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Ozone
has been used in reef aquaria for many years. It is claimed to
have many benefits, ranging from increased water clarity to
decreased algae. It has never, however, risen in popularity to
the point where a seeming majority of reef aquarists use it.
Many reasons likely prevent its widespread use, including its
cost, complexity and safety concerns for both the aquarist and
the aquarium's inhabitants. Speaking only for myself, my reasons
for never having used it in my first ten years of maintaining
reef aquaria were driven primarily by concern over ozone
byproducts' toxicity in the aquarium, and the lack of a
perceived need.
Back in the early to mid 1990s there was a fair amount of
emphasis on ozone and other oxidizers as a way to raise the
water's (the oxidation reduction potential). The ORP, in turn,
was incorrectly described as a good way to measure the water's
"cleanliness." So aquarists raised ORP. Then ozone and other
oxidizers (such as permanganate) fell out of favor for a variety
of reasons, not the least of which was the overall trend toward
less technological approaches to reef maintenance.
It appears, however, that the use of ozone may be on the
upswing. In a recent (December 2005) survey I did of experienced
reef aquarists, the results were equally split between those who
had never tried it, and those who were presently using it or who
had in the past and would do so again in an appropriate
aquarium. For most people who had used it, the emphasis is now
on water clarity, not ORP as some surrogate of something that
was vaguely defined but that was supposed to be beneficial.
This article is the first in a series that addresses the myriad
issues around the use of ozone in reef aquaria. The articles
should help aquarists understand why ozone is used and what
molecular level processes take place when using ozone. Together,
they should help aquarists determine for themselves if ozone is
something they want to use, and if so, how to do so.
After
a brief introduction to how ozone is used and some of its
claimed benefits, this first article proceeds to describe what
ozone is and how it reacts with seawater. It also relates
ozone's perceived benefits to the actual chemical and
biochemical changes that it can cause. In a sense, it provides
the mechanistic framework for understanding why ozone does what
it does, helps aquarists understand its limitations and details
the changes in the aquarium water that ozone will cause, whether
they are apparent to most aquarists or not (and, in fact, many
are not).
The
subsequent articles in this series will address the types of
equipment necessary to effectively and safely use ozone, and the
benefits that accrue upon initiating ozone in an aquarium system
(mine) that had been operating for many years without it.
What is Ozone Supposed to Accomplish in a Reef Aquarium?
I've
asked many aquarists what they believed dosing ozone
accomplished in their aquaria. The list is always headed by
increasing water clarity, but also includes other possibilities.
Below, in no particular order, are the sorts of claims that are
made:
1.
Increased water clarity (even if it had been very clear
before ozone)
2. Increased light penetration
3. Decreased yellowness
4. Decreased algae
5. Decreased cyanobacteria
6. Decreased skimmate production
7. Increased skimmate production
8. Increased ORP
9. Reduced nitrate
10. Decreased pathogenic bacteria
11. Reduced circulating toxins
12. Cleaner (more pure) water
Some
of these make perfect sense, and the chemical and biochemical
mechanisms that cause them through ozone's use will be detailed
in the subsequent sections of this article. Others may not be
correct assertions (decreased pathogenic bacteria, for example)
and these issues are also discussed.
Some
instances of apparent problems and perhaps underlying issues
with the use of ozone are subtle enough that most aquarists
never notice them. Bleached corals, for example, are obvious and
have been reported. Perhaps the bleaching that has been
experienced is related to a rapid increase in light penetration.
But suppose that some small invertebrates in the aquarium were
less prone to successfully reproduce due to residual bromate in
the water. Or that the incidence of fish cancers from bromate (a
suspected carcinogen) increased from, say, 1% to 2% for some
particular fish species. How many reef aquarists would notice
those changes, or attribute it to the ozone, even if it were
true?
On the
other hand, many aquarists might not particularly care about
such subtle issues, and want the water to be clearer regardless
of hypothesized problems. In any case, the data such as they are
will be presented and aquarists can decide for themselves if
ozone use is a practice they want to pursue or not. At the end
of the last article in the series, where I present the results
in my aquarium, I'll comment on whether I think it is desirable
to continue using it or not in my system.
How is Ozone Used in Reef Aquaria?
How
ozone is used will be the primary topic of the second article in
this series, but in order to understand many of the issues
presented in this article, it is necessary to have a rudimentary
understanding of how ozone is used.
The pathway for ozone entering an aquarium starts with an
ordinary aquarium air pump. The air travels out of the pump and
often into an air dryer. The air's moisture is removed as it is
absorbed by very
hygroscopic
solids. Not all aquarists perform this step, but removing the
air's moisture has at least two benefits as the air passes into
the next stage of the process. The next stage is a small device
that generates ozone. The method used by most ozone generators
is to pass the air through a high voltage electric discharge
that breaks apart some of the oxygen (O2) molecules, and when
they recombine, some ozone (O3) is formed (a second, less
effective method uses UV light to accomplish the same process,
either by passing air or the water itself past a UV light
source). Moisture in the air reduces the amount of ozone formed
in the generator, and it also results in the formation of nitric
acid (HNO3; from water and nitrogen gas in the air). This nitric
acid can reduce pH and alkalinity, and provides nitrate to the
aquarium (which will be discussed in further detail next month).
After
the ozone-containing air passes out of the ozone generator, it
usually is sent into some sort of mixing chamber where aquarium
water and the gas are mixed well, and are kept in contact for at
least a few seconds. Aquarists often use skimmers or specially
made ozone reactors for this purpose, and selection of suitable
materials is a concern as the ozone can degrade some types of
plastic, rubber and tubing. The amount of ozone delivered varies
widely. Many manufacturers recommend on the order of 0.3 to 0.5
mg/hour per gallon of aquarium water, but many aquarists use
less, or do not use it all of the time. They believe that using
less ozone achieves their need for clearer water, reduces the
need for more expensive equipment and air dryers, reduces
concerns about toxicity due to byproducts and reduces its
negative impact on skimming.
Inside
the contact chamber, the ozone reacts with many different
chemicals in the seawater including organics, ammonia, iron and
other metals, bromide and iodide. It may also interact with
viruses, bacteria and other organisms drawn into the chamber.
The ozone itself survives for only a few seconds in seawater,
but it leaves other reactive oxidizers (called ozone produced
oxidants, OPO; for example, hypobromous acid, BrOH) in its wake.
These can further react with organics and other materials and
are also potentially toxic, so they should be removed before the
water is released to the aquarium. Much of ozone's benefits
happen in this chamber, where, for example, the water is made
"clearer" as certain pigments in dissolved and particulate
organic molecules are destroyed.
Water
leaving the reactor is optimally passed over an amount of
activated carbon sufficient to remove the remaining ozone
produced oxidants. The carbon catalytically (and also
noncatalytically) breaks down these oxidants before they enter
the aquarium. The air passing out of the reactor also contains
ozone, and is also best passed over activated carbon to reduce
the aquarist's concern for airborne ozone's toxicity.
In
order to ensure that not too much ozone enters the aquarium,
aquarists should monitor ORP (the oxidation reduction potential)
in the aquarium's water. For those aquarists using a small
amount of ozone, monitoring may be adequate. For those aquarists
using large amounts of ozone, an ORP controller is important. It
can be used to shut off the ozone if the ORP rises above a set
point (that point being either an emergency shut-off point that
is rarely, if ever achieved, or a target ORP where the generator
is actually running only part of the time, and only when the ORP
controller says that ORP needs to be raised to the set point).
For
comparison to other studies reported in this article, reef
aquarists typically use up to about 0.3 ppm ozone in the
"contact chamber" and have contact times on the order of a few
seconds before the water passes into the aquarium. This value of
0.3 ppm ozone is based on adding ozone at a rate of 100 mg/hour
(a typical addition rate suggested by ozone generator
manufacturers for a tank of about 200 gallons) to a contact
chamber (like a skimmer) that has a flow of 333 L/h; 100 mg/h /
333 L/h = 0.3 mg/L). Higher flow rates, lower ozone addition
rates or incomplete transfer of the ozone into the water will
give lower ozone concentrations in the contact chamber or
skimmer.
Conclusion
Ozone
has many effects when used in a reef aquarium. The most useful
of these is the degradation of organic materials. Most
importantly, and quite coincidently and fortunately for
aquarists, the colored organic pigments in marine aquaria are
very sensitive to ozone. For this reason, ozone can remove
seawater's color quite readily, and much more effectively than
it removes the overall load of organic material. Its effects on
water clarity described by most aquarists range from minimal to
very dramatic, with most aquarists reporting significant
beneficial effects.
Another big effect of ozone is the bioavailability of the
organics in the water. Many organics in the aquarium are not
readily metabolized by bacteria, and such materials may last for
hundreds or in the ocean. Ozone, however, has the ability to
make many organic materials more readily absorbed and
metabolized by bacteria. So in a sense, ozone triggers a
bacterial attack that can reduce the load of circulating organic
materials. This reduction in organic materials may also usefully
apply to circulating toxins released by the aquarium inhabitants
in an effort to kill each other with chemicals.
Ozone
and its byproducts can, in high enough doses, kill many
pathogens. The levels of ozone encountered in reef aquaria,
however, may be inadequate to have any significant effect on
total bacterial populations. Viruses are more susceptible than
bacteria to ozone, and they may be effectively inactivated by
typical use. Larger pathogens and parasites are much harder to
kill and are not likely to be killed by ozone in reef aquaria.
Ozone
also has a dark side. When reacted with seawater, ozone produces
a variety of highly oxidized halogens such as BrOH and BrO3-. If
the ozone produced oxidants are not largely removed with
activated carbon, they may enter the aquarium and be hazards to
the most sensitive organisms in the aquarium (which are likely
eggs or early stage larvae).
Finally, ozone alters a variety of other inorganic materials in
ways that may or may not be important. It alters the aquarium's
redox balance, raising the ORP (which may mean as little as
altering the ratios of different forms of manganese in
solution). It may permit more rapid conversion of ferrous ion to
ferric ion, and may increase its bioavailability, but perhaps
decrease the lifetime of strongly complexed iron such as EDTA
iron. Ozone also oxidizes ammonia to nitrate. While that is
likely beneficial, it may alter the relative effectiveness of
different nitrogen export pathways (macroalgae vs.
denitrification, for example). It may drive the speciation of
iodine toward iodate and away from iodide. Is that good or bad?
I expect neither, although others have different opinions, but
it is a good poster child for the many things that happen in
reef aquaria when using ozone that normally take place without
any notice or recognition of them by the aquarist.
So
with all things considered, is the use of ozone in a reef
aquarium worthwhile? Many aquarists answer with a resounding,
"Yes!" I'll leave that question unanswered until additional
information is detailed in the next two articles discussing what
equipment and methods are most useful for applying ozone to
aquaria, and reporting on what impact it had in my aquarium. |
