Tap water is the most convenient source of water for houseplants and gardens, but it often contains chemical additives used for purification. Municipalities add disinfectants, primarily chlorine and chloramine, to kill harmful pathogens and ensure water safety. While safe for human consumption at regulated levels, these additives can potentially harm the beneficial microbial life in your soil and may affect sensitive plants. Understanding the specific chemical in your water is the first step toward determining the most effective method and time required for treatment before watering your plants.
The Chemical Distinction Between Chlorine and Chloramine
Municipal water systems rely on two different halogens to disinfect the public water supply: chlorine and chloramine. Chlorine is the more traditional disinfectant, a highly reactive chemical that quickly kills bacteria and viruses. When added to water, chlorine forms hypochlorous acid and hypochlorite ions, which are collectively known as free chlorine. This form is relatively unstable and dissipates quickly when exposed to air or sunlight.
Chloramine is a compound formed by combining chlorine with ammonia. Water utilities increasingly favor this compound because the ammonia bond makes it significantly more stable and longer-lasting than free chlorine. This stability allows the disinfectant to maintain its effectiveness over the long distances of the water distribution system. Unlike volatile free chlorine, chloramine is far more resistant to simple evaporation, meaning letting water sit out will not effectively remove it.
Calculating Evaporation Time for Free Chlorine
The process of chlorine removal through evaporation is known as off-gassing, where the dissolved chlorine escapes from the water’s surface into the atmosphere. For water containing only free chlorine, a significant reduction in concentration can be achieved by simply letting the water sit uncovered. Most of the free chlorine will dissipate within a 12-to-24-hour period at room temperature.
The rate of off-gassing is directly tied to physical factors that influence the movement of the chlorine molecules. Increasing the water temperature accelerates molecular movement, which causes the chlorine to escape more rapidly. Maximizing the water’s surface area, such as by using a wide, shallow bucket instead of a deep vase, provides more points of contact between the water and the air.
Agitation or aeration also speeds up the process dramatically by constantly circulating fresh, chlorine-rich water to the surface. Adding an aquarium air stone or simply stirring the water occasionally can reduce the required off-gassing time from a full day to just a few hours. Completely eliminating the residual parts per million (PPM) concentration may take as long as four to five days, depending on the initial concentration and conditions.
Alternative Methods for Rapid Water Treatment
When simple evaporation is impractical or when chloramine is present, faster and more complete treatment methods are necessary.
Boiling and Chemical Neutralization
One rapid technique is boiling the water, which forces the chlorine to off-gas almost instantly. A 15-to-20-minute boil is sufficient to remove both chlorine and chloramine. However, this method is energy-intensive and only practical for small volumes of water, which must then be cooled completely before use.
A chemical solution offers the fastest method for treating large volumes of water, as certain chemicals instantly neutralize both chlorine and chloramine. Ascorbic acid, commonly known as Vitamin C, is a non-toxic option that neutralizes these compounds through a chemical reaction. A highly concentrated one-gram tablet of ascorbic acid can neutralize the chlorine and chloramine in up to 100 gallons of water, with the reaction occurring almost immediately upon mixing. The reaction converts the chlorine into harmless chloride ions. Alternatively, commercial aquarium dechlorinators are widely available and contain chemical compounds like sodium thiosulfate, which are highly effective at neutralizing disinfectants in a matter of minutes.
Filtration Systems
For a long-term solution, specialized filtration systems are highly effective. A standard activated carbon filter removes chlorine, but a more advanced catalytic carbon filter or a reverse osmosis (RO) system is required to reliably remove the more stable chloramine.