Boiling water is a common method considered for removing the chlorine used by municipalities to disinfect drinking water supplies. Chlorine, often added to kill bacteria and viruses, can leave behind an unpleasant taste and odor. The effectiveness of boiling depends entirely on the chemical form of the disinfectant present. Understanding this difference is important, as boiling works well for one type of disinfectant but proves largely ineffective for another.
The Process of Removing Free Chlorine
Boiling water can significantly reduce the concentration of free chlorine, which includes chlorine gas, hypochlorous acid, or hypochlorite ion typically used as a disinfectant. This removal occurs through volatilization, where the compound transitions from a liquid state into a gaseous state and escapes into the air. Chlorine is highly volatile, meaning it has a very low boiling point compared to water. As the water heats up, the chlorine molecules gain energy and start escaping from the surface.
Simply reaching a rolling boil is not enough for a noticeable reduction; the water must be boiled continuously. It is recommended to maintain a full, rolling boil for 15 to 20 minutes to achieve a significant reduction, often removing 80% or more of the initial concentration. Using an open pot with a wide surface area and stirring the water can accelerate this off-gassing process. After boiling for the recommended duration, the water must be allowed to cool completely before consumption.
The Challenge of Chloramine
The effectiveness of boiling changes drastically if the water system uses chloramine instead of free chlorine. Chloramine is a more stable disinfectant compound created by combining chlorine with ammonia. This stability allows the disinfectant to remain active longer as it travels through the distribution pipes. Consequently, boiling is an unreliable method for its removal.
Chloramine is significantly less volatile than free chlorine, so it does not readily convert into a gas and escape when heated. Extended boiling, often 30 minutes or more, is required for even a minimal reduction, making it an impractical method for daily water treatment. Furthermore, boiling can break down chloramine into its components, specifically releasing ammonia, which is not removed by heat. This breakdown can also lead to the formation of other chemical byproducts, complicating the water chemistry.
Contaminants Concentrated by Boiling
While boiling removes volatile substances like free chlorine, it poses a problem for non-volatile contaminants. As water is heated and turns into steam, the water evaporates, but dissolved solids or heavy chemical compounds remain behind. This evaporation reduces the total volume of water while the amount of non-volatile substances stays the same, increasing their concentration in the remaining liquid. Contaminants such as heavy metals, including lead, arsenic, and mercury, have high boiling points and are not removed by heat.
Minerals like calcium and magnesium, which cause water hardness, also become more concentrated, leading to increased scaling on cookware. Nitrates and salts, often found in water from agricultural runoff or natural deposits, are similarly concentrated. A concern also arises with disinfection byproducts, specifically Trihalomethanes (THMs), which form when chlorine reacts with organic matter. Although THMs are volatile and can be removed by boiling, the heat can accelerate the reaction between residual chlorine and organic matter, potentially creating more THMs during the process. This fluctuation underscores that boiling is not a comprehensive water purification solution.