Most reef aquarists
use artificial seawater mixes, and these always need to be
made with freshwater. Likewise, all reef aquaria need their
evaporated water to be replaced with freshwater in some fashion.
Consequently, all aquarists need access to suitably pure freshwater.
water often contains contaminants, including chlorine,
copper, and a variety of other undesirable compounds, that
make it unsuitable for aquarium use. There are several ways
to purify tap water to make it suitable for reef aquaria,
and the best method for achieving this is likely the combination
of reverse osmosis and deionization (RO/DI) purification.
This article describes what these multistage systems are
comprised of, what each stage accomplishes, and how to make
the most of an RO/DI system. Specifically, the contents are:
What Contaminants are Present
in Tap Water?
Tap water can contain a variety of
undesirable impurities. Some are present in the freshwater
source before it is collected by a municipal water supply
company. These can include nitrate,
certain potentially toxic metals (such as chromium), and a
variety of organics (Tables 1-4).
(year of report)
County, Georgia (2002)
York City (2002)
Falls, SD (2002)
supply (year of report)
York City (2002)
supply (year of report)
York City (2002)
4. Chromium in Tap Water
supply (year of report)
Some "contaminants" are intentionally added to
the water to make it suitable for human consumption; these
include chlorine and chloramine,
as well as silica
(added to some water supplies to raise the pH and reduce corrosion
and release of copper and lead into drinking water). From
the aspect of aquarists, chloramine
can be among the worst of these offenders, with many water
supplies targeting levels of 2-4 ppm chlorine equivalent (ppm-Cl).
Some organisms are sensitive to chloramine at levels far below
this concentration. 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. Consequently,
chloramine must be removed before using tap water that contains
it. Unfortunately for aquarists who want to use tap water,
it is long-lived, and will not rapidly dissipate the way that
Lastly, some contaminants are more likely to come from the
pipes in the aquarist's home than from the water supply itself.
Consequently, these contaminants do not depend on the quality
of the source water as much as the pH of the water and the
nature of the pipes within the home. Many aquarists are fooled
into thinking that their town has very clean water, so they
need not worry about anything in their tap water. Unfortunately,
that can be untrue. Chief among these that are of concern
to aquarists is copper. Copper is allowed by the U.S. EPA
to be present in drinking water at levels exceeding 1 ppm.
Some homes in recent surveys have been found to exceed 1.3
ppm (Table 5).
5. Copper in Tap Water (tested in homes)
(year of report)
level (ppb), 10% of
homes above this level
(depends on district)
County, Georgia (2002)
York City (2002)
Falls, SD (2002)
Figure 1. The RO/DI system that I have been using
for my reef aquarium needs for the past ten years.
That level is well above the threshold for toxicity
to marine organisms, and is about a hundred times higher
than I found in my aquarium the last time I tested it for
copper (about 10-15 ppb copper).
What is an RO/DI System?
At a minimum, an
RO/DI system consists of a reverse osmosis membrane through
which the water flows (and is purified), followed by a deionizing
resin that removes any residual charged compounds. Typically,
other necessary parts ensure that these two main parts function
properly. These other parts may include sediment filters,
activated carbon filters, pressure gauges and conductivity
or total dissolved solids (TDS) monitors. Figure 1 shows the
RO/DI system that I have been using for nearly 10 years. It
has each of these components and, other than changing filters
when needed, hasneeded no maintenance. Not all aquarists are
so lucky, however, and sometimes the RO membranes become clogged
or broken and need to be replaced. Maintenance issues are
discussed later in this article. The function of each of these
components is described below.
Sediment filters are the first thing
that incoming tap water encounters, and they do just what
their name implies: filter out sediment. If this sediment
were to get past the filter, the sediment would then rapidly
clog the activated carbon filter, making it much less useful
and potentially risking damage to the RO membrane (as described
below). The sediment filter also prevents sediments from getting
through that might coat the inside of the reverse osmosis
membrane. Typical sediment filters used for this application
have pore sizes in the 0.5 to 1 mm
(micrometer, or millionth of a meter) range. If the water
contains a lot of sediment, it is sometimes useful to have
a series of sediment filters with steadily decreasing pore
size, increasing the lifetime of the filters.
The next filter in line typically
contains activated carbon. The primary purpose of this activated
carbon is to break down the chlorine and chloramine in the
tap water (Figure 2). If not removed, these compounds may
damage the RO membrane. They will also pass through the remaining
filters (RO and DI) fairly readily, and so can end up in the
final filtered water.
Figure 2. A schematic representation of an activated
carbon granule. Both chlorine
(as hypochlorite) and chloramine are catalytically broken
down on the carbon surface.
The reactions within the activated carbon that break down
these compounds rely on having enough active surface area
for these catalytic reactions to take place. If the sediment
filter is faulty, sediment may clog the activated carbon and
reduce its ability to break the compounds down. Chlorine and
its hydration product in water, hypochlorite ion (ClO-),
are broken down into chloride ion (Cl-)
and oxygen as shown in reactions 1 and 2 below. Equation 1
occurs at the water supply site where the water is chlorinated,
and equation 2 takes place on the activated carbon in the
RO/DI filter (where C* stands for the activated carbon and
CO* stands for the activated carbon with an attached oxygen
+ H2O à
OCl- + 2H+ + Cl-
+ C* à
Cl- + CO*
Some of the oxidized activated carbon remains, and some breaks
down to produce oxygen (O2):
None of the products of reactions 2 or 3 is of concern to
The reaction of chloramine (NH2Cl)
on activated carbon is a bit more complicated, and produces
ammonia (NH3), chloride (Cl-),
and nitrogen gas (N2). Equations 4
and 5 show the processes.
NH2Cl + H2O + C*
à NH3 + Cl-
+ H+ + CO*
2NH2Cl + CO* à
N2 + H2O+ 2H+ + 2Cl-
Unlike the breakdown of chlorine, one of the byproducts of
chloramine breakdown, specifically ammonia, is of significant
concern to reef aquarists. Ammonia will pass fairly readily
through an RO membrane (discussed later in this article),
but if there is a DI system, it should effectively remove
The carbon filters used in an RO/DI system will also remove
certain organic chemicals from the water. Most of these are
large enough not to pass through the RO membrane, but a few
may be small enough to get through, yet still be reasonably
well bound by the carbon. An example of something bound by
the carbon that might otherwise get through the RO membrane
is trichloromethane (CHCl3). In general,
however, I do not believe that this binding is important for
reef aquarium applications using normal potable water as the
Often the carbon filter also acts as a secondary sediment
filter, and can therefore become clogged over time. A pressure
drop at the RO membrane that is not solved by replacing the
sediment filter may be due to a clogged carbon filter. In
some cases, clogged filters may be cleared somewhat by backwashing,
if the system permits reverse flow. Both carbon and sediment
filters can become clogged with bacteria as well. In such
a situation, some aquarists dry the filters out, thereby opening
the pores, and then reusing them. I have never backwashed
or dried any of my filters.
Reverse Osmosis Membranes
Reverse osmosis membranes consist
essentially of a sheet of porous organic polymer. A variety
of different materials is used commercially, including cellulose
acetate/triacetate blends (sometimes called CTA), thin film/thin
layer composites (sometimes called TLC or TFC), and modified
polysulfone (sometimes called SPSF). The relative advantages
of each of these materials are detailed in other
articles, but some important points are:
CTA membranes are inexpensive and resistant to oxidation
TFC membranes are more costly, but have high impurity
rejection. They must be protected from chlorine and chloramine.
SPSF membranes are generally optimal only in special
situations, such as very soft source water.
If the membrane's pore sizes are made just a bit larger than
water molecules, then water can pass through them, but larger
compounds cannot. Size in this case is a somewhat simplified
idea. Many ions are smaller than a water molecule (Figure
3), but it turns out that charged ions (such as sodium, Na+)
in solution contain several very tightly bound water molecules.
Removing all of these attached water molecules requires a
lot of energy, so when passing through a porous membrane,
they act as if they are as large as the whole hydrated assembly
(Figure 4). These larger assemblies cannot pass through an
RO membrane as readily as they could without the tightly bound
water molecules (Figure 5).
Figure 3. Comparative sizes of a water molecule (H2O;
and a bare sodium ion (Na+;
Figure 4. Comparative sizes of a water molecule (H2O;
right) and a sodium
ion with tightly bound water molecules (Na+;
Figure 5. A schematic representation of an RO membrane,
large enough for water molecules to pass through, but not
large enough for
assemblies of sodium ions with their tightly bound water molecules.
The more charges an ion has, the more water molecules are
attached and the harder they are to remove. It has recently
been suggested that the ratio of the hydrated volumes
of two ions approximates the ratio of the square of the charges
of the same two ions. So, for any simple inorganic X, Y, and
Z, X+ is one-quarter the
size of Y++, and X+
is one-ninth the size of Z+++.
The same holds true for negatively charged ions.1
For these reasons, the relative order of rejection by RO membranes
is typically trivalent > divalent > monovalent, as shown
7. Typical Rejection Rates of Ions From RO Membranes
monovalent ions (Na+,
divalent ions (Ca++,
trivalent ions (Fe+++,
Since RO membranes purify based on size, they are subject
to some obvious limitations. Certainly, anything that is very
large cannot pass though them. In this category would be bacteria
(although they may colonize both sides of the filter, they
cannot pass through it), viruses, large organic molecules
such as proteins, and inorganic mineral particulates that
were small enough to pass through the sediment and carbon
filters (often called colloids).
Also, in order to get a sufficiently fast flow of water through
the membrane, membrane pores are actually significantly larger
than a water molecule. For this reason, some of the molecules
of compounds that are somewhat larger than a water molecule
can still get through (sodium ion, for example, is not perfectly
However, at the small end of the spectrum a number of compounds
can pass through a reverse osmosis membrane to some extent
and are, therefore, of concern to reef aquarists. These include
carbon dioxide (CO2), ammonia (NH3),
hydrogen sulfide (H2S, especially a
concern with well water) and silicic
acid (Si(OH)4, which is the uncharged
and predominate form of silicate at pH values below 9.5).
All of these should be trapped by a functioning DI resin (discussed
below), but can still be a concern.
In the case of CO2, for example, there
can be a lot of it in certain well waters, and DI resins may
become rapidly depleted because the CO2
so readily passes through RO membranes (how to deal with this
is discussed later in this article). As another example, ammonia
that comes from chloramine in the water can be significant,
and is one reason that RO/DI is greatly preferred to RO alone
in those situations where chloramine is added to the tap water.
In the case of silicic acid, some types of RO membranes can
be better than others at excluding it, even before it gets
to the DI resins. For example, a thin-film polyamide membrane
might let only 0.3%
of the silicic acid pass, while a similar cellulose acetate
membrane might let 12.7%
of it pass.
In order to function properly, the RO membrane is coupled
with a flow restrictor that allows pressure to build on the
upstream side of the membrane, rather than letting the water
simply run out of the unit and down the drain. This pressure
helps force the water molecules (and other small molecules)
through the membrane. After passing through the pores, the
water then continues on to the DI resin.
Many systems will include a pressure gauge that measures
the line pressure ahead of the RO membrane. It is the pressure
across the RO membrane that forces water through. At low pressure,
the water may simply run past the restrictor and down the
drain. Most membranes need at least 40 PSI or so to get reasonable
flow and purification. In my system, the pressure drops over
time as sediment clogs the sediment and carbon filters. I
use this gauge as an indicator that the filters before the
membrane need to be replaced. Some RO/DI manufacturers (e.g.,
sell kits that allow the membrane to be flushed with water,
permitting loose sediments and calcium/magnesium carbonate
that can clog it to be washed away.
Various factors, such as temperature
and pressure, impact not only the flow rate through the
membrane but also the purity of the resulting water. Lower
temperatures make the water more viscous and less likely to
flow through the small pores, reducing the production of purified
water. The effect of temperature on purity is much smaller,
with purity decreasing slightly at higher temperatures. Higher
line pressure across the RO membrane results in higher rates
of production and quality, although a pressure that is too
high can damage the membrane. Any backpressure on the effluent
will degrade performance. Very high TDS
(total dissolved solids) in the source water also leads
to higher osmotic backpressure, reducing the membrane's effectiveness.
As a rough guide, every 100 ppm of TDS produces 1 psi of osmotic
For those interested in a great many more details on reverse
osmosis membranes and their engineering applications, a big
library of articles is available online at General
Electric's website: "What is Reverse Osmosis."
The final filter in an RO/DI system
is the deionizing resin. A DI resin traps all charged molecules
passing through it, and leaves uncharged (neutral) molecules
free to pass through. Water, for example, passes through it,
as would other uncharged inorganic molecules such as oxygen
(O2), nitrogen (N2)
and chloramine (NH2Cl, if any remained
from the previous filters). Uncharged organic molecules also
pass through a DI resin, including ethanol (CH3CH2OH),
methanol (CH3OH), methane (CH4),
carbon tetrachloride (CCl4), and methylene
Ions such as sodium (Na+),
copper (Cu++ or Cu+),
ammonium (NH4+), phosphate
(PO4---), silicate (Si(OH)3O-),
and acetate (CH3CO2-)
all get caught.
All atoms or molecules that are in rapid and significant
equilibrium with their charged forms will be caught and removed
just as their charged forms are. These include ammonia (NH3)
caught as ammonium, silicic acid (Si(OH)4)
caught as silicate, carbon dioxide caught at least partially
as bicarbonate (HCO3-)
or carbonate (CO3--), and
acetic acid (CH3CO2H)
caught as acetate, etc.
To catch these ions, the resin consists of porous beads that
have fixed charges attached to them. The counterions to these
fixed charges start off as H+
and OH- in a fresh resin.
Normally, there are different beads intended to bind cations
and anions. In a mixed bed DI resin, the beads are mixed together
in a single filter. In a separate bed system, each bead type
will be in a different filter, thereby potentially allowing
the DI to be recharged (a process that is discussed later
in this article).
Figure 6 shows a cation-binding resin bead ready to bind
sodium (the fixed charges on the resin are not shown, only
the replaceable H+ ions).
As the sodium ions enter the bead, they bind to the fixed
negative charges, and H+
is released as they swap places. The chloride ions pass through
this bead unchanged since they are not attracted to the negatively
charged sites in the bead. After all of the sodium ions that
entered the bead are bound, the product is a hydrogen chloride
solution that passes on to the next bead in the bed (Figure
Figure 6. A cation binding bead of a DI resin, shown
ready to swap
an internal H+ for the incoming
Figure 7. A cation-binding resin, showing how a sodium
ion (Na+) gets
exchanged for a proton (H+).
This bead consequently converts a
sodium chloride solution into a hydrogen chloride solution.
Figure 8 shows an anion-binding resin bead ready to bind
chloride (the fixed charges on the resin are not shown, only
the replaceable OH- ions).
As the chloride ions enter the bead, they become bound to
the fixed positive charges, and OH-
is released as they swap places. As the chloride ions that
entered the bead are bound and OH-
is released, the H+ and
OH- ions combine to form
water molecules (H2O). In this way,
none of the original sodium and chloride ions remain in solution,
and only pure water passes out of the DI resin chamber (Figure
Figure 8. An anion-binding bead of a DI resin, shown
ready to swap
an internal OH- for the
incoming Cl- ion.
Figure 9. An anion-binding resin, showing how a chloride
ion (Cl-) gets
exchanged for a hydroxide ion (OH-).
This bead consequently
converts a hydrogen chloride solution into water alone.
Deionizing Resin Depletion
Eventually, all of the H+
and OH- originally installed
in a DI resin will become depleted, and ions will pass through
unchanged (Figure 10). When charged ions begin to pass through
the DI resin, the effluent's conductivity rises. Many RO/DI
systems use an inline
conductivity meter to alert users when ions are starting
to appear, indicating that the resin needs to be replaced.
Without such an inline meter, users need to periodically monitor
the effluent’s conductivity (in mS/cm
or ppm TDS; details are given in the tips section on what
conductivity to target for resin replacement).
Figure 10. A cation-binding resin shown nearly depleted
of H+, and
allowing sodium ions to pass through.
Some DI resins incorporate color changes to indicate when
the DI is depleted. Such indicators are typically pH indicating
dyes that change color when the pH in the interior of the
beads shifts from the very high or low pH values when OH-
or H+ are the dominant counterions,
to more neutral values when other ions dominate (such as Na+
or Cl-). Such color changes
may be less effective than measuring the effluent’s
conductivity for indicating early breakthrough of ions. The
color change may not indicate that some beads or parts of
beads may become depleted before others due to channeling
of the ion flow. Consequently, I would not rely exclusively
on such color changes unless they have proven to accurately
predict the rise in conductivity of the effluent for a given
brand of DI filter and bead.
Several issues arise relating to the depletion of the DI
resins that aquarists need to be aware of. Primary among these
is that when a DI resin becomes depleted, that does not
simply mean that the water passes through just as it came
from the RO effluent. It may actually be much worse from
an aquarist’s perspective. The reason for this is that
while the DI resin is functioning properly, all ions will
be caught. But when it is depleted, not only the new ions
are coming through and might show up in the product water,
but so are all the ions that ever got into the DI resin in
the first place. The total concentration of ions coming out
of the exhausted DI resin will not be raised as compared to
the RO's effluent, but which ions are released may be very
In the DI descriptions above, I did not address the fact
that some ions will show a greater preference for attachment
to the resin than will others. When the resins are not depleted,
it does not matter what the ions’ affinity is, as all
are bound. But in a depleted scenario, when there are more
ions than ion binding sites, those with a higher affinity
for the resin will be retained, and those with a lower affinity
will be released. It turns out that silicate
is found at the lower end of affinity for anion resins.
Consequently, if the DI resin has been collecting silicate
for a long period and is then depleted, a large burst of silicate
may be released.
Perhaps even more of a concern is ammonia. In a system with
chloramine in the tap water, the DI resin will serve the important
function of removing much of the ammonia produced by the chloramine
breakdown. Ammonia has a poorer affinity for many cation-binding
resins than do many other cations (e.g., calcium or magnesium).
Consequently, when the DI resin first becomes depleted, a
big release of ammonia from and through the DI resin is likely.
I recently had a DI resin become depleted, and the effluent
contained so much ammonia that I could easily smell it.
Other complications can also impact resin depletion. One
potentially important issue is that the anion and cation-binding
sites may not become depleted at the same time. Figure 10
shows this scenario when both types become depleted together,
with sodium and chloride in the effluent. But, it is possible
for one to become depleted first, and in that case, the pH
of the effluent can swing far from neutral. Figures 11 and
12 show what happens when a lot of carbon dioxide is present,
as is the case with some well waters. Initially, it is mostly
bound as bicarbonate, and the effluent is essentially pure
water. Note, however, that as the bicarbonate is removed,
the anion binding resin is being taken up with bicarbonate,
while the cation-binding resin is unchanged and is therefore
not being depleted.
Figure 11. A DI resin, shown ready to bind carbon dioxide
that has dissociated
into H+ and bicarbonate
Figure 12. A DI resin, shown binding carbon dioxide
Eventually, the anion-binding sites become fully occupied
(Figure 13). At that point, additional ions coming through
(such as sodium and chloride) are no longer equally swapped
out to produce pure water. The sodium is swapped for H+,
but the chloride does nothing, potentially leaving the effluent
water with a very low pH.
Figure 13. A DI resin that has been depleted by carbon
dioxide (Figure 12), shown
binding sodium but not chloride, resulting in highly acidic
A similar effect can be hypothesized for silicic acid in
the RO permeate:
H+ + Si(OH)3O-
The effect on pH of the DI resin’s initial depletion
would be similar here to the effect of carbon dioxide in the
The same can happen in the opposite sense with ammonia. If
a lot of ammonia gets through the RO membrane (as is the case
when chloramine is present), the ammonia will be bound in
the DI resin as ammonium:
NH4+ + OH-
The ammonium depletes the cation-binding resin, while the
OH- does not impact the
anion-binding resin. Eventually, then, the cation-binding
capacity can become depleted before the anion binding is depleted,
passing through is converted into Na+
and OH-, with a potentially
[[As an aside, my RO/DI effluent always seems to have
a high pH (9-10) even before its conductivity rises
significantly. While there are many complications to
measuring pH in pure water, where pH kits and meters
do not function well, I cannot so easily dismiss these
readings as being purely artifact, although they may
be. I have wondered for years what might be causing
it, and have not yet found any clear answer. However,
if the above process is happening in my system even
on a small scale, it might explain the results (my tap
water contains chloramine). A sodium hydroxide solution
with a pH of 9 has only 10-5
moles/L of sodium hydroxide, or 0.4 ppm sodium hydroxide
by weight. Is that all or part of the high pH that I
observe in my effluent? I’m not sure.]]
Final Effluent pH
Aside from the issues discussed above
concerning the effluent’s pH when the DI resin becomes
depleted, the final pH coming out of an RO/DI system should
not significantly concern reef aquarists. Many aquarists with
low pH problems have asked, for example, if their aquarium’s
low pH may be caused by their replacing evaporated water with
RO/DI water that they measure to have a pH below 7. In short,
the answer is no, this
is not a cause of low pH nor is it something to be generally
concerned about, for the following reasons:
1. The pH of totally pure water is around 7 (with the exact
value depending on temperature). As carbon dioxide from the
atmosphere enters the water, the pH drops into the 6’s
and even into the 5’s, depending on the amount of CO2.
At saturation with the level of CO2
in normal (outside) air, the pH would be about 5.66. Indoor
air often has even more CO2, and the
pH can drop a bit lower, into the 5’s. Consequently,
the pH of highly purified water coming from an RO/DI unit
is expected to be in the pH 5-7 range.
2. The pH of highly purified water is not accurately measured
by test kits, or by pH
meters. There are several different reasons for this,
including the fact that highly purified water has very little
buffering capacity, so its pH is easily changed. Even the
acidity or basicity of a pH test kit’s indicator dye
is enough to alter pure water’s measured pH. As for
pH meters, the
probes themselves do not function well in the very low
ionic strength of pure freshwater, and trace impurities on
them can swing the pH around quite a bit.
3. The pH of the combination of two solutions does not necessarily
reflect the average (not even a weighted average) of their
two pH values. The final pH of a mixture may actually not
even be between the pH’s of the two solutions when combined.
Consequently, adding pH 7 pure water to pH 8.2 seawater may
not even result in a pH below 8.2, but rather might be higher
than 8.2 (for complex reasons relating to the acidity of bicarbonate
in seawater vs. freshwater).
Recharging DI Resins
When DI resins are present as mixed
bed filters, they are essentially one-time use devices that
must be thrown away when depleted. These are the types supplied
with many commercial RO/DI systems, and are what I use. Separate
bed DI resins have certain advantages, however. In particular,
if they are kept in different cartridges, they can be recharged.
Trendy deals of the Makro Specials and exclusive price range.
The recharging process is essentially the reverse of the
deionizing process (Figures 6-8). A strong acid (usually hydrochloric
acid, HCl) is used to swap H+
for all of the positively charged ions on the cation-binding
resin (Na+, K+,
etc.). Likewise, a strong base (usually sodium hydroxide,
NaOH) is used to swap OH-
for all of the negatively charged ions on the anion binding
resin. Both hydrochloric acid and sodium hydroxide are readily
available and inexpensive. They are, however, potentially
dangerous to work with. The detailed procedures and safety
precautions are beyond the scope of this article, but can
be found elsewhere
Tips on Using RO/DI
If you have a pressure gauge on the RO membrane, change
the sediment filter when the pressure drops to about 40
PSI. If the pressure does not rise adequately after replacing
the sediment filter, change the carbon filter.
If you do not have a pressure gauge, change the sediment
and carbon filters when the output of purified water becomes
Avoid running the RO/DI if your town’s water system
is flushing fire hydrants in your area, as the sediment
and carbon filters will clog very fast.
If the sediment and carbon filters do not seem to get
clogged, at least replace the carbon filter periodically
to ensure that chlorine and chloramine are being broken
down appropriately. Both of these can be measured with
an inexpensive chlorine test kit if you want to be sure
they are being removed.
If you have especially hard water, you might consider
running it through a water softener before the RO/DI filter.
That process removes calcium and magnesium by swapping
sodium for them. While aquarists are not generally worried
about calcium and magnesium in their source water, they
can foul RO membranes by forming calcium and magnesium
carbonate precipitates inside it. That precipitation will
reduce flow and eventually make the membrane unusable.
Make sure that your waterline pressure is appropriately
high (at least 40-60 PSI). In some situations where the
pressure is low, an inline pump may be desirable before
the RO/DI system to boost the pressure.
If there is excessive carbon dioxide in the source water
(such as well water), you might consider degassing the
water first to remove some of the carbon dioxide, and
thereby reduce the rate of depletion of the DI resins.
Note, however, that this option is not inexpensive, and
typically involves repressurizing the water with a pump.
If you are evaluating an existing RO membrane and can
collect water from the tap and after the RO membrane,
the conductivity (in mS/cm
TDS) should drop by a factor of more than 10 across
it (to as much as 100), relative to the tap’s water.
If the drop is less than a factor of 10, it is not working
properly, and may have holes in it.
Monitor the DI resins by measuring the effluent’s
conductivity, either with an inline meter (set to its
most sensitive level), or by measuring the effluent manually.
If you are using a TDS or conductivity meter, then the
measured value should drop to near zero, or maybe 0-1
ppm TDS or 0-1 mS/cm. Higher values indicate that something
is not functioning properly, or that the DI resin is becoming
saturated and needs replacement. That does not necessarily
mean, however, that 2 ppm TDS water is not OK to use.
But beware that the flow of impurities and the conductivity
may begin to rise fairly sharply when the resin becomes
saturated. Do not agonize over 1 ppm versus zero ppm.
While pure water has a TDS well below 1 ppm, uncertainties
from carbon dioxide in the air (which gets into the water
and ionizes to provide some conductivity; about 0.7 mS/cm
for saturation with normal levels of CO2,
possibly higher indoors) and the conductivity/TDS meter
itself may yield results of 1 or 2 ppm even from totally
pure water by not being exactly zeroed properly. Also
note that the first impurities to leave the DI resin as
it becomes saturated may be things that you are particularly
concerned with (such as ammonia if your water supply uses
chloramine or silica if there is a lot in the source water).
If you recharge your DI resins yourself, be very
careful with the acid and base used, as they can
RO/DI is likely the most effective
way for reef aquarists to adequately purify all tap waters
they are likely to encounter. Most aquarists will find that
a standard system consisting of a sediment filter, a carbon
filter, a reverse osmosis membrane, and a deionizing resin
cartridge is perfectly suitable. It is also useful to have
the ability to measure the conductivity of the effluent inline,
and to measure the pressure across the RO membrane.
In some cases, it may be desirable to degas the incoming
tap water if it contains excessive carbon dioxide. It may
also be desirable to have two DI cartridges in series if there
is a lot of silica or other easily penetrating problem ions
in the RO permeate. If you have well water that smells bad
or has other peculiarities, it may be worth discussing the
issue with a top-of-the-line RO/DI system manufacturer to
make sure that you are treating the water appropriately.
Whatever brand or setup you use, be sure to change the filters
at appropriate intervals.