Boiling water is a highly effective method for disinfection, but it requires a heat source, time, and fuel, making it impractical in many travel or emergency scenarios. Water purification is the process of making water safe for consumption by removing or neutralizing harmful elements like pathogens, suspended solids, and chemical contaminants. The need for alternatives has led to the development of various portable and low-cost methods that rely on chemical reactions, physical barriers, or solar energy. These techniques offer flexibility and accessibility when a traditional heat source is unavailable.
Chemical Disinfection Techniques
Chemical disinfection involves using additives to kill or inactivate bacteria, viruses, and protozoa in water. The most common and accessible chemical is household chlorine bleach, which contains sodium hypochlorite as its active ingredient. For clear water, the typical recommended dosage is 8 drops of regular, unscented bleach (containing 6% to 8.25% sodium hypochlorite) per gallon of water.
The treated water must be thoroughly mixed and allowed to stand for a minimum of 30 minutes to ensure adequate contact time for the chlorine to neutralize pathogens. After this period, the water should have a faint chlorine odor; if no smell is present, the dosage should be repeated, followed by an additional 15-minute waiting period. If the source water is noticeably cloudy or murky, the recommended dosage of bleach should be doubled to account for the organic material that consumes chlorine.
Iodine is another chemical disinfectant, often available in tincture form or commercial purification tablets. It is highly effective against many bacteria and viruses because it acts as a strong oxidant, damaging the cellular structure of the microorganisms. However, iodine leaves a distinct, sometimes unpleasant, taste in the water and is unsuitable for certain individuals. People who are pregnant, have pre-existing thyroid conditions, or have an iodine allergy should avoid using iodine for water purification.
Commercial purification tablets, such as those based on chlorine dioxide or iodine, simplify the process by providing a pre-measured dose for a specific volume of water. Chlorine dioxide tablets are particularly favored as they produce fewer taste and odor issues than traditional chlorine or iodine. These chemical methods kill pathogens but do not remove sediment, heavy metals, or most chemical pollutants.
Mechanical Filtration Systems
Mechanical filtration systems purify water by physically removing suspended solids and microorganisms using a porous barrier. These systems are categorized based on the size of the pores, measured in microns, which determines the contaminants they can block. Basic emergency filtration, such as passing water through a fine cloth, only removes large sediment before a secondary disinfection step is performed.
Commercial portable filters use membranes with specific micron ratings to remove progressively smaller particles. Microfiltration (MF) membranes typically have pore sizes ranging from 0.1 to 0.5 microns, effective for blocking bacteria and protozoa like Giardia and Cryptosporidium. However, microfiltration pores are too large to reliably capture viruses, which are substantially smaller.
Ultrafiltration (UF) systems use membranes with a smaller pore size, generally between 0.01 and 0.1 microns, offering a higher level of protection. These smaller pores allow UF to remove bacteria, protozoa, some viruses, and larger organic molecules. Unlike chemical disinfection, filtration does not change the water’s chemistry, but a filter rated for viruses or chemical removal is often necessary for comprehensive purification.
Many portable filters incorporate activated carbon, a highly porous material used to improve the water’s taste and odor. Activated carbon works by adsorption, attracting organic compounds, chlorine, and some chemical contaminants to its surface. While it enhances water quality, activated carbon does not effectively kill or remove pathogens, serving only as a complement to the physical membrane barrier.
Solar and Ultraviolet Treatment (SODIS)
Solar and ultraviolet treatment, specifically the Solar Water Disinfection (SODIS) method, is a simple, non-chemical, and low-cost way to purify water. The method utilizes the sun’s energy to neutralize pathogens by exposing water stored in clear plastic bottles to direct sunlight. Purification is achieved through two main effects: UV-A radiation and heat from infrared radiation.
The UV-A radiation damages the DNA and RNA of microorganisms, preventing them from reproducing. Simultaneously, infrared radiation heats the water; if the temperature rises above 50°C, the disinfection process is significantly accelerated due to a synergistic effect between heat and UV light.
For the process to be effective, bottles must be placed horizontally in full, direct sunlight for a minimum of six hours on a sunny or partly cloudy day. If the sky is heavily overcast, the bottles must remain exposed for two consecutive days to ensure sufficient pathogen inactivation. A limiting factor for SODIS is turbidity; the water must be relatively clear before treatment, as cloudiness prevents the UV-A rays from penetrating the water.