Mixed System Design Ratios

[TCLynx]

Well, Kobus Ratios thread is gone but perhaps I can get Daniel to find some of the info from that old thread to re-post here and we can perhaps get some discussion going on the design ratios and stocking numbers for different configurations of mixed systems.

 

Ok so actually it had been a blog by Kobus.  I'll be re-posting the original blog post plus some of the comments that Daniel found for me.

[TCLynx]

• Posted by Kobus Jooste on November 11, 2010 at 7:28am

In this blog I will try to tie together many of the thoughts that have been flying around on my blogs and discussions that followed. I will start by revisiting some important issues that influence my thoughts around aquaponic design, followed by the presentation of arguments in favour of experimenting with component ratios. There has been much talk around these matters. My frustrated thought at one point was something down the line of “I understand that there is a need to have some basic guidelines for people when they start up in aquaponics, but then give them some safe beginning set-up PLUS all the reasons for having the ratios to begin with.” Then the post of Dr Lennard’s calculator came along, and the comments that I saw about it made a very big light go on in my head. I think there are at least two ways people set about trying to rationalize ratios, and if you are from a different mindset, the other ways of doing things appear to be, well, WRONG. If you come from an aquaculture background and spent some time looking at the evolution of AP systems, in particular UVI, you may consider an aquaponic system to be an aquaculture filtration set-up allowing macrophytes in where you may have had to start employing denitrification stages. If you got your grounding in horticulture / hydroponics and you look at some home kits on the market now, they look very much like a hydroponic set-up with fish tossed into the nutrient reservoir. My belief is that it is possible then to say that people in aquaponics either think like a aquaculturalist or a hydroponist.

I am an aquaculturalist. When I see fish and a filter, I think oxygen supply, water flow rates, filter type, media type and surface area required. I think feeding rates and fish food composition. That is why I have an immediate affinity to how UVI and Dr Lennard’s component ratio calculations are explained. Not because I think that these are the only ones that can work, but because I think like these guys do. If you allow yourself to think like an aquaculture person first, and only let the horticulture thoughts peek through once the water hits the plants, you will look at the water movement through filter stages (media or otherwise) and aeration processes in your set-up from the following perspective:

1) How much nutrients have been added into the system.

2) How much metabolites will this result in the water column, and how much sludge can you expect

3) What is the oxygen demand of your fish, and what will the oxygen demand be of the biological agents responsible for nitrification.

4) What type of media am I using.

If you do not address these questions logically, your system design will stumble at the fish tank stage, regardless of how happy the plants may appear. Can you see how these issues above also form the basis of the spreadsheet from Dr Lennard? Your thought process starts at the type of food you opt for, especially in terms of the protein content. Your feed input should be scaled to fish size – again from aquaculture ratios – and the feed application should be done is such a way as to prevent wastage. Ideally you want to deal with metabolites and excrement only, not uneaten food. You could argue at this point that this line of thought is the same as going for a fish to grow bed ratio. This would have worked for my desire for detail if it was not for the fact that through variable water flow rates and different types of media, you could support different numbers of fish with the same tank and grow bed. This may or may not be an “advanced” issue, but in terms of system components, I believe that people should know where they can stretch their system to one day. Nothing is as irritating to me as realizing that what I have got cannot be expanded on modularly, but have to be replaced. There I am a fan of the theory of buying a big tank and adjusting your stocking rates as you expand. If your system turn-over rates are geared for correct flow rates (such as implied in the calculator), then there should be no difference between different stocking ratios in the same tank, but then with different media bed volume to match the fish.

About fish food composition and where the different components go I have written about before, and so have a ton of others that know more than me. While I have papers with differing views about exactly how much of the fish food components go where, you can work on safe figures for waste solids Ammonia as well as dissolved TAN. The figures are also available for the oxygen demand for the nitrification processes, making it possible to calculate exactly how much oxygen will be needed to make your water “safe” in terms of Ammonia and Nitrite. Add to this your fishes’ oxygen demand and you have a clear picture emerging of how much oxygen, in mg/L, that will be sucked from your water on a daily basis. This is one of those “hidden” reasons for having extra water volume in a system. Not just to alleviate cramped conditions for fish, but to have some more oxygen available. Just having a 300 liter sump with no fish in it can have an extra 2400 mg of oxygen at saturation point in my greenhouse.

Getting back to the spreadsheet for sizing your system, we have now looked at much of why the different components are there, apart from the choice of media. Most of us are on gravel, but if you should fill that 300 liter sump with a high surface are artificial media you are going to see a clear reduction in gravel bed needs should space be an issue. It can also be a bit of ecological redundancy if you so desire. Having said all that though, promoting only a single “golden rule” ratio to go by in the end is an incorrect assumption. There are many different set-up out there, and in time I think efforts such as Dr Lennard’s spreadsheet will get refined for those uses. In my mind, there are no one set of ratios, but rather some important operational targets. These are:

1. Required water quality parameters and

2. plant nutrient needs.

THAT IS ALL. Component ratios are based on equipment used, surface area of media, fish stocking rates, food type, and water flow rates to mention but a few. The correct ratio flows around in a three dimensional space and is influenced by your decisions at critical design stages. It is far better to understand these choices than to memorise something somebody said somewhere. Now, in order for people starting out in aquaponics, some people have become fixated on finding rules and ratios without having the foggiest notion of why these ratios are applied, or indeed to which design types they apply best. To go back to an engineering comparison: You have a 1 ton vehicle that you must propel forward at 120 km/h. You have so many choices here that which engine you pick in the end does not matter. 1000 cc for economy, V8 for brute force, Rotary for acceleration , Diesel for torque. In the end, the question was not about efficiency or cost, but simply to reach 120. People will get all emotional about their choice of engine, but if they do not acknowledge that their opinion is based on personal preference and that there are multiple possible options, that person is being closed minded. Now that we have the basics out of the way, we can look at ease of operation and efficiencies, about emissions and lifespan reliability, but still, there are more than one way………………….

I see issues with all the different ratios and rules that I have encountered, even the latest one from Dr Lennard (I’m not getting confused by the metric / Imperial issue – my notes say feed the adult fish 1.5% of their weight daily and his feed input increases in whole numbers, requiring me to do some subtractions) The following points highlight just some of the factors that I consider to be important in my objections:

• · If we go back to founding statements of the technology, it is often described as the marriage of aquaculture practice with hydroponics. To this end I want to ask: Why have we shifted significantly away from RAS filtration component ratios during this evolution of the aquaponics method. Having between 3 and 5:1 tank to filter ratios is not uncommon there. Does not flushing the gravel beds daily swing the component ratios so drastically to the other side in that people now operate 1:4 tank to grow bed systems?
• · I want to know if most people discount the network of plant roots in an NFT system completely when they calculate the surface area of their aquaponic biofilter. We know from Lemna water treatment research that these little plants (duckweed) become a nitrifying bacteria substrate as the colony matures. The same should apply to the roots of our raft and NFT plants. I am now heading back to my previous posts linking my understanding of aquatic system productivity with this question. In a way, I am starting to think that I want to have a “dirty NFT” set-up. This will mimic what is known locally as the River-Estuary-Interface of REI. This is the nutrient powerhouse of our systems, where nutrient laden water from rivers flow into densely vegetated sections at the head of estuaries. The fines pack onto the exposed roots of aquatic plants, where bacteria mineralizes and nitrify the stuff and sends it downstream. Take a look at the picture of strawberry roots inside an experimental “dirty NFT” line of my greenhouse. Now why can we not assume that the entire surface area under water inside the 110 mm pipe will not become a huge “media mat” that can potentially have a far greater surface area than a media bed of the same proportions.
• · Many of the ratios / rules indicate only bed to fish ratios, without taking into account the flow rate of water. I have always worked around rules of thumb in terms of water turn-over (filtration and aeration) coupled with nutrient input into the system. Just having a bed to tank / fish ratio does not explain enough to me.

For me, the next challenge in getting the ratio wars settled, is to define all possible variants of aquaponics, and devise rules and ratios for them independently. I’m breaking many rules for a straight-up set up, but my combo has water quality readings that tell me what I am doing can’t be that bad. I’m not saying that my system is optimal (yet), just that all the rules obviously do not apply to this design. My water quality monitoring says so, not my ego!

[TCLynx]

Comment by George J. Thurmon on November 13, 2010 at 10:57pm

There I am a fan of the theory of buying a big tank and adjusting your stocking rates as you expand. If your system turn-over rates are geared for correct flow rates (such as implied in the calculator), then there should be no difference between different stocking ratios in the same tank, but then with different media bed volume to match the fish.

This is something I've mulled over for a while so it was good, somewhat of a relief, to see it here in print. My tank will likely be over sized initially compared to media beds. Later I'll be able to increase stocking density and add media beds if I wish or need to do that. If I never expand, the extra water volume will still be nice to have with regard to dissolved oxygen and possibly more stable water temperature.

[TCLynx]

Comment by Kobus Jooste on November 13, 2010 at 11:34pm

George - I have seen this in a number of different Aussie designs and cannot find fault with starting with a 1000 liter tank and a single 200 liter grow bed. Stock the fish and set the flow rates as if you have only that one grow bed, and you should be fine. They add up to four similar sized beds onto that tank before it is required to add another fish tank. I like the "modularity" of such an approach. Commercial systems are pretty much designed for getting max output from a specific surface area, but home systems develop organically with the owner's interest. If the interest is low, then you do not expand, but if you like it, you buy some more grow beds and fish and off you go. Imagine having to toss your entire system every time you want to expand a bit!