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:
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.