(PART-1)
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."
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.
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 BELOW.
-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.
Simplifying and summarizing these "skimming laws" allows us to focus our efforts to ensure we have optimal skimming. A list of concerns are below 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
-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.
(cont. to part-2)
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."
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.
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 BELOW.
-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.
Simplifying and summarizing these "skimming laws" allows us to focus our efforts to ensure we have optimal skimming. A list of concerns are below 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
-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.
(cont. to part-2)