# Filtration efficiency



## joescaper1 (Feb 14, 2013)

Hello all,

First time poster here.

I have a question floating around in my head. It has been for quite some time. I have watched more videos and read more posts than I can count, but have not seen this question addressed.

If a tank is being filtered by a wet/dry/sump system, there is an overflow that takes surface water, runs it through the filter, the water falls into the sump, and returned to the surface, how effecient can this filtration actually be? I figure a more effecient flow pattern would be to return filtered water to the bottom of the tank.

Is there a reason (all the videos I have watched use this surface to surface flow pattern) for not using the surface to bottom flow pattern?

Thank you all for your insight.
Joe


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## nodima (Oct 3, 2002)

Not all overflows take water only from the surface. Also, even if the water is returned to the surface, it is entering with enough force that it creates current and moves downward, causing some current in the water.

The main reason to avoid the bottom flow is the danger of draining the tank when (not if) a power failure occurs. A secondary benefit is less visible equipment in the water.


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## b3w4r3 (Dec 14, 2012)

Have you have ever seen the oily looking slick that can build up on top of the water? That is basically proteins and organic compounds. Surface skimming helps direct this slick to the filter where it can be broken down more efficiently. The slick can prevent proper gas exchange as it creates more surface tension on top of the water.


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## joescaper1 (Feb 14, 2013)

@ nodima,
I do not believe I have seen an overflow that does not take water from the surface only. (If you could direct me to a video that shows this, I would really appreciate it.) Also, most systems have heaters in the sump, therefore, the water temperature returning is warmer than the temperature of the display tank. This water will rise, and since it is already on the surface it will run out of the overflow without having a chance to pass through gills. If only 20% of the water does this, a 10X per hour system is now reduced to 8X per hour efficiency. I just can't see the wisdom of evacuating water and returning water at the same level.

A siphon break eliminates the possibility of the tank draining during power or pump failures. Pipes can be hidden.

@ b3w4r3,
Understood. I run power heads through an undergravel filter that exit water across the surface, thus eliminating stagnation.

Thank you.
Joe


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## clhinds78 (Jul 27, 2012)

Regulra water changes and gravel vacuuming help with debris that is not picked up by the surface skimmer as does running powerheads for circulation. If there is enough water movement in the tank there are really no dead spots.

Once the water in the tank is heated to the proper temperature the water coming from the return really isn't that much warmer than the tank's water and the tank's temp stays pretty even throughout. Again, good circulation within the aquarium helps with this as well.


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## yamadog (Oct 7, 2012)

I have a six foot 135 gallon tank. My overflow box is in the far right side of the tank, the returns are on the far left. The return is t'd and returns water at two points: one is about two inches from the left side of the tank and points toward the front of the tank, 45 degrees downward. The second points toward the right side of the tank, also 45 degrees downward. I have zero debris in the tank. you can actually see small particles flow across the bottom and up the right hand side of the tank into the overflow. I also have no scum buildup on the surface. Skimmer boxes definitely take water from all levels of the tank, not just the top. Jason


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## joescaper1 (Feb 14, 2013)

Thanks to everyone for the replies. My questions are answered.

Joe


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## nodima (Oct 3, 2002)

joescaper1 said:


> @ nodima,
> I do not believe I have seen an overflow that does not take water from the surface only. (If you could direct me to a video that shows this, I would really appreciate it.) Also, most systems have heaters in the sump, therefore, the water temperature returning is warmer than the temperature of the display tank. This water will rise, and since it is already on the surface it will run out of the overflow without having a chance to pass through gills. If only 20% of the water does this, a 10X per hour system is now reduced to 8X per hour efficiency. I just can't see the wisdom of evacuating water and returning water at the same level.
> 
> A siphon break eliminates the possibility of the tank draining during power or pump failures. Pipes can be hidden.
> ...


Here is a diagram for the Aqueon version. Mine are Perfecto, but work the same way.

http://www.reefcentral.com/forums/showt ... ?t=1466310


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## joescaper1 (Feb 14, 2013)

nodima,

Thank you. My tank is not drilled, I have a hang in tank overflow (a modified version of uaru joey's) which would take a 4" diameter pvc standpipe modification to achieve the same effect (I utilize this concept in the bio portion of my wet dry filter, another modified version of uaru joey's). My original question was asked to gather any negative input on a bottom return system. Since no one has voiced any opinions which show a negative impact on the health of the fish, I see the bottom return as an acceptable system of filtration.

Again, thank you for the information.

Joe


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## clhinds78 (Jul 27, 2012)

joescaper1 said:


> nodima,
> 
> Thank you. My tank is not drilled, I have a hang in tank overflow (a modified version of uaru joey's) which would take a 4" diameter pvc standpipe modification to achieve the same effect (I utilize this concept in the bio portion of my wet dry filter, another modified version of uaru joey's). My original question was asked to gather any negative input on a bottom return system. Since no one has voiced any opinions which show a negative impact on the health of the fish, I see the bottom return as an acceptable system of filtration.
> 
> ...


A few have said that you could have flooding issues. If your pump stops working the tank will begin to back flood into your sump you will have major flooding issues. The nice thing about a return near the surface is that only a small amount of water will backflow into the sump and the sump should be able to handle that volume of water.

P. S. I love uarajoey's videos!


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## Narwhal72 (Sep 26, 2006)

I agree that you could have flooding issues even with the siphon break, or a check valve installed.

Siphon breaks can become plugged with algae and fail. Check valves can become fouled and also fail. Last thing anyone would want is for an entire tank to drain onto the floor.

Something else to keep in mind. When you have the returns at the surface the water shoots away from the overflow, down the front glass, up the back glass, and down the overflow. When you have the returns at the bottom the water shoots across the bottom, up the front glass, across the surface, and down the overflow. In either case the net effect is the same. However the debris kicked up by the bottom jet has a chance to settle out as it crosses the surface which it doesn't going the other way.

Also since you are mixing filtered and unfiltered water in your aquarium (no matter where you return it) the turnover rate remains the same. T=9.2(v/f), T=time in hours for 99.99% of the tank volume to pass through the filter one time, 9.2 purity coefficient, v = volume of tank in gallons, f=flowrate in gallons per hour.

Andy


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## k7gixxerguy (Jan 12, 2012)

I can think of a possible huge problem with a low in tank return. Most of us run sand as substrate and with the volume of water that a wet dry turns over, I dont think you could not end up with a big sand dune on the opposite side of the tank as the return regardless of how you aim it if its near the bottom. I am speculating on this, but seeing how much my filtration moves my sand in my 125 with a spray bar from a fx5 and the returns from my Mag drive 9.5 shooting across the top of the water, it only seems logical that it would be worse even if the returns were angled up towards the overflow on the other side of the tank, which by your description of rational to do it this way would make me think that you wanted to aim it across the floor to stir up detritus and get it up to the overflow.


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## jcabage (May 29, 2012)

One of the local reef guys in my area drills 5 returns for each of his tanks. Two at the top, two on either side of the lower back wall, and one in the middle. He claims that it helps keep stuff out of the rocks (which is probably true). I believe he relies on check valves as flood prevention, which obviously isn't a great choice. I haven't heard of him ever having an issue though.

Although keeping detritus suspended and therefore filtered is a legitimate concern, this can be accomplished with very little effort through proper placement of powerheads to minimize dead zones.

The idea of water moving at 7-900+ gallons per hour out of a couple 1/2" tubes at the top of the tank and it not circulating throughout the tank is not exactly logical. Andy gave you some sweet math to debunk this in addition to logic too.

I would say adding returns at the bottom of the tank offers big risk for minimal gain. "Don't fix it if it aint broken", as any true Tennessean might say


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## joescaper1 (Feb 14, 2013)

@clhinds78
With the system I have set up, back flooding is not a possibility.

@narwhal72,
Check valves are unreliable(personal experience not in an aquarium situation), so I do not use them. The siphon break in my setup is 2" above the waterline; there is no possibility (I know, see "Titanic") that it will fail. That is the joy of physics.

I have no issue with the math involved in the turnover rate, my issue is with sending water fresh from the return straight down the overflow. The concept is: Any water passing through the filter that does not need to be filtered(at that time), reduces the efficiency of the filtering system. My 80% example was an attempt to say that 20% of the water did not need to be filtered at that time, so the efficiency of the filtration system(not the flow rate or turnover rate) becomes 80%. If the return is at the bottom of the tank, then 100% of the water flowing out of the overflow needs to be filtered. Theoretically the system would then function at 100%.

@k7gixxerguy,
It is about an inch above the bottom. I have no problem with blowing sand(In one tank I have approximately 2" of black sand and I do not like it, vacuuming is difficult). It is set up in an L shaped manifold, made from 3/4" pipe. The pipe that runs completely back to front is capped at the end, 6 - 3/8" holes are drilled into the pipe, or 50% more capacity through the holes than the 3/4" pipe. This reduces the water speed(in linear inches per second) by 1/3.

The reason is strictly to ensure the maximum amount of water removed by the overflow has been unfiltered for the longest period of time.

@jcabage
From my way of looking at the situation it is no risk for maximum gain.  
If anyone has any information that this is bad for the fish, please let me hear about it.

Thank you all.
Joe


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## Narwhal72 (Sep 26, 2006)

> @narwhal72,
> Check valves are unreliable(personal experience not in an aquarium situation), so I do not use them. The siphon break in my setup is 2" above the waterline; there is no possibility (I know, see "Titanic") that it will fail. That is the joy of physics.
> 
> I have no issue with the math involved in the turnover rate, my issue is with sending water fresh from the return straight down the overflow. The concept is: Any water passing through the filter that does not need to be filtered(at that time), reduces the efficiency of the filtering system. My 80% example was an attempt to say that 20% of the water did not need to be filtered at that time, so the efficiency of the filtration system(not the flow rate or turnover rate) becomes 80%. If the return is at the bottom of the tank, then 100% of the water flowing out of the overflow needs to be filtered. Theoretically the system would then function at 100%.


Since your overflow will be at the top of your aquarium that means that your siphon break will be above the lip of the aquarium (if it is indeed 2" above the waterline). You must realize that during normal operation water from the return is coming out of the siphon break right? You wouldn't want water spraying over your lights. Typically the siphon break is installed right at the water line.

Your theory is based on an assumption that 20% of the water from the return is going down the overflow. By any stretch that's not good science or engineering. Since the return is entering the tank and shooting water away from the overflow I would say that your assumption is incorrect. Even more, if you have any décor pieces (rocks, etc..) that the bottom return would hit it could deflect water upwards and back to the overflow.

In any case, I have already provided the mathematical formula for turnover rate in a mixed container.

Andy


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## joescaper1 (Feb 14, 2013)

20% was an example, even if it is .01%, it is still an unnecessary reduction in effeciency. IMHO

Joe


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## b3w4r3 (Dec 14, 2012)

It's always going to be more efficient to skim the surface where proteins collect than to intake water from anywhere else in the tank. As far as recycling filtered water back through the overflow, that water still has to cross "x" distance where it mixes with tank water and diffusion quickly balances the mixtures.

Just look at the typical HOB filter... It dumps water straight down where the intake tube is, and they still manage to keep aquarium water safe for fish.

None of the typical aquarium filters are going to be 100% efficient, but most of them will maintain water that is safe for the fish.


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## joescaper1 (Feb 14, 2013)

@moderator,
Please, lock this thread.


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## Narwhal72 (Sep 26, 2006)

Conversations don't end just because you don't like what you hear joe. I see no reason why the thread should be locked. Others may benefit and still wish to discuss it. You do not have to participate if you don't want to.

It's obvious that you had a preconceived notion of what you wanted to do. Several people have come forward and said that your setup could result in possible catastrophic problems without any real benefit and noone has supported your theory or position.

I have to ask, if you are not going to listen to any advice why did you even ask the question?

Andy


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