Tap water purification involves treating water to remove contaminants that affect its safety, taste, or appearance. While municipal systems deliver water that meets health standards, many homeowners seek purification for personal preference, such as eliminating the taste of chlorine. Water treatment is also necessary in emergency situations when the regular supply is compromised or the source is unknown. Methods range from simple heat treatments for immediate safety to advanced filtration systems for long-term home use. Understanding these techniques allows a person to choose the most appropriate solution for their specific water quality concerns.
Emergency and Microbial Safety Methods
Boiling is the most reliable method for neutralizing biological contaminants, such as bacteria, viruses, and parasites, in an emergency. Heating water to a rolling boil for a minimum of one minute effectively inactivates these pathogens, making the water microbiologically safe to drink. At elevations above 6,562 feet (2,000 meters), water should be boiled for three minutes because lower atmospheric pressure causes water to boil at a lower temperature. This method does not eliminate heavy metals, chemicals, or dissolved solids; in fact, boiling may concentrate them slightly as water evaporates.
Chemical disinfection provides an alternative when boiling is impractical, typically using unscented household liquid chlorine bleach containing sodium hypochlorite. For generally clear water, the recommendation is to add eight drops of 6% bleach or six drops of 8.25% bleach per gallon of water. If the water is cloudy or very cold, the dosage should be doubled to ensure adequate disinfection. After mixing thoroughly, the water must stand for at least 30 minutes; a slight chlorine odor should be detectable, confirming successful treatment.
Improving Taste and Removing Common Chemicals (Activated Carbon Filtration)
The most common household method for improving the aesthetics of tap water is activated carbon filtration, which primarily works through adsorption. Activated carbon is derived from materials like coconut shells or wood, processed to create a massive internal surface area with many micro- and mesopores. As water passes through, organic compounds, chlorine, and other contaminants are attracted to and trapped on the carbon’s surface through weak forces.
Activated carbon reduces contaminants like chlorine, volatile organic compounds (VOCs), and some pesticides, which cause unpleasant tastes and odors. The removal of chlorine often results from catalytic reduction, where the carbon alters the chlorine molecules into a benign form. The filter’s effectiveness depends on the contact time between the water and the carbon, meaning a slower flow rate often results in better purification.
Two primary forms of activated carbon filters exist: Granular Activated Carbon (GAC) and carbon block filters. GAC filters consist of loose carbon granules, offering a higher flow rate but potentially allowing some contaminants to bypass the media. Carbon block filters are made by compressing powdered carbon into a solid, dense block, which provides a greater surface area and forces water through a more tortuous path. This increased density and contact time make carbon block filters more effective at removing finer particulates and a wider range of chemicals than GAC filters, though their flow rate is slower.
Carbon filters are available in several practical applications for home use, with point-of-use systems being the most common for drinking water. Pitcher filters and faucet-mounted filters are simple and convenient, typically using GAC or a loose carbon block, but they offer the lowest flow rate and capacity. Under-sink systems install directly onto the cold water line and generally use larger, higher-capacity carbon blocks, providing better flow and more comprehensive contaminant reduction for daily consumption.
Comprehensive Contaminant Reduction (Reverse Osmosis and Distillation)
For the most thorough removal of dissolved solids, heavy metals, and challenging contaminants, advanced physical separation techniques are necessary. Reverse Osmosis (RO) uses pressure to force water through a semi-permeable membrane. This membrane has pores small enough to block dissolved inorganic solids and large organic molecules, effectively separating pure water from nearly all contaminants, including lead, arsenic, nitrates, and fluoride, achieving up to 99% reduction of dissolved impurities.
An RO system typically includes pre-filters, such as sediment and carbon filters, to protect the delicate membrane from clogging by larger particles and chlorine. A drawback of the RO process is its slow production rate and significant water waste, as a portion of the incoming water flushes rejected contaminants away. Furthermore, the system removes beneficial minerals, which can result in water with a flat taste, sometimes requiring a post-filter to reintroduce them.
Distillation is a phase change process that mimics the natural hydrologic cycle to produce highly purified water. Water is heated to its boiling point, creating steam that leaves behind virtually all non-volatile contaminants, including heavy metals, salts, minerals, and most biological organisms. The steam is then cooled in a separate chamber, where it condenses back into liquid water. This process is highly effective at removing inorganic compounds and dissolved solids, often achieving over 99.5% purity.
The main disadvantages of distillation for home use are high energy consumption, since it requires boiling water, and slow output compared to filtration systems. Additionally, some volatile organic compounds (VOCs) with a lower boiling point than water can vaporize along with the water and re-contaminate the product, potentially necessitating an additional post-carbon filter. While both RO and distillation produce highly pure water, RO is more energy-efficient and faster for continuous home drinking water supply. Distillation is favored when extremely low levels of dissolved solids are required.