Protein skimmer - WO!skimmerProtein Skimmer

A protein skimmer or foam fractionator is a system used mostly in saltwater aquarium to remove organic compounds from the water before they break down into nitrogenous waste.
Protein skimmer 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 saltwater aquarium, 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.

Let's simply say that the air bubbles inside the skimmer is body strip the water of undesirable waste by-products. How the bubbles accomplish this is a neat trick that needs some explanation. 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! No pretty rainbow of color here...nope. Only the vilest and nastiest looking sludge imaginable ride our skimmer's bubbles. It was discovered long ago in waste treatment plants that by injecting high volumes of air bubbles into a column of waste water, the resulting effluent was purer and much cleaner than before. How could this be? Actually, quite simple. Surface tension. Surface tension? What's that? The interaction between the oxygen bubble and the surrounding water creates a kind of friction between the two. This friction in turn "charges" the molecules in the water.
Playing on the old law of physics, "opposites attract," the charged gunk molecules "stick" to the bubbles, riding them up the column of water. Once the bubbles reach the surface air they burst, depositing their hitchhikers into a collection cup. This collection cup keeps the accumulated gunk from slipping back down into the water column inside the reaction chamber. Due to the very nature of saltwater, this process is possible. Freshwater protein skimming just isn't feasible at our level as the technology to make it possible just isn't practical at the hobby level.

Playing on the old law of physics, "opposites attract," the charged gunk molecules "stick" to the bubbles, riding them up the column of water. Once the bubbles reach the surface air they burst, depositing their hitchhikers into a collection cup. This collection cup keeps the accumulated gunk from slipping back down into the water column inside the reaction chamber. Due to the very nature of saltwater, this process is possible. Freshwater protein skimming just isn't feasible at our level as the technology to make it possible just isn't practical at the hobby level.

Co-current flow systems

Bubble size is a fundamental ingredient to a successful protein skimmer and various methods are used to create this "perfect" bubble. Originally, lime wood was and still is used to create the froth. The European hobbyists were amongst the first to recognize the importance of skimming their fish tanks. Specifically, the Germans marketed some of the finest models in their day, and still do. Tunze and others brought protein skimming to our shores with the original design. This was called Co-Current skimming.

The basic, Co-Current protein skimmers used an open-ended tube or cylinder with the bubble source mounted at it's base. As with uplift tubes utilized with under gravel filter plates, Co-Current skimmers used the volume of air bubbles rising in the column to bring the system water into contact within the chamber body. The water was "drawn" up into the cylinder from below the water's surface and once the bubbles burst at the collection cup, the treated or stripped waters simply "fell" back down into the fish tank.

Above is a typical co-current protein skimmer, either hang-on or sump mounted.

Counter-Current protein skimming
This basic concept is more correctly known as an aspirating skimmer, since some skimmer designs using an aspirator do not use a "Needle-Wheel"/"Adrian-Wheel" or "Pin-Wheel". "Adrian-Wheel"/"Pin-Wheel" describes the look of an impeller that consists of a disk with pins mounted perpendicular (90°) to the disc and parallel to the rotor. "Needle-Wheel" describes the look of an impeller that consists of a series of pins projecting out perpendicular to the rotor from a central axis. "Mesh-Wheel" describes the look of an impeller that consists of a mesh material 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 fish tanks 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 water pump vs. the same water pump with a different impeller when it is only pumping water.