The question of whether chlorine remains in purified water is a common concern for consumers. Water supplied to homes is treated with disinfectants to ensure safety, but subsequent purification processes are designed to strip away virtually all remaining additives. Understanding the distinction between standard tap water and purified water, and the methods used to achieve this purity, provides a clear answer regarding the presence of chlorine. Purification involves steps specifically intended to eliminate the chemicals added for municipal safety.
Understanding How Water Becomes Purified
“Purified water” refers to water treated beyond standard filtration, aiming to remove nearly all dissolved solids, contaminants, and chemicals. This high level of purity is typically achieved through intensive, multi-stage processes that exceed basic tap water standards. The goal is to produce water that meets or exceeds pharmacopeial standards for quality. The two most common methods used are Reverse Osmosis (RO) and Distillation.
Reverse Osmosis
Reverse Osmosis forces water under pressure through an extremely fine, semi-permeable membrane. This membrane blocks most dissolved inorganic and organic molecules, effectively serving as a molecular sieve that rejects substances larger than water molecules.
Distillation
Distillation involves boiling the water to create steam, which is then condensed back into a liquid state. Since most contaminants, including minerals and salts, have a significantly higher boiling point than water, they are left behind in the boiling chamber. Both techniques are highly effective at removing impurities, resulting in water with extremely low total dissolved solids (TDS).
Why Chlorine is Added to Public Water Supplies
Municipal water systems introduce chlorine as a powerful disinfectant for public health. This practice, known as chlorination, eliminates waterborne pathogens like bacteria and viruses. Chlorine is added at the treatment plant to neutralize harmful microorganisms before the water enters the distribution network.
A small, controlled amount of chlorine is intentionally left in the water as it travels through the pipes. This persistent level, called “chlorine residual,” prevents the regrowth of bacteria or recontamination within the pipe system. The U.S. Environmental Protection Agency (EPA) regulates that the maximum residual disinfectant level should not exceed 4 parts per million (ppm).
Residual chlorine levels are typically maintained between 0.2 and 0.5 mg/L to ensure continuous microbial protection. This residual chlorine can exist as free chlorine (hypochlorous acid) or combined chlorine (chloramines), which form when chlorine reacts with ammonia or organic compounds.
Effectiveness of Purification Methods in Removing Chlorine
Purification methods used to create purified water are extremely effective at removing municipal chlorine residual. Free chlorine is a volatile compound with a lower boiling point than water, allowing it to easily evaporate or “off-gas.” This volatility makes distillation a highly effective removal method, as the boiling process causes the chlorine to dissipate.
Reverse Osmosis systems typically employ a carbon pre-filter before the water reaches the semi-permeable membrane. Activated carbon filters remove chlorine through adsorption, trapping chlorine molecules on the carbon’s surface area. This pre-filtering is necessary because chlorine can degrade and damage the RO membrane itself.
Removal of chloramines, the combined forms of chlorine, is slightly more complex but successful with advanced purification. While the RO membrane rejects dissolved solids, the activated carbon pre-filter handles both free chlorine and chloramines. True purified water, having gone through these rigorous processes, should contain negligible to zero chlorine, eliminating chemical taste and odor.