Pond Boss Forums AERATION Systems questions Air diffuser selection

I'd like to start a topic to discuss types of air diffusers to use. I noticed that many folks on this board use membrane type diffusers and I've even seen some claims that they are the most efficient. I'm not sure what is meant by efficient but I'd like to learn more about why some of you have chosen the membrane systems. I've got one of each in my pond. The pond consists of two parts separated by a long narrow channel. So I put an air stone diffuser in one side and a good fine pore membrane diffuser in the other. Most lake management companies I consulted with did not recommend membrane type diffusers at all for ponds with fish because they are not efficient in transfering oxygen. I would think this means that one would then need more power and more expense to do the job. Some data I looked up says that a particular membrane diffuser placed 10 feet deep with a 10 cfm compressor will support a BOD equivalent to 950 lbs of fish and cost $.85 per pound per year to run. In contrast a mediocre air stone diffuser with the same compressor will support 1350 lbs of fish and cost $.57 per pound per year. That's 40% more fish for 30% less cost. The air stone sounds more efficient to me! In my pond I can see a world of difference. My air stones require only 2 cfm compared to my membrane that I'm running at 3 cfm. They are both at same depth and the air stones create a boil on the surface that is twice the diameter of that created by the membrane (move more water) and the bubbles coming from the air stones are visibly more numerous and much smaller. Comments?

BZ,

Very good and well thought out questions. You can tell that you are an engineer.

The efficiency rating you are quoting are the actual efficiency in clean water test. All O2 is removed and the diffuser is turned on to see how quickly this water reaches saturation at temperature.

The airstones are more efficient in this application. If Aquaculture is your deal and you plan on doing the required maintenance on the stones and replace them once the acid eats away at their integrity, then the stone is your choice.

When the companies you previewed say that the membrane is more efficient, they are speaking in turnover rates i.e. lifting the water from the bottom to the surface in GPM.

I have personally experienced both types of diffusers in a laboratory setting and in GPM circulation at equal depths and cfm, the membrane type out performed the stone.

This is due to the size of the bubble. The membranes by one particular company start out at .5mm and go down or up depending on the cfm you are providing. The stones start out at 3mm and do the same with the amount of cfm provided.

In either case, it was explained to me by another of the companies that the oxygen transfer in the bubble to water is less than 5% unless you are using pure 02.

In a lake situation like yours, once the water reaches saturation at temperature, it boils down to which diffuser will maintain that constant bubble size and circulation rate with the least amount of maintenance.

With you method of placing each type in your pond and not measuring any water quality before, during and after installation, it is like taking a polaroid snap shop of the moment compared to an xray. What you see on the surface may be very different at the bottom.

I would suggest testing for oxidation reduction potential, 02, temp profile, nitrites, nitrates, ammonia, Biochemical oxygen demand and bottom grabs for muck type and benthic organisms to truly know which diffuser is performing better.

Most pond owners do not have the equipment needed to do these test so unfortunately it is up to the company to truthfully give you the information you need to make the informed choice of what type of diffuser is best for your application. Also a forum page like this one helps to give a conduit to ask the questions you did and allows me to give my answer from my background in aeration for over 14 years.

Cary, thanks for the reply. You've obviously got a lot of experience and I've got more questions. It's interesting that according to your experience the membrane diffusers move more water. It was explained to me, by several other aquaculture companies, that smaller bubbles both move more water and dissolve more air because there is effectively more surface area in contact with the water. The smaller bubbles carry more water with them because of surface tension and the fact that more bubbles can physically trap more water between them and carry it up. Is this true? Then again I have wondered why membrane diffusers are used in waste water treatment over air stones. The key to waste water treatment as I understand is both water movement and good oxygenation. I've reasoned that perhaps they use them solely for maintenance reasons and if there were airstones that never clogged they'd use them? You also seem to verify that in terms of just getting air into the water the stones are better and that also makes sense if they produce smaller bubbles. On the maintenance issue I've had both types in my pond for going on 5 years with no problems with clogging, maybe I'm just fortunate.

I'm going on three years with an airstone diffuser in place without cleaning (have figured I will just replace it with a rubber membrance type). It still bubbles, but it looks like maybe the bubbles are bigger now (?). What passes for logic would indicate that small holes in the stone woud clog first and bigger holes would stay open - it would keep working but less efficiently, with bigger bubbles.

You know as I've thought about this I realized that I have a different situation and different goals perhaps than you Cary are used to dealing with. In regard to your comments about the water getting saturated and after that water movement is more important. I think my situation is different. I installed a 1/2 hp 5cfm aerator because I plan to expand my pond. It has already been expanded 3 times to get where it's at. My pond has two larger oval sections connected by a narrow channel. So I put a diffuser in each main section hooked up to the same pump. Since I don't need all the air that this system puts out my goal is to run the system for the least time possible to save cost. In this scenario my need is to keep the DO up high enough while avoiding running 24/7 and therefore I do not have the situation you describe where diffuser efficiency doesn't matter. The more efficient air stones should allow me to run less. My pond might be cleaner if I ran all the time since max DO and max movement tend to allow everthing to work better. But, when my system is on it's large output really moves things around. This is my situation and hence my contention that air stones are better for me than rubber diffusers. I run my system for two 4 hour periods each day. I don't have equipment to measure DO but I've never had a DO problem that shows itself. This is all seat of the pants and I'm afraid I'm way behind many of you in the pondmeister science.

Airstones are not always more efficient than membrane diffusers and vice versa. In measuring efficiency and comparing diffusers you have to know the bubble size of each diffuser. Some brands and or styles of stones AND some membrane diffusers produce large bubbles. Bubble size will vary from one manufacurer to another. Not all air stones are created equal. Some membrane diffusers vary based on design and construction. The thickness of the membrane, the size of the pores (slits) and the amount of cfm to the diffuser will affect bubble size. Lots of variables to consider when discussing this topic.

Clogging of the diffusers is caused by mineralization of dissolved solids AND biological growth at the pore sites. Water chemistry, biological inhabitants ( periphyton ) and water clarity that are specific to each pond will determine how fast a diffuser will clog. Diffuser design can slow down but not eliminate the clogging affects.

We have airstone diffusers on our farm ponds. We didn't go with membrane diffusers because we want to be able to seine over them without problems hanging up.

From what I've seen, and we check DO twice daily, is that direct oxygen transfer with airstones (and probably with membrane diffusers as well) do very little to prevent a daily depletion, and do very little to actually raise DO. By daily depletion I mean low oxygen at night. Since we have the ability to manage cfm at each pond, I've actually experimented with the diffusers and checked DO accordingly. No real difference in DO even when diffusers operating at up to 5 cfm in 1/2 acre pond. Mechanical aeration like agitators or paddlewheels are most efficient at directly adding O2 to a pond.

However, the bottom diffuser systems cycle water and will prevent the dreaded "turn over" if used correctly. A turn over can be much more severe than a daily depletion. Agitators and paddlewheels won't destratify like bottom diffuser systems unless the pond is very shallow.

Moral of the story...I wouldn't expect a bottom diffuser system to actually raise DO in most situations, regardless of efficiency. I would expect bottom diffuser systems to prevent sudden turnover.

Overton.. interesting comments. If I understand what you're saying it means that diffusers of any type only serve to prevent turnover and I would also assume you see a benefit in also preventing low DO layers? When you experimented and measured DO levels were you measuring only near the surface? Perhaps air stones on the bottom add oxygen but not so much at the surface? And how about distance from the diffuser? Seems like that would be hard to figure how to compare since each type probably carries the water and the air different distances. I think a real scientific way to compare would be to take readings at several depths and numerous locations throughout the pond. This way you could map out DO levels with each type system. I'd really be interested to know if that's what you did.

bz,
It has long been stated here that bottom aeration directly adds very little DO to the pond. As Overton states here: To further explain his last post, he stated he has lowered the output of the compressor which would produce a much smaller boil, (fewer bubbles of air) & raised it to max.
All this while checking DO & there was no change in DO.
As Todd, Bill Cody, Ted & others have stated the benifit of bottom aeration is in mixing water & therefore preventing stratification & low DO in the bottom of the pond.
It is an understandable misconception that bottom aeration directly adds DO.
It is used however because bottom aeration is much cheaper in operation in accomplishing complete turnover of water in the pond.
The air bubbles, as they move up through the water column, cause the water to circulate bringing the bottom water to the top. This constant movement of bottom to top water eventually circulates the entire pond.

Good basic information so far. Tests by employees at OvertonFisheries verifies that bottom diffusers do not add "a lot" of DO to the overall pond oxygen budget.

When reading about efficiency of diffusers one has to be aware that efficiency of a diffuser can apply in at least two areas. Efficiency of the transfer of dissolved oxygen to water AND or efficiency of the amount or volume of water that a bottom aerator can move per hour based on a given cfm of air.

The main benefit of bottom aerators is they can "fairly cheaply" and efficiently move the water with poor quality out of the bottom zone where it developed this poor quality. Bottom aeration gets the deeper "bad" water to the illuminated surface zone where it naturally gets "refreshed" and oxygenated by several means including but not only limited to oxygenation by algae and higher plants and diffusion of several gases by mixing with the atmospheric air.

I'm having trouble understanding why, if the above is true, then overall average DO levels in a pond should rise with aeration. Moving low DO water to the surface where it is oxygenated should increase measurably the DO levels in a pond. The only explaination for not doing so is that the water that moves to the bottom looses its DO making the pond overall stay the same...and if that is the case, why do we aerate?

With all respect to the posters, I question the belief that DO does not indeed increase with aeration. If you move large volumes of water (i.e. move the entire pond water volume a couple of times a day) to the surface where it is oxygenated, the overall pond DO levels has to increase. Perhaps we are not measuring overall average DO, only surface or shallow water DO.

Please explain where I'm wrong? Thanks.

ML,

Shooting from the hip here so take it for what its worth. Using bottom diffusers as my reference, could it be that by changing low quality water to that which is suitable to support more organisms (zooplankton and phytoplankton) the NET effect would NOT result in a significant increase in the DO levels of the pond??

Russ

It does. Overall average DO levels do increase with aeration.
The misconception has been that the bubbles produced by the diffuser directly "mix" with the water raising DO levels. It is understandable one would come to this conclusion but the bubbles just impart flow to the water causing the effect as Bill describes so well.

Unless I'm missing something, there are only two outcomes to the question of does aeration raise DO levels in a pond? Yes or no.

It either does or it does not. If it does, then the means by which it accomplishes that whether it is by the bubbles or by moving bad water to the top where it can get oxygen or both may be of interest in determining and assessing various systems for efficiency.

However, from these posts it appears that we have two differing opinions from two people that are highly respected on these matters, Todd and Bill.


Todd has what sounds like extensive measurements that say it does not raise DO levels in his ponds.

However, if I understand Bill Cody correctly, he is saying that "Bottom aeration gets the deeper "bad" water to the illuminated surface zone where it naturally gets "refreshed" and oxygenated. If it gets water that was not oxygenated, oxygenated, then it is raising DO levels.

So, Todd and Bill which is it? Does aeration raise DO levels in a pond or not?

Seems like a relatively easy question to answer with specific actual measurements. Hopefully, this isn't one of those "regional" dependent questions.

I will ramble some more here to try and explain or confuse this topic. I did not take the proper time to best organize my thoughts.

Note that "overtonfisheries" (OF) mentions that no "real appreciable rise in DO" or "no real rise in DO". He does not say no increase in DO.

ML - you have to realize that several things are goin on here. Plus I will assume some things for the discussion.

The tests by O.fisheries were not in laboratory settings nor were they controlled, replicated tests. I think he is saying that from his tests in his particular type of ponds that basically bottom aeration does not significantly or noticably increase the DO in water of the overall pond when he measured DO using the same diffuser and with different CFM inputs. There are several possible reasons for his conclusions.

Water in the upwelling boil usually experiences "some" slight oxygen increase depending on depth (distance bubble traveled) and bubble size. Atmospheric air is only abt 20% oxygen. I think this DO increase will often not test greater then 1ppm due to depth of the pond and the current DO conditions at the time of test. Such things as temp and percent DO saturation of the water will some have impacts on this.

The upwelling water is fairly quickly diluted into the surface waters which are often oxygen rich. Dilution begins to recharge the dissolved oxygen in the upwelling water. DO recharging is fairly slow. And again the speed at which this happens will depend on current surface water chemistry and the condition of upwelling water. If surface water contains dangerously low DO (low % saturation) the reoxygenation of the upwelling boil will be minimal and usually quite slow (greater than 6-12hrs). Pond size, water flow (boil size / volume) and other things will have big affects on this.

I assume "OFisheries" tested DO in primarily the surface waters at 1ft-2ft deep. Many hatchery ponds are shallow and bubbles are not in contact with water very long to allow for much oxygen transfer as bubbles rise to the surface. He did not mention the DO in bottom waters.

MLark quote - "The only explaination for not doing so is that the water that moves to the bottom looses its DO making the pond overall stay the same...and if that is the case, why do we aerate?"
Once water has a high % saturation of DO, the DO loss should not be rapid or great during the water's return movement to the bottom UNLESS the DO "sink" or DO reserve is low at the bottom. The goal of bottom aeration is to build up and or maintain elevated DO conditions at the mud water interface in the deep zones of the pond through constant or frequent redilution or water renewal of the deep zone waters. Once you get adequate DO at the bottom you want to try and maintain DO at 3-4ppm or above. This level of DO favors "healthy" biological life processes.

Continual or frequent mixing will do this water renewal for you. The rate of DO loss at the mud water interface will determine how long you mix (aerate) or the required frequency of the mixing. Most people overmix, to be on the safe side.

Moving low DO to the surface will dilute and lower the DO levels near the diffuser and in areas surrounding the diffusers. Same thing happens with water temperatures. Renewing the DO naturally in the upwelling bottom water does take some time which is why bottom aeration usually will not stop or help a fish suffocation problem. How fast reoxygenation occurs depends on numeous things such as photosynthesis potential condition, wave action, water clarity/stain, time of day, weather conditons, current pond biological oxygen demand, AND water turnover rate.

Bottom line is, I think most of the reoxygenation of the bottom water is done by dilution and photosynthesis in the surface waters and this takes time. Most (biggest percentage) of the oxygen in the surface waters (illuminated zone) is generated by photosynthesis, algae and or underwater rooted plants. That is why when you have a die off of phytoplankton or submerged rooted weeds your pond can SOMETIMES quickly produce a fish die-off, especially if DO is only present in the illuminated zone (surface water layer).

Let's not get too deep here. I think a controlled labratory experiment would reveal a rise in DO levels, but it must be impossible to control all parameters in the outside world in order for a test like this to be scientifically sound.

Mechanical aeration forces oxygen into the water, no doubt it directly oxygenates pond water.

Bottom diffusers mix water more than anything. Assuming you have no bottom diffuser, the oxygen gradient may reveal 10 ppm 02 at surface and 1 ppm at bottom. This gradient is due to oxygen production by phytoplankton at surface and statification. With bottom diffuser you may see lower DO levels at bottom and higher DO levels at surface as these two layers are mixing. Mechanical aeration in deep ponds usually only adds oxygen to the surface strata of the pond.

I might add that with this mixing effect DO levels are more stable. We have a 1/4 acre crappie pond right now without aeration of any sort that is fluctuating from oxygen supersaturation during the day to depletion at night.

Also, our ponds contain more biomass of fish than most recreation ponds and heavier plankton blooms.

All that should be drawn from my measurements is as follows:

If you have bottom diffuser in a deep pond and don't have dense plankton bloom (12" or less vis) then you'll probably not have oxygen depletion.

Mechanical aeration like surface aerator will not prevent a turn over in a deep pond.

Most ponds are "deep ponds" compared to our 4-5 ft max farm ponds. In these deep ponds I believe bottom diffuser is still most efficient, most economical, and overall best way to go.

I don't regret for one second having bottom diffusers in our ponds, but we still have to use surface aeration on emergency basis....like today with cloudy/muggy weather.

You guys are way over my head here. But from reading your discussions it appears that perhaps the way a bottom diffuser may increase DO if at all is by bringing water to the top so it can get some oxygen either from algea or from the atmosphere. I wish I had a DO meter to play with (too expensive)! The more of this stuff I understand the more I enjoy my hobby.

Bz,

I'm with you. The only thing I know is what I see in my pond, and the pond water looks better, more alive, and appears to be a healthier, better place for the fish than the same pond without aeration.

Whatever the cause of that (DO or mixing or whatever), I like aeration. Interesting thread, Bz. Thanks.

These have been some great posts as Ive checked in on this thread the past few nights to see how it was coming. I think everyone is pretty much on the same page. I will add just a few thoughts and try not to muddy the water too much.Bottom diffusers of any type will increase the DO levels on an average unless you are already a very high percentage of saturation of DO from top to bottom. I say as an average as the upper saturated DO level in most water will come down some when being aerated(with bottom diffused aeration) as the lower low DO water is pushed up.The mention that diffusers of any type (airstones and membranes) put little DO in the water but add to the lift or vertical current to expose the lower water to the surface is accurate. One of the differences in diffusers and aeration systems in general is the ability to "lift" the water to the surface to be oxygenated.There is a lot of "ROT" (rules of thumb)that come into play. One of them would be that a warm water fish pond that is high nutrient and mature may need to be turned over 1-2 times per day to achieve full benefit from aeration.This is more than what it would take to keep most fish alive if DO was the only concern.If one wanted to maintain 0-2ppm then no aeration in the mentioned pond would in time be these levels on the bottom for certain.At 2-4 ppm a few kinds of fish and insects can survive and 4-7 ppm is acceptable for most all warm water fish.7-11 is great for cool and cold water fish. As far as a percent saturation basis goes figure that below 60% is considered poor and water may be too warm or bacteria may be using up the DO.60-79% saturation is acceptable for most aquatic organisms and 80-100% is excellant. With this in mind it can require different turnover rates for different goals. A case study is posted at www.vertexwaterfeatures.com on Heron Cay in Vero Beach Fl and levels of DO.BOD,nitrogen,total phosphate and (ORP) and seechi reading are available on a before and after aeration basis. This is a 21 acre lake with 11 Airstations.I believe this lake is being turned over between .7 and once per day.These results are what is to be expected and are not smoke and mirrors.If you are familiar with Aquatic Eco Systems in Apopka Fl you will see them "Guarantee" they will keep the bottom DO at 5 mg/l as mandated by the Clean Water Act when requested to do so with bottom aeration.(pg 35 2005 Master Cat)In 83 F water that is about 65% saturation. Last Wed I installed 6 diffusers in a 5.91 acre pond in NW Ohio that averaged .7 ppm DO in 12 ft of water (15ft max depth) and just this morning got the startup period over and is now running 24/7 In 3-4 weeks I will have at least 6 ppm or 75% DO in this 12 ft depth and about 7-7.5 in the top 2 feet.This pond is being turned over 1.5 times per 24 hour day.If you know your desired turnover rate once twice or whatever all you need to know is the total gallons in your pond and the turnover rates at given cfm's at different depths per diffuser. Placement of diffuser to assure you are not simply turning the same water over and over and leaving shallows or inlets unaerated come into play.(sizing a system)BOTTOMLINE >>Bottom diffused aeration contributes to the process that allows higher DO levels (than previous) to become established from the bottom of the pond up. Your surface DO will be lower with bottom diffused aeration than it was without aeration. Airstones at given CFM's lift less water then some membrane diffusers and more then some other brands of membrane diffusers.It is difficult to overaerate with bottom diffused aeration but you can overcirculate (cold water fish and high temps dont mix)The higher DO water at the surface does lose some of the DO but the more you turn the more likelyhood the bottom DO will increase. Thus the importance of how many times to turn your water per day. Again great posts from all, Ted PS Bob Lusk (How bout spell check for Xmas)

You can call me a knucklehead if you want but as an engineer I believe something after I see two things. One is the physical evidence, second is the numbers that verify what I think I’ve seen. You all have conveyed enough personal experience that indicates air diffusers don’t add much DO to a pond. This idea is new to me. I have no reason to doubt the physical evidence that you have kindly conveyed but forgive me I still need some numbers. Let’s take a hypothetical situation where we have a 1 acre pond, average 6 feet deep, deepest point 10 feet with a bottom diffuser, 2cfm with average bubble size of 0.5mm. I think we’d all agree that aerators work to whatever degree they might by increasing the surface area of the water exposed to air. So my thought is that it might be revealing to look at how much surface area is exposed to the air pumped through the bottom diffuser in the hypothetical pond. It’s easy to do some math and figure out how many air bubbles are created by 2cfm air and what their surface area is. My scuba diving experience tells me that small air bubbles travel upward in water at about 1 foot per second. This allows me to figure how many air bubbles are rising through the water at any given time. The surface area of these bubbles turns out to be about 1240 sq. ft. This is equivalent to the surface area of a square pond that measures 35 x 35 feet. This is approximately 3% of the original 1 acre pond area. I reason therefore that the 2cfm diffuser is equivalent to adding 3% more surface area to the 1 acre pond but it of course doesn’t change the pond volume. It turns out you can accomplish the same thing by making the theoretical pond 2 inches shallower and adding 3% more area. Even my gut tells me that this is not going to help add significant amounts of DO. But what about the numerous sources I’ve found that show how to calculate the amount of oxygen a diffuser system can add to water and the fact that tank farmers use this to size their aerators? This seems to contradict the idea that diffusers don’t increase DO. Well, the key is size/volume. If our pond were much smaller the diffuser may add significant DO. Let’s say the pond were only 35 by 35 feet to begin with. Now this same 2cfm diffuser effectively doubles the surface area of water exposed to air. In this case the diffuser might add significant DO to the water. We could accomplish the same thing by making the pond twice as big but half as deep. I think we’d all agree that this would improve the ability of the pond to pick up oxygen. This thought process seems to explain not only your observations but also the claims that some ponds do get a DO benefit from diffusers. The key is surface area to pond volume. We’ve all had an aquarium where fish died when the air pump was shut down. My rationale even explains what many of you have described as the main benefit of a diffuser in a large pond. That is that a bottom diffuser needs to move a lot of water to be effective. The numbers prove that the diffuser cannot put much oxygen into the pond directly but I would imagine that giving more of the water time at the surface allows it to pick up air naturally (some day I'll tackle the numbers on that). My conclusion is that surface area is what gives you more DO and that a diffuser cannot possibly increase the surface area enough to make a difference. But it can bring up to the surface water that normally would not be exposed to air. So I’m with you now, we need to move more water, or make our ponds lots bigger. Hmm, which would I rather do?

bz,

No knucklehead that I can see.

I came to the conclusion after reading all this that we are not measuring average DO across the entire water column...if we did it would show a definite increase in DO...that's my theory

If you bring bad water to the top, where it adsorbs more oxygen and you do that over a long time...well my instincts say we are not measuring average DO across the water column properly or it would show an increase in that average DO. We are probably only measuring DO at specific shallow water locations.

My two cents worth on this subject. When I was a kid I had a fish aquarium. If bubbles were coming out of the stone in the bottom the fish were ok--if not the fish would not last long.

Meadowlark,

I take a different slant to this on why you don't see an appreciable increase in D.0. levels in a pond with diffusers. It's because with continual mixing your D.O. levels have reached equalibrium from top to bottom.

I'm with the rocket scientist, the water column in enriched, no if's, and's, or but's.

In laymans terms...the most efficient, most economical, and most reliable method of merging stratification layers is using the sun. Solar powered systems will replace the high dollar electrical units in production now...stay tuned, I'm working on this one !

Eastland,

Remember your friends...I'll be the first in line to buy one of those solar powered systems...in fact, I'll take 3.

(all transactions subject to price and final approval of the parties of the first part, i.e. the one who writes the check)