Protein skimming is a type of filtration method that uses some unique properties of air bubbles, and their interaction with certain dissolved compounds in the water, to remove dissolved substances from the water. Mechanical filtration strains out particulate matter from the water. But there are still many types of compounds dissolved in the water that can negatively affect water quality. Much of this material is classified under the term, Dissolved Organic Carbon or DOC. A foam fractionator removes the DOC, among other things, that a traditional mechanical filter does not. The accumulation of DOC in an aquarium can, among other things, inhibit the nitrifying bacteria and increase the biochemical oxygen demand (a way of measuring water pollution), thus lowering water quality. Furthermore, certain heterotrophic bacteria convert some DOC into substances, such as ammonia and carbon dioxide, and consume vital oxygen in the process. In many cases, activated carbon can be used to adsorb some of the DOC but carbon does not get everything. Protein skimming can compliment activated carbon and vice versa. This article will cover some of the basics about protein skimming. A protein skimmer is a device that concentrates and removes dissolved material from aquarium water, using particular properties of air bubbles in a more or less controlled environment. As air bubbles rise through the water, areas form on the bubble that attract certain types of molecules and compounds dissolved in the water. By producing millions of bubbles and controlling the contact time between the bubble and the water, these dissolved substances can be concentrated on the bubble and extracted from the water. This is the foaming part of the skimmer. When the bubbles rise and burst they has taken some of the dissolved material with them to the water surface, and when captured effectively, the material is removed from the water.The two keys to effective protein skimming are air bubbles and surfactants. DOCs are surfactants – compounds whose surface is defined as “active.” This means that when a surfactant compound is in water, its non-polar end – labeled hydrophobic or “water hating” – seeks the surface (the air). Normally, the only “air surface” in the aquarium is the surface of the water. However, if bubbles are added to the water, more air surface is created. More air surface means more surfactants (DOC) are attracted from the water and removed.
Bubble size and contact time determine how effective and how fast a skimmer will work. Smaller bubbles have more surface area than larger ones. Also, the longer the bubble stays in the water, the longer its contact time with the surfactant.
The name “protein skimmer” is essentially misleading. These devices do not skim the water surface and they, in fact, remove more than just protein. A more appropriate name for a protein skimmer is “foam fractionator” – due to the fact that the bubble concentrations (foam) serve to separate (fractionate) dissolved material from the water. But protein skimmer is the more common name and so the one used here.
Most protein skimmers have, in one fashion or another, the following parts – the contact column that serves to concentrate the bubbles in a small area. It basically consists of a tube, such as PVC pipe or Plexiglas tubing. A device to generate bubbles in a simple skimmer may use an airstone, while more complex units use powerheads or venturis. There needs to be a water inlet and outlet and, finally, a collection cup where the concentrated foam collects.
Some units may add additional features, but the basic design is the same. Certain skimmer models employ a venturi air injection system instead of an airstone to produce bubbles without an air pump. These skimmers pump aquarium water through an injector via a water pump or powerhead. The injector has a narrowed pathway in its center and an additional opening that admits air into the unit. Differential pressure is generated at the other end of the restriction that causes air to be sucked into the water stream. Venturi-operated skimmers can be very effective and tend to be smaller than other skimmers, a positive consideration where is not much space to place the skimmer.
While skimmers can be made relatively easily by anyone with some basic plumbing skills, commercially available units are generally better because they do not need the constant attention of homemade units, nor do you have to spend days trying to figure out the optimal running conditions. The difference between commercial brands of skimmers is most evident in the ways they move water and generate bubbles.
There are two basic types of protein skimmer: co-current and counter-current. Current pertains to water flow direction, while “co-” or “counter-” indicates whether the air is moving with or against the water current, respectively. Either of these units will require an airstone and air pump (powerhead/venturi in some cases) to generate the bubbles needed for operation.
The simplest skimmers, although not that common anymore, fit inside the aquarium, hanging from the top lip of the tank. An airstone placed at the bottom of the column produces the foam that is collected at the top. This type has no water pump or hoses. A more common version nowadays hangs on the outside back of the aquarium. Water is pumped into the unit, either co- or counter-current, and returns to the tank via a spillway at the top. Perhaps the most commonly used skimmer is one located under the aquarium in the filtration sump. In this mode, the skimmer draws water from the sump, concentrates the dissolved material, and returns the water back to the sump. These devices can be effective and simple, plus the unsightly skimmer residue is hidden in the cabinet. No particular placement is superior to another. Selection is usually a matter of individual space and budget restraints.
With all skimmers, there is a certain amount of maintenance. Too much air pumped into the contact chamber will cause excess foam to be produced, and results in a lot of water being pumped into the collection cup. In this case, the foam is said to be ‘too wet’ meaning there is too much water being removed and too little DOC. On the other hand, if the contact chamber has too little air or water flow then not much foam will be produced and what is produced will be rather dry. In either case, adjustments have to be made to the water and airflow to get everything back in sync. The most difficult/frustrating aspect of protein skimmers is that their performance seemingly changes daily (or more frequently) depending upon many parameters of the aquarium system. Units located in sumps can be affected by the water level in the sump. Units hung inside the aquarium are affected by the water level in the tank.
To retain its effectiveness, a protein skimmer must be clean to allow the bubbles to form at the top and flow to the collection cup. After awhile a greenish brown sludge will begin to form on the walls of the skimmer, especially near the top. This should be brushed cleaned regularly. The airstone should be replaced each few months. For a venturi skimmer, the venturi should be cleaned often to prevent the buildup of calcium or other deposits.
What systems need a skimmer? For the most part, skimmers are not used on freshwater systems because producing the correct size bubble is difficult. This does not mean skimmers won’t work on freshwater systems, it just means that they will not be as efficient or fast.
Most often, the primary protein skimmer user is the saltwater hobbyist. For a fish-only tank, a skimmer can be considered an option but for a reef aquarium it is a must. Even if your reef aquarium has a biological and chemical filter, a protein skimmer is vital to maintaining the lowest possible organic levels. In fact, one reef-keeping approach – the Berlin Method – relies more on the protein skimmer than any other piece of filtration equipment.
In a Berlin reef system, the skimmer is used to remove DOC before bacteria can break the DOC down into ammonia. But there is more to a Berlin system than just a protein skimmer. The organisms growing on the live rock that are a vital part of any reef system also remove some of the ammonia present. Because they also require large amounts of light (of the proper wavelength) for growth, an efficient, reliable lighting system is also necessary. A fair amount of water current is also mandatory. Finally, due to the fact that the emphasis of a Berlin system is on coral, not fish, the fish population is kept light and fed sparingly (if at all) with prepared feeds.
As stated earlier, protein skimming and activated carbon compliment each other and both should be used to remove as much of the organics as possible. Studies show that foam fractionation does not remove all types of organics …nor will it remove 100% of any one of them. The same can be said of activated carbon.
Skimmers should be run 24 hours a day. But be aware that there will be some periods of the day when the skimmer will produce more foam than others, but that is normal. Shortly after feeding or doing a water change or adding certain additives the skimmer will foam greatly for a while.
Protein skimmers, or foam fractionators, can prove a welcome addition to many types of aquarium filtration systems. By understanding the basic operative mechanisms and functional nuances of these devices, you should be able to improve your water quality and system filtration.
©1998 Timothy A. Hovanec