Leaving Your Tap Water to Sit Overnight Probably Does Not Work Anymore. Here Is Why.
For years, I kept a jug of tap water on the kitchen counter and filled it the night before, letting it sit uncovered until the morning. The logic was solid: chlorine is a gas, it evaporates at room temperature, and leaving water uncovered overnight removes most of it along with the taste and smell it produces. I had read this in multiple places, tried it, and found that the water genuinely tasted better the next morning, which confirmed the approach worked. It did work, for a while, and then at some point the improvement became less noticeable, and the water from the overnight jug started tasting more or less the same as water straight from the tap.
What had changed was not my water jug or my taste buds. My water supplier had changed the disinfectant they used, switching from chlorine to chloramine, a change that water utilities across many countries have been making for several decades. Chloramine does not evaporate at room temperature. The overnight jug trick that works for chlorine does almost nothing for chloramine, and most people who follow this advice do not know whether their water contains chlorine or chloramine, which determines whether the advice actually applies to them.
The Difference Between Chlorine and Chloramine
Both chlorine and chloramine are used to disinfect municipal water supplies, and both leave a taste and smell in the water that some people find unpleasant. They work through similar mechanisms, killing bacteria and pathogens to make water safe to drink, and they are both effective at their primary purpose. The difference that matters for home water treatment is in their chemical stability.
Free chlorine is relatively volatile. At room temperature in an uncovered container, it dissipates into the air over a period of hours, which is why the standard advice to leave water uncovered overnight or for several hours genuinely works for chlorine-treated water. Chloramine, which is a compound of chlorine and ammonia, is specifically valued by water utilities because it is more stable than free chlorine, lasting longer in the distribution pipes as water travels from the treatment plant to homes. This stability is exactly what makes it harder to remove at home. A covered or uncovered container left at room temperature for twenty-four hours will retain most of its chloramine. Boiling, another commonly recommended method for chlorine removal, is largely ineffective against chloramine for the same reason: the compound is more stable under heat than free chlorine is.
How to Tell What Your Water Contains
Most people do not know whether their tap water is treated with chlorine or chloramine, and the answer varies by location and even by season, since some utilities switch between the two depending on conditions. The most reliable way to find out is to contact your local water supplier and ask directly, or to look for the Consumer Confidence Report that most municipal water suppliers are required to publish annually, which lists the disinfection method used along with chemical levels. Chloramine is sometimes listed as combined chlorine, monochloramine, or just described as the disinfectant type in the annual water quality report.
Home water testing kits that measure both free chlorine and total chlorine can also indicate the presence of chloramine indirectly: if total chlorine is measurably higher than free chlorine, the difference represents chloramine. These kits are inexpensive and available from hardware stores and online suppliers, and they provide a more immediate answer than waiting for an annual report.
What Actually Removes Chloramine
Activated carbon filtration is the most practical and effective method for removing both chlorine and chloramine from tap water at home, and unlike the evaporation approach, it works equally well on both compounds. Activated carbon has an enormously high surface area relative to its volume, and chlorine and chloramine molecules adsorb onto that surface as water passes through, removing them from the water effectively. Pitcher filters using activated carbon, under-sink carbon filters, and whole-house carbon filtration systems all work on this principle. The key consideration when choosing a filter is to confirm it is rated for chloramine removal specifically, since some activated carbon filters are certified only for chlorine and may be less effective against the more stable chloramine compound.
Vitamin C, specifically ascorbic acid, neutralises both chlorine and chloramine almost instantly on contact, which is why it is used by aquarium keepers to dechlorinate water for fish tanks where even trace amounts of either compound can be harmful. Vitamin C powder or ascorbic acid tablets dissolved in water provide an immediate chemical neutralisation that works regardless of which disinfectant is present. The quantities needed for drinking water are very small; a small amount, typically around a quarter of a gram per litre, is generally considered sufficient, though exact amounts vary by product and water chemistry, and the effect is immediate rather than requiring hours of evaporation time. This method is less commonly known as a domestic water treatment approach than it is in the aquarium community, but it is straightforward, inexpensive, and effective for both chlorine and chloramine. Knowing what is actually in the product you are using and what the mechanism behind it is produces better outcomes than following advice that was accurate in a different context.
The Sourdough and Fermentation Angle
For most drinking and cooking purposes, the chlorine or chloramine levels in properly treated municipal water are at concentrations too low to cause any health concern, and the decision to treat tap water is primarily about taste and preference rather than safety. There is one specific context, however, where even low levels of chlorine and chloramine have a meaningful practical effect: fermentation. Both compounds are biocidal by design, and in concentrations sufficient to kill the harmful bacteria in a municipal water supply, they also suppress the beneficial bacteria and wild yeasts used in sourdough baking, home fermentation of vegetables, and kombucha production.
Sourdough bakers who struggle with sluggish or inactive starters, and home fermenters who find their vegetable ferments developing off flavours or not progressing normally, often find that switching to filtered or dechlorinated water produces an immediate improvement in fermentation activity. This is a specific, practical reason to remove disinfectants from water that applies to anyone involved in home fermentation, independent of any taste or health preference. For sourdough in particular, using chloramine-treated tap water without dechlorination consistently suppresses starter activity, while using the same water passed through an activated carbon filter or treated with vitamin C produces notably more active starters and more reliable fermentation.
Conclusion
The overnight jug on the kitchen counter is not useless. If your water supplier uses free chlorine as their disinfectant, it works genuinely well for exactly what it is supposed to do. If your supplier uses chloramine, which is now the case for a significant proportion of municipal water supplies, the jug is doing almost nothing. Knowing which situation applies to you takes five minutes to find out and changes which approach is actually useful. For chloramine-treated water, an activated carbon filter or vitamin C neutralisation provides what the overnight jug does for chlorine: a simple, inexpensive, effective treatment that produces better-tasting water and removes any impediment to fermentation without requiring specialist equipment.
