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.
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.
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.
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.
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.
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.
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.
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.
- Delbeek, J. Charles, Julian Sprung (1994). Reef Aquarium, The, Volume 1. Coconut Grove, Florida: Ricordea Publishing.
- Frank Marini. Skimming Basics 101: Understanding Your Skimmer. Reefkeeping ... an online magazine for the marine aquarist. Retrieved on 2006-06-14.
- Frank Marini. "Bite the Bullet" The Evolution of the Precision Marine Bullet 2 Skimmer. Reefkeeping ... an online magazine for the marine aquarist. Retrieved on 2006-10-4.
- Randy Holmes-Farley. What is Skimming?. Reefkeeping ... an online magazine for the marine aquarist. Retrieved on 2006-10-4.