Rainwater is often viewed as naturally pure, yet collecting it for drinking requires careful preparation and treatment. Rainwater collected directly from a roof or other surface is not potable without first undergoing purification. Contaminants are collected as the water falls and travels through the collection system, making treatment necessary before consumption.
Sources of Contamination in Rainwater
Rainwater begins to pick up contaminants while still in the atmosphere, absorbing airborne pollutants such as dust, soot, and industrial chemicals. Atmospheric pollution is a particular concern in urban or industrial areas where the air quality is lower.
The collection surface, typically a roof and its gutters, introduces significant biological and chemical contamination. Bird droppings, decaying organic matter, and insect parts wash into the collection tank, introducing bacteria like E. coli and protozoa such as Giardia. Additionally, roofing materials like composite shingles, metal flashing, and gutters can leach heavy metals, including lead, copper, and zinc. These chemical contaminants cannot be removed by simple disinfection methods.
The Necessity and Process of Boiling
Boiling is one of the most reliable methods for disinfecting water contaminated with biological pathogens. The high heat effectively kills or inactivates waterborne organisms, including bacteria, viruses, and parasites like Cryptosporidium. This method addresses the microbial threats that are the most immediate cause of waterborne illness.
To properly disinfect water, it must be brought to a full, rolling boil for at least one minute. At elevations above 6,500 feet, the boiling time should be extended to three minutes to compensate for the lower boiling point. After boiling, the water should be allowed to cool and then stored in a clean, covered container to prevent re-contamination. Boiling only eliminates biological threats and does not remove chemical pollutants or heavy metals.
Other Methods for Making Rainwater Safe
Beyond heat treatment, several other methods can be used to purify collected rainwater, often in combination for comprehensive safety. Chemical disinfection involves adding a measured amount of household bleach, specifically unscented sodium hypochlorite. The standard emergency dose involves adding about eight drops of 6% bleach per gallon of water, which must then stand for at least 30 minutes to allow the chlorine to inactivate the pathogens.
Physical filtration is a common method, primarily used to remove sediment and larger protozoa. To effectively remove microscopic cysts like Giardia and Cryptosporidium, a filter with a pore size of 0.5 to 1 micron is generally recommended. Activated carbon filters can also improve the water’s taste and odor by removing certain chemicals, but filtration alone may not eliminate all viruses due to their extremely small size.
Ultraviolet (UV) light treatment uses specialized lamps to inactivate the DNA of microorganisms, preventing them from reproducing and causing infection. This method is effective against bacteria and viruses, but the water must first be pre-filtered to remove any sediment. Sediment can shield pathogens from the UV light, reducing the system’s overall disinfection performance.
Safe Collection and Handling Practices
Improving the safety of harvested rainwater begins with the collection system itself. Installing a first-flush diverter is an effective preventative measure that automatically discards the initial, dirtiest runoff from the roof. This initial flow contains the highest concentration of accumulated debris, dust, and fecal matter that settled on the roof between rain events.
Regular cleaning of gutters and collection surfaces is necessary to minimize the accumulation of organic material and sediment. Storage containers, such as barrels or cisterns, should be opaque to block sunlight, which prevents the growth of algae and other microorganisms. Finally, all stored, treated water must be kept in containers with tightly sealed lids to prevent new contaminants from entering the clean supply.