Municipal water suppliers routinely add chlorine to public drinking water to maintain safety and prevent the spread of waterborne diseases. This chemical acts as a powerful disinfectant, killing harmful bacteria, viruses, and other pathogens. Chlorine’s ability to remain active in the water as it travels through pipes protects public health until the water reaches the tap. While this practice has significantly reduced illnesses, the presence of chlorine can impart an unpleasant taste and odor. Furthermore, the chemical can be detrimental to sensitive biological systems, such as aquarium fish or beneficial soil microbes in plants, necessitating its removal before use.
Passive Dechlorination: The Time Needed for Evaporation
The most straightforward way to remove chlorine from water is to simply let it stand exposed to the air. This passive method relies on chlorine’s volatile nature, meaning it easily converts to a gas and “off-gasses” into the surrounding atmosphere. For a typical volume of water in an open container, the majority of the free chlorine will dissipate within 12 to 24 hours at room temperature. Achieving complete removal may require the water to sit for up to 48 hours, depending on the initial concentration and environmental factors.
The speed of this process is directly related to the temperature of the water and the surface area exposed to the air. Warmer water causes the chlorine molecules to become more energetic, accelerating their escape into the gas phase. Increasing the surface area, such as using a wide, shallow vessel, significantly speeds up the process. Agitation, such as stirring or using an aquarium air stone, also dramatically reduces the necessary time by promoting the movement of chlorine gas to the surface.
While most chlorine leaves within a day, removing higher concentrations could take several days without assistance. Increasing the surface area and water temperature minimizes the time required to eliminate the chlorine. This method is effective only if the water utility uses free chlorine as its primary disinfectant.
The Difference: Why Chloramine Won’t Evaporate
A major complication for the passive method is the widespread use of chloramine, a disinfectant compound created by combining chlorine with ammonia. Chloramine is much more chemically stable than free chlorine, allowing it to maintain its disinfecting power for longer distances within the water distribution system. This stability means that chloramine does not readily “off-gas” from standing water like free chlorine does.
Leaving chloramine-treated water out for 24 hours will not effectively remove the disinfectant, and it could take several days for a noticeable reduction to occur. This lack of volatility ensures the water remains sanitized all the way to the consumer’s tap. Relying on evaporation is ineffective and potentially harmful, especially for sensitive uses like aquariums or brewing.
The only reliable way to know which disinfectant your local water utility uses is to contact them directly or review their annual water quality report. If the report indicates the presence of chloramine, or if letting the water stand overnight does not remove the chemical odor, an active removal method is necessary.
Active Methods for Immediate Dechlorination
When time is a factor or chloramine is present, active dechlorination methods offer immediate solutions. The most common and rapid technique involves chemical neutralization using a reducing agent, such as sodium thiosulfate. This ingredient is found in many commercial water conditioners and aquarium dechlorinators. These products chemically bond with both chlorine and chloramine, instantly converting them into harmless compounds like chloride ions. This method is highly effective, neutralizing the disinfectant within seconds, and is the only reliable way to quickly treat chloramine.
Another highly effective active method is filtration, specifically using activated carbon filters. Carbon has a porous structure that physically and chemically adsorbs the chlorine and chloramine molecules as the water passes through. This process is utilized in many home filtration pitchers, faucet attachments, and whole-house systems, providing clean, dechlorinated water on demand.
A thermal method, boiling, can also be used to remove chlorine, though it is impractical for large volumes. The high temperature rapidly increases the volatility of the chlorine, causing it to evaporate much faster than at room temperature. Boiling water for about 15 minutes is generally sufficient to remove free chlorine, but it is less effective for the more stable chloramine compound.