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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications

Rainwater Harvesting And Utilisation

An Environmentally Sound Approach for Sustainable
Urban Water Management: An Introductory Guide for Decision-Makers


What Must be Considered From Quality and Health Aspects in Utilising Rainwater?

In the past, it was believed that rainwater was pure and could be consumed without pre-treatment. While this may be true in some areas that are relatively unpolluted, rainwater collected in many locations contains impurities. Particularly during the last three decades, “acid rain” has affected the quality of the collected water, to the point where it now usually requires treatment.

Rainwater quality varies for a number of reasons. While there are widely accepted standards for drinking water, the development of approved standards for water when it is used for non-potable applications would facilitate the use of rainwater sources.

In terms of physical-chemical parameters, collected roof water, rainwater and urban storm water tend to exhibit quality levels that are generally comparable to the World Health Organisation (WHO) guideline values for drinking water. However, low pH rainwater can occur as a result of sulphur dioxide, nitrous oxide and other industrial emissions, hence air quality standards must be reviewed and enforced. In addition, high lead values can sometimes be attributed to the composition of certain roofing materials – thus it is recommended that for roof water collection systems, the type of roofing material should be carefully considered.

A number of collected rainwater samples have exceeded the WHO values in terms of total coliform and faecal coliform. The ratios of faecal coliform to faecal streptococci from these samples indicated that the source of pollution was the droppings of birds, rodents, etc.

Currently, water quality control in roof water collection systems is limited to diverting first flushes and occasional cleaning of cisterns. Boiling, despite its limitations, is the easiest and surest way to achieve disinfection, although there is often a reluctance to accept this practice as taste is affected. Chlorine in the form of household bleach can be used for disinfection, however the cost of UV disinfection systems are usually prohibitive. One promising area of research is the use of photo-oxidation based on available sunlight to remove both the coliforms and streptococci.

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