This article about protein skimmers covers a bit about what they are, how they work, and a look at the design philosophy of a number of skimmer designers.
Now before I start, a quick reminder that I am something of an agnostic when it comes to the subject of skimmers, so you can be assured I have no bias when it comes to make or mode of operation.
Also, I feel I have to yet again repeat my usual proviso, used previously in connection with lighting and water flow: there is no “Best”, only what is most appropriate. When it comes to protein skimmers, there is only the piece of equipment that is best suited to the needs of your particular reef set up. For example, you can have two highly regarded skimmers, one from each of two different manufacturers, if your system doesn’t have a sump and only one of the skimmers is usable without a sump, then that will be your choice. It doesn’t make it the “best skimmer” but of the two skimmers it is the most appropriate for your reef.
What is a Protein Skimmer?
So what is a protein skimmer? A protein skimmer is a filtration device that removes certain dissolved organics, and other potential pollutants from water, through the effect of fine air bubbles on these substances. It can be considered primarily to be a form of chemical filtration but in operation, some incidental mechanical filtration will also be taking place.
How Does a Protein Skimmer Work?
The principle behind why a skimmer works is that certain molecules called surfactants (surface-active molecules) are attracted to the surface of air bubbles. Fortunately for marine aquarists, many of these are the molecules of organic waste we’d like to remove from our aquaria. They are polar at one end (hydrophilic) and non-polar (hydrophobic) at the other; the hydrophobic end is attracted to the surface of an air bubble, where it can be away from water, adhering to the bubble it will rise to the surface helping to form the organic foam that is collected in the skimmer cup. The incidental mechanical filtration comes about as inevitably some fine particulate material, plankton etc. can be pulled into the skimmer and be removed by either becoming “gummed up” with the organic waste or just by being pumped up the skimmer column in the same manner as water is pumped up an old fashioned under gravel filter air up-lift.
At its simplest, a protein skimmer works by pushing fine air bubbles, through water, up a column. The air bubbles collect molecules of organic waste from the water and as they reach the top of the column they are pushed out of the water and up into a collection cup. The simplest of skimmers therefore is just a cylindrical tube, suspended in water, with an air diffuser positioned at the bottom of the tube and a collection cup at the top of the tube above the water level. This is a design that works but it is by no means a very efficient design.
Beyond this very basic skimmer configuration, skimmers have evolved over time with a number of design improvements being made, effectively producing different classes of skimmer.
The basic design described above, utilising an air pump and air diffuser, is usually termed a co-current skimmer; the air travels in the same direction as the water flow. By changing the design, so water travels down the column as the air goes upwards we have a more efficient design with increased contact time, a counter-current skimmer.
Venturi skimmers can be co-current or counter-current, these do away with the air pump and draw air in through a venturi valve. With a venturi valve, pressurized water from the pump enters the inlet where, by passing through a tapering aperture, it is constricted as it goes into the injection chamber, changing the flow into a high-velocity jet stream. This increase in velocity results in a decrease in pressure, which sucks air in through the air inlet. As the jet stream is diffused through the expanding injector outlet a multitude of small diameter bubbles are produced, mixed in with the water.
Needle/pin wheel skimmers draw air in through a dedicated pump that uses a special design of impellor, the needle/pin wheel, to “chop” the air up, forming very fine bubbles of a size smaller than that produced by a venturi. These impellors are formed with numerous narrow projections instead of the usual six to eight flat blades commonly found in pumps. These projections can be radial in design or mounted parallel to the pumps axis of rotation, depending on manufacturer. It’s these projections that chop up the air.
Downdraft skimmers, not very common in the UK, this design uses a tall vertical column, filled with bio-balls, connected to a sump with an internal baffle plate. Water is introduced at the top of the column, it then repeatedly smashes down over the bio-balls, and by the time the water reaches the sump at the base, the water is a sea of white foam. The baffle plate creates dwell time, allowing the organic laden foam to rise up a wide-mouthed tube into the collection cup. This is really only suitable for very large tanks and for dealing with large volumes of water.
So, what factors affects the efficiency of a skimmer?
Here is a list of some of the different variables that need to be taken into consideration in the design and selection of a skimmer:
- Volume of water passing through the skimmer in a given time.
- The number of times the volume of the aquarium travels through the skimmer in a given time.
- The height of the skimmer column.
- The diameter of the skimmer column.
- The volume of air being introduced into the skimmer column.
- The diameter of the air bubbles being used.
- The length of time the bubbles are in contact with the water.
All these variables can interact with each other, giving an immense number of different possible combinations of parameters around which a skimmer could be designed.
There is little in the way of hard scientific information available to the aquarist, most writing in magazines and books is more in the form of simplified explanation of the principles involved, plus some description of what the writer believes to be the key parameters. Usually the emphasis is put on the volume of air being used, the height of the skimmer column, the contact time, etc. For this article I decided to try and take a different approach. Instead of just summarising my own knowledge, and possibly making false assumptions about what is important, I asked the real experts, the guys who actually design and build the skimmers we use, to contribute their opinions. The following is based on a series of interviews carried out in 2005.
Dr. Manfred Schluter, the designer of the ab AquaMedic range of skimmers:
We want to get a high airflow rate with small bubbles so that we have a huge air/water surface. We want to keep these air bubbles in contact with water for the longest possible time and we want to make the skimmers as high as possible. This increases the time the bubbles spend rising in the water column and also the pressure at the bottom. Higher pressure means higher solubility of gases, faster gas exchange and also smaller bubbles, as the air is compressed.
To achieve this, the skimmer would be tall, with the air bubbles injected in at the lower end. Skimmers like this are very effective (e.g. our Turboflotor 5000 Twin). But for many applications, they are too tall. Customers want shorter skimmers that fit under the aquarium in a cabinet. Here, we do not get the long retention time of the air bubbles in the water. In order to still get good skimming results, we have to increase the flow rate of the air and also the turnover of the water. The Turboflotor 5000 Shorty injects, with one pump, as much air as the Twin with 2 pumps.
The turnover of the water is another parameter. As a rule of thumb, for the tall skimmers, with aquariums up to 3 – 5000 litres, a turnover time of once per hour gives good results. For the shorter skimmers with medium sized aquariums, the turnover should be increased to 2 – 3 times per hour to receive the same skimming results.
This is the reason, that we recommend for both the Turboflotor 5000 Twin (153 cm high) and the Turboflotor 5000 Shorty (50 cm high) the same flow rate of water – approx 3.000 l/h).
What is the ideal size of the bubbles? This depends on the counter current and the water flow. They should be as small as possible, but big enough to rise under the conditions of the specific skimmer. A tall skimmer with a large volume and comparatively low water flow can use much smaller bubbles, than a short skimmer with high water flow and short retention time of water in the column. Here, if the bubbles get too small, they “escape” through the outlet. To make it more complicated. The bubble size is not completely defined by the physical parameters of the skimmer (Needle wheel construction, air diffuser), but also by the chemical composition of the water. Water that is highly loaded with organics produces smaller bubbles, than water low in organics. This means, if you connect a new skimmer to an older, or poorly skimmed aquarium, at first the bubbles are smaller and you may get problems with bubbles entering the aquarium. But after some days, when the concentration of organics is reduced, the bubble size increases and bubbles no longer enter the aquarium. Compared to the concentration of organics, the change of bubble size caused by the salt concentration is rather small – at least, if we look at seawater of, say, a specific density from 1.0020 to 1.0030.
Bubble size will also change within a skimmer. The smallest bubbles are at the deepest point with the highest pressure. As they rise, the diameter increases, giving more area for organics to attach to. This is the reason, that the bubbles should not be pressed downwards again, once they have been up. On their way down, the bubble size and the surface decrease again, and the attached organics may be lost.
If you ask Wolfgang Weidl, the designer of the Deltec range of skimmers, for his main criteria when it comes to skimmer design you’ll get an answer to the effect, “There are three main criteria to be taken into consideration – Air, Air, and Air!”
Looking at the way the Deltec skimmers are designed you’ll see fairly short skimmer bodies with large volumes of air being introduced, characterised by collection cups with large diameter necks to accommodate the high air flow. As a rule of thumb, they find that an airflow of 500 to 600 litres per hour for each 1000 litres of water volume works well. Although Deltec do make tall skimmers, mainly for the commercial market, the short-bodied skimmer tends to be the one of choice for the hobbyist because of the necessity to position it out of sight in a cabinet under the aquarium.
Wolfgang also pointed out a number of useful bits of information for you to consider when choosing a skimmer.
When it comes to manufacturers sizing their skimmers according to aquarium volume, just bear in mind that it may not be so much the tank volume that’s important, rather it’s the organic load of a system that’ll dictate the size of skimmer required.
Be wary of the dangers of overskimming. If your tank and skimmer combination is working well for you, don’t think you can make it even better by increasing skimmer size. Overskimming can lead to the removal and oxidation of trace elements, resulting in a nutrient poor aquarium that can be detrimental to your animals unless appropriate steps are taken.
When a skimmer is operating correctly, the lower forty percent of the of the neck of the collection cup should contain an even, cream like foam, with no signs of turbulence. Above this the bubbles should be larger, starting to break up, and showing the colouration from the organic load being removed.
Beware of turbulence in a skimmer. This brings about the danger that the organic pollutants will be returned to the system rather than being removed.
In conversation with Mario Ratz, designer of the Ratz range of skimmers, at the UK trade show last year I was made aware of a couple of design issues that may effect the efficiency of needle/pin wheel type skimmers. I was interested to learn that there may well be a phenomenon taking place when needle/pin wheel impellors are used that enhances the skimming capacity of these designs. The theory is that the chopping motion of the specially designed impellor results in an electrostatic force being imparted to the bubbles being produced and that this in turn increases the ability of the air bubbles to attract hydrophobic molecules to their surface. Think about how a plastic ruler can be charged with static electricity by stroking it with a cloth and how it then has the ability to attract small items to its surface.
One thing Mario was a little concerned about was the orientation of the pumps used in this design of skimmer. The worry being that if the pump was positioned with the air being introduced at the bottom of the pump that there was the possibility of air being trapped high up in the pump, excluding water and potentially leading to failure through overheating.
Tunze, another German company have always taken a slightly different tack with their designs, here Roger Vitko, of Tunze USA, explains.
Our company was one of the first to develop a protein skimmer. In the early days of the aquarium hobby the main focus was soft corals and macroalgae which needed a certain level of nutrients and plankton to thrive. To avoid overskimming we incorporated a recirculating loop. The water simply recirculated inside the skimmer. By osmosis proteins diffuse into the skimmer for removal. This effect is very simple to understand. Imagine adding sugar to a glass of tea, even if you don’t stir it the tea will be evenly sweetened given sufficient time. Dissolved matter is evenly dispersed throughout a solution and for this reason even without active flow through, the skimmer will remove wastes that diffuse in to keep the solute levels even in the solution, the solute levels will always be lower in the skimmer due to the skimming action so waste is constantly diffusing in to even the concentration gradient and is then removed. By avoiding contact of the pump with the aquarium water the plankton are not exposed to the destructive shearing forces. Because they are not dissolved, but rather living suspended in the water, they do not diffuse in or enter the skimmer in any great number. This year we are introducing a new skimmer line which also has the option to run as a traditional open loop skimmer.
Well there you have it, a lot of information to chew over. And a lot of different approaches. As one of these designers said, “All skimmers work, it’s just a matter of how well”.
I’d like to thank every one who contributed to my request for information to enable me to put all this together. The amount of space given over to any particular manufacture doesn’t reflect on the importance of the contributions, many people made similar comments about some aspects of skimmer design which it would have been pointless to repeat.
If you’re looking to purchase a skimmer but are also thinking of upgrading to a larger reef in the future, then consider buying a skimmer with the capacity to work on a greater volume of water.
When you’ve got a skimmer keep it maintained, that doesn’t just mean emptying the collection cup! A well maintained, inexpensive air driven skimmer can easily outperform a far more expensive pump driven skimmer where maintenance has been neglected.
Protein Skimmer Maintenance.
It goes without saying that every time you empty the collection cup of a skimmer that you should clean both the cup (so you can see what waste is being produced) and the inside of the cup’s reaction pipe. When you clean the cup it’s worth leaving the skimmer off for half an hour or so; this gives an opportunity for any build up of salt in the air injection nozzle/venturi to dissolve. If you notice any reduction in air flow this is the area to look at, if leaving the skimmer off for a short period doesn’t work try either injecting R.O. water down the air intake or stripping down the pump and soaking the air injection nozzle/venturi in vinegar.
Every few months clean the inside of the reaction tube and if there is a build up of coralline algae allow affected parts to soak in vinegar to aid removal.
Skimmer Pumps and Power Heads.
All centrifugal pumps associated with your skimmer will need occasional cleaning and servicing to maintain performance. Pump pre-filters should be cleaned regularly as required to ensure maximum water flow at all times. Periodically strip all pumps to enable cleaning of the impellor and impellor chamber where you’ll probably experience a build up a bio-film of some sort. Check the impellor shaft for wear (or corrosion) and replace as necessary. Remove any animals who have set up home in the interior of the pump.
Some makes of pump – Aqua-Bee and AquaMedic – may have a concealed pump flap in the part of the pump housing leading to the outlet; this flap and the surrounding housing should be cleaned of any calcium deposits to ensure that the flap can move freely.
Wooden air diffusers need to be changed every four to six weeks depending on make, depth of water, and the pressure output of the air pump. When changing, check the end of the airline for any salt build up.
Replace any airline showing signs of deterioration e.g. hardening, cracking, or discolouration.
Tip – When you replace a wooden diffuser make sure you’ve got air running through the new one before immersing it. From the moment a diffuser is placed into water it starts absorbing water, the wood then swells, constricting the pores that the air travels through, hence restricting airflow and resulting in poor skimming.
Air pumps: Wooden air diffusers produce a lot of backpressure, which can lead to diaphragms failing. If your pump starts to become noisy this is usually the cause. Either strip and examine the pump regularly or make sure you have a spares kit handy ready for when it fails.
Protein Skimmers and Ozone.
Ozone is usually introduced into a system via the skimmer, being drawn in instead of atmospheric air. Ozone can improve skimmer efficiency by around ten percent.
Ozone is potentially dangerous both to you and the animals you keep. Treat it with respect and follow the manufacturers instructions at all times. If carbon is used with your ozoniser make sure you change it at the specified intervals. Periodically replace all airline used with an ozoniser, making sure you use airline hose of the correct grade. Ozone will rapidly degrade unsuitable plastics – do not use plastic airline valves in conjunction with ozone they will melt!
From our research when coming up with the Reef Scientific ozoniser I came to the conclusion that most people are applying too much ozone. I’d suggest starting off with a very low level of ozone and then gradually increase it over 10 to 14 days until you reach the point where the skimmer no longer produces foam, now gradually back off the amount of ozone until you reach a satisfactory compromise.
©2005 – 2010