Introduction

The majority of unwanted organic wastes found in our aquariums collect at the surface of a gas-liquid interface. These "surface-active" materials, including fish wastes, uneaten food, and decomposing matter are what we attribute to increasing our tanks pollution. We associate this extra waste with increases in nuisance algaes, cyanobacterial blooms, and having a generally unhealthy tank appearance. Therefore, when we add new fish, and/or increase the volume of foods in our tank, we hope to remove an equal amount of potential pollution. The good news is we can reduce some of these potential wastes by removing them from the water before they cause problems. One method of removing organic wastes from a fish tank is through foam fractionation (skimming).

The protein skimmer originated in the wastewater treatment industry many years ago. It was used primarily to reduce the organic load before the water reached the activated sludge reactors. This technique exploits the affinity of organic waste to adsorb to air bubbles. In basic terms, organic-waste-laden aquarium water is reacted in a column of air bubbles, the waste sticks in the foam, and the foam collected. Foam is what is produced when one passes a gas through a liquid that contains high levels of surfactants.

In the following sections of this article I would like to explore the basics of protein skimmers (also known as foam separators, foam fractionators, or air-strippers), discuss why foam fractionation works, and describe a few of the current skimmers which are commonly available to the hobby.

So What are We Trying to Remove?

Fish poop, of course. Honestly, proteinacious waste only makes up a small portion of what we are trying to eliminate. According to Millero [1], the concentration of proteins found in natural seawater (as amino acids) is about 0.02 to 0.25 ppm (20 to 250 ug/l). These make up 2-3% of the dissolved organic compounds found in our tank water. However, data presented by Shimek [2] in which 23 hobbyist's tanks were surveyed reported protein concentrations of 0.00+ 0.00, suggesting that these tanks were devoid of any detectable proteins. Other organic wastes, which are more readily detectable, are primary and secondary amines, some amino sugars, alcohols, and fatty acids. Lipids (fats) were measured at 1.361 ± 0.940 ppm with a maximum of 3.20 and a minimum of 0.00 in the same study [2]. So, then, what are protein skimmers removing? More importantly, why do we even call this device a "protein skimmer" when there's no protein to remove?

Dissolved Organic Compounds (DOCs)

DOCs are the waste molecules skimmers are designed to remove; these are produced as byproducts from the breakdown of biological materials. This pollution arises from not only the deliberate input of foods in our tank but also from decaying organic matter (bacteria, algaes, etc). DOCs are bipolar molecules; these surfactants are attracted to air/water interfaces, i.e., bubbles. A bipolar molecule contains one or more atoms attracted to air, and one or more atoms attracted to water. A skimmer exploits this difference in the following manner:

"As an air bubble moves through the column of organic-laden water, the electrically charged protein molecules (which contain electrically polar and electrically nonpolar regions) are attracted to the air/water interface of the bubble. The polar regions of the molecule (made up of nitrogens, oxygens, etc.) are attracted to the air/water interface and these polar "tails" stick out away from the air bubble into the water column. The nonpolar regions stick out into the air bubble because it does not "like" to be in contact with the polar solvent (i.e. water). If you could look at this bubble under high enough magnification down to the molecular level, the entire air bubble would look like a fuzzy ball with protein tails and other electrically charged tails sticking out from the surface of the air bubble. The polar regions outside of the air bubble stabilize the air bubble very much like a soap bubble in your kitchen sink or your washing machine. This is the reason why foam begins to build up at the surface of the skimmer. As the protein-laden bubble reaches the top of the protein skimmer, the proteins begin to accumulate which creates a stable foam bubble. These stable foam bubbles take a long time to pop. Thus, the proteins slowly are concentrated at the top of the skimmer where they are slowly pushed through the "throat" of the protein skimmer and into the collection cup."
This description, written by Shane Graber, can be found in its entirety here.

The longer that the DOCs are in contact with the bubbles, the more of them will attach to the bubbles, the more of them will be removed. Longer contact times allow for less adherent molecules to be attracted and "stuck" to an air bubble. Other compounds besides DOCs can be removed as well. These may be VOCs (volatile organic compounds), POC (particulate organic compounds), uneaten fish food, trace elements, secondary metabolites from soft corals, bacteria, macro- and micro-planktons, coral eggs and sperm and other similar compounds.

Okay, So How Do We Make a Protein Skimmer?

Theoretical concerns VS reality
If a foam fractionator were constructed in its simplest form, it would look like a tall tube in which tiny air bubbles are injected into the bottom center. Waste laden water would be pumped into this tube with the intent of the waste adhering to the air bubbles. If you recall in the above DOC section, air bubbles act as an adsorbent, and waste molecules are attracted to this adsorbent and are removed from the water column as foam. With this description, let's explore the parts of a simple skimmer.

A skimmer has at least three parts:

1. The skimmer body, where most of the contact between the DOCs and water takes place.
2. The foam separation area, or riser tube, where the organic laden foam is separated from the water.
3.
A collection cup, where the foam spills over the riser tube and is drained away.(image below)


The above description can take many shapes and forms, and the myriad of hobbyist skimmers available on the market suggests this is true. However, my basic description above doesn't look like many of the skimmers on the market today, and this is where hobbyists get confused. With so many skimmers on the market, which one is the "best one" for their tank? The bottom line is there is NO one best skimmer. Many hobbyists are looking for the perfect skimmer, one that will remove all the organic waste in the tank water, a skimmer which is compact in size, inexpensive to run, and requires virtually no maintenance. In reality, what skimmer to use is truly a guessing game. What we do know is that different skimmer designs equate to different levels of effectiveness at removal of waste. The confusing part comes when hobbyists are confronted with manufacturer's advertising, claiming their skimmer is superior over all the rest because of some 'new' skimming technology. Claims such as, "The "Cycloskim2000" which is 18" tall, will easily skim your 100gal fish tank, and only requires 1 watt of electricity to operate," are rampant in manufacturer's advertisements. So, how do we separate what's truth and what's advertising? For me, one of the least complicated ways to understand which skimmer to buy is to understand a few of the theoretical basics of skimming and then apply this theory into functional reality. In chapter 9 of the textbook Aquatic Systems Engineering: Devices and How They Function, P.R.Escobal 2000 [3] Escobal presents a few theoretical concerns that should "shape" the efficiency of a skimmer. These are presented in original form in Table #1.

2nd Law: The bombardment rate (number of times a clean air bubble bumps into a drop of water) depends on the duration of the tank water exchange and the diameter of the skimmer.
3rd Law: Increased skimmer length or height only raises the value of the absolute contact time but does not affect the bombardment rate.
5th Law: The airflow rate entering a skimmer should produce a full upward blossom of bubbles without excessive turbulence, and is theoretically determined as a function of skimmer diameter, length, bombardment rate and absolute contact time.
6th Law: The value of bombardment rate within the skimmer, its length, diameter and airflow must all be properly chosen for optimum operation.
Table #1


Simplifying and summarizing these "skimming laws" allows us to focus our efforts to ensure we have optimal skimming (Table #2). This is a list of concerns, and there are four parameters that need to be addressed.

Skimmer Design Concerns

The water flow rate through the skimmer
The height of the skimmer
The amount of air pumped into the reaction chamber of the skimmer
The diameter of the skimmer

Table #2


1) For optimum skimming, water flow thru the skimmer should be sufficiently slow as to allow interactions of an air bubble and organic waste. The best designs for this are skimmers that employ water moving against the flow of bubbles. These are called counter-current skimmers. However, slow is a relative term.
2) Make the skimmer reaction chamber as tall as possible to maximize the contact time that the water has with the air in the skimmer. Pump as much air into the reaction chamber as possible.
The key to injecting air is twofold: a) maintaining the smallest sized air bubbles, and b) reducing any potential turbulence of the air bubbles in the skimmer reaction chamber.
3) The diameter of the skimmer must be increased in proportion to the amount of water being skimmed. The larger the amount of water to skim, the wider the diameter of the skimmer should be.

Remember that these are theoretical concerns; and often a theory is just that, a theory. Reality is often times different. As an example, let's assume that you want to build the most efficient skimmer for a 100 gal-reef tank. Using the skimming laws above, the skimmer should have a five to six foot tall, eight inches wide reaction column, with a turnover of two tank volumes/day. It would have dense thick foam consisting of 0.2 to 0.5mm bubbles, injected in a counter current fashion against the water entry. So, while this might be the most efficient skimmer, it is also, in my opinion, impractical for many hobbyists. This is where we can put aside the theory as directly written and understand some real world examples.

Can sufficient skimming be achieved by using a shorter, thinner skimmer or one that has a higher flow rate? Yes. Will it be as efficient as our theoretically-defined skimmer above? Maybe not, but it will be sufficient for the needs of many hobbyists. Manufacturers of skimmers have gone to great lengths to optimize their skimmer function and even though many of the above "skimming laws" are not met at face value, their skimmers work efficiently. In attempting to bypass tall reaction column heights, manufacturers have devised clever ways to lengthen the reaction time: the use of triple pass flow, and downward air injection tubes of equal length to reaction tubes which thereby effectively double, or triple, reaction column lengths. Water swirling patterns will also effectively increase reaction times. To maximize bubble density, manufacturers have incorporated air-injecting spray heads, venturi valves, and other foam generating heads (such as a Beckett valve) into their designs. The use of a foaming head effectively removes the requirement for airstones and an air pump. When matched to a powerful water pump, many of these foaming heads produce much larger quantities of air than any airstone/air pump combination can achieve.

Quote: "If you study Escobal, you will find that residence time, diameter and air volume all have effects on skimmer operation. Nothing in his book explains why the Hot-1 works at all, but users like it." CWA46, Nashville, TN.

Let's examine a few of the skimmer types available on the market today and how they are optimized to skim efficiently.


Air driven counter-current (CC) skimmers
Air driven skimmers are the original type of skimmer designed for the home aquarium, and fortunately they are one of the most efficient. However, they also require the most frequent maintenance, and can be costly. These skimmers use wooden air diffusers and a powerful air pump to produce their bubbles. These skimmers have excellent foam volume and consistency when operated with new (unclogged) airstones. According to Theil [4], the minimum recommended height of an air driven CC skimmer is 28", and this skimmer should have two to three air woods (airstones) for every 4" of skimmer width. Therefore, a 6" diameter skimmer should have four to six air woods. Depending on the organic load of the tank, airstones should be replaced monthly to every two months, and air pump diaphragms checked and replaced when weakened. His reasoning for a minimum height of a CC skimmer is due to the fact that the water and air bubbles must interact over a set distance and time, and a skimmer shorter than 28" does not meet these criteria. Long-time hobbyists consider this skimmer type to produce the most consistent quality foam. Interestingly, this skimmer style strictly adheres to our "skimming laws," and when tuned properly it seems to yield the best results. This is a common skimmer for DIY'ers, as this skimmer is easy to construct and requires minimal construction skills.

Venturi skimmers
A venturi skimmer uses a venturi injector to create air bubbles. These skimmers tend to be shorter than air driven CC skimmers, and require a powerful pump to drive the venturi valve. For an explanation of how a venturi valve works, see the explanation box found in this article [5]. One reason why venturi skimmers are often utilized in home aquariums is that venturi valves produce tremendous amounts of bubbles. One of the more common venturi injectors is a Mazzei® injector (image right). When matched with a high performance pump, these injectors create large volumes of fine bubbles. The bubble quality tends to be very good, and this aerated mixture is often injected into a reaction chamber in a swirling fashion to maximize contact times. A 1" Mazzei® injector can pull up to 240cubic ft/hr of air. The Lifereef VS2 is an example of a skimmer which utilizes this injector. Another type of venturi skimmer is a Beckett-head skimmer. The Beckett foam generating spray head is designed for increasing the amount of air that is mixed into the water. Skimmers incorporating a Beckett-head often utilize the Beckett in a higher position than the water level and this extra distance allows for an increase in reaction time. Additionally, manufacturers have begun to tangentially inject aerated water into swirling patterns, which also increases contact time. What is most obvious with a Beckett-head is the density and consistency of the foam; it far exceeds any of the other current aeration strategies. The Precision Marine BulletXL skimmer is an example of one utilizing this injector(image left).

Downdraft™ skimmers click here for full size picture
The downdraft™ skimmer is designed quite differently from all other skimmers; it utilizes a jet of water sprayed at high velocity into a column containing bio-balls. This column contains media that effectively disrupts the jet spray and forms bubbles. After the bubbles flow through this "bioball column" they enter a sump box, and are deflected upwards to the foam collection riser in the neck of the skimmer.

In this skimmer configuration, the lengthy water path through the bioball column, across the sump, and up through the neck of the skimmer, effectively increases the reaction time. Additionally, the disruption of the jet spray results in copious amounts of bubbles and foam. These skimmers require a very powerful pump to run correctly, and tend to be fairly large units. A skimmer incorporating this design is the ETSS® Gemini 800.

Aspirating impeller skimmers (Air Shredding) click here for full size picture
This style skimmer offers a unique twist on an inexpensive aeration system. Essentially, directing air into the impeller cavity of a powerhead or small pump creates bubbles. The regular impeller (Figure #7b below top), a specialized needle wheel (Figure #7c below left), or pegged wheel (Figure #7d below right) then finely chops up these air bubbles into very fine bubbles like those found in an airstone driven skimmer. These units are unique in that they have a very slow water flow, generate large amounts of foam, and are cheap to run because they use low power pumps. A concern which has arisen with these skimmers is the durability of these specialized impellors. Because these needlewheel impellors place the weight load on the periphery of the impellor (and not close to the central shaft), these impellors have higher failure rates than do normal vaned impellors. Two skimmers that utilize specialized needlewheels are the Turboflotor 1000 and the Euro-Reef. A skimmer that utilizes a traditional impellor in an air shredding strategy is the German Klaes Skimmer, which is reviewed here.

(Figure #7b)
Regular impeller
(Figure #7c)
Specialized needle wheel
(Figure #7d)
Pegged wheel

To Summarize the Current Skimmer Types:

Air driven counter-current: low water flow, good foam production (with new air stones), good bubble size, maximum contact times (with taller units), frequent maintenance and requires frequent water height adjustments (called tuning).

Venturi: good water flow, good foam production, moderate contact time, requires a powerful pump, valve tends to clog (Lifereef has a self-cleaning venturi valve).

Beckett-head skimmer:
high water flow, maximum bubble production, moderate contact time (swirling patterns will increase this), requires a very powerful pump, Beckett-head requires cleaning.

Downdraft™: good water flow, excellent bubble production, excellent contact time. Units tend to be tall and bulky (the ETSS 1000 is over 60” tall), require powerful pumps to create air bubbles.

Aspirating skimmer: low to moderate water flow, good foam production, good bubble size, excellent contact time, cost-efficient, requires specialized impellor or needlewheel. The needlewheel impellors have been shown to be a weak point.

A few words about maintaining skimmers: to ensure optimal skimming, periodically clean the skimmer. Empty the collection cup frequently, and wipe clean the organic buildup that collects around the body and neck of the skimmer click here for full size picture(image left). This organic sludge is what the skimmer removes from the water and this material shouldn't be allowed to fall back into the water. Clean the air valves often, and remove any built-up salt crust or particulate matter clogging the air intake. Also, be aware that small snails and rock fragments may become caught in the restrictions of many venturi valves and pump intakes and must be removed if the skimmer is to perform at peak efficiency. If airstones and an air pump are being used, ensure its bubble production is optimal, replace the airstone at regular monthly or bimonthly intervals and check the air pump diaphragm to ensure maximal air is being produced. Deep skimmers require consistently high pressure from the pump diaphragm to work properly. If the skimmer requires a water pump to generate bubbles, make certain the pump drives the venturi or other air injection device at maximum efficiency; weak pumps cause skimmers to work poorly.

In Summary

Protein skimmers can be an effective method in reducing the problems related to waste accumulation in our tanks. The application of skimmers have been associated with reduced waste (reduction of DOCs), reduced algal growth, and increased water quality. Many of the present skimmers on the hobbyist market are too small (height and diameter) and/or underpowered to effectively remove sufficient amounts of organic waste. As a rule of thumb, taller skimmers tend to work better than short, squat ones. The hobbyist needs to ensure bubble production is maximized (smaller bubbles work better than big bubbles), and consequently, the water in the reaction column should be milky white in appearance. The foam produced in the skimmer should be continually moving up towards the collection cup so that is being collected and removed from the system. One final word on skimmate consistency: this topic is often a subject of debate (i.e., thick, mud colored, and dry versus watery and iced-tea colored (image above right)), and it would bear further discussion in another article. I'll leave the topic for now by saying: you want to collect skimmate (any skimmate), and any skimmate is better than no skimmate at all.

For additional infomation see: www.proteinskimmers.com



If you have any questions about this article, please visit my author forum on Reef Central.

References:

1)-Millero, F.J., Sohn, ML, Chemical Oceanography, CRC Press, Boca Raton 2nd edition 1996. ISBN:0-8493-8840-6.

2)-Shimek R., It's (In) The Water. Reefkeeping online magazine, 2002

3)-Escobal P.R., Aquatic Systems Engineering: Devices and How They Function, Dimension Engineering Press, 2000. ISBN: 1888381108

4)-Theil A.J. The new Marine Fish and Invert reef Aquarium.,Theil Infobase 1998

5)-Marini F.C., "Bite the Bullet"-Evolution of the Precision Marine Bullet 2 Skimmer. Reefkeeping online magazine, 2002


Photo Credits:

1 - Basic skimmer - Drawing by Frank Marini
2 - Air driven CC (Precisionmarine.com)
3 - Venturi skimmer (Precisionmarine.com)
4 - Mazzei injector photo (Precisionmarine.com)
5 - PM BulletXL skimmer (Precisionmarine.com)
6 - Downdraft skimmer - Photo courtesy of Ryan Baker
7 - Air shredding skimmer
7b-Regular impellor - Photo courtesy of Nicolas Will
7c-Specialized needle wheel - Photo courtesy of Nicolas Will
7d-Pegged wheel - Photo courtesy of Nicolas Will
8 - Organic build-up on neck of skimmer - Photo by Frank Marini
9 - Skimmate - Photo courtesy of Jess Hull




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Skimming Basics 101: Understanding Your Skimmer by Frank Marini, Ph.D. - Reefkeeping.com