You can usually figure out what's in a bottle of water by the stated water type -- the FDA regulates the use of terms like "spring water" and "purified water." There are six primary bottled-water types, and several more variations that combine a couple of the basics:
- Artesian water: Artesian water comes from an artesian well, which draws water from a confined aquifer (an underground, porous rock or sand formation that bears water and is under pressure from a layer of rock or clay above it). The pressure from the confining layer forces the water from the aquifer upward. The level of the water supply the artesian well is drawing from must sit above the uppermost layer of the aquifer.
- Mineral water: Mineral water is spring water that has at least 250 parts per million (ppm) of total dissolved solids (TDS) in it. These "dissolved solids" are minerals like calcium, magnesium, sodium and potassium, and they must be present in the water at the source, not added later. By contrast, regular spring water typically has about 50 ppm of TDS.
- Naturally sparkling water: Naturally sparkling water comes from a spring or artesian well and has natural carbonation in it. The carbonation may be removed during processing and then replaced, but the carbonation levels after replacement must be the same as the level of carbonation at the source.
- Purified water: Purified water comes from either a protected underground source or from a municipal drinking-water supply (public tap water). It has been "demineralized" -- treated by one or more of several processes to remove dissolved solids. If the water comes from a municipal water source that uses chlorine as a disinfectant, the manufacturer may further treat the water to remove the chlorine.
- Spring water: Spring water comes from a protected, underground water source from which the water flows to the surface on its own. It may be collected either at the surface spring or through a sanitary, protected hole drilled directly into the source feeding the spring. If the water is collected through a hole drilled into the source, it has to have exactly the same composition as the water in the surface spring.
- Well water: Well water is drawn from a protected well that taps directly into an unconfined aquifer.
Of the basic types of bottled water, the only one required by definition to undergo further treatment once it's drawn from the source (or purchased from the municipal water supply) is purified water. The most common treatment processes for purified water include:
- Filtration: The water is sent through filters or membranes whose holes will only let through extremely small particles, typically those smaller than 1 micron. Contaminants larger than 1 micron, including many germs and inorganic solids, can't make it through.
- Distillation: The water is vaporized. Since minerals don't vaporize, all that's left after the vapor recondenses is demineralized water.
- Reverse osmosis: The water is forced through semipermeable membranes not porous enough to let minerals or other contaminants through. For more in-depth information, see How does reverse osmosis work?
- Ozonation: Ozone gas (the same type found in the atmosphere), typically created by subjecting oxygen to electrical current, is an antimicrobial agent -- it kills microorganisms. The water is infused with ozone (03) molecules as a disinfecting process, and the molecules naturally break down and leave the water fairly quickly. As an additional benefit, when the ozone molecules degrade to 02 and molecules, this leaves free oxygen ions to bond with other contaminants like iron and sulfur. When the oxygen bonds to these molecules, it turns them into oxides, which are insoluble. These now-insoluble contaminants are then filtered out.
- UV-light treatment: Ultraviolet (UV) light is a disinfecting agent -- it kills microorganisms, including bacteria and viruses, although some microorganisms are less affected by it than others. The process uses no chemicals. Instead, the water is subjected to intense UV light inside a chamber. The UV light damages microorganisms at the cellular level, either killing them or causing them to lose the ability to replicate. A virus that can't replicate is harmless.
Aquafina is an example of purified drinking water. It begins as municipal tap water and then undergoes a treatment process that Aquafina calls the "HydRO-7(tm) purification system." Here is the HowStuffWorks interpretation of Aquafina's marketing-friendly description of the HydRO-7(tm) process:
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This animation shows the ideal -- nothing's left at the end of the purification process except H20. But according to Aquafina, what's left is actually 4 parts per million (ppm) of total dissolved solids -- still significantly less than the FDA regulation stating that purified water should have no more than 10 ppm of TDS.
In the next section, we'll examine some of the regulations affecting bottled water and find out how they compare to EPA standards for tap water.