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An
Article by George J. Reclos, Takis Tsamis & Andreas Iliopoulos
THIS ARTICLE APPEARED IN THE MARCH 2002
ISSUE OF FAMA
Undoubtedly, the most critical stage in keeping aquaria successfully is
controlling the amount of nutrients present in the water column. Our main
purpose is to control the amount of the so-called “Nitrogen” compounds in
our tank.
Nitrogen
(N2)
is an abundant gas in the atmosphere, accounting for almost 80% of its
total volume, therefore there is much more nitrogen than Oxygen or
Hydrogen. It can be found in most organic compounds, as it has the
ability to take part in many chemical reactions. Although Nitrogen itself
can’t sustain life, it is absolutely essential for the existence of life.
Well-known “Nitrogen” compounds are proteins, DNA and RNA.
All
living organisms “excrete” Nitrogen compounds, which are formed during the
various biological processes that take place in their body. One of the
best known such compounds that we, humans, excrete is urea (a byproduct of
protein metabolism). Fishes eliminate Nitrogen compounds via their
urogenital and respiratory systems.
Perhaps a
way to control the amount of Nitrogen compounds introduced into our water
would be by reducing or even stopping feeding. However, fish also cast out
a lot of it from their gills. The more the stress, the more
Nitrogen will be released. This adds more stress to fishes as the
presence of ammonia or nitrites further induces stress, thus entering an
endless cycle. Normally, non - toxic Ammonium (NH4+)
found in acidic water, or toxic Ammonia (NH3)
found mainly in alkaline water, is oxidized by Nitrosomonas spp.
to Nitrite (NO2־),
which can be more toxic than Ammonia itself, especially in acidic water
where it will form the deadly Nitrous acid (HNO2).
Further nitrification by Nitrobacter spp. produces
Nitrates (NO3־)
that are used from plants as a fertilizer along with other radicals like
Phosphates (PO4
־³)
and Silicates (SiO4-2). During this nitrification
procedure oxygen is required, hence the nitrifying bacteria are called
“aerobic”. The opposite procedure is called denitrification and takes
place in the absence of oxygen, by anaerobic bacteria (usually found in
“live” rock and/or “live” sand). During denitrification, nitrogen and
nitrogen monoxide gases are produced as a final product. These gases are
first released into the water, and later into the atmosphere via gas
exchange, or absorbed by a kind of bacteria called Cyanobacteria.
Cyanobacteria, or “blue – green” algae can come in more colors than
cyanon (the Greek word for blue). Usually they are colored reddish,
greenish, brownish, bluish or even black. They are single cellular
organisms and form ugly mats in the tank. They are something between algae
and bacteria, and they are called “grease” and “smear” algae. When their
environment is oligotrophic (little amount of suitable food is
present), Cyanobacteria are nearly harmless, as they cannot
grow in critical quantities, but when their environment is rich in
nutrients (Phosphates, Nitrates, Silicates, among others) they grow very
fast. The worst of them (from the hobbyist’s point of view) are the red
and brown ones. Cyanobacteria have the ability to fix atmospheric
Nitrogen, which they can consolidate in their tissues when dissolved in
water. When those – full of fixed nitrogen - tissues are eaten by
herbivores, nitrogen is converted to the well-known Ammonia and then to
Nitrites and finally Nitrates. See diagram 01.
Diagram 1.
Schematic presentation of the Nitrogen Cycle.
The more
alkaline the water, the more toxic Ammonia is, and this is a
very good reason we should try to avoid the accumulation of it, especially
in systems that require an alkaline pH like marine and African Rift Lake
cichlid tanks. In contrast, the more acidic the water, the more deadly the
nitrites become. Nitrifying bacteria consume a lot of inorganic carbon to
grow and to deal with the transformation of Ammonia to Nitrites. All the
carbon used comes from the break down of carbonates, bicarbonates and
other radicals (negatively charged compounds) that are useful in keeping
an adequate buffering capacity in our water. This affects alkalinity,
and may result in dramatic drops of the pH. We should understand that pH
shocks might occur when the buffering capacity of our water is reduced
hence it can’t prevent rapid pH changes any more.
Nature
had solved this particular problem long before aquarists had even realized
it. Waves, tides and the coastal substrata are the equipments used to
eliminate organic substances that contain Nitrogen, among other things.
You can see it on the coastline, where the waves break – it is the “foam”
formed there. This foam comes from “washing” those compounds and consists
mainly of proteins, rich in Nitrogen.
The human
made equivalent of this natural process is the Protein skimmer.
Structure
- Function
There are
two types of skimmers, the air - driven and the motor - driven
ones. They are usually manufactured of plastic, acrylic or glass fiber.
They both remove fats and fatty acids, organic acids, amino acids,
lipids, phosphates, phenols, carbohydrates, iodide, metals complexed with
proteins and, of course, proteins from water. The list of removed
compounds is far longer, as metals complexed with proteins can also be
found in detritus and plant or animal originating infiltration products.
Some of the vitamins are also a source of nitrates. Tap water
itself usually contains an amount of nitrates, too. And, since nitrates
are nutrients, the Nitrogen cycle goes on again.
Protein
skimmers consist basically of a cylindrical contact column and a
motile collection cup. Inside the contact column water is mixed
with air, and in the collection cup swarms proteins in the nature of foam.
·Air
- driven
skimming devices are usually placed within the tank, so water enters and
exits the contact column from underneath. Inside the contact column, an
air stone is placed. It is fed with air from an air pump to produce as
many fine bubbles as possible. This can be achieved with the use of a
wooden block or very high quality air stones and powerful air pumps.
·The
motor - driven skimming devices are a little more complicated.
Water enters the contact column from a water inlet and exits from a water
outlet, both drilled on the cylindrical body of the contact column. They
may be fed from a water overflow, or by a water pump. Another water pump
circulates the water in the contact column. This pump is equipped with a
Venturi valve. The Venturi valve is a small part of the device
located – usually - at the bottom of the contact column. It directs water
into a pipe, through a constriction of the pipe. Before the constriction,
there is a high pressure and after it, a low pressure. The low pressure
after this constriction forces air to be sucked in from one or more ports
in the Venturi forming very fine bubbles.
Since the
Venturi reduces the volume turnover of the water pump, needle wheels are
used in the place of classic impellers. They have a star-like shape and
are very powerful, as they can rotate at more than 3000 rounds per minute.
There are
two types of protein skimmers. One operates by co – current flow of air
and water, while the other operates with counter – current flow.
2. Typical counter current
flow protein skimmer. Water moves against the blow of the bubbles
Why and
how it works?
Organic
surface – active molecules (surfactants) are attracted to the air bubbles’
surface, as air bubbles act like an adhesive interface for them. These
molecules are polarized with one “hydrophilic” and one
“hydrophobic” side. While their first site (hydrophobic) allows them
to be dissolved in organic solvents (they “fear” water), the other one
(hydrophilic – “water loving”) allows them to stay in contact with water.
So air bubbles collect molecules during their contact with water inside
the contact column because they offer organic matters the two things they
like: a water surface and a non-water surface (the air).
Ever blow bubbles as a kid? Remember all the rainbow colors on them? Just
as the soap clung to the giant bubbles you were creating, so too does all
the junk and other organic gunk in your aquarium water. Those pretty
rainbow colors were the light refracting off the soap film...you could
actually see it! In our skimmers, the bubbles are microscopic, and the
results can only be "seen" after they burst and deposit their "films" into
the collection cup!
The most important issue in skimming is the water/air interface. This
works by using surface tension and the different polarities of the “waste”
molecules. Roughly, it can be seen as the blood/air surface, which is
created in our alveoli in our lungs. Alveoli create a relatively huge
surface in which blood (a very thin film of blood) becomes exposed to air
so the “waste” (mainly carbon dioxide) can be exchanged with oxygen. So,
the larger the “exchange” surface, the better the exchange rate, therefore
the higher the efficiency of the skimmer. The shape of the bubble is also
very important. The sphere is known to be the geometrical shape which
offers the largest possible surface / volume ratio which is very important
(that is why most cells have a spherical shape whenever possible).
Speaking of cells, they divide when they have grown too much, because they
automatically realize that their volume has become too large for their
surface to cope. Cells prefer to have a smaller spherical volume with a
large surface, so they can exchange nutrients with their environment more
efficiently. This is also the case with protein skimmers. The size of the
bubbles is consequential. What we need is as many bubbles as possible with
as small a volume as possible. This forces the manufacturers of skimmers
to use media or constructions that will lead to the formation of a large
number of tiny bubbles. When the size of the bubbles become too large or
the number gets reduced, the efficiency of the skimmer is greatly reduced,
as well. At this time, the aquarist should replace the air stone, or clean
the air pathways. This is also one of the reasons skimmers are much more
effective in the marine aquarium, while its usefulness in freshwater tanks
is limited. If you take a normal air pump with a normal air stone you will
immediately notice that the size of the air bubbles produced in salt water
is much smaller (and the number much larger) when compared to a freshwater
tank. This is, of course, due to the higher density of the salt water.
This is not to say that a skimmer will not work in a freshwater tank.
Actually, all you have to do is to use more powerful air pumps and as many
wooden air blocks as possible in freshwater tanks, while at the same time
ensure that the air stones you use will produce the smallest possible
bubbles. Therefore, by using air pumps rated for much larger tanks and
wooden blocks, one can make a skimmer work efficiently in freshwater,
also. The reason the skimmer is mandatory in marine tanks is that large
and frequent water changes are not desired, as well as being expensive.
So, the freshwater aquarist has the option of large and more frequent
water changes which takes care of excess wastes, making the protein
skimmer an optional extra. The water, when entering the contact column,
has a high concentration of surfactants. Their concentration is reduced as
water is driven to the top of it since most of them stay attached on the
air bubbles. So all this protein foam is formed in the collection cup.
This foam consists mainly of nitrates, which if they had stayed in the
water would have mineralized and accumulated in the tank.
continued in next page
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