Skim The Cream
Skim The Cream
Written by Saturday, 24 July 2004 00:00
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Undoubtedly, the most critical stage in keeping aquaria successfully is controlling the amount of nutrients present in the water column. Our main purpose is to control the amount of the so-called “Nitrogen” compounds in our tank.
Nitrogen (N2) is an abundant gas in the atmosphere, accounting for almost 80% of its total volume, therefore there is much more nitrogen than Oxygen or Hydrogen. It can be found in most organic compounds, as it has the ability to take part in many chemical reactions. Although Nitrogen itself can’t sustain life, it is absolutely essential for the existence of life. Well-known “Nitrogen” compounds are proteins, DNA and RNA.
All living organisms “excrete” Nitrogen compounds, which are formed during the various biological processes that take place in their body. One of the best known such compounds that we, humans, excrete is urea (a byproduct of protein metabolism). Fishes eliminate Nitrogen compounds via their urogenital and respiratory systems.
Perhaps a way to control the amount of Nitrogen compounds introduced into our water would be by reducing or even stopping feeding. However, fish also cast out a lot of it from their gills. The more the stress, the more Nitrogen will be released. This adds more stress to fishes as the presence of ammonia or nitrites further induces stress, thus entering an endless cycle. Normally, non - toxic Ammonium (NH4+) found in acidic water, or toxic Ammonia (NH3) found mainly in alkaline water, is oxidized by Nitrosomonas spp. to Nitrite (NO2־), which can be more toxic than Ammonia itself, especially in acidic water where it will form the deadly Nitrous acid (HNO2). Further nitrification by Nitrobacter spp. produces Nitrates (NO3־) that are used from plants as a fertilizer along with other radicals like Phosphates (PO4 ־³) and Silicates (SiO4-2). During this nitrification procedure oxygen is required, hence the nitrifying bacteria are called “aerobic”. The opposite procedure is called denitrification and takes place in the absence of oxygen, by anaerobic bacteria (usually found in “live” rock and/or “live” sand). During denitrification, nitrogen and nitrogen monoxide gases are produced as a final product. These gases are first released into the water, and later into the atmosphere via gas exchange, or absorbed by a kind of bacteria called Cyanobacteria. Cyanobacteria, or “blue – green” algae can come in more colors than cyanon (the Greek word for blue). Usually they are colored reddish, greenish, brownish, bluish or even black. They are single cellular organisms and form ugly mats in the tank. They are something between algae and bacteria, and they are called “grease” and “smear” algae. When their environment is oligotrophic (little amount of suitable food is present), Cyanobacteria are nearly harmless, as they cannot grow in critical quantities, but when their environment is rich in nutrients (Phosphates, Nitrates, Silicates, among others) they grow very fast. The worst of them (from the hobbyist’s point of view) are the red and brown ones. Cyanobacteria have the ability to fix atmospheric Nitrogen, which they can consolidate in their tissues when dissolved in water. When those – full of fixed nitrogen - tissues are eaten by herbivores, nitrogen is converted to the well-known Ammonia and then to Nitrites and finally Nitrates (see diagram 1 below).
The more alkaline the water, the more toxic Ammonia is, and this is a very good reason we should try to avoid the accumulation of it, especially in systems that require an alkaline pH like marine and African Rift Lake cichlid tanks. In contrast, the more acidic the water, the more deadly the nitrites become. Nitrifying bacteria consume a lot of inorganic carbon to grow and to deal with the transformation of Ammonia to Nitrites. All the carbon used comes from the break down of carbonates, bicarbonates and other radicals (negatively charged compounds) that are useful in keeping an adequate buffering capacity in our water. This affects alkalinity, and may result in dramatic drops of the pH. We should understand that pH shocks might occur when the buffering capacity of our water is reduced hence it can’t prevent rapid pH changes any more.
Nature had solved this particular problem long before aquarists had even realized it. Waves, tides and the coastal substrata are the equipments used to eliminate organic substances that contain Nitrogen, among other things. You can see it on the coastline, where the waves break – it is the “foam” formed there. This foam comes from “washing” those compounds and consists mainly of proteins, rich in Nitrogen.
The human made equivalent of this natural process is the Protein skimmer.
Structure - Function
There are two types of skimmers, the air - driven and the motor - driven ones. They are usually manufactured of plastic, acrylic or glass fiber. They both remove fats and fatty acids, organic acids, amino acids, lipids, phosphates, phenols, carbohydrates, iodide, metals complexed with proteins and, of course, proteins from water. The list of removed compounds is far longer, as metals complexed with proteins can also be found in detritus and plant or animal originating infiltration products. Some of the vitamins are also a source of nitrates. Tap water itself usually contains an amount of nitrates, too. And, since nitrates are nutrients, the Nitrogen cycle goes on again.
Protein skimmers consist basically of a cylindrical contact column and a motile collection cup. Inside the contact column water is mixed with air and in the collection cup swarms proteins in the nature of foam.
Air - driven skimming devices are usually placed within the tank, so water enters and exits the contact column from underneath. Inside the contact column, an air stone is placed. It is fed with air from an air pump to produce as many fine bubbles as possible. This can be achieved with the use of a wooden block or very high quality air stones and powerful air pumps.
The motor - driven skimming devices are a little more complicated. Water enters the contact column from a water inlet and exits from a water outlet, both drilled on the cylindrical body of the contact column. They may be fed from a water overflow, or by a water pump. Another water pump circulates the water in the contact column. This pump is equipped with a Venturi valve. The Venturi valve is a small part of the device located – usually - at the bottom of the contact column. It directs water into a pipe, through a constriction of the pipe. Before the constriction, there is a high pressure and after it, a low pressure. The low pressure after this constriction forces air to be sucked in from one or more ports in the Venturi forming very fine bubbles.
Since the Venturi reduces the volume turnover of the water pump, needle wheels are used in the place of classic impellers. They have a star-like shape and are very powerful, as they can rotate at more than 3000 rounds per minute.
There are two types of protein skimmers. One operates by co – current flow of air and water, while the other operates with counter – current flow (see diagrams below).
Why and how it works?
Organic surface – active molecules (surfactants) are attracted to the air bubbles’ surface, as air bubbles act like an adhesive interface for them. These molecules are polarized with one “hydrophilic” and one “hydrophobic” side. While their first site (hydrophobic) allows them to be dissolved in organic solvents (they “fear” water), the other one (hydrophilic – “water loving”) allows them to stay in contact with water. So air bubbles collect molecules during their contact with water inside the contact column because they offer organic matters the two things they like: a water surface and a non-water surface (the air). Ever blow bubbles as a kid? Remember all the rainbow colors on them? Just as the soap clung to the giant bubbles you were creating, so too does all the junk and other organic gunk in your aquarium water. Those pretty rainbow colors were the light refracting off the soap film...you could actually see it! In our skimmers, the bubbles are microscopic, and the results can only be "seen" after they burst and deposit their "films" into the collection cup! The most important issue in skimming is the water/air interface. This works by using surface tension and the different polarities of the “waste” molecules. Roughly, it can be seen as the blood/air surface, which is created in our alveoli in our lungs. Alveoli create a relatively huge surface in which blood (a very thin film of blood) becomes exposed to air so the “waste” (mainly carbon dioxide) can be exchanged with oxygen. So, the larger the “exchange” surface, the better the exchange rate, therefore the higher the efficiency of the skimmer. The shape of the bubble is also very important. The sphere is known to be the geometrical shape which offers the largest possible surface / volume ratio which is very important (that is why most cells have a spherical shape whenever possible). Speaking of cells, they divide when they have grown too much, because they automatically realize that their volume has become too large for their surface to cope. Cells prefer to have a smaller spherical volume with a large surface, so they can exchange nutrients with their environment more efficiently. This is also the case with protein skimmers. The size of the bubbles is consequential. What we need is as many bubbles as possible with as small a volume as possible. This forces the manufacturers of skimmers to use media or constructions that will lead to the formation of a large number of tiny bubbles. When the size of the bubbles become too large or the number gets reduced, the efficiency of the skimmer is greatly reduced, as well. At this time, the aquarist should replace the air stone, or clean the air pathways. This is also one of the reasons skimmers are much more effective in the marine aquarium, while its usefulness in freshwater tanks is limited. If you take a normal air pump with a normal air stone you will immediately notice that the size of the air bubbles produced in salt water is much smaller (and the number much larger) when compared to a freshwater tank. This is, of course, due to the higher density of the salt water. This is not to say that a skimmer will not work in a freshwater tank. Actually, all you have to do is to use more powerful air pumps and as many wooden air blocks as possible in freshwater tanks, while at the same time ensure that the air stones you use will produce the smallest possible bubbles. Therefore, by using air pumps rated for much larger tanks and wooden blocks, one can make a skimmer work efficiently in freshwater, also. The reason the skimmer is mandatory in marine tanks is that large and frequent water changes are not desired, as well as being expensive. So, the freshwater aquarist has the option of large and more frequent water changes which takes care of excess wastes, making the protein skimmer an optional extra. The water, when entering the contact column, has a high concentration of surfactants. Their concentration is reduced as water is driven to the top of it since most of them stay attached on the air bubbles. So all this protein foam is formed in the collection cup. This foam consists mainly of nitrates, which if they had stayed in the water would have mineralized and accumulated in the tank.
Effectiveness
Protein skimmers are considered the most effective chemical filtering devices, because they remove most of the organic compounds from the water by physical means (the only media that protein skimmers use are water and air), before they begin to break down into toxic compounds. So they give a helping hand to biological filters. Additionally, some bacteria and phytoplanktonic organisms are mechanically trapped in the foam and are removed alongside the organic matter. The adequate skimming, which removes all these organic nutrients, is the most sufficient way to control the growth of Cyanobacteria as well. It is not wise to use algaecides and antibiotics (erythromycin for instance) to control algae and Cyanobacteria, as an alternative to using a skimmer. Algae and Cyanobacteria will be killed, but they can return if there are enough nutrients in the environment. Therefore, it is preferable to work at the basis of the problem: elimination of excess organic matter.
Factors that dictate the efficiency of a protein skimmer are bubble size, airflow rate, and the active contact time of the bubbles in the skimmer. The desired bubble size is between 0.5 and 1.0 mm. This maximizes the surface area of the bubbles, without considerably lowering their buoyancy. Air flow rate dictates the relative ratio of water to air bubbles in the protein skimmer. The recommended airflow rate for protein foam skimmers is around 1.8 cm/sec per cm2 of cross-sectional area of the main cylinder of the protein skimmer. Having the correct flow rate will increase the skimmer's efficiency by creating good foam without compromising the active surface area of the bubbles. The active contact time of the bubbles is generally controlled by two factors - the height of the protein skimmer's main cylinder, and the flow rate of the water through the skimmer. There is no specific "golden" value for these since they are a strong function of the particular characteristics of each protein skimmer and aquarium setup.
Size - Installation
The size of a skimming device is, of course, related to the size of the tank we want to install it on. Generally, we can say that a 120 cm tall skimmer, with a diameter of its contact column of about 15 cm and a water flow rate of about 1.200 liters per hour can be used with a 600 liter aquarium. A skimmer should process at least the equivalent of one tank full of air and one tank full of water per hour. For most tanks, the water rate is easy. The air rate, however, is not. Most counter current (explained below) skimmers are under-supplied with air. If you have a sealed skimmer where the air can only exit from one fitting, it’s easy to measure the flow rate. Simply take a large plastic bag (something in the 2 gallon size works well), empty it, and place it over the air exhaust port. Time how long it takes to fill and do the math.
Three basic styles of skimmers exist: counter current air driven, venturi driven, and down-draft (see diagrams). All styles work fine, all have tradeoffs. All require some tuning. Expect to spend some time over the first month or so learning how to keep your skimmer tuned.
The place we shall install a skimmer is not so critical. We can place it inside the tank (internal skimmer), or outside it (hung or put in the sump). If it is the right skimmer for the tank and is the right size, it will produce foam either way. If it is placed in the sump, then it must be placed on the compartment that the water enters the sump. We recommend installing the skimmer before the sump, and, more specifically before the chemical and after the mechanical media. The reason for this is that many (visible or not) particles of wastes will be removed during mechanical filtration before entering the skimmer; therefore the chemical filtration compartment (usually activated carbon) will have less of a load to handle, prolonging its usefulness. The outlet of the skimmer can supply the biological media with skimmed water, after that. It is better when the water entering the skimmer is collected from the top of the water column of our tank and even better if it is collected from the drilled overflow of our tank. Make sure you have a very good design with valves and watertight piping, and take extra care with the water level and flow (see diagrams).
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