Water found in the forest, even from seemingly clear streams or springs, is unsafe for immediate consumption and requires purification to prevent serious illness. Natural water sources are often contaminated with microscopic pathogens, including bacteria, viruses, and protozoa. These pathogens cause waterborne diseases like giardiasis and cryptosporidiosis, leading to severe gastrointestinal distress, vomiting, and debilitating diarrhea. Ingesting untreated water can rapidly lead to dehydration and incapacitation, making effective purification techniques a fundamental skill for survival.
Pre-Treatment: Straining and Filtering Sediment
Before any disinfection method can be fully effective, the water must first be cleared of large particles and suspended sediment. This initial physical filtration, known as pre-treatment, significantly reduces the water’s turbidity (cloudiness). Highly turbid water can shield pathogens from chemical disinfectants or boiling temperatures, making the purification process less reliable.
You can improvise an effective filter by pouring raw water through a clean piece of cloth, such as a bandana or multiple layers of cheesecloth. This straining removes visible debris like silt, leaves, and mud. For more thorough filtration, construct a multi-layer filter using a container like a plastic bottle with the bottom cut off. The layers are arranged from coarse to fine: gravel, coarse sand, finer sand, and finally, crushed charcoal to absorb impurities and improve taste.
Pour the water slowly through this improvised system, collecting the filtered liquid in a separate, clean container. This process does not disinfect the water or remove microscopic pathogens. However, removing these physical contaminants ensures the subsequent disinfection step can directly target the remaining microorganisms, protecting the efficiency of your chosen purification method.
Essential Disinfection: The Boiling Method
Boiling water is the most reliable method for disinfecting water in a survival situation because the high temperature kills all major classes of waterborne pathogens. The heat effectively inactivates bacteria, viruses, and protozoan cysts like Giardia and Cryptosporidium, which are known for their resistance to some chemical treatments.
To effectively purify water, bring it to a full, rolling boil for a minimum of one minute at sea level. A rolling boil means large, continuous bubbles are rising vigorously from the bottom of the container. If operating at an altitude above 6,500 feet (2,000 meters), the boiling point is lower due to reduced atmospheric pressure, requiring a longer exposure time.
At these higher elevations, maintain the rolling boil for a minimum of three minutes to ensure complete pathogen inactivation. This method requires a fire and a metal container, which can be a drawback if those items are unavailable. Once boiling is complete, allow the water to cool and store it in a clean, covered vessel to prevent recontamination.
Chemical Purification Using Tablets and Additives
When boiling is not feasible due to a lack of fire or fuel, chemical purification offers a portable and lightweight alternative for disinfection. Commercial water purification tablets use active ingredients like chlorine dioxide or iodine to inactivate pathogens through oxidation or by disrupting cellular functions. Chlorine dioxide is preferred as it is effective against a wider range of pathogens, including Cryptosporidium oocysts, and leaves a less noticeable aftertaste than iodine.
For effective treatment, follow the manufacturer’s directions precisely regarding dosage and contact time, which is usually one tablet per liter of water. The waiting period is necessary for the chemical reaction to fully penetrate and destroy the microorganisms, often requiring 30 minutes for most bacteria and viruses. Colder or dirtier water may require up to four hours to inactivate protozoan cysts. Shaking the container periodically helps ensure the chemical reaches all surfaces, including the threads of the bottle cap.
In an emergency, common household bleach, which contains sodium hypochlorite, can be used as a disinfectant. Only plain, unscented liquid bleach (5% to 9% sodium hypochlorite) is safe for this purpose. Add 2 drops of bleach per liter of clear water, or 4 drops per liter of cloudy water, and allow a 30-minute contact time before consumption.
Emergency and Alternative Purification Techniques
For scenarios where heat and chemicals are unavailable, alternative methods can be employed, though they often require more time or specific environmental conditions. Solar Water Disinfection (SODIS) uses the sun’s natural ultraviolet (UV-A) radiation and heat to destroy pathogens. The technique involves filling a clear plastic polyethylene terephthalate (PET) bottle with pre-filtered water and laying it horizontally in direct sunlight for a minimum of six hours on a sunny day.
The UV rays penetrate the water, damaging the DNA of bacteria, viruses, and protozoa, rendering them incapable of reproduction. If the day is overcast, the exposure time must be extended to two consecutive days for adequate disinfection. SODIS is a simple, zero-cost method, but it is limited by weather conditions and cannot treat large quantities of water quickly.
Another emergency option is distillation, which involves a solar still. The still uses solar energy to evaporate water, leaving behind contaminants such as pathogens, heavy metals, and salts. The water vapor then condenses on a cooler surface and is collected as pure water. While resource-intensive and slow, producing only a small amount of water per day, distillation is the only field method that reliably removes both biological and chemical contaminants, making it a viable solution for highly contaminated or brackish water sources.