Chlorine vs Chloramine in Tap Water
How free chlorine and chloramine differ in tap water, why utilities use chloramine, why it does not gas off, and how dechlorination releases ammonia that fish and filters must handle.
Two disinfectants, one purpose
Municipal water is disinfected to stop waterborne diseases such as cholera and typhoid fever. A UF/IFAS Extension aquaculture lesson notes that almost all US public water systems use chlorine, and that many also add ammonia as a secondary disinfectant. Chlorine and ammonia combine to form chloramine, a compound that is highly toxic to fish and other aquatic animals. According to the US EPA, utilities have used chloramines since the 1930s, and more than one in five Americans drinks water treated with them.
When chlorine gas dissolves, it forms hypochlorous acid (HOCl) and hydrochloric acid; HOCl in turn dissociates into the hypochlorite ion (OCl⁻), and the balance between the two shifts with pH. Water-treatment terminology calls HOCl and OCl⁻ together "free available chlorine". When HOCl reacts with ammonia it forms monochloramine (NH₂Cl), and with more chlorine dichloramine (NHCl₂) and nitrogen trichloride (NCl₃). These chloramines are called "combined chlorine", and free plus combined chlorine is reported as total residual chlorine. In utility chloramination the chlorine-to-ammonia ratio is controlled so that monochloramine dominates; Wikipedia gives the formation reaction as NH₃ + NaOCl → NH₂Cl + NaOH.
Why utilities switched to chloramine
The main driver is disinfection byproducts. Free chlorine reacts with natural organic matter to form trihalomethanes and haloacetic acids, some of which are linked to cancer and organ damage. The EPA began regulating disinfection byproducts in 1979, and monochloramine has a much lower tendency than free chlorine to convert organic matter into chlorinated compounds such as chloroform. The EPA states that some systems adopted chloramines as a secondary disinfectant specifically to meet byproduct rules.
The second reason is persistence. Chloramine is a weaker and slower-acting disinfectant than free chlorine, but it is far more stable, so a residual survives through long distribution networks to the consumer's tap. Wikipedia cites an EPA limit of 4 ppm for chloramine in public water, based on a running annual average. Monochloramine is also more stable against light than hypochlorite.
Stability: why standing and aeration are not enough
Free chlorine is a volatile gas that leaves water with time and agitation. In the UF/IFAS exercise, vigorous aeration of chlorinated tap water for 24 hours greatly reduces or eliminates the chlorine reading, whereas chloraminated water aerated for the same period still shows chlorine and keeps its full ammonia reading. The lesson explains that chlorine is combined with ammonia precisely so that it stays active and does not gas off easily.
Tampa Bay Water, a regional US supplier, warns that monochloramine may take weeks to dissipate and that letting water stand is not a safe way to remove it. A hatchery-oriented review of dechlorination methods likewise lists "does not remove chloramines" among the disadvantages of heavy aeration.
Dechlorinator chemistry: thiosulfate and the ammonia problem
Sodium thiosulfate (Na₂S₂O₃) is a reducing agent: it reduces hypochlorite and chlorine to chloride and is itself oxidised to sulfate. It is used to dechlorinate water in treatment plants and for aquariums, pools and spas. In the UF/IFAS test, adding it to chlorinated water brings the chlorine reading to 0.0 ppm within minutes.
With chloramine the result is different. Thiosulfate removes the chlorine part of the molecule, so the chlorine reading still falls to zero, but the ammonia that was bound in the chloramine is released and stays measurable. Whether that ammonia is dangerous depends on pH: total ammonia exists as toxic un-ionized ammonia (NH₃) and much less toxic ammonium (NH₄⁺), and the SRAC ammonia fact sheet states that the toxic fraction rises about tenfold for each one-unit increase in pH, and also rises with temperature. Released ammonia is therefore a greater risk in alkaline water than in acidic water. Thiosulfate also consumes dissolved oxygen, so treated water should be aerated before use.
- Free chlorine only: a reducing dechlorinator, or prolonged aeration, removes it.
- Chloramine: a reducing dechlorinator removes the chlorine; the released ammonia must then be bound or removed, for example by zeolite (an ammonia-adsorbing mineral), by a conditioner whose label states it treats chloramine and ammonia, or by an established biological filter.
- Activated carbon: granular activated carbon removes chloramine only with sufficient contact time and fresh, high-grade media; carbon beds need periodic replacement or reactivation.
- Reverse osmosis can also remove chloramine under suitable conditions, but performance must be monitored.
Practical dosing of conditioners and treatment of large water changes are covered in the separate dechlorination guide (dechlorination-guide).
Testing tap water
Free chlorine and total chlorine are measured separately; the difference between total and free chlorine approximates the combined chlorine, mostly monochloramine. Colorimetric DPD kits and test strips are available for both. A simple approach from the UF/IFAS lesson is to test fresh tap water for both chlorine and ammonia: chlorine with no ammonia points to free chlorine, while chlorine plus ammonia indicates chloramination. After a dechlorinator, an ammonia reading that persists confirms that chloramine was present. Some ammonia-binding conditioners interfere with certain ammonia tests, so readings after treatment should be interpreted with care.
Impact on the biological filter
Residual chlorine harms not only fish but also the nitrifying bacteria of a biological filter, which is why water is dechlorinated before it reaches the aquarium. Ammonia released from chloramine is the same substrate that nitrifying organisms oxidise to nitrite and then to nitrate; drinking-water research documents exactly this process in chloraminated distribution systems. In an aquarium, a mature biofilter can therefore process modest amounts of released ammonia, while a new or disturbed filter cannot, so large water changes with chloraminated water carry more risk in uncycled or recently treated systems.