Protein skimmer

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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.

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[edit] 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 organic molecules, then removed with the organic. Oil molecules(fats) can cause a skimmer foam to collapse, handicapping its efficiency to produce a dry foam.

[edit] 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 them 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.

The two primary factors, thus, in a skimmers ability to produce skimmate -- the foam product of skimming-- are (1) the total air/water surface area determined by the number of bubbles, and (2) the dwell time, or the amount of time these bubbles have to react with the water. The Protein skimmer is a reactor to maximize these two.

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 are in contact with the water for a longer period in a counter-current flow system, protein skimmers of this type are more effective at removing organic wastes.[1]

[edit] Co-current flow systems

A relatively new distinction among Co-currrent flow systems is the recirculating skimmer. This is a special type of Co-current flow system discussed separtely below.

[edit] 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 or wooden 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.

[edit] 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.

[edit] 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 a 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. They also typically operate with much smaller pumps, saving energy costs.

[edit] Counter-current flow systems

[edit] Downdraft

The Downdraft skimmer is a proprietary design belong to AETech Inc. that uses at least one tower with plastic media such as bio balls inside to mix water and air in the body of the skimmer producing foam that collects protein waste. The foam rises into a collection cup for easy disposal. There are no moving parts internal to the skimmer. This type of skimmer is extensively used in public aquariums.[citation needed]

[edit] Beckett skimmer

The Beckett skimmer is a type of downdraft skimmer that uses a foam nozzle to produce the flow of air bubbles. Although effective, the downside to this system is its noisiness and higher electricity consumption. Beckett designs usually depend on a strong, pressure rated pump for effective action. This can raise their operating costs due to the typically more expensive pumps they require and the power they consume.

[edit] 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.

[edit] Recirculating Co-Current systems

Since a skimmers effectiveness is by and large determined by its bubble count and dwell time, a recent trend is to change the method by which the skimmer is fed 'dirty' water from the aquarium as a means to increase this time. Typically, a skimmer has one inlet: the pump that pulls water in from the aquarium and injects it with air is the only way into the skimmer, releasing the mix into the reaction chamber. With a recirculating design, the one inlet (the 'feed') receives the dirty water to process, while the pump driving the air into the reaction chamber is set up seprately in a closed loop on the side of the skimmer: it simply pulls water out of the skimmer and blasts it back injected with air--thus 'recirculating' it. The feed in a recirculating design typically injects a smaller amount of dirty water than co/counter-current designs. Where a typical skimmer may inject around 800 gallons per hour (GPH) into the skimmer, a recirculating skimmer would inject 200 GPH. The dirty feed can come directly from a tank overflow by gravity feed or via a small separate pump.

The effect of this decrease in dirty water injection into the skimmer effectively increases the dwell time--each unit of water is exposed to more air for longer, giving the proteins a longer opportunity to polorize

[edit] References

  1. ^ P. R. Escobal: Aquatic Systems Engineering: Devices and How They Function, Dimension Engineering Press, 2000, ISBN 1888381108

[edit] Further reading

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