Chloramine is a disinfectant put into
many municipal water supplies. In recent years it has often
replaced chlorine for two main reasons. The first is that
it is much longer lasting, so it continues to provide a disinfectant
action in supply pipes, where chlorine typically loses its
capacity to disinfect. The second is that it does not react
with organics nearly as readily as does chlorine. The reaction
products of chlorine and organics (chlorinated organics) are
very toxic to people, and water supply operators elect to
use chloramine to reduce this toxicity.
Unfortunately for aquarists, dealing with
chloramine in tap water is not as easy as dealing with chlorine.
The chlorine in tap water can be eliminated just by letting
the water sit for a few days prior to use. This is not the
case for chloramine, and aquarists MUST take active steps
to eliminate it.
This article describes what chloramine
is, what it does that is a problem in aquaria, how to test
for it, and how to rid water of chloramine. It also reports
on a survey study of aquarists using reverse osmosis/deionization
systems (RO/DI) for purification of their water. There has
recently been considerable concern and debate over whether
such systems will adequately remove chloramine in all normal
circumstances, even among manufacturers and distributors of
such systems. The results of the survey described here will
help aquarists understand how effective such systems are at
What is Chlorine?
Before beginning to discuss what chloramine
is, understanding chlorine is useful. Chlorine, as Cl2,
is a greenish yellow gas at room temperature. It is sometimes
used as a disinfectant
in water supplies, and it is also used to make chloramine,
as described below. When dissolved in water, it forms dissolved
Cl2, and it also reacts with water
to form HOCl (hypochlorous acid; pKa = 7.5), and HCl (hydrochloric
acid). The HOCl and HCl will also dissociate into H+,
Cl-, and OCl-
(hypochlorite), with the extent of dissociation depending
Cl2 + H2O
HOCl + H+ + Cl-
Since chlorine and hypochlorous acid/hypochlorite
are in equilibrium in water, it doesn't really matter which
ones are added to attain a disinfectant medium. So, for example,
one can add chlorine gas, hypochlorous acid, or hypochlorite,
and attain similar results. In fact, according to the U.S.
Protection Agency (EPA),
"The term free residual
chlorine most accurately refers to elemental chlorine, hypochlorous
acid (HOCl) and hypochlorite ion (OCl-)."
In light of that, many water supplies (including
the Massachusetts Water Resources (MWRA) that serves my area)1
choose to use sodium hypochlorite (bleach, NaOCl) to provide
this same OCl- as the primary
What is Chloramine?
Chloramine is formed through the reaction
of dissolved chlorine gas (forming hypochlorous acid) and
ammonia in tap water. Chloramine is a term that actually describes
several related compounds: monochloramine NH2Cl
(Figure 1), dichloramine, NHCl2 and
NH3 (ammonia) +
NH2Cl (monochloramine) + H2O
NH2Cl + HOCl à
NHCl2 (dichloramine) + H2O
NHCl2 + HOCl à
NCl3 (trichloramine) + H2O
Figure 1. A 3-dimensional model of monochloramine showing
the relative size
of the chlorine, nitrogen, and hydrogen atoms.
form in most water supplies (where the pH is 7 or above)
is monochloramine, and that form will be chosen for most discussions
in the remainder of this article. and that form will be assumed
to exist in the remainder of this article. Nevertheless, water
supplies may contain mixtures of these compounds, and the
exact proportions of the various species present depend on
the pH and the relative concentrations of chlorine and ammonia
How much chloramine is used by water supplies
varies quite a bit. In the case of the water that I use, for
example, the MWRA uses chlorine for primary disinfection (via
sodium hypochlorite), and then later uses chloramine to provide
lasting disinfection as it is sent into pipes.1
In the latter case, the amount of chloramine added may not
be as high as if it were used for the primary disinfection.
Most aquarists in the greater Boston area (including myself)
found chloramine levels of less than 0.5 ppm-Cl in their tap
water when tested at a recent Boston Reefers Society function.
Elswhere, however, chloramine levels can be significantly
higher, ranging up to several ppm-Cl. The maximum
allowed by the EPA is 4 ppm-Cl , and some
water supplies target 2-4 ppm-Cl . The amount seen at
the tap will also depend on distance from the treatment plant,
and how long the water has been sitting in pipes.
A note on concentration units. In this
article (and in links to the EPA and other sites), all concentrations
are given as ppm-Cl. That means ppm of chlorine mass, regardless
of what form the chlorine is in. It is analogous to units
of NO3-N (nitrate nitrogen) that are
often used for nitrogen species. That complication is necessary
since the various chloramines may be present as mixtures,
and it also facilitates comparison to chlorine and other oxidants.
So 1 mg/l of monochloramine would be reported as 0.69 ppm-Cl
of monochloramine because the chlorine comprises 69% of the
mass of monochloramine. The unit does not imply that there
is any free chlorine present.
Toxicity of Chloramine to Marine Organisms
The great majority of reported toxicity
tests involving chloramine use freshwater organisms. Nevertheless,
there is adequate testing reported for a variety of marine
organisms to know that it is very toxic to such organisms.2-7
A complication in marine systems is that chloramine and chlorine
react with substances in seawater, forming other reactive
chlorine species. In the case of chlorine, these are often
simply referred to as chlorine-produced oxidants (CPOs). For
example, monochloramine is known to react with bromide in
seawater over a period of hours to form bromochloramine (Br-NH-Cl).8
Consequently, identifying the exact species causing toxicity
is often difficult.
What is the mechanism of toxicity? The
mechanism has not been established for many invertebrates,
but in fish the mechanism is well known. Chloramine passes
through the gills of fish and enters the blood stream. There,
it reacts with hemoglobin, forming methemoglobin. In fathead
minnows (Pimephales primelas) exposed to 1 ppm-Cl of
monochloramine, for example, about 30% of the hemoglobin is
converted into methemoglobin. The fish then suffer from anoxia
(low oxygen in their tissues) because they have lost some
of their hemoglobin, which is responsible for carrying oxygen
in the blood.9
While knowing the exact species causing
the toxicity is important to physiologists
studying the phenomenon, it is not so important to aquarists.
The important thing that aquarists need to know is how low
the concentration needs to be before toxicity is not displayed
by any organisms that are present in the aquarium. Most toxicity
tests are designed with unmistakable upper endpoints, often
death. Table 1, for example, shows the concentration that
kills half of the exposed individuals in a few days, called
Table 1. Toxicity
of chloramine and CPOs to marine species.
to kill half of the individuals in 48-96 h
to kill half of the individuals in 168 h
fuscus (pipe fish)4
virginica larvae (oyster)4
americanus larvae (lobster)5
americanus larvae (lobster)6
americanus larvae (lobster)6
ppm-Cl caused respiratory distress
In its assessment of chloramine toxicity
to marine invertebrates, Environment Canada (the Canadian
equivalent of the United States Environmental Protection Agency,
EPA) determined the Estimated
No-Effects Value (ENEV) based on this type of data to
be 0.002 ppm-Cl for marine and estuarine environments.
How much chloramine should one allow into
an aquarium? That, of course, depends on what is in the aquarium.
In the absence of knowing the toxicity of chloramine to every
inhabitant of the aquarium (or of even knowing the identity
of every inhabitant), it seems prudent to have chloramine
levels far below those where the most sensitive organisms
are killed, and that chloramine concentration is somewhere
well below 0.005 ppm-Cl. The value suggested by Environment
Canada seems like a reasonable maximum.
There is, however, substantial uncertainty
in deciding exactly which levels are acceptable and which
are not, since there is so little data available. Perhaps
the acceptable levels for daily exposures during the entire
lifetime of an organism needs to be even lower than this value.
After all, some organisms live quite a long time, and presumably
we are interested in preventing all toxicity, not just death.
It is apparent from the data in Table 1 that the longer the
exposure, the lower the toxic levels become. In the end, we
are limited by the available data and also by the ability
of aquarists to measure chloramine itself.
This target of 0.005 ppm-Cl or less does
not necessarily imply that all water used for aquaria must
be that low. For example, an aquarium that tops off 2% of
the tank volume daily (to replace evaporated water) will not
have a chloramine concentration equal to the top off water.
It will, however, have fresh chloramine added every day. Even
if the chloramine added each day is broken down in the aquarium
before the next addition (something that is likely, but not
demonstrated for aquaria), then if the top off water contained
4 ppm chloramine, the aquarium would be boosted to 0.08 ppm
every day. That level appears to be well above the danger
zone for many invertebrates. Consequently, aquarists need
to be aware of the chloramine levels in water that they use
to replace evaporated water. Similar, and even more stringent,
concerns would apply to water used for water changes or in
setting up a new aquarium.
There are many kits suitable for measuring
chloramine, with varying limits of detection. Many are not
suitable for testing the low levels necessary for reef aquaria.
The kit that I prefer for measuring low levels of chloramine
is the Hach
CN-70 (part # 1454200). It is capable of measuring total
chlorine and free chlorine. Chloramine is found by the difference
between these two values. It has a low range scale that runs
from 0 to 0.7 ppm, and a high range that runs from 0 to 3.5
ppm. The low range can detect 0.01 ppm chloramine. It costs
about $64 (with shipping) and is good for many tests. The
colorimetric kit is very easy to use: reagent is mixed with
the water to be tested and compared to a color wheel.
Removing Chloramine From Water: Chemical Reducing
There are two primary ways to remove chloramine
from tap water. The first is through the use of inorganic
reducing agents such as thiosulfate. Thiosulfate (S2O3-
-, which actually looks like -OSO2S-)
is an inorganic chemical that is typically dissolved in water,
usually as the sodium salt. When added to water containing
chloramine, a reaction takes place, destroying the chloramine.
The electrochemistry of sulfur compounds can be complicated,
and different researchers report different products of this
reaction (extrapolated from reactions with chlorine itself,
not chloramine). The products have been suggested to include
sulfate (SO4- - and HSO4-),10,14
elemental sulfur (S),10
and tetrathionate (S4O6-
may depend to some extent on the conditions, including the
pH and the relative amounts of compounds present. John F.
Kuhns (inventor of Amquel below) has indicated that he believes
that the reaction resulting in sulfate is the most frequently
observed. The reaction for this process is shown below:
+ 4HCl + 4NH3
Thiosulfate is also equally suited to
dechlorinating free chlorine in water, and it has gained wide
use in marine and freshwater aquaria. Unfortunately, the ammonia
that is produced as a result of the reaction is still toxic.
Consequently, thiosulfate alone is not always adequate for
eliminating toxicity from chloramine.
Other products, such as hydroxymethanesulfonate
a known ammonia binder15
patented for aquarium uses by John F. Kuhns16
(sold as Amquel
by Kordon and ClorAm-X
by Reed Mariculture, among others) can be used to treat chloraminated
water because they both break down chloramine and bind up
The reaction of ammonia with hydroxymethanesulfonate
is mechanistically complicated, possibly involving decomposition
to formaldehyde and reformation to the product (aminomethanesulfonate;
shown below).15 The simplified
overall reaction is believed to be:
NH3 + HOCH2SO3-
Even more complicated is the reaction of
hydroxymethanesulfonate with chloramine, or chlorine (as Cl2
or HOCl). In this case, the products that are formed have
not been established.
So are these useful products? That is,
do they eliminate all toxicity from chloramine and provide
none of their own, either by themselves or through their degradation
products? I cannot answer that question. Almost certainly,
using them is better than not using them if there is chloramine
in the water. Is the toxicity eliminated for even the most
sensitive larval invertebrates? Again, I don't know. Without
knowing what the degradation products are, or without detailed
testing on a variety of very sensitive invertebrates, I don't
know how one would conclude that they are satisfactory (or
not). Maybe such tests exist, and if so, I'd be pleased to
hear of them. In the end, my recommendation is to remove chlorine
and chloramine in other ways, such as through an RO/DI system
as described below.
Removing Chloramine From Water: Activated Carbon
Another method for removing chloramine
from water is with activated
carbon (as is contained in most RO/DI systems). In a two
step process, the carbon catalytically breaks the chloramine
down into ammonia, chloride, and nitrogen gas
C + NH2Cl
+ H2O à
C-O + NH3
+ Cl- + H+
C-O + 2NH2Cl
C + N2
stands for the activated carbon, and C-O
stands for oxidized activated carbon. In this case, as was
found for thiosulfate, the product includes ammonia, which
is not bound significantly by activated carbon. Consequently,
treatment of water with activated carbon will need to be followed
up by some method of eliminating the ammonia.
In the case of a reverse osmosis/deionizing
system (where carbon is usually part of the prefiltration
prior to the RO membrane), the ammonia is partially removed
by the reverse osmosis system. The extent of removal by the
RO membrane depends on pH. At pH 7.5 or lower, reverse osmosis
will remove ammonia from 1.4 ppm-Cl monochloramine to less
than 0.1 ppm ammonia. The DI resin then removes any residual
ammonia to levels unimportant to an aquarist.
Removing Chloramine With Activated Carbon: Does
it Really Work?
There has been much debate over whether
commercial RO/DI systems used by aquarists are actually removing
chloramine in adequate quantity. The concern is not whether
they can theoretically do so, but whether the actual units
allow sufficient contact time between the water and the activated
carbon for the units to do an adequate job.
I have been using a Spectrapure
RO/DI system (CSP25DI) for years, and my water does contain
chloramine, so naturally I was interested to know if it was
up to the task. In discussing the issue with Charles Mitsis,
President of Spectrapure, he said that my water was among
the most difficult to successfully remove chloramine from
because the pH was high, and he was not sure that the unit
was adequate. The reasons for being concerned were that:
||Monochloramine is the most difficult
of the three chloramine species to remove because it is
small (allowing it to pass through a reverse osmosis membrane).
|| Monochloramine is the most chemically stable
of the chloramine species, so is the hardest to break
down (as on activated carbon).
over the other forms in tap water at pH above 7 (dichloramine
predominates at pH 4-7).
||The pores of the activated carbon
may become plugged with sediment over time, reducing the
effectiveness of the carbon at breaking apart chloramine.
||At high pH, the pores of the
RO membrane can swell, resulting in poorer rejection of
With this as the backdrop, I set about
organizing a round of testing by aquarists to see if their
commercially-available systems were adequately removing chloramine.
First, I selected a single, high quality
test method for participants to use: the Hach CN-70 kit described
above. I then asked aquarists to test several things:
||The free and total chlorine
in their tap water after letting it run for a while.
|| The free and total chlorine
in their RO reject water.
||The free and total chlorine in
their finished RO/DI water.
||The pH of the tap water.
In my case, for example, I had the following
Total Chlorine: 0.4-0.5 ppm one day, 0.08 ppm on a second
Free chlorine: <0.01 ppm (effectively all of the total
chlorine was chloramine)
RO Reject water:
Total Chlorine: 0.02 ppm
Free chlorine: <0.01 ppm
Final RO/DI water:
Total Chlorine: <0.01 ppm
Consequently, within the capabilities of
the Hach test kit (0.01 ppm), there is no chloramine getting
through the system. A small amount does appear to get past
the carbon to the RO waste water, but it does not get through
the RO membrane and DI resin.
A similar set of data (more or less complete)
was collected from about 20 aquarists in different parts of
the country. These included systems that were stated to have
a capacity of 25-100 gallons per day, the higher volume systems
being especially interesting because the contact time with
the carbon might be shorter. All but one had similar results
to those reported here. The anomalous report produced the
Total Chlorine: >3.5 ppm
Free Chlorine: >3.5 ppm
Filtered Tap Water: (single cartridge
under sink, cold water side)
Total Chlorine: 0.7 ppm
Free Chlorine: 0.38 ppm
RO water: (11 month old cartridges)
Total Chlorine: 0.16 ppm
Free Chlorine: 0.06 ppm
RO/DI water: (11 month old cartridges)
Total Chlorine: 0.04 ppm
Free Chlorine: 0.02 ppm
RO/DI water: (Fresh cartridges)
Total Chlorine: <0.01 ppm
Free Chlorine: <0.01 ppm
In short, his tap water chloramine (and
chlorine) levels were quite high. His old carbon and sediment
cartridges were not quite up to the task, but when replaced,
were adequate to remove all of the chloramine. Note that the
11 month old cartridges were still producing 0-1 ppm TDS RO/DI
Lessons Learned and Suggestions:
||Most RO/DI systems seem capable
of removing chloramine adequately for aquarists.
|| The carbon cartridge may become less
useful over time, and it is possible that the chloramine
removal effectiveness of a system may be lost before the
DI appears to need changing.
||Cheap sediment cartridges
may expose the carbon cartridge to unnecessary fouling,
which may permit chloramine to pass through the system.
Cartridges should be replaced as soon as the pressure
drops significantly, even if RO/DI water is still being
produced at a reasonable rate or purity as measured by
total dissolved solids.
||Testing for chlorine and
chloramine is easy, so any concern is easily reconciled.
||One Hach kit provides several
dozen test results. Our local Boston Club bought some
kits and had a "water testing day." The kits
can also become part of the "library" of a local
club for aquarists to use once in a while to see if their
systems are functioning. That way, the cost to each aquarist
Chloramine in tap water should be a significant
concern to aquarists. Its peculiar properties make it well
suited to disinfection of water supplies, but also make it
a potential toxin in aquaria. In order to render the water
safe for use, aquarists need to use one of two systems for
purification: an inorganic reducing agent combined with an
additive that binds ammonia (or a single product that does
both), or an RO/DI system. Chloramine is toxic enough that
it would seem prudent for aquarists to spend the time and
money necessary to ensure that they do not unduly stress their
organisms. This activity includes setting up appropriate purification
systems, and may also include testing the water to ensure
that those systems are functioning properly.