Protein Skimmer: A General
Definition
A protein skimmer or foam fractionator is a device used
mostly in saltwater aquaria to remove organic compounds from the water
before they break down into nitrogenous waste. Protein skimming is the
only form of aquarium filtration that physically removes organic
compounds before they begin to decompose, lightening the load on the
biological filter and improving the water's redox potential. Although
the process of foam fractionation is commonly known for removal of waste
from aquaria, it is, in fact, a rapidly developing chemical process
used in the large-scale removal of contaminants from wastewater streams
and the enrichment of solutions of biomolecules.
Function
Protein
skimming removes certain organic compounds, including proteins and
amino acids, by using the polarity of the protein itself. Due to their
intrinsic charge, water-borne proteins are either repelled or attracted
by the air/water interface and these molecules can be described as
hydrophobic (such as fats or oils) or hydrophilic (such as salt, sugar,
ammonia, most amino acids, and most inorganic compounds). However, some
larger organic molecules can have both hydrophobic and hydrophilic
portions. These molecules are called amphipathic or amphiphilic.
Commercial protein
skimmers work by generating a large air/water interface, specifically by
injecting large numbers of bubbles into the water column. In general,
the smaller the bubbles the more effective the protein skimming is
because the surface area of small bubbles occupying the same volume is
much greater than the same volume of larger bubbles. Large numbers of
small bubbles present an enormous air/water interface for the organic
molecules which are hydrophobic and those which are amphipathic to
collect on the bubble surface (the air/water interface).
The diffusion of organic
molecules is hastened by water movement, which effectively brings more
organic molecules to the air/water interface and allows the organic
molecules to accumulate on the surface of the air bubbles. This process
will continue until the interface is saturated, unless the bubble is
removed from the water or it bursts, in which case the accumulated
molecules are released back into the water column. However, it is
important to note that further exposure of a saturated air bubble to
organic molecules may continue to result in changes as compounds that
bind more strongly may replace those molecules with a weaker binding
that have already accumulated on the interface. Although some aquarists
believe that increasing the contact time (or dwell time as it is
sometimes called) is always good, it is incorrect to claim that it is
always better to increase the contact time between bubbles and the
aquarium water. As the bubbles increase near the top of the protein
skimmer water column, they become denser and the water begins to drain
and create the foam that will carry the organic molecules to the
skimmate collection cup or to a separate skimmate waste collector and
the organic molecules, and any inorganic molecules that may have become
bound to the organic molecules, will be exported from the water system.
In
addition to the proteins removed by skimming, there are a number of
other organic and inorganic molecules that are typically removed. These
include a variety of fats, fatty acids, carbohydrates, metals such as
copper and trace elements such as iodine. Particulates and other
detritus is also removed, along with phytoplankton and bacteria. This
removal is highly desired by the aquarist and is often emphasized by the
placement of the skimmer before other forms of filtration; lessening
the burden of the filtration system as a whole. There is at least one
published study that provides a detailed list of the export products
found in protein skimmer skimmate.
Design
All
skimmers have key features in common: water flows through a chamber and
is brought into contact with a column of fine bubbles. The bubbles
collect proteins and other substances and carry them to the top of the
device where the foam, but not the water, collects in a cup. Here the
foam condenses to a liquid, which can then be easily removed from the
system. The material that collects in the cup can range from pale
greenish-yellow, watery liquid to a thick black tar.
For a
skimmer to function maximally, the following things must take place:
1.
A large amount of air/water interface must be generated.
2. Organic
molecules must be allowed to collect at the air/water interface.
3.
The bubbles forming this air/water interface must come together to form a
foam.
4. The water in the foam must partially drain without the
bubbles popping prematurely.
5. The drained foam must be separated
from the bulk water and discarded.
Also under considerable recent
attention has been the general shape of a skimmer as well. In
particular, much attention has been given to the introduction of cone
shaped skimmer units. Originally designed by Klaus Jensen in 2004, the
concept was founded on the principle that a conical body allows the foam
to accumulate more steadily through a gently sloping transition. This
reduces the overall turbulence, resulting in more efficient skimming.
While research into the specific benefits of the design are still being
measured, early reviews of many conical skimmers have been positive
overall.
Overall, protein skimmers can be classed in two ways
depending on whether they operate by co-current flow or counter-current
flow. In a co-current flow system, air is introduced at the bottom of
the chamber and is in contact with the water as it rises upwards towards
the collection chamber. In a counter-current system, air is forced into
the system under pressure and moves against the flow of the water for a
while before it rises up towards the collection cup. Because the air
bubbles may be in contact with the water for a longer period in a
counter-current flow system, protein skimmers of this type are
considered by some to be more effective at removing organic wastes.
Co-current
flow systems
Air stone
The original method of protein
skimming, running pressurized air through a diffuser to produce large
quantities of micro bubbles, remains a viable, effective, and economic
choice, although newer technologies may require lower maintenance. The
air stone is most often an oblong, partially hollowed block of wood,
most often of the genus Tilia. The most popular wooden air-stones for
skimmers are made from limewood (Tilia europaea or European limewood)
although basswood (Tilia americana or American Linden), works as well,
may be cheaper and is often more readily available. The wooden blocks
are drilled, tapped, fitted with an air fitting, and connected by air
tubing to one or more air pumps delivering at least 1 cfm. The wooden
air stone is placed at the bottom of a tall column of water. The tank
water is pumped into the column, allowed to pass by the rising bubbles,
and back into the tank. To get enough contact time with the bubble,
these units can be many feet in height.
Air stone protein
skimmers may be constructed as a DIY project from pvc pipes and fittings
at low cost and with varying degrees of complexity.
While this
method has been around for many years, many regard it as inefficient for
larger systems or systems with large bio-loads.
Venturi
The
premise behind these skimmers is that a venturi pump, or aspirator, can
be used to introduce the bubbles into the water stream. The tank water
is pumped through the venturi, in which fine bubbles are introduced,
then enters the skimmer body. This method was popular due to its compact
size and high efficiency but venturi designs are now more likely to be
included in other skimmer designs rather than as a simple venturi
design.
Counter-current flow systems
Aspirating:
Pin-WHeel/Adrian-Wheel, Needle-Wheel, Mesh-Wheel
This basic concept
is more correctly known as an aspirating skimmer, since some skimmer
designs using an aspirator do not use a "Pin-Wheel/"Adrian-Wheel" or
"Needle-Wheel". "Pin-Wheel"/"Adrian-Wheel" describes the look of an
impeller which consists of a disk with pins mounted perpendicular (90ยบ)
to the disc and parallel to the rotor. "Needle-Wheel" describes the look
of an impeller which consists of a series of pins projecting out
perpendicular to the rotor from a central axis. "Mesh-Wheel" describes
the look of an impeller which consists of a mesh material that is
attached to a plate or central axis on the rotor. The purpose of these
modified impellers is to chop or shred the air that is introduced via a
venturi apparatus or external air pump into very fine bubbles. The
Mesh-Wheel design is fairly new and, while providing excellent results
in the short term because of its ability to draw in more air and create
finer bubbles with its thin cutting surfaces, it is still being
developed and will likely continue to evolve over a few more years.
This
style of protein skimmer has become very popular and is believed to be
the most popular type of skimmer used with residential reef aquariums
today. It has been particularly successful in smaller aquariums due to
its usually compact size, ease of set up and use, and quiet operation.
Since the pump is pushing a mixture of air and water, the power required
to turn the rotor can be decreased and may result in a lower power
requirement for that pump vs. the same pump with a different impeller
when it is only pumping water.
(Information taken largely from Wikipedia)
