Skimmer 2 minute dwell time. Anyone have any proof of this?

[sherm71tank]

Well, is there any out there? Please, keep it real. I'm not asking about recirc vs. beckett or venturi. I don't care about that. Just the dwell time. I am looking for any documentation to support a 2 minute dwell time removing the mysterious "stubborn particle" better than shorter dwell times.

 

[hahnmeister]

Or I would like to throw into the fray : Is dwell time the time it takes for one bubble to make it to the surface (the time that the passing water is exposed to one bubble), or is it the time that the water is exposed to multiple bubbles? As in, if a shorter skimmer cant have individual bubbles that last 2 minutes, can it be made up for by keeping the water in the skimmer (like a recirc skimmer) for 2 minutes?

 

[sherm71tank]

Edit: in the interest of keeping this on track.

 

[sherm71tank]

I really don't want to taint the conversation with subjectives. Lets take an ideal skimmer, large enough to have one bubble travel with the same water for 2 full minutes before the water has to exit the skimmer and bubble pop.

 

[mr.wilson]

Sorry to go off topic, but the frustrating thing is that manufacturers don't post dwell times with the specs. The consumer is forced to do these test on their own dime. I would love to hear a qualified response to your question, but I don't think there's one out there.

I don't want to cloud the water by defining terms, but for what it's worth; dwell time is the amount of time the water is exposed to (any) bubbles. Bombardment rate is the amount of air, and at what speed (ratio), it's exposed to water.

Air flow meters give deceiving results as they measure the amount of air drawn into the pump. They don't measure the surface area of the bubbles (bubble size) or the water to air ratio. Many skimmers are marketed with air flow rates as the key parameter. This simply isn't the case. Introducing too much air to water causes bubbles to merge with each other, lowering stability, and surfactant migration.

It's like the old sating goes "horsepower sells the car, while torque makes the car go fast". Air flow sells protein skimmers, while dwell time skims proteins.

I choose to use tall protein skimmers with relatively slow flow-through rates; however, if flow through rates are too slow, bubbles will rise rather than spin in the contact chamber, thus shortening the bombardment duration, while maintaining the same bombardment rate. Spiral motion helps increase bubble duration and water dwell time, but for some reason this feature has been lost somewhere along the way. The big names don't go through the trouble of adding an elbow (inside the contact chamber) on the influent, so there's no first in first out (FIFO).

The use of dyes with influent water, such as malachite green or methylene blue, can help you get a visual of how long the dwell time is, but with no FIFO, the water mixes, and you can't get a fair measurement.

 

[jdieck]

<a href=showthread.php?s=&postid=8712752#post8712752 target=_blank>Originally posted</a> by mr.wilson

I choose to use tall protein skimmers with relatively slow flow-through rates; however, if flow through rates are too slow, bubbles will rise rather than spin in the contact chamber, thus shortening the bombardment duration, while maintaining the same bombardment rate. Spiral motion helps increase bubble duration and water dwell time, but for some reason this feature has been lost somewhere along the way. The big names don't go through the trouble of adding an elbow (inside the contact chamber) on the influent, so there's no first in first out (FIFO).

You might be onto something here. There are a lot of variables to consider than just the dwell time.
The molecule attachment to the air water interface at the surface of the bubble is very fast, it does not require minutes to do so. On the other hand the translation of a molecule trough the water takes a very long time and depending on the size of the molecule it will take a lot more than just two minutes.
If you have a bubble travelin with the water the bubble surface will trap the molecules in the immediate surounding almost imediately and from there the rate at which the moleculaes are traped reduces exponentially so the bubble surface will not be able to trap as many molecules as it could even if you give it dewlling times of hours.
What is needed here is to have a way to bring the molecules to the bubble, in other words bring new water to the air bubble so rather than a bubble traveling WITH the water you look for a bubble traveling through the water or viceversa.
Regularly this effect happens within the skimmer box rather than at the skimmer riser column as it is generally assumed. IMO the riser is basically to separate the molecule ladden bubbles from the rest so in reality dwel time at the riser may become irrelevant, rather I would look more to have enough agitation an time in the box to fully saturate the buble interface and less time of the bubble spent in the riser as to increase the speed of interface saturation while getting rid of saturated bubbles as fast as posible.
So in summary, it is more important IMO to bring water to the bubbles rather than just how long the bubbles are in contact with the water.

Another variable is wheather to remove all available molecules dissolved in some amount of water or partilly remove available molecules from a larger amount of water.
In other wors you are looking to remove as much contaminants from the tank as possible, not as much contaminants from the water in the skimmer.
Basically we will be removing from the tank the same amount of contaminants if we remove half the contaminants of twice the amount of water passing the skimmer than all contaminants from half the amount of water. The skimmer that removes half, might not be as efficient power wise but skimming wise will be equally efficient. IMO it will be my preference to remove half the contaminants of three times the amount of water even not that power efficient. This will be a skimmer with less dwell time but in total will be removing 50% more contaminants that one that removes all contaminants from 1/3 the amount of water.

So looking at those variables I would say that an specific dwell time would be a measurement of skimming efficiency without considering all the other factors involved in the skiming process.

 

[mr.wilson]

Sherm71tank,
To answer your question, you will find formulas for turn time, bombardment rate, volume of air, bubble rise time, air flow rate, and dwell time, in P.R. Escobals "Aquatic Systems Engineering: Devices and how they Function". They're not for the layperson, but they answer your question. Skimmer manufacturers don't use formulas as profit lies in hype, not science.

Jdieck,
On the surface, it appears to be a "six one way, half a dozen the other" with respect to bombardment rate and dwell time, but there are other factors to consider here.

1) Bubble stability: If you have a rapid bombardment of air "hitting" the passing water, you will have a fast "bubble rise time". A fast rise time, with a subsequently short contact time, will yield bubbles of lesser stability, as they've collected fewer surfactants. Unstable bubbles will merge together and prematurely pop before the surfactants (surface active agents) can be collected at the top of the neck. Coated, stabilized, bubbles are less efficient at binding proteins, but they are a superior delivery system.

2) Hydrophobic (surface active), semi-hydrophobic, and hydrophylic (water soluble) proteins: Wet skimming will remove more hydrophylic proteins, as water soluble agents are easily collected and exported before the bubbles get the chance to pop. The rapid action of wet skimming (high bombardment ratio) can actually remove POC (particulate organic carbon), as particles are juggled from one bubble to the next as they merge in the riser tube. Conversely, dry skimming will remove more hydrophobic proteins, as the increased amount of air interface allows for stable migration. A longer dwell time will give phydrophylic proteins more time to bond to bubbles, thus removing more DOC (dissolved organic carbon). The added benefit of wet skimming is the passive water change that occurs. You just have to keep an eye on salinity and make sure you don't flood your floor. It doesn't matter how efficient the skimmer is if your wife throws it out on the front lawn with the rest of your stuff.

3) Fractionation method: The smaller the bubble, the greater the surface area (air/water interface) and stability. Wood air diffusers generate ideal bubbles < 1mm. Downdraft, venturi, and needlewheel technology form much larger, less efficient bubbles. The mechanical method required to make these aspirated bubbles also causes more aggitation in the contact chamber, diminishing the contact time and bubble stability. These mechanical methods do however provide a faster rise time. The bombardment rate isn't however increased, as air pumps and diffusers introduce more air than aspiration methods, with no additional water flow required.

4) Ozonation: Onoze injection increases the protein skimmers overall efficiency through oxidation of organics, but bubble stability decreases with the extra atom of oxygen, lowering its' ability to export POC & DOC. Contact time should be longer when using ozone, firstly to allow the bubbles to be coated (stabilized, and to allow the ozone gas to break down larger organics. These larger pigmented organics are the ones we want to remove for water clarity.

In summary, if we want to remove the maximum amount of proteins, in all forms, we want a long dwell time and rapid bombardment rate.

 

[jdieck]

mr wilson:
The more variables the merrier

 

[Fredfish]

The original experiments on dwell time and efficiency were done for the sewage treatment industry and are well documented. Some of those references may be posted here on RC in very old threads or they may be found in usenet archives that are pre-RC.

THE most efficient skimmers are the counter current air driven type. The primary reason they are not popular is because they require regular maintenance. There was a big thread here last year on air driven skimmers.

From a practical perspective, all these skimmers seem to work reasonably well, as do skimmerless methods of nutrient management. Pick your poison.

As a side note, Eric Borneman has been looking at skimate to figure out what exactly it is. So far, a lot of it seems to be paticulates stuck together by coral/fish mucus with some disolved nitrates and phosphates.

Fred

 

[hahnmeister]

Id challenge that the most efficient skimmers are the air driven type, well, sort of. I can see how they are from the wattage standpoint, but one thing you can do with a recirculating needlewheel is plumb the pump 1/2 way up the skimmer with its output facing down, and the intake at the bottom of the skimmer just above the outlet. While this makes bubbles very similar to the airstone skimmer, and can be set up to run counter-current, I also like the fact that you can create an even greater downdraft between the mixing pump outlet and inlet by spacing them out in a narrow skimmer body. You can trap smaller bubbles in a limbo in this area, and even on a skimmer as short as 2' tall you can have bubbles that linger in this zone for well over 2 minutes. You just cant create that large of a downdraft with airstone skimmers... unless you have a large throughput... but then you are losing out in the interface time. So from the height standpoint, I would say that recirc needlewheels, if set up right, can be more efficient from the dwell time standpoint vs. height of the skimmer.

 

[mr.wilson]

Sorry for straying again sherm71tank, but your answer still lies within the discussion.

I stand by my old friend the air-driven skimmer for the following reasons.

1) Bubble size is less than half that of aspirated methods (downdraft, venturi, beckett, needlewheel & pinwheel). Smaller bubbles are more stable and have greater surface area with less premature merging, and a slower assent to the top of the skimmer.

2) Air pumps will inject about 7.5psi of air, while the aspirated methods inject about 3.5psi.

3) Wood air diffusers need replacement every two months with little or no adjustments in-between, while other methods require daily nursing. Becketts and venturis need more frequent cleaning due to crystallization at the intake orifice. I don't mind spending $12.00 a year on replacement diffusers. It's a lot cheaper than running an Iwaki 100RLT, 24/7.

4) Counter-current slow flow design adds to dwell time. Recirculating needlewheel pumps chop-up bubbles that have attached proteins, and cause too much turbulence.

5) They are easy to DIY. This is the main reason why manufacturers abandoned them along with the formulas that prove their worth. Why sell $1.00 air diffusers and $50.00 air pumps, when the market will accept $100.00 venturis and $300.00 pumps. Venturis entered the market as no maintenance replacements, a statement that proved to be false. Beckett fractionators are noisy and go on strike when you put your hands n the tank. Needlewheels are maintenance-free, but draw a lot of power, generate large bubbles, and have all the usual drawbacks of a submersed pump (stray current, noise, vibration, heat transfer, short life is saltwater, and possibility of electrical shock).

 

[hahnmeister]

I dont want to argue with you (I know you arent trying to either), but I just say that because Ill just agree to disagree with you. Your points are valid, but Im thinking of ways around what you say. For instance, the asperating skimmer may only be 3.5psi at most, but if your skimmer is only 2' tall vs. 6', that may be all you need. And if its such a concern, you can always force feed a needlewheel. You are correct about bubble size with respect to most skimmers like becketts and venturis, but Needlewheels make some pretty fine bubbles IME. The advantage of using NW over airstone is minimal cleaning of course, and I have never seen a needlewheel that needed daily nursing/adjusting like with becketts and their like. Counter current doesnt have to be too turbulent with a needlewheel (and so what even if it was, according to Randy, some turbulence is a good thing... too many are of the 'paranoid about turbulence' pro-bubbleplate everything group it seems)... bubble plates and proper setup are key.

And I gotta call you out on one thing though... you say that needlewheels draw alot of power? Compared to what? Airstone maybe, but thats it, and even at that, they arent exactly hogs. Needlewheels are a fraction of the wattage of a beckett, venturi, downdraft, etc. And to add in those ideas about stray current, noise, heat transfer, short life, electrical shock... where are you getting this from?!?!?! Many needlwheels are external as well as internal, running on either submerged or external pumps. Look at the needlewheel darts as an example, or the Red Dragons. There is minimal heat transfer and the likelyhood of shock??!!?!? Short life in saltwater? Yeah, maybe back in the 80's or something. I havent heard of any of those problems on current needlewheels.

 

[sherm71tank]

mr.wilson, jdieck, hahnmeister, Fredfish, thanks for the input. I didn't want this thread to get into a skimmer debate. I still don't have an answer to the 2 minute protein. I don't think I ever will.

 

[mr.wilson]

I would post Escobals formulas, but you really have to read the whole chapter and have a math background to use them.

If you have the skimmer, you can dye the water and measure how long it takes to flow through the skimmer. This process will compensate for water that bypasses or lingers. If you want to measure any skimmer on the market, you can do a rough calculation, assuming that the water flows through in a first in first out basis.

The Aqua-Medic Turbo Floater 5000 has a volume of 15 gallons. If you were to have a throughput of 15 GPH, then you would have a one hour dwell time (defined as the amount of time the water is in the skimmer, exposed to passing air bubbles). In order to achieve a two minute dwell time, you would set your throughput at 30 times faster, which is 450 GPH. Once again, we're looking at the average time exposed, not the guaranteed amount of time.

Escobals' formulas cover every aspect mentioned in our meandering discussion. Here are some shortcuts for measuring the volume of a particular skimmer.

http://www.online-calculators.co.uk/volumetric/cylindervolume.php
http://www.easysurf.cc/metric/cnver6.htm#cig4

 

[sherm71tank]

mr.wilson, I am not arguing that with you. I am only interested in which particles/ proteins/ DOC's or whatever they are that need this 2 minutes to extract. What are they? Where are they named? Thank you for your time.

 

[hahnmeister]

Also, I would like to define the dwell time as the time it takes for one bubble to escape the skimmer (and the water that it encounters in that time), not the time that the water has in the skimmer, as it is crossed by multiple bubbles. FWIW, the more I reread Randy's article, the more it makes sense though.

A few months ago I posted in Randy's and also tried to talk with Steve Pro to get me in the right direction. My argument was that how is a 120minute dwell time even functional considering the water is going one way, and the bubble is going another... it is highly unlikely that one bubble is going to interact with just one protein or a few for an extended period of time, let alone 2 minutes. What Randy's article does suggest is this, and correct me if I am wrong, please, or throw in your suggestions:

Okay, so a bubble isnt going to interact with one protein or molecule for an extended period. Rather, as it rises, it encounters many, and its surface attracts many molecules on its path.. some more strongly attracted to the boundary than others. So a bubble starts out with a mix of strong and weakly attracted molecules at its surface, but as it ages, the weaker attracted ones get bumped off, and replaced with stronger ones.

This suggests that the longer a single bubble is in the water, the stronger it will be once in the collection area, er, its surface will be covered with more hydrophobic molecules than a shorter dwelling bubble. So it will collect in the cup as a more stable bubble, and most likely dry into a much darker skimmate (unless you skim wet).

The downside of this is that the less phobic molecules are less able to 'hitch a ride' as the average dwell time for a bubble increases... they get bumped off as stronger molecules replace them on the maturing bubble. So in a way... a shorter skimmer with more air production, although producing a wetter, lighter colored skimmate, could be seen as a better method??? It seems to suggest that.

So there is this problem of what to do with the less attracted molecules still, right? Well, this must be where the bombardment rate, and interface time come into play. With a single-pass skimmer, it might take out all of the more attracted proteins, but its shorter interface time means that many of the less phobic proteins and such are left in the water.... so.. what do we do... we slow the throughput... and if the skimmer isn tall enough, then we make it a recirculating. This might be more of where the 120second thing comes in rather than the dwell time (I always wondered that myself... how the two relate). Simply put, the recirculating skimmer, or counter-current extends the interface time, allowing enough time for not only the stronger attracted molecules to hitch a ride, but then also the less attracted ones as well. This would explain the greater output of the recirculating skimmers I have seen... they arent skimming more in volume, they are skimming more types... or the less-attracted (perhaps this is what we read as 'stubborn' molecules).

This also suggests that a shorter skimmer, with a shorter dwell time, might not make as thick or dark of a skimmate foam, but that foam has a wider spectrum of proteins in it... as the shorter dwell time means there is less of a chance for less attracted molecules to get bumped off by stronger ones. So rather than a tall narrow skimmer, the fat beast (like a BK) can make up for its shorter dwell time with shear interface volume and time.

Now that Randy has written the article, I think its time to revisit him with some of my age-old skimming questions, and maybe he can comfirm this as well.

 

[sherm71tank]

I have proposed that a recirc skimmer works better because the water is "scrubbed" cleaner of the "easier" bonding proteins leaving the more difficult proteins with cleaner bubbles to attach to. I didn't get the feeling Randy thought it held much water.

 

[hahnmeister]

Well, its sort of what he is suggesting in his article. He didnt make any direct conclusions, but if you start to 'connect the dots'... its a much different scenario than what Escobal suggests, but it does make sense in its own way.

 

[mr.wilson]

Sorry, I misread your original post. The information you seek can be found in this article http://www.reefkeeping.com/issues/2006-08/rhf/index.php#2a and on this website http://www.hawkfish.org/snailman/skimmer101.htm

Hydrophilic proteins like phenols can be partially hydrophobic or can bind to POC that attach to bubbles. A longer dwell time allows for better migration & bonding of these molecules.

Surfactants are polar at one end and non-polar at the other. The polar end is hydrophilic while the non-polar end is hydrophobic. Many of these surfactants are proteins, but not all of them are. A longer dwell time allows for a greater chance for air bubbles to find the non-polar end of surfactants.

An efficient design will seperate proteins and heavy metals from water on the first pass.

Rubin et al. (1963), proved that foam fractionation removes DOC by two mechanisms...

1) surface-active materials are absorbed physically at the surfaces of rising air bubbles in a contact column.

2) non-surface-active compounds may combine chemically with surface-active material and be collected.

 

[hahnmeister]

"It was estimated by Escobal that some proteins take upwards of 2 minutes contact time with air to attach properly." < from the snailman skimmer thread.

So it is Escobal... Ill have to crack my copy open and find that...

So its not the dwell time, or the time it takes for a single bubble to rise, but the interface time, or the time that the water hangs around getting blasted with bubbles. Well... that makes things alot easier.

Well, then either way you look at it... my theory based on Randy's, or sticking with Escobal's science... if its the idea that it just takes longer for the easier to skim proteins to get removed so that then the lesser attracted ones can have a bubble to 'catch a ride', or if some proteins just need a longer interface time before they can get removed because they are just more 'stubborn', or whatever, the bottom line is still the same. The longer the water stays in the skimmer, the cleaner it will become.