Water purity is often defined in two ways. Scientifically, pure water is only H₂O, meaning all dissolved solids (TDS) have been removed, resulting in a reading near zero. For household use, however, “pure water” usually refers to purified water—water that is safe and pleasant to drink because harmful contaminants and pathogens have been filtered out. The necessary level of purification depends entirely on the water’s intended purpose.
Emergency Safety: Boiling and Chemical Disinfection
When a water source is compromised, the immediate concern is eliminating disease-causing microorganisms. Boiling water is the most reliable method for microbial disinfection, requiring a rolling boil for at least one minute to neutralize bacteria, viruses, and protozoa. This process is highly effective against biological threats, but it does not remove chemical pollutants, heavy metals, or mineral salts, which remain concentrated as the water evaporates.
If boiling is not possible, chemical disinfection offers a temporary solution for making water potable. Unscented household bleach (sodium hypochlorite) can be used, typically requiring 6 to 8 drops per gallon for clear water. After adding the chemical, the water must sit for at least 30 minutes to allow the disinfectant to work. These methods are strictly for pathogen removal during emergencies and should not be relied upon for long-term contamination issues.
The Gold Standard: Water Distillation
Distillation produces water with a very high purity level. The process begins by heating water to a boil, converting the liquid into steam. This steam leaves behind virtually all contaminants, including minerals, salts, heavy metals, and non-volatile organic compounds.
The purified steam is then collected and cooled in a separate chamber, causing it to condense back into liquid water. This resulting liquid is chemically pure, often achieving a TDS reading of 1 to 5 parts per million (ppm). While highly effective at demineralizing water, distillation is an energy-intensive and slow process, making it less practical for daily high-volume consumption.
Modern Home Purification Systems
For convenient, high-volume purification, modern home systems, particularly Reverse Osmosis (RO) units, are the standard choice. An RO system works by forcing water under pressure through a semi-permeable membrane with microscopic pores. These pores block dissolved inorganic solids, including salts, nitrates, and many heavy metals, while allowing purified water molecules to pass through.
The RO process is usually preceded by multi-stage filtration, using a sediment filter to remove larger particles and an activated carbon filter to adsorb chlorine, pesticides, and volatile organic compounds. While RO water is very pure, it may retain trace amounts of certain low-molecular-weight contaminants and typically has a slightly higher TDS than distilled water. A drawback of RO is that it generates wastewater containing the concentrated contaminants, which flows down the drain.
Matching Purity Levels to Intended Use
The required purity level of water must align with the specific application to ensure performance and safety. For drinking water, an RO system or advanced multi-stage filter is often preferred, as the trace minerals they retain contribute to a more palatable taste. An optimal TDS range for flavor generally falls between 50 and 150 ppm. Distilled water, while safe to drink, often tastes “flat” due to the complete absence of dissolved solids.
Appliances that use heat and steam, such as clothes irons, CPAP machines, and humidifiers, require high-purity water to prevent damaging mineral buildup. For these uses, distilled water is the only suitable choice because it contains almost no dissolved solids. Emergency situations demand rapid microbial safety, making boiling or chemical disinfection the most appropriate and fastest method, despite their inability to remove chemical contaminants.