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.

[Harold Sukhbir]

Of course, you gave some great points on redundancy vs failure points TC. I was leaning more to the trend of the ongoing blog discussion. Kobus on the one hand is adding an extra pump and utilizing cascade/trickle and getting respectable DO levels, on the other hand, I made recent additions of 2 pumps, 1 air pump, and a swirl which sucks huge amounts of air to the bottom of a 14" deep raft and the same happens with the out flow pipe of the raft returning to the FT, I have the same high end water temps as Kobus(24C-28C) and my DO is around 6-7ppm. This only shows to me the merit of that method of aeration as compared to air pumps or air bubbles in the water column.

[TCLynx]

I like the idea of getting most if not all of our aeration from water flow and gravity, I'm going to point out that avoiding additional means of aeration because it could be another failure point is not actually protecting your system from devastation by failure.

 

Let me explain,

say system one has a single pump that is providing extra flow to be able to not only circulate enough water for filtration but to also spray or cascade extra water to provide enough supplemental aeration to keep the system going on a backyard scale.

 

Then system two has two pumps.  One is a water pump sufficient to provide enough water flow for the needed filtration and some small amount of aeration in the process and another pump (could be water or air whichever is available and efficient in your location) providing supplemental circulation/aeration to bring the system up to a reasonable backyard level of dissolved oxygen.

 

Which system is more prone to failure?  Perhaps system two because there are more components.  However, which system is more likely to have a sudden fish loss due to a single pump failure that isn't noticed within a few hours?

It can be a great backup to have some redundancy by say plugging things into separate circuits if that option is available.  Or in some cases the single pump might be too big to run on a battery backup while one might be able to afford a battery backup to run one of the pumps from system 2.

 

So is adding another device really creating more failure or more redundancy?

[Harold Sukhbir]

Hi Kobus, Didn't realize your cascade/trickle method generated such high DO levels! Using gravity to add oxygen, as we have inherent in AP design already, seems a logical step. Logical because as we include additional mechanical devices we open ourselves up to extra failure points and add maintenance to our systems. I'm not sure that this may be the appropriate place, but quite by accident I've recently discovered a method for adding huge amounts of aeration through the use of gravity. Whether the design targets low tech situations or not, i think the method produces enviable Do levels. Well done!

[Kobus Jooste]

What I mean with "the tanks we typically use in aquaponics" is the depth and volume.  Typically 1 meter to 1.2 meters deep.  The reason why I ask whether you have data on fine bubble diffusers' ability to dissolve oxygen into a water body is because I would like to know if there is data explaining whether the air bubbles entrain water from the bottom to the surface for X% and diffuse directly into the water for Y%.

 

I tried to set the scene very carefully for the conditions under which I operate my systems, but perhaps it was not clear enough.  Tilapia in a 1000 liter tank at max stocking rates for AP, and fast cycling gravel beds.  In winter I am at saturation and now, high summer, with the water at 26 degrees Celcius and after five feeds (they love to eat and I'm always around most days) my DO was 5.2 - around 65% saturation.  This is not a commercial unit, where I know this DO level will not be acceptable.  I design for rural, low tech environments.  We do not have mail order aquaculture supply stores here, and we have to develop simple methods of doing things.

[Ryan Chatterson]

Im not sure what you mean by percentage of air diffusion...Lbs of O2 per hour? I don't doubt that in your situation you are able to get good results from a water pump as your primary means of aeration, I just wanted to point out that it is very dependent on the specific build you did, your fish stocking density, temperature, etc and there are a lot of other factors that come into play. I just couldnt say " in the type of fish tanks we practice aquaponics, we can operate perfectly well in the manner I described" because everyones set up is different. Id say its more of a "in the type of fish tanks I practice aquaponics in, I can operate perfectly well in the manner I described."

I think you do great stuff Kobus, ive just seen a lot of failures and dead fish from water pump only aeration methods and wanted to point out that it is very dependent on the exact set up. :)

[Kobus Jooste]

Ryan - can you please quantify for me what percentage of air diffusion you believe happens from bubble to water if you are using air stones at the base of a tank.  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.  I can take my water to oxygen saturation like this.  I accept that you may have knowledge of more effcient air pumps than the piston-type I used before, but I am still comfortable with my assertion that in the type of fish tanks we practice aquaponics, we can operate perfectly well in the manner I described.

[Ryan Chatterson]

"That trade-off is there, but as I explained, the capacity of a 45 Watt pump gives more aeration than a 165 Watt air pump, all the while it is doing the water circulation as well. Makes perfect sense for me"

45w vs 165w isnt a very fare analysis. This is highly dependent on water depth and your method of getting air into the water (cascading through a degassing tower, inline venturi, waterfall, spray bar, etc). On the other end, type of diffuser and bubble size makes a huge difference in O2 exchange when using an air pump. In a deeper tank, watt for watt a water pump couldnt come close to the aeration performance of a piston/linear air pump or even a high pressure blower when you get into larger volumes or water. At shallow depths surface aeration is more efficient but as you get deeper into the water column, bottom based aeration takes the cake. 750gal/hr pump @ 45w (which through a 3/4" venturi produces 0.28cfm/hr of semi large bubbles) vs a linear air pump (50w) producing 2.6cfm or small size bubbles...you'll definately see a trade off as you get deeper into a tank.

[Kobus Jooste]

LOL Harold.  They'd be a bit bulky there in my 1000 liter tank!

[Harold Sukhbir]

Kobus I've never noticed an AP pic with a paddle wheel in it, probably got a divorce from this marriage?

[Kobus Jooste]

That is a very important point to underline and it is one I tried to make in my response to greener.  If you look at Dr Lennard's calculator, the water flow per hour recommendations are typically between 1/3 and 1/2 of tank volume per hour.  I do way much more than that, but as the pumps are cheap and they suck very little power, I was comfortable making the shift.  I'm yet to get serious management problems from my choice of method.  Greatest problem turns out to be the rainwater! :)