Potable water is water that is safe to drink and use without risk to your health. The World Health Organization defines it as water free from harmful microorganisms, dangerous chemicals, and unacceptable physical characteristics like excessive cloudiness or unusual taste. In practical terms, if you can drink it straight from the tap or bottle without getting sick, it’s potable.
What Makes Water Potable
Three categories of safety determine whether water qualifies as potable: microbiological, chemical, and physical.
Microbiological quality is the most critical factor. Water must be free of disease-causing bacteria, viruses, and parasites. Historically, germs in drinking water caused devastating outbreaks of cholera and typhoid. Today, the biggest concern in treated water systems is bacteria like E. coli and total coliforms, which signal that sewage or animal waste may have contaminated the supply.
Chemical safety covers long-term exposure risks. Contaminants like lead, arsenic, mercury, nitrates, and pesticides all have maximum allowable levels. In the United States, the EPA sets legally enforceable limits called Maximum Contaminant Levels for dozens of substances. Nitrates, for example, are a particular concern in agricultural areas where fertilizer runoff seeps into groundwater.
Physical characteristics matter too, though they’re less about health and more about whether you’d actually want to drink the water. The EPA sets secondary (non-enforceable) guidelines for things like iron, which causes a metallic taste and rusty orange staining above 0.3 mg/L, and chloride, which gives water a salty taste above 250 mg/L. Water pH should fall between 6.5 and 8.5. Below that range, water tastes bitter and metallic and can corrode pipes. Above it, you may notice a slippery feel and mineral deposits. Total dissolved solids, a broad measure of minerals and salts in water, should stay below 500 mg/L.
How Tap Water Becomes Potable
Municipal water treatment typically follows four steps, each targeting a different type of contaminant.
Coagulation: Treatment plants add salts containing aluminum or iron to the raw water. These chemicals cause dirt, debris, and tiny particles to clump together into heavier masses called flocs.
Sedimentation: Because flocs are heavier than water, they sink to the bottom of large settling basins. This step separates out the bulk of solid material.
Filtration: The clearer water on top passes through layers of sand, gravel, or charcoal. These filters catch remaining bacteria, parasites, viruses, dust, and dissolved chemicals. Some facilities use ultrafiltration membranes with pores so small that only water molecules and tiny dissolved salts pass through. Reverse osmosis, another membrane-based method, removes even more contaminants.
Disinfection: The final step kills any germs that survived filtration. Most plants add chlorine, chloramine, or chlorine dioxide. Some use ultraviolet light or ozone instead of, or alongside, chemical disinfectants. A small amount of chemical disinfectant often remains in the water as it travels through pipes to your home, preventing recontamination along the way.
Potable vs. Non-Potable Water
Non-potable water is any water not treated or tested to drinking water standards. It can look perfectly clear and still carry invisible pathogens or chemical contaminants. Common categories of non-potable water include:
- Greywater: Wastewater from showers, bathroom sinks, and laundry. It’s not safe to drink but can be reused for irrigating trees and shrubs, where soil acts as an additional filter.
- Blackwater: Wastewater from toilets, and in some states, from kitchen sinks and dishwashers. This carries the highest contamination risk and requires full treatment before any reuse.
- Untreated surface water: Rivers, lakes, and streams. Globally, 106 million people still drink directly from untreated surface sources.
Roughly 1 in 4 people worldwide, about 2.1 billion, lack access to safely managed drinking water. In wealthy countries, potable water is easy to take for granted because treatment infrastructure handles the work invisibly.
How to Test Your Water
If you’re on a municipal system, your water utility tests continuously and publishes annual quality reports. But if your home uses a private well, testing is your responsibility. The CDC recommends testing well water at least once a year for total coliforms (bacteria that indicate contamination), pH, total dissolved solids, and nitrates.
Depending on your area, your local health department may also recommend testing for lead, arsenic, mercury, volatile organic compounds, radium, or pesticides. Many state and county health departments offer affordable testing, or you can use a certified private lab. When collecting a sample, follow the lab’s instructions carefully, since improper collection can produce misleading results.
Making Water Potable in an Emergency
When your regular water supply is compromised by a natural disaster, a boil-water advisory, or a broken treatment system, you have a few reliable options.
Boiling is the simplest method. If the water is cloudy, filter it first through a clean cloth, paper towel, or coffee filter. Bring clear water to a rolling boil for one minute. At elevations above 6,500 feet, boil for three minutes. Let it cool and store it in clean, covered containers.
Chlorine bleach works when boiling isn’t possible. Use regular, unscented household bleach with a sodium hypochlorite concentration between 5% and 9%. For one gallon of clear water, add 8 drops of bleach (about half a milliliter). Stir well and let it stand for at least 30 minutes before drinking. If the water is cloudy, cold, or colored, double the amount of bleach. The water should have a faint chlorine smell after treatment. If it doesn’t, repeat the dose and wait another 30 minutes.
Water purification tablets, available at camping and outdoor stores, are another option. They use chlorine or iodine compounds and come with dosing instructions on the packaging. Neither boiling nor chemical treatment removes chemical contaminants like heavy metals or pesticides, so these methods work best when the concern is biological contamination from bacteria, viruses, or parasites.