Quality Considerations in Utilizing Rainwater


It is generally believed that rainwater can provide clean, safe and reliable water which can be consumed without pre-treatment. This however may be used in some areas that are relatively unpolluted. Rainwater collected in many locations contains impurities. Therefore, in order to ensure quality of water, the collection systems will have to be properly built and maintained and the water shall also have to be treated appropriately for intended uses. Once rain comes in contact with a roof or collection surface, it can wash many types of bacteria, molds, algae, protozoa and other contaminants into the cistern or storage tank. Indeed, some samples of harvested rainwater have shown detectable levels of these contaminants. Health concerns related to bacteria, such as salmonella, e-coli and legionElla, and to physical contaminants, such as pesticides, lead and arsenic, are the primary criteria for drinking water quality analysis. Falling rain is generally free of most of these hazards. But, if the rainwater is intended for use inside the household, either for potable uses such as drinking and cooking or for non-potable uses including showering and toilet flushing, appropriate filtration and disinfection practices should be employed. If the rainwater is to be used outside for landscape irrigation, where human consumption of the untreated water is less likely, the presence of contaminants may not be of major concern and thus treatment requirement can be less stringent or not required at all. Depending on where the system is located, the quality of rainwater itself can vary, reflecting exposure to air pollution caused by industries such as cement kilns, gravel quarries, crop dusting and a high concentration of automobile emissions.

In many parts of the globe, Acid Rain has also 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 nonpotable 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 Organization (WHO) guidelines 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 need to 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.

*pH is the measure of acidity or alkalinity. In a scale from 0 to 14, 7 is neutral, values less than 7 represent more acid conditions, values greater than 7 represent more basic or alkaline conditions. The determination of whether water is acidic, neutral, or basic, is referred to as pH, which is a measure of the hydrogen ion concentration in water. The desired pH of potable water is pH 7, while the scale ranges from values of less than pH 7 down to pH 1 as increasingly acidic and greater than pH 7 up to pH 14 as increasingly basic. Soda pop and vinegar have a pH of about 3.0


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