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An
Article by George J. Reclos, Takis Tsamis & Andreas Iliopoulos
THIS ARTICLE APPEARED IN THE MARCH 2002
ISSUE OF FAMA
Effectiveness
Protein
skimmers are considered the most effective chemical filtering devices,
because they remove most of the organic compounds from the water by
physical means (the only media that protein skimmers use are water
and air), before they begin to break down into toxic compounds. So
they give a helping hand to biological filters. Additionally, some
bacteria and phytoplanktonic organisms are mechanically trapped in the
foam and are removed alongside the organic matter. The adequate skimming,
which removes all these organic nutrients, is the most sufficient way to
control the growth of Cyanobacteria as well. It is not wise to use
algaecides and antibiotics (erythromycin for instance) to control algae
and Cyanobacteria, as an alternative to using a skimmer. Algae and
Cyanobacteria will be killed, but they can return if there are
enough nutrients in the environment. Therefore, it is preferable to work
at the basis of the problem: elimination of excess organic matter.
Factors
that dictate the efficiency of a protein skimmer are bubble size, airflow
rate, and the active contact time of the bubbles in the skimmer. The
desired bubble size is between 0.5 and 1.0 mm. This maximizes the surface
area of the bubbles, without considerably lowering their buoyancy. Air
flow rate dictates the relative ratio of water to air bubbles in the
protein skimmer. The recommended airflow rate for protein foam skimmers is
around 1.8 cm/sec per cm2 of cross-sectional area of the main
cylinder of the protein skimmer. Having the correct flow rate will
increase the skimmer's efficiency by creating good foam without
compromising the active surface area of the bubbles. The active contact
time of the bubbles is generally controlled by two factors - the height of
the protein skimmer's main cylinder, and the flow rate of the water
through the skimmer. There is no specific "golden" value for these since
they are a strong function of the particular characteristics of each
protein skimmer and aquarium setup.
Size -
Installation
The size of a skimming
device is, of course, related to the size of the tank we want to install
it on. Generally, we can say that a 120 cm tall skimmer, with a
diameter of its contact column of about 15 cm and a water
flow rate of about 1.200 liters per hour can be used with a 600
liter aquarium. A
skimmer should process at least the equivalent of one tank full of air and
one tank full of water per hour. For most tanks, the water rate is easy.
The air rate, however, is not. Most counter current (explained below)
skimmers are under-supplied with air. If you have a sealed skimmer where
the air can only exit from one fitting, it’s easy to measure the flow
rate. Simply take a large plastic bag (something in the 2 gallon size
works well), empty it, and place it over the air exhaust port. Time how
long it takes to fill and do the math.
Three basic styles of skimmers
exist: counter current air driven, venturi driven, and down-draft (see
diagrams). All styles work fine, all have tradeoffs. All require some
tuning. Expect to spend some time over the first month or so learning how
to keep your skimmer tuned.
3. Typical co-current flow
protein skimmer. Water and Air move to the same direction.
The place
we shall install a skimmer is not so critical. We can place it inside the
tank (internal skimmer), or outside it (hung or put in the sump). If it is
the right skimmer for the tank and is the right size, it will produce foam
either way. If it is placed in the sump, then it must be placed on the
compartment that the water enters the sump. We recommend installing the
skimmer before the sump, and, more specifically before the chemical and
after the mechanical media. The reason for this is that many (visible or
not) particles of wastes will be removed during mechanical filtration
before entering the skimmer; therefore the chemical filtration compartment
(usually activated carbon) will have less of a load to handle, prolonging
its usefulness. The outlet of the skimmer can supply the biological media
with skimmed water, after that. It is better when the water entering the
skimmer is collected from the top of the water column of our tank and even
better if it is collected from the drilled overflow of our tank. Make sure
you have a very good design with valves and watertight piping, and take
extra care with the water level and flow (see diagrams).
4. Environmental Tower
Skimmer. Down-Draft Skimmer design.
For better
results
·If
you want effective skimming you’ll have to acquire the right size
of skimmer, and make sure that flow is at least two times your tank
volume per hour.
·Also,
it must produce very fine bubbles (sizes between 0,5 mm and 0,8
mm are preferred) and a contact time that is as long as possible. The
finer the bubbles are, the more surface is provided, and more time is
needed for the bubbles to rise (more contact time). The taller the contact
column, the longer the contact time.
·When
we buy a skimmer, we have to check thoroughly if we can easily disassemble
it completely.
·Check
the collection cup and the “neck” (foam riser cylinder). They must be
easily removable to allow for frequent service. The neck has to be long
enough, so that the collected foam will dry out and be properly gathered.
·Check
tubes, plumbing, inlets and outlets. The inlets and outlets have to have
sufficient lumen for the quantities of water they administer.
·Take
a good look at every valve on the system. Skimmers equipped with gate
valves are easier to be adjusted with very fine controls (rather than
the ones with ball bulbs). Ball bulbs are fine when used as
gateways (just to open or close a system), but they are a bit inadequate
for making fine adjustments.
·When
you have to deal with an air – driven skimmer with a long contact tube, be
sure that the air pump that supports it has enough power to pump enough
quantity of air into it.
·Use
more air stones. You should at least double the number of air stones
recommended by the manufacturer. Remember that it is impossible to
over-skim a tank. Wooden blocks make very fine bubbles. Skimmers with a
diameter more than 10 cm work better with two wooden
blocks, at least. First turn on the air pump, and then put the air
stone into the water. It is harder for an air pump to push the water
through a presoaked air stone (it is even harder with dense wooden
blocks).
·Venturi
systems do not need frequent replacement, but they do need some searching.
Choose the ones with an installed adjust valve to get proper abundance of
properly sized bubbles. The ones with a water pump installed (which powers
the Venturi) fill the skimmer with a dense body of air bubbles (they are
sold under the name “Venturi skimmers”). The pumps that are
installed on such skimmers are equipped with needle wheels
(impellers with many sharp flaps) for more efficient water – air mixture.
·Air
stones and the air pump to supply them present a "what came first the
chicken or the egg?" dilemma. Sure, you'd like to maximize your air volume
while minimizing bubble size; but they're mutually exclusive goals. The
best you can do is to have the most airflow you can with ideal, consistent
bubble size (@ 1/2 millimeter). Glass and wood air stones work great when
new/clean. Keep your eye on them and have at least two sets, one in use,
and the other for cleaning/air-drying to extend their functional/useful
life.
5. A co-current protein
skimmer. Due to its short reaction chamber, the contact time is minimal.
This is the least effective of all protein skimmer designs and is mainly
aiming at small tanks. Models with longer reaction chambers are available
but require much more powerful air pumps.
6. Hang-on skimmer design
What
to care for
Skimmers
are very sensitive to changes occurring within them. Dust, insecticide or
aromatic sprays and paint vapors affect their function. These substances
decrease the foam production. Human body fat is also a factor that
will suddenly decrease the foam produced by skimmers. Feeding
also decreases foam production temporarily. This happens because oily or
fatty substances, when introduced into the system, lower the tension of
the water surface. So it is not strange to observe a decrease in the
produced foam for several hours after feeding our fish or putting our
hands in the tank water.
The
operation of skimmers is also affected by its mode of operation.
Temperature, pH, water and air flow rates,
dimensions of the contact column, surface tension,
specific gravity, as well as other factors such as the properties
of the bubbles (size, abundance and distribution of air bubbles, as
well as the method used to produce them) affect the performance of the
skimmer.
We have
to clean the device and empty the collection cup regularly. Both the dry
(standard) and the wet (protein) foam are poisonous because of their high
concentration in toxic compounds. Make sure that none of it is dropped
back into the tank, either because of a mistake or due to an improperly
designed construction. Some commercial skimmers are equipped with an
overflow on the collection cup. Through this overflow, skimmed protein
foam is drawn away from the collection cup. You can drill a hole (12/16 is
enough lumen) if there is no overflow in your skimmer and with a tube you
can dispose the wet foam into a container or directly into your domestic
drainage system.
Lots
of turbulence at the top of the contact column prevents the correct
separation of the dry foam and skimmed water, thus decreasing its
performance. The pH affects foam formation, too. As pH changes throughout
the day, the skimmer operates with a varying efficiency. The higher the
pH, the stronger the bonds of the organic molecules to the bubbles’
surface.
Because
of the differences in the structure of various proteins, some of them are
skimmed easier than others since the pH also affects their ionization (the
further away from their isoelectric point the more ionized they become),
which is directly linked to their degree of electrostatic attraction.
Specific
gravity also affects skimming, because dissolved salts increase the
stability of the bubbles (greater water viscosity).
Last, but
not least, when temperature increases, surface tension is decreased. The
higher the temperature the more readily foam breaks, and its formation is
dry and stable.
7. Stand-alone skimmer
design.
Disadvantages
Protein
skimmers, in general, remove trace elements from the water, so we have to
replenish them. This is accomplished with regular water changes, which
introduces them back into the tank with the introduction of fresh tap
water.
The air –
driven protein skimmers have some disadvantages when compared to the motor
– driven ones. Their main problem is because they are not able to supply a
constant and stable quality of air bubbles.
8. An example of a
Sump-mounted Venturi protein skimmer.
continued in next page
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