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Note that I have recently upgraded the simulator to take into account a number of other factors, hopefully should be another article up in the not too distant future covering this.

Introduction

In early January 1999, there was a discussion on the best place to have a skimmer located within a reef aquarium system, and which one was more efficient. In order to sort out what most people had been saying for quite sometime without any factual basis, it was time to simulate the skimmer/aquarium system to actually find out what was happening.

The simulator uses a simple stepwise calculation method. What this means it that is moves the water around the system in packets, then calculates the state of the system, then moves move water around again. As long as the time step employed is small enough, then the simulation is close to what actually happens in a continuous system.

Note that most of the results attained from this simulator are nothing special, they are just confirmation of what is basically common sense. But it is always nice to have something to back up thoughts like this.

Systems Simulated

There are seven different system arrangements that the program simulates. Two of them are ideal cases, so are not really applicable to real life. The remaining five are more realistic, and should not be too far away from what really happens in a reef aquarium.

Fully Mixed Tank Only

This simply consists of a tank with a skimmer directly attached. The tank is fully mixed, which means as soon as some water is added back to it from the skimmer it is mixed totally with the rest in the tank. This is an ideal case, so is a long way from what actually happens. The the ideal cases is considered, as they are an easy place to start, and they do tell you a fair amount of what is happening.

Partially Mixed Tank Only, Return Same End

Now the tank has been divided into four equal parts, with flow between each section. The skimmer takes water from one end, and then returns it to the same end of the tank. This is basically what most hang on the back type skimmers are arranged like.

Partially Mixed Tank Only, Return Opposite End

Same tank set up as previous one, but now the skimmer return goes back into the tank at the opposite end of the tank. This is the arrangement that the majority of people recommend to have in an aquarium, and it will be demonstrated actually why shortly.

Fully Mixed Tank and Sump

The tank and the sump are both fully mixed, this is the ideal case. Water that enters the tank or sump are instantly mixed with all the other water in that tank. Once again, not a very realistic system, but it does give a starting point.

Partially Mixed Tank and Sump, Return Same End

The tank is again divided into four equal sized compartments, and the sump into two. The water from the sump is returned to the same end of the tank. If the skimmer takes water out of the sump faster that the return rate, then water is simply recycled within the sump.

Partially Mixed Tank and Sump, Return Opposite End

This is how a typical tank is arranged, water is draw into the sump at one end, the skimmer takes water from the sump and returns it to the sump, and water is then returned to the tank at the opposite end of the tank. And the last one is...

Partially Mixed Tank and Sump, Return Opposite End, Tank Fed Skimmer

This arrangement is the same as the last, but instead of water being removed from the sump then fed to the skimmer, it goes straight to the skimmer. If the skimmer rate is larger than the return rate, then water is drawn from the sump to make up for it.

The last two systems are what the discussion was initially all about, would it be better to take water directly from the overflow into the skimmer then into the sump or from the sump?

Definable Parameters

The following system parameters can be defined within the simulation program, and it is quite flexible with what can be done/specified.

Tank Volume (litres)

The amount of water that the display tank holds.

Sump Volume (litres)

The amount of water that the sump tank holds.

Return Rate (litres per hour)

The flowrate of water from the tank to the sump, and from the sump back to that tank.

Skimmer Rate (litres per hour)

The flowrate of water through the skimmer.

Tank Rate (litres per hour)

The flowrate of water within the tank, excluding the skimmer or return pumps i.e. circulation pumps/powerheads.

Initial DOC (milligrams per litre)

The initial concentration of dissolved organic carbon material (DOC) in the system.

Skimmer Efficiency (%)

The percentage of the total amount of DOC the skimmer has access in a single pass it can remove.

Skimmer Exit DOC (milligrams per litre)

The exit concentration of DOC from the skimmer.

Increase DOC in Tank (milligrams per litre)

This is a feature that can be used to see what happens when there is DOC input into the system. The value of this parameter sets how much the DOC concentration in the tank is increased.

Increase Period (minutes)

This determines how often the Increase DOC in Tank occurs.

Time Step (minutes)

The time between calculations in the simulation.

A couple of those parameters should be stirring questions like ... how do we know what the skimmer efficiency is? Or even what is the level of DOC in the system? Well, the short answer at the moment is that there is no idea .... OK, maybe a little with the skimmer efficiency as some initial work has been done on this. But this does not stop any use of the results attained. It is still possible to make comparisons, and say something intelligent about what is happening, even though the actual specific values might not be real or even close.

Simulator Results

Now it is time to go through some of the results that the simulator produces. It should all start to make a bit more sense from here on in. Firstly, the base systems is as follows, i.e. the common values better each of the graphs shown unless stated otherwise.

Base System

• Tank Volume : 1,000 lt
• Sump Volume : 500 lt
• Return Rate : 5,000 LPH
• Skimmer Rate : 5,000 LPH
• Tank Rate : 5,000 LPH
• Initial DOC : 100 mg/l
• Skimmer Efficiency : 30%
• Skimmer Exit : 10 mg/l
• Time Step : 0.30 min

Now the thing with those system values are that they are scaleable. What this means is that say there is a tank that is only 100lt. Well then if you skimmer then moves 500LPH, then the graphs still apply, i.e. five times the tank volume per hour is cycled through the skimmer. Time for some graphs .....

Tank and Sump Simulations

Base Tank and Sump Systems Comparison

This is a graph of the four different base systems simulated with the tank and sump. As can be seen from this, the end that the return lines goes to makes a real difference to how fast the DOC is removed i.e. the pink and yellow lines. And with the same skimmer rate, it is much better to first send the water through the skimmer rather than into the sump i.e. the yellow and blue lines. So, the take home message from that graph is the return should be located on the opposite end of the tank, and take water from the overflow and put it directly through the skimmer.

Tank Only Simulations

Base Tank Only Systems Comparison

Once again, the return at the opposite end of the tank makes a difference. Why is it that the ideal, fully mixed, systems are worse off than those that are partially mixed? The reason for this is that the amount of DOC that is removed is determined by it's concentration. In the fully mixed system the DOC available to the skimmer is reduced faster than that is a partially mixed system. As a result it is then removed from the system slower.

Skimmer Efficiency

Effect of Skimmer Efficiency

The more efficient the protein skimmer is, the faster the DOC concentration goes down. And that effect is not that surprising. One thing that should be noted though, is that again the degree of improvement decreases for a given increase in efficiency. So there will be a region reached where there will no longer be a substantial improvement, i.e.spending more on a better skimmer would be better off spent on some other things that can be done to make it work better.

Tank Circulation Rate

Effect of Tank Circulation Rate

This would be expected to make a bit of a difference, but in actual fact it does not unless the return is at the same end of the tank. When the return is at the opposite end, it fractionally decreases the amount that can be removed as the water is being mixing in the system faster. So the rate the water is moving around that tank does not make any significant difference, unless the return is located on the same end. This is applicable to systems that have hang on the back type skimmers.

Return Rate

Effect of Sump Return Rate

It actually turns out that it is best to the the return rate around the same value as the skimmer rate. Why this is the case yet is not known yet, have to look into it more, as it contradicts the result when the skimmer rate is increased. This is shown by the fact that the yellow line, highest return rate, is above the line for the return rate half of this.

Skimmer Rate

Effect of Skimmer Feed Rate

With the skimmer taking the feed from the sump, running the skimmer faster and faster improves the speed at which DOC is removed. Note that as higher and higher skimmer rates are used, then it will start to slow down the amount of improvement from a given rise in flowrate. So there is a region limit here where if a bigger pump is used then money and power is being wasted to get only a little more efficiency. This is the one that goes against what is in the above graph with the Return Rate. Some possible reasons for this have been suggested, but it will have to be looked into it more before anything can be said about it.

Tank and Sump Relative Sizes

Effect of Tank and Sump Relative Size

If the system total volume is kept the same, but change the ratio of the tank to the sump volume, what happens? Well, basically not much at all with this particular base system. It really doesn't matter too much.

Skimmer Exit DOC

Effect of Skimmer Exit DOC

No surprises here, the lower the DOC from the skimmer, then the better off things are.

Tank Fed versus Sump Fed and Double Skimmer Rate

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Comparison Between Tank Fed and Sump Fed Skimmer

Another arrangement that people tend to use for their skimmer is to have it running faster than the return. Therefore water is recycling around in the sump. This graph shows that if the skimmer is operating at the same rate as the return pump, then things are better off to have the water coming directly from the tank to the skimmer. If skimmer is then run at a higher rate, then improvemnents are made. So lets say there is a certain sized pump running a skimmer. If it is close to the rate of the return pump, then it is more advantageous to feeding water directly to the skimmer. If on the other hand, there is a choice between the skimmer fed from the tank and the skimmer operating at double the rate then go for the latter.

Note that the numbers used in the simulations are used to illustrate relative values and effects. They are not necessarily applicable to a real system. What is important is it is a qualitative study, showing a direct comparision between different situations.

Questions and Discussion

How long did it take to set up and perform the calculations for each model?

The development of each particular model took about 30 minutes. It started with the easiest model, then got more complex with each one. This makes it very simple to build up to the next one, taking one step at a time.

If a reef aquarium doesn't have a sump what does it mean, what is applicable?

If there is no sump, then what is needed is to either have the return to the other end of the tank i.e. not be able to be mixed quickly with water it is taking in ... or have a lot of circulation in the tank if the return must be near to the intake. That will make it perform better. The aim here is is for there to be as higher concentration of DOC entering the skimmer intake. And both of those steps gotowards that goal.

Tank circulation rate does not seem to effect system export efficiency, but it isstill important, isn't it?

No it doesn't effect the rate at which the DOC is removed and reduced in the system for the cases shown above. But this does not mean it is not important, it is important for other reasons than skimmer efficiency and how fast it can remove DOCs. It will be important though if the skimmer return is located at the same end that it's intake is. Then it is important to increase the DOC concentration in the intake water by mixing the more pure water straight from the skimmer with that in the rest of the tank.

Most graphs show that once the DOC reaches 20 the graph bottoms out ... does that mean that the cleaner the water gets the harder it is to remove the DOC?

Yes, that is what it means. And there are various reasons for that. The way a skimmer operates, or the way the author belives it does and how it can be simulated, is as follows:
Lets say 100 mg/l of DOC goes into the skimmer. It then has the ability to remove down to say 10 mg/l i.e. that will be the exit concentration. It will not be able to remove all of it, that is just not possible. But on top of that the efficiency of the skimmer has to be included. In these simulations a value of 30% has been used, which is a number that someone came up with in a study recently. So, with that 100 mg/l water, it can get it down to 10 mg/l, but it is only 30% efficient.
What does that mean? Well, it can only get 30% of the way to the lowest exit concentration. In this case, that means that exit concentration will be 73 mg/l ... 30% of (100-10) is 27 ... so it can remove 27 mg/l in passing that water through it. So each time the water passes through, it gets closer to the final value, but not quite there.
A good practical way to visualise it is to stand at the end of a room. Now move 1/2 way to the front of the room. Then repeat that again, move 1/2 to the front of the room. You are now located 1/4 of the way from the front. Repeat that hundreds of times. Will you reach the other wall? No, you get closer and closer, but never make it. It if very similar to that.

What would be a good range of values for skimmer efficiency?

That is not an easy one to answer. There really needs to be some testing done on that. The only one that has been heard of is that the skimmers were 30% efficient. Not much more than that can be said as not a lot of work has been done on it.

Would it be possible to bound the thought experiment though? Say that a small skimmer is X efficient and a downdraft skimmer is Y efficient?

Well, with this simulator it is actually a little simpler than that. The efficiency is linked to the bombardment rate. Have a look at Escobal's book, he covers it in detail. So it would then be possible to get a relative measure of how efficient each type of skimmer is. It is then possible to feed in a certain flowrate through it, i.e. a size of skimmer, and the efficiency will be similar. What is meant here is a small counter current skimmer should be just as efficient or the same efficiency as a large one. That is all assuming that they are desgined in the same manner. Then it can be said that may be the downdraft is twice as efficient as a air driven skimmer, or something like that. The problem is it is not really known, or there is no measure of how different they are.

All graphs have shown that after about 200 minutes most systems are equal no matter what you do ... in the long run does it matter how the system is set-up since most removal comes within the first 100 minutes?

Good to see this was picked up. The reason that it doesn't make any difference is because the end point that is being aimed for is the same. In the case of most of the graphs shown above, it was a minimum DOC concentration of 10 mg/l. Increasing the skimmer rate, return rate, etc. changes the speed at which it gets there, but does not move the end point. Now, the thing that does make it important is when a generation of DOC is added to the system. This is where it stands out. As it would be expected, what comes out is the faster that the DOC levels are reduced, the better off the system is. As an example, say the skimmer is 15% efficient, and compare that with one that is 45% efficient. Now, starting with 100 mg/l in both cases, and add 1 mg/l to the tank every 1 minute. What you find is the 45% efficient case levels off within 200 min or so to around 40 mg/L. But with only 15% efficiency, then it is roughly double, around the 80 mg/l mark.