Rainwater, often perceived as pure, begins as evaporated moisture, leaving impurities behind. As it falls through the atmosphere and collects on surfaces, its cleanliness changes significantly. Rainwater purity depends on various environmental and collection factors, which are important to understand for different uses.
Factors Influencing Rainwater Purity
Rainwater purity is influenced by atmospheric conditions and the surrounding environment. As raindrops fall, they absorb gases and collect particulate matter present in the air, including dust, pollen, soot, industrial emissions (sulfur dioxide, nitrogen oxides), and vehicle exhaust.
Geographical location plays a role, with urban areas having higher levels of atmospheric pollutants. The initial portion of rainfall, known as the “first flush,” washes accumulated contaminants from the air and collection surfaces. This first flush can contain bird droppings, ash, and air pollution, making it more contaminated than later rainfall. Natural rain has a pH around 5.6 due to dissolved carbon dioxide, forming a weak carbonic acid. Pollutants like sulfur dioxide and nitrogen oxides can lower the pH further, leading to acid rain, which typically has a pH below 5.0.
Determining Rainwater Safety for Use
Directly consuming untreated rainwater carries health risks. Even if clear, rainwater can contain contaminants picked up during its descent and collection. These include bacteria, viruses, and parasites from sources like animal droppings, leaves, and other organic debris.
Chemical contaminants are also a concern. Rainwater can leach heavy metals such as lead, copper, and zinc from roofing materials, gutters, and piping, especially if old or containing lead flashing. Modern rainwater can also contain per- and polyfluoroalkyl substances (PFAS), potentially exceeding health guidelines. Due to these microbial and chemical hazards, drinking untreated rainwater is not recommended.
Techniques for Rainwater Purification
Making rainwater safe for consumption or other sensitive uses requires specific purification techniques. Basic filtration, using mesh screens and sediment filters, removes large debris and particulate matter. For potable uses, more advanced methods address microbial and chemical contaminants.
Boiling collected rainwater for at least one minute effectively kills most bacteria, viruses, and parasites.
Chemical disinfection, using agents like chlorine or iodine, eliminates microorganisms.
Ultraviolet (UV) treatment systems use UV light to inactivate bacteria, viruses, and protozoa by disrupting their DNA without adding chemicals.
Multi-stage filtration systems combine sediment filters, activated carbon filters (for odors, tastes, chemicals), and then UV or reverse osmosis for comprehensive purification.
Proper collection and storage are also important. This includes using clean catchment surfaces, diverting the first flush, and storing water in opaque tanks to prevent algae growth and insect access.
Common Uses for Collected Rainwater
Collected rainwater has many practical applications beyond drinking, especially when full purification is not feasible. Common uses include gardening and irrigation, as rainwater is less acidic than tap water and can contain beneficial nutrients for plants. It is also suitable for flushing toilets, reducing municipal water consumption, and for laundry, cutting indoor water use.
Other non-potable uses include washing cars, cleaning outdoor surfaces, and filling ponds or water features. For these, basic filtration to remove sediment and debris is often sufficient. While some treated rainwater can be used for bathing, any application involving human contact or consumption requires thorough purification to mitigate health risks.