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.

[TCLynx]

no argument here Kobus.  (I only mention it cause systems are sometimes designed with the bare minimum water flow to turn over the tank volume once per hour and if it's intermittent flow a re-think of the pump for the purpose might be appropriate to avoid needing to also have the air pump.)

[Kobus Jooste]

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.

[TCLynx]

There is a trade between air or water pump to make sure you get the constant aeration needed.  An air pump might not be necessary but then you might have to go for bigger water pumps to get the extra flow needed everywhere.

[Kobus Jooste]

Please do not risk your fish though - as I stated, my system is designed around the needs of the species I chose and tilapia can take a lot lower DO.  In the end, you need to make sure that you agitate your FT water enough, whether by bubbler or by cascade.  I move a lot of water in the tilapia system to cause enough agitation.  Is your set-up capable of doing this?

[Greener]

Great post Kobus, thanks for sharing this.  And perfect timing -- I was just about to order air pumps!  I'm now thinking I might start without them, see what my DO levels are like and then decide.  I'll be raising catfish so it could be risky.  I might be better off with a compromise.. need to consider the cost/benefit a little more.  Anyway thanks again.

[Kobus Jooste]

I think economics are going to be a big driver in both home and commercial aquaponics.  With food prices hitting alarming levels everywhere (our frozen veg prices are up 300% is just a few seasons) farming cheaply is going to become a major issue.  I share your desire to get "cheap" operating systems established, but at the same time are being pressurized by people that want to see entire families living off home systems.  I think that we are not quite there yet in terms of protein production

[Raychel A Watkins]

Really great explanation Kobus.   I had an experience where the blower went off one day and I did not know it.  I lost quite a few fish but I never lost one fish in a tank that had cascading water.  I would love to shut down the blower because I feel it is to great an expense.  I am a little hesitant to do so out of fear.  But I am going to digest what you have written and see if I cant get more cascading water in some systems and then measure the DO for awhile and see if I can exist that way.  I have read with  interest what has been said about the air in the rafts and I put a stone in two different places and so little or no difference.  I am with you I will sacrifice a little more time in growth verses the cost.  One of my ideas is to produce organic (non certified) products at a decent p;rice.  Here a non organic papaya is 89 cents per pound.  An organic can be over 3 dollars a pound.  People just can't afford that much if they are feeding a family.  I have a hope to teach people to eat better they can only do it if they can afford it.  Thanks for the blog it is very helpfull.