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有一種幾乎尚未被開發的水資源，就是經過處理的城市廢水，可用在灌溉或補充地下水。回收一項「廢棄」的產物且使其成為可靠的供水水源，可帶來巨大的利益。「回收廢水」是要利用污水中的營養物質為農作物提供養分，並防止它們污染水道。「回收廢水」也可以延後擴建和更換造價昂貴的新排水系統的需求，也消除廢水被排入河流和海洋所造成的問題。它透過減少自河流與湖泊中抽取的水量，從而保護了淡水生態系統。回收的廢水還有助於恢復因抽水過度而遭受損害的水生生態系統。利用回收的廢水，而非從數百公里外的地方將水運來，這樣可以節省很多能源。
以色列擁有世界上最先進的廢水回收系統；70%的污水目前都經過處理，並用於灌溉。官員們預計，到2010年，該國總供水量的五分之一將來自回收的廢水。以色列有許多的「再利用水」專案，大都採行不同的處理方法，其中一種是用藻類活性生物體來處理廢水。廢水一開始被儲存在一系列的水池中，進行厭氧微生物和需氧微生物處理，處理過的水就足以灌溉農作物了。
印度的加爾各答開闢了一些渠道，將許多未經處理的污水排放到一個飼養著魚類的天然潟湖系統中去。該市3千英畝的潟湖每年生產約6千公噸的魚，供市區的消費者食用。這些魚是安全可食用，因為潟湖裏複雜的生物性的相互作用，可將污水中有害的病原菌除去。
根據《追蹤》雜誌（1995年5~6月），南非葛蘭姆斯市政府建造了示範性附屬系統，利用細菌和其他微生物的來處理廢水。這些水池裏種植出來的藻類，為種類繁多的家畜提供了營養豐富的飼料。
隨著廢水處理技術的提昇，再利用處理過廢水的應用技術也得到了改進。已有少數城市（但其數量正在增加）開始使用經高度處理的廢水去補充飲用水的供應。例如，納米比亞首府溫荷克市，是非洲南部第一座將回收的廢水用於公共供水的城市，而且已如此進行15年以上了。
經高度處理的廢水並不能直接用管道輸送至供水系統。最常見的模式是，首先讓廢水流入湖泊、蓄水池或地下蓄水層。天然水和回收的廢水經過混合後，再按照正常程序處理，然後就可以作為飲用水被分配到社區中去。
在美國國家研究院在其1998年的一份報告中特別提到，政府和水資源管理人員在規劃廢水利用時絕對不能「走捷徑」。在決定將經過處理的廢水加入到城市供水系統之前，他們必須先充分估計：處理過的廢水中所可能存在之污染物對於人體健康的影響，並建立起各種監管、測試以及處理的綜合系統；經過處理的水，仍可能含有目前測試或處理程序尚無法確定的污染源。
減少用於處理污水過程中的水量，相對地也能夠獲得很多水。處理廢水是一種需要大量集中用水的過程，而如果沒有污水處理這個過程，這種最常使用的系統就不可能持續地擴展，而滿足目前30億人口的需求。像利用濕地這樣的天然水處理系統，往往可以代替現代的水處理技術。透過氧化池和曝氣池進行廢水回收（以供應農業用水），這種方法並不像一般所認為的需要很多的土地；但是，氧化池仍要求一定數量的土地，這仍然造成了此法推行的障礙 - 尤其是在市區。此外，它還能減少污染，降低對於化肥的需求，而且往往透過小規模、低成本技術-以當地傳統、可分散式經營且對生態無害為基礎-就可以達成。
版權歸屬 國際河網IRN，環境資訊協會（陶俊 譯， 吳國玢、李瑞玉 審校）
中英對照全文：http://e-info.org.tw/issue/water/2001/is-water-irn01102201.htm |
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A largely untapped source of water for irrigation and groundwater recharge is treated municipal wastewater. Recycling a "waste" product into a reliable water supply has huge benefits. Recycling wastewater makes use of the nutrients in sewage to feed crops and keeps them from polluting waterways. It postpones the need to enlarge and update costly new sewage discharge systems, and eliminates the problems from discharging wastewater into rivers and oceans. It protects freshwater ecosystems by reducing the amount of water extracted from rivers and lakes. Recycled wastewater can also be used to help restore aquatic ecosystems harmed from over-extraction. Using recycled wastewater instead of importing water from hundreds of kilometres away can also result in significant energy savings.
Israel has the most advanced system of wastewater recycling. Currently, 70 percent of sewage is treated and used for irrigation. Officials predict that by 2010, one-fifth of the nation's total water supply will come from recycled wastewater. Israel uses many different treatment schemes for its many water-reuse projects. One method relies on algae-activated organisms to treat the wastewater. The wastewater is initially stored in a series of ponds in which the anaerobic and aerobic treatment is sufficient to irrigate crops.
Calcutta, India, channels much of its raw sewage into a system of natural lagoons, where fish are raised. The city's 3,000 hectares of lagoons produce about 6,000 metric tons of fish a year for urban consumers. The fish are safe to eat because the complex biological interactions in the lagoons remove harmful pathogens from the sewage.
The municipality of Grahamstown, South Africa has built demonstration pending systems to treat wastewater using bacteria and other microorganisms, according to On Track (May-June 1995). Algae produced in these ponds provide a rich feed for a range of livestock (see graphic).
As the technology to treat wastewater has improved, so have the applications for the use of the water. A small but growing number of cities are beginning to use highly treated wastewater to supplement drinking water supplies. Windhoek, Namibia, for example, was the first city in Southern Africa to use recycled wastewater in its public supply and has been doing so for more than 15 years.
Highly treated wastewater cannot be piped directly into a water supply. Most commonly, wastewater is used to augment the drinking-water supply by adding it first to a lake, reservoir, or underground aquifer. The mixture of natural and reclaimed water is then subjected to normal water treatment before it is distributed as drinking water for the community.
A 1998 report by the US-based National Research Council notes that governments and water managers must not take shortcuts in planning to use wastewater. Before deciding to add reclaimed wastewater to urban water supplies, they must first fully assess health impacts from likely contaminants and develop comprehensive systems for monitoring, testing, and treatment. Reclaimed water may contain sources of contamination that cannot be determined through current testing or treatment processes.
There is also much water to be gained by reducing that used for sewage treatment. Treating waste is a hugely water-intensive process, and the commonly used systems cannot be sustainably expanded to serve the three billion people now without access to sewage treatment. Natural water treatment systems such as using wetlands often can be an alternative to modem water treatment technologies. Recycling waste for agricultural purposes by using oxidation ponds and aerated lagoons does not require as much land as is often assumed; however, the land requirement of oxidation ponds is a stumbling block for their use - particularly in urban areas. Moreover, it decreases pollution, reduces the need for fertilizers, and often can be accomplished with small-scale, low-cost technology that is based on local traditions, is decentralized and ecologically sound.
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