricultural drainage and industrial waste water contaminated with nitrates, pesticides and organic matter; to improve the quality of drinking water that is high in dissolved minerals; for municipal waste water treatment; and to improve taste, odor and color of drinking water.
In most cases, desalted water is not the sole source of a community's water supply - though this may change as the cost of desalted water goes down (and especially for coastal areas that are very short of water). It is usually combined with water from less expensive sources. In 1991, desalting plants in approximately 120 countries worldwide had the capacity to produce 15.54 billion litres a day. In many areas of the Caribbean, North Africa and the Middle East, desalted water is used as the main source of municipal supply. At this time, Saudi Arabia ranks first in total capacity with about 24 percent of the world's capacity.
The most common concerns about desalination are that the process is too expensive and consumes too much energy. In some places, desalinized water costs many times more than conventional local water sources (on Namibia's dry northern coast, for example, water from a new desalination plant is expected to cost 35 percent more than local groundwater). However, technical break-throughs are beginning to lower the price (although still not to the artificially low levels that the agriculture industry is used to paying for water). Cost comparisons for desalted water are often made to existing water supplies, which generally did not include a full, fair cost-benefit analysis when they were developed. To be fair, comparisons should be made to the cost of developing other new sources (and all costs should be included in the analysis, such as environmental and social costs). Given that scenario, desalting may be found to be financially and environmentally competitive with building dams, aqueducts and other new water infrastructure.
The amount of salt to be removed greatly affects the cost of desalting, as does the method used to remove salts. The more salts to be removed, the more expensive the desalting process. The capacity of the desalting plant also impacts costs, with larger plants generally being more economical. The most significant factor in desalinated water is energy. Energy for most current technologies amounts to about 30-40 percent of the total cost. Other factors include the amount and type of treatment required, treatment process selected, disposal of the removed salts (concentrate), regulatory issues, land costs and conveyance of the water to and from the plant. |
