Purified water is a common term on bottled water labels, but it often leads to confusion about what exactly is—or is not—in the bottle. Unlike natural spring or tap water, which contains a variable mix of dissolved solids, purified water has been mechanically treated to remove nearly everything except the water molecules themselves. Understanding the processes used for purification helps clarify the resulting composition and its implications for daily consumption.
The Processes That Create Purified Water
True purification is achieved through methods designed to strip water of almost all dissolved substances, including minerals, chemicals, and contaminants. The two primary techniques used to produce this high level of purity are distillation and reverse osmosis (RO).
Distillation involves boiling water into steam and then condensing the steam back into a liquid. This process leaves behind all non-volatile compounds, such as salts, heavy metals, and many minerals, which have higher boiling points than water.
Reverse osmosis, by contrast, uses high pressure to force water through a semi-permeable membrane. The microscopic pores of this membrane allow water molecules to pass through while blocking larger particles, including dissolved inorganic solids and a wide range of contaminants.
Mineral Content and Composition
Purified water contains minimal, often negligible, amounts of minerals, which is the direct result of the rigorous treatment it undergoes. The mineral content of any water is measured by its Total Dissolved Solids (TDS), which represents the concentration of inorganic salts and organic matter present, expressed in parts per million (ppm). While typical tap water can range from 100 to 400 ppm, high-quality purified water often has a TDS level below 10 ppm, sometimes registering near zero.
Purification removes both desirable minerals, like calcium and magnesium, and undesirable contaminants. The goal of the process is to achieve a defined standard of purity. Because water with an extremely low TDS can taste flat, some manufacturers will introduce a small, measured amount of minerals back into the final product for improved palatability, a practice known as remineralization. This final water is still labeled as purified, but it has a slightly higher mineral content.
Health Implications of Demineralized Water
The consumption of demineralized water often raises concerns about potential negative health effects, particularly regarding the loss of dietary minerals. However, the vast majority of essential minerals, such as calcium and magnesium, are sourced from food, not from drinking water. For the average person with a balanced diet, the trace amounts of minerals found in tap or spring water contribute only a small fraction of their daily nutritional requirement.
The human body is highly adept at maintaining a stable internal mineral concentration, or homeostasis, regardless of the mineral content of the water consumed. The kidneys are responsible for balancing these concentrations, reabsorbing needed minerals when levels are low and excreting excess when levels are high.
The primary health function of water is hydration, and purified water performs this role effectively. The World Health Organization suggests that water with a very low TDS, under 50 ppm, may be less ideal because it can be less thirst-quenching and may lack the beneficial effects of calcium and magnesium intake. However, focusing on a robust diet remains the most reliable strategy for achieving proper mineral intake. The most noticeable difference for consumers is often the taste, as the removal of all dissolved solids can result in a flavor that is perceived as neutral or “flat”.