Once freshwater has been used for an economic or beneficial purpose, it is generally discarded as waste. In many countries, these wastewaters are discharged, either as untreated waste or as treated effluent, into natural watercourses, from which they are abstracted for further use after undergoing "self-purification" within the stream. Through this system of indirect reuse, wastewater may be reused up to a dozen times or more before being discharged to the sea. Such indirect reuse is common in the larger river systems of Latin America. However, more direct reuse is also possible: the technology to reclaim wastewaters as potable or process waters is a technically feasible option for agricultural and some industrial purposes (such as for cooling water or sanitary flushing), and is a largely experimental option for the supply of domestic water. Wastewater reuse for drinking raises public health, and possibly religious, concerns among consumers. The adoption of wastewater treatment and subsequent reuse as a means of supplying freshwater is also determined by economic factors. Human excreta and wastewater contains useful materials. These are water, organic carbon and nutrients and should be regarded as a resource. In their natural cycles, they are broken down by micro-organisms and become accessible to plants and animals, thus sustaining natural ecosystems. When improperly disposed, these substances can cause pollution. This is because the organic materials exert oxygen demand, and the nutrients promote algal growth in lakes, rivers and near-shore marine environments. Human excreta and wastewater also contain pathogens. Reuse of the wastes must ensure that public health is maintained. Planned reuse is the key to wastewater reuse. Planning for reuse ensures that public health and protection of the environment are taken into account. Reuse of treated wastewater for irrigation of crops, for example, will need to meet (i) standards for indicator pathogens, and (ii) plant requirement for water, nitrogen and phosphorus. WHO and others have developed standards for reuse of wastewater for various purposes. Further details of these standards can be found in the Regional Overviews in the Source Book, published by IWA and IETC. It must be pointed out, however, that requirements for water and nutrients are plant-specific and site-specific (dependent on soil type and climate), and information on these requirements need to be obtained from local information sources
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