The oxygen debate

[Kobus Jooste]

In previous blogs, posts and discussions, I have often stated that I am a fan of mimicking nature, and of attempting to recreate the aquatic ecosystems I study in my AP set-ups.  I do not think that my thought patterns are likely to go mainstream any time soon, but I am happy to share my experiences with people on this forum.

 

First up, I want to put my views into context.  As an ecologist with a keen interest in aquaculture, as well as a fish keeper for the last 25 odd years, I have a large amount of experience with the type of equipment that we typically associate with hobby and industry systems.  With aquaponics being a marriage between aquaculture and hydroponics, it is also natural that people look at the technology used in these methodologies and naturally assume, or anticipate, that they will all be required in aquaponics.  As with all marriages though (especially ones where both parties have accumulated some “stuff” over time before settling into the new partnership), some things fall by the way side eventually (Sports gear, hunting trophies, semi-precious and interesting rock collections…………).  My very first attempt at aquaponics is a good example of the newlywed scenario – all the clutter in there and the couple still deciding what stays and what goes.  The picture below is of my first unit.  Sump, fish tank, aerator, beds and some crops.

 

 

One of the things I immediately (or almost) wanted to toss was aeration.  At 165 Watts running 24/7, the air pump was providing oxygen to the fish tank and rafts, which was a plus, but it was tripling my energy consumption and not tripling anything else as far as I could see.  This has always been my way of thinking.  Cost benefit analysis meets ecologist.  I’d rather have slightly slower growth than paying for all that power to noise and bubble conversion.  Before I get to the important reasons for my mind shift, I would like to recap some of the aspects of air and water that helped me along.  As a young ecology lecturer, I was loaded up with all the boring lecture modules the senior lecturers did not want, but one thing to look forward to was animal adaptive physiology.  Here we dealt with animal respiration in air and water, and gas exchange between the two media.  While I had not touched the subject matter for a few years, something in the back of my mind started nagging me.  I dug up one of my favourite books from the University era – Knut Schmidt-Nielsen’s Animal Physiology.  To condense this trip down memory lane to a few salient points:

 

1. The amount of oxygen that will dissolve into a water body is directly dependant on the amount of atmospheric pressure experienced by the water body
2. The amount of oxygen that will dissolve into a body of water is directly dependant on the partial pressure of the gas, and the volume of the gas in the atmosphere (% composition)
3. To complete Henry’s Law, you have to throw the temperature of the water into the equation.

 

Considering the fact that oxygen is not a rapid dissolver, and that we are working with normal atmospheric air concentrations of oxygen at normal atmospheric pressure (I’m almost at sea level, but unless you are seriously high up somewhere the change in partial pressure will not be mind blowing), that blower going wild in the background and ruining the sound of trickling water is only going to assist the movement of around 4.5 ml of oxygen into a liter of water at 15 °C (100 mm Hg pressure).  In the end, the natural saturation points are a bit higher, with the best DO levels I can hope for sitting at around 8 mg/L for winter temps and between 5 and 6 mg/L in summer.  What an utter waste, considering that you are basically only disturbing the water / air interface to allow the saturated surface water to make way for water depleted in oxygen from deeper down in the water body.

While the information above was highly condensed to aid in getting a few points across, the impact for me was very important.  I can use the fact that I already have moving water in my system to replace the air blower’s role in moving water around.  PLEASE note that I completely disregarded the amount of plant growth that I may be sacrificing in large rafts with this argument, as I am experimenting with smaller systems here.  I currently have three systems (one still waiting for its grow bed) that is devoid of aerators and are going fine.  The smallest is a 300 liter aquarium, and the largest is a 2 500 liter circular tank.  These are pictured below.

 

 

 

In all my systems, trickling or cascades from return flows are the only surface agitation I have.  I have taken the IBR tank micro system to oxygen saturation level with just the cascades.  My argument is simple.  We create agitation of the water surface, an air / water interface or altered partial pressures / oxygen mixes to get oxygen into water.  In AP, we typically do not use pressure vessels or pure oxygen, thus it is all about agitation and the air / water interface (plus your temperature).  I have kept ornamentals, koi and tilapia happy with my way of doing things. 

 

So what is my way exactly?  Take a look at the close-up of the IBR tank’s water surface.  All that disturbance is the result of a large amount of water pump driven agitation. 

 

 

Water pumps are very efficient these days.  A 45 Watt unit can move over 3 cubes an hour, and you can use all of the redundancy to create cascades in your fish tank.  You must also remember that every time your gravel / media bed drains, you create a huge air / water interface.  Aquaculture don’t have this feature at all.  I have two ways of getting the cascades built into your system.  One is trickle towers with no plants, or the other is just to turbo-charge your media beds.  In a small space such as my home system, the grow beds flood and drain every 5 – 8 minutes all day long.  The plants are alright with that.  On a larger system, you can have towers above your fish tanks that fill and drain or trickle if you want to have slower rotation on the gravel bed cycles.  On commercial scale, I have ideas but these have not been tested, so I will keep them to myself for now.  On a small scale, however, I think I have done enough to convince myself that with moderate AP stocking densities ( I run my home system at the maximum recommended stocking rate of Dr Lennard’s calculator) aerators are at most a useful back-up if the pump goes belly-up or to run off a battery as a power failure back up unit.  I do not consider them vital on daily operation for my systems any more.  My systems reach a water temperature in summer of 30 °C, and I do not loose fish.  Plant grow well enough for me.  This set-up will have to be tested on trout or catfish that are less robust, but for what I do, I do not need an aerator at all.

[Raychel A Watkins]

I agree Kobus and I did not shut off my blower but I am trying to get to the point where I have confidence in the aireation that I do have from enough cascading water.  Keep up the good posts.  I always learn something.  I learned from you about the cascading water and saw it proved out in my own tanks when the blower went off.  Only tanks with cascading water had no deaths.

[Kobus Jooste]

to continue:

but: also understand that these rules of thumb are safe starting guides and not rigid.  We find new approaches all the time and if we banish them to specialist discussions, no-one will learn.  I'd hate to suggest that someone would just run off and shut off his blower because he saw my system does not have one.  Then he or she did not read and did not understand and did not learn.  people don't just go plugging gas-burning cars into electric outlets after they read that a car can run on electricity do they?

[Kobus Jooste]

O how I'd love to be able to potter around a mountain stream now!

 

But seriously - Harold's comment below highlights a thought of mine that I will try to get out VERY diplomatically, as I am a huge fan of the friendly nature of this forum and the people in it.  People are not supposed to be considered stupid.  I dislike "Do not try this at home, kids" labels being slapped onto my theory the moment it goes into a general post rather than being inside a discussion group.  With this I do not mean comments that question my arguments or relay an alternative position with clear arguments.  I mean the "people that are new here might just think they can toss aeration if they read this" concerns.  The people that are new here should be encouraged to learn how things WORK, not be told what to do and slavishly follow because they do not know a nitrate from a lime flake and they just want the fish to be happy and know nothing about water chemistry or animal physiology.  Only from a good understanding of what I am dealing with did I start messing with things.  My training helps, but I did not just wake up one day and decide to screw the theory and do it the maverick way.

 

Theory is just that.  Like people going ballistic at the word "evolution".  It is a theory. Not a rule, not a Law, but a theory and as such, not holy grail.  It is the summary of CURRENT UNDERSTANDING, with the acknowledgement that in the future, it can be augmented or even totally rejected.  No science is rigid and even Einstein's mass-energy equivalence (E = mc2) has been questioned, with sound arguments as to why (the speed of light through a vacuum (c) is being questioned as a constant).  Are we to slip back to a time where dinosaur fossils were considered "placed here to test our faith" or are we going to accept the startling realization that such discoveries brought?

 

I would like for people to get to a point where we accept that rules of thumb are good, but

[nathaniel taylor]

I am of the study nature type as well. While snorkeling in streams, I spened a fair amount of time watching. I too find this fascinating and look forward to your developments.

[Harold Sukhbir]

At this point in my system design i find that the calculator loses its compatibility. The net/swirl filter removes how much solids exactly? how much nutrient stays in the system from allowing solids to sit in the net for 7 days? The worms in the beds mineralizes how much solids? How much more nitrification does the pea sized gravel generate as opposed to 3/4 inch or hydroton?

The calculator, while being a great tool, may not be able to be adequately sufficient for use in hybrid systems and their different combinations and the many varied applications of these.

The info you gave helps to gather a clearer image of  oxygen transfer in water.

[Kobus Jooste]

Harold: In terms of how I treat oxygen diffusion into water, it is all about the surface area of the FT coupled with the degree of disturbance thereof and the level of oxygen saturation at the surface.  Whether you draw from the bottom, pump from the bottom or have an air stone at the base, the effect for me is the same - you move a column of water most likely depleted in oxygen up to the surface, where you most likely can have a body of water that is close to saturation level.  If you just leave a tank full of water alone, the surface layer will become saturated in time, and because the movement of air into water is so slow, you will get stratified oxygen levels going from saturated at the top to low at the base of the tank.  The key is to disrupt this, and it works with blowers or with aggitation.

 

Another reason I am not all that fond about air stones in a smaller tank is the issue of solids suspension.  We know that in bio-floc theory, having these particles suspended in the water column becomes the goal.  In my design, I want them on the bottom of my tank or in the sump filter.  Having air stones at the base of my tank will put the lot into suspension and mess with the circulation patterns I have set up.  The way it works now, the large particles get moved from the base of the tank into the grow bed, and the fines get trapped in the sump.

[Kobus Jooste]

While it seems as if you would like me to state for about the 5th time that I have a fast-cycling system, I would like for some people reading the thread to jump in and tell me about their interpretations of Dr. Lennard's calculator.  If I plug in my system's values, including the high protein content of my food, I still only get a required flow rate of 400 liters per hour.  Now I do not know if there is made mention in any of the accompanying documentation for his calculator on minimum aeration requirements, but if I make my system run on his recommendations, it will take almost 3 hours to turn my system over once.

 

there seem to be a slight syagnation of thought IMO in many ways when there is an attempt to transfer aquaculture operating theory to aquaponics.  While it would certainly be great to turn a system over at least once an hour (mine is faster) it is clearly not considered the rule in all situations.  From past discussions I know that you are cautious about inexperienced people taking a post out of context but at this point in time, I would like to know if your adherance to wanting to drive your points home are for the sake of new operators or because you do not like to see theory breaking away from "standard methodology" being put forward?

 

I am all for standard theory, but my theory standardizes solely around critical water conditions.  All comers to the industry should learn to test for and maintain these at optimal levels, based on the fish they keep.

 

Critical considerations are:

1. Nitrogen management - people need to know about safe levels of all the different forms, and the interplay of temperature, pH and NH - variants

2. pH and related alkalinity management, and how this influences nutrient availability and the bacteria that drives the nitrification process

3. DO levels related to the type of fish that are kept, and all the processes that demand oxygen in the system such as mine

[TCLynx]

I totally agree with you guys about trying for simple and easy parts and options for your locations.  Notice I didn't say the second device needed to be an air pump.  I was just pointing out that some division and redundancy need not be seen as "more failure points" but as "backup".

 

Kobus, in your systems would you say that there is always water moving in your fish tanks?  This might be important to point out to the people who are designing systems on a timer operation who are still under the impression that they don't need any form of supplemental aeration (supplemental aeration might be an air pump but it could also be a secondary water pump creating water movement or running a spray bar.)  Some one who designs their system with only the bare minimum one water turn over in the hour and that happens in say a 15 minute time block and they have a siphon on the bed to drain it quickly to boot will leave the fish tank with no aeration for most of the hour.  It is these systems that I feel need something supplemental to provide the constant water movement.

[Harold Sukhbir]

Kobus said; "My pump is mounted in the base of the tank, thus sucks the oxygen depleted water up, dumps it into flood-and drain beds where some oxygenation takes place, and then that water gets cascaded into the tank on the surface where a massive amount of aggitation happens."

 

This is good information. We can design an AP system from just looking at some pics and reading the rules of thumb, which is great by itself. Designing AP with purpose, with understanding the operating principles/science applied in all areas crosses a line toward greater efficiency. The above quote actually draws a picture of the efficient movement of oxygen in a system. So i get that oxygen is actually pulled from the surface through the water column to the pump and during the transit get's depleted only to repeat the cycle. Some Ap'ers may have available to them a surface pump and may rig a surface entry/exit design for convenience but can achieve better effect by piping the outlet to the center base of the FT. I think someone should start a blog about the hardware/principle relationships of varying designs. I have learned something here, may be common knowledge for others but we cannot take it for granted that by physically describing an operation the reader is aware of the principle underlying it.

[Kobus Jooste]

TCL - If you have a look at some of Harold and my exchanges in the mixed design aquaponics you will see that I am contemplating multiple pumps at the moment.  This whole exercise reported on here had a simple beginning - wanting to cut power use.  It is not as much an attempt to run a whole system off a single device.  I put my ecology background into practice and found a way to make the water pump run the circulation and the aeration.  I'm not advocating the tossing of the blower for all and sundry.

 

I support your arguments for redundancy, but typically in my case, and in the areas that I am planning designs for, back-up is more likely going to have to be solar or generator, rather than more hardware.  Over in the US, I suppose almost any settlement is only an overnight shipment away from component and spare part nirvana.  Over here, I have been in places where you wait 7 days for a permanent marker and getting a bread is a 80 km trip.  Components will be in very short supply, and spare parts likely will not exist.  Power supply is eratic and so is water.  Under these conditions, one tries to make a system function as simply as possible.  For me, this sytem was to prove that we can slice the power use, component need and spare part headache of blowers.  People report very efficient air pumps in this thread, but looking at the UVI design, 1/2 hp water pump and 2 1/2 hp air blowers give you a clear idea of why I went after reducing air pump use.