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
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.
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 attracted to the air/water
interface. Protein skimmers work by injecting numerous tiny
bubbles into the water column. The small bubbles present an
enormous air/water interface for the protein molecules to
cling to. The longer the bubble resides in the water, the
more proteins it is able to attract. The action of a protein
skimmer is often compared to the action of waves producing
sea foam.
The term protein skimming is misleading because
this form of filtration removes other substances besides
proteins. These other substances include
fats and fatty acids, carbohydrates, metals such as copper
complexed with proteins, and certain trace elements such as
iodide, in addition to particulates and other detritus,
phytoplankton, and bacteria.
Although
copper and other metals can be removed through protein
skimming, the element is attached to organics, then removed
with the organic. Fat can cause a skimmer foam to collapse,
handicapping its efficiency.
Design
The basic skimmer design consists of a main chamber
through which the water to be filtered flows. Air is
injected into the column as fine bubbles. The surface of the
bubbles collect proteins and other substances and carries
them to the top of the device where a collection cup allows
the foam to leave the main chamber. Here the foam condenses
to a liquid, which is removed from the system. This material
can range in color from a light tea to black tar.
Every skimmer operates either by co-current flow, in
which the water flows upward in the column along with the
bubbles, or by counter-current flow, in which the water
flows downward against the direction of the bubbles.
Counter-current skimmers are more efficient because of the
greater contact time the water has with the bubbles.
Air stone
The original method of protein skimming, it is not
completely obsolete, although many newer technologies have
eclipsed this method. The air stone is a ceramic block with
an air hose attached that runs to a small air pump. The
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. While this method has been around for many years,
many regard it as inefficient for larger systems or systems
with large bio-loads.
Downdraft
Pumped tank water enters through a small vertical column
next to the main column, that contains a plastic media that
shreds the water, entrapping air in to the stream. The
result is a milky white appearance of very fine bubbles. The
stream enters a mixing box and is allowed to rise within the
reaction chamber. This skimmer design was popular for larger
skimmers but has become less prevalent with the growth of
needlewheel and Beckett skimmer.
Venturi
The premise behind these skimmers is that a
venturi valve, 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.
Needlewheel
This skimmer design is more correctly known as an
aspirating skimmer, since many skimmers in this family do
not use needlewheels (needlewheel describes the look of the
modified water pump impeller which appears to have a number
of needles sticking from it) but instead use an water pump
impeller with a number of pegged rods protruding from it.
The purpose of these modified impellers is to chop or shred
the air that is introduced via a venturi or external air
pump into very fine bubbles. This style 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.
Spray Induction
This method is related to the downdraft, but uses a pump
to power a spray nozzle, fixed a few inches above the water
level. The spray action entraps and shreds the air in the
base of the unit, which then rises to the collection
chamber.
Beckett
This type of skimmer uses a foaming jet fountain nozzle
manufactured by the Beckett Corporation in a protein skimmer
design similar to the downdraft skimmer. Instead of using
plastic media to produce the needed small bubbles as in a
traditional downdraft design, a Beckett skimmer uses the
Beckett 1408 foam nozzle. A Beckett skimmer can use very
powerful water pumps to produce great amounts of foam and
the Beckett skimmer design is often used for protein
skimmers for large reef aquariums. Some larger Beckett
skimmers are designed with 2 or more Beckett injectors and
can use multiple water pumps to further improve the amount
of foam produced and support even larger aquariums or
further reduce the level of dissolved organic compounds in
the aquarium. Beckett skimmers have borrowed from both
downdraft and venturi designs (the Beckett foam head is a
modified 4 port venturi) to produce a hybrid that may be
more effective than it's predecessors. However, while they
can be quite powerful, Beckett skimmer designs requirement
for powerful water pumps to take full advantage of their
foam producing ability can mean that they sometimes use more
electricity and may produce more noise than the smaller
water pumps typically used in needlewheel skimmer designs.
While there is debate over the merits of different skimmer
designs many people consider Beckett skimmers to be the most
powerful available for aquariums and they are particularly
popular for larger aquariums.
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