If rainwater is used for drinking, it is often filtered first. Filtration (such as reverse osmosis or ultrafiltration) may remove pathogens which may be present in rainwater (such as cryptosporidium and giardia), or for chemical contamination in some areas. While rain water does not contain chlorine, contamination from airborne pollutants, which settles onto rooftops, may be a risk in urban or industrial areas. Many water suppliers and health authorities, such as the New South Wales Department of Health, do not advise using rainwater for drinking when there is an alternative mains water supply available. However, reports of illness associated with rainwater tanks are relatively infrequent, and public health studies in South Australia (the Australian state with the highest rainwater usage rate) have not identified a correlation. Rainwater is generally considered fit to drink if it smells, tastes and looks fine. However some pathogens, chemical contamination and sub-micron suspended metal may produce neither smell, taste and not be visible to the eye.
There are certain paints and roofing materials which may cause contamination. In particular, a Melbourne Water publication advises that lead-based paints never be used. Tar-based coatings are also not recommended, as they affect the taste of the water. Zinc can also be a source of contamination in some paints, as well as galvanised iron or zincalume roofs, particularly when new, should not collect water for potable use. Roofs painted with acrylic paints may have detergents and other chemicals dissolve in the runoff. Runoff from fibrous cement roofs should be discarded for an entire winter, due to leaching of lime. Chemically treated timbers and lead flashing should not be used in roof catchments. Likewise, rainwater should not be collected from parts of the roof incorporating flues from wood burners, overflows or discharge pipes from appliances such as air-conditioners or hot-water systems.
Maintenance includes checking roofs and rain gutters for vegetation and debris, maintaining screens around the tank, and occasionally desludging (removing sediment by draining and cleaning the tank of algae and other contaminants) which should be carried out every three to four years. It should be noted that the warranty on most plastic or lined tanks are void if any damage is caused by this procedure.
