Closing the loop

Is anyone achieving closing the biological loop of the system? ie:

  • Growing enough fish food to avoid relaying in external inputs.


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This would imply that nothing is harvested from the system.

It seems so, then fish food as an external input is paramount:
1 - Procedence of the feed and ecological footprint should be taken into account. In Europe many feeds brands come from deforesting amazonas. In Spain, for instance, feeds are brought from brasil to grow pigs. The footprint is huge.

2 - Aquaponics provides for fish which is a great source of proteins. If the feed is external, isn’t it the only difference to hydroponics having a protein source?

3 - If I need to buy the feed to an external provider. What is the difference to go to fishmonger and buy fish.

Please take this comments as a personal reflection, no intention about attacking this wonderful community.


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Totally agreed, sourcing the fish feed in a sustainable manner is one of the major issues and challenges of aquaculture in general and thereby also for aquaponics.

Fish feed contains fishmeal from wild catch. But this is not as black and white as it seems at first glance. European fishing laws prohibit discarding bycatch. This fish is not suitable for direct human consumption, but it can be used for fish feed production. The same goes for slaughtering waste of the fishing industry. This waste stream can also be redirected into producing fish feed.

As of recently EU regulations have been changed to (re)allow for using blood meal and bone meal in animal food production. It had been prohibited due to the Creutzfeld-Jacob disease topic. With this change in regulations another waste stream can be repurposed in a sensible manner.

AFAIK fish feed producing companies like Skretting and Aller Aqua are researching into replacing animal protein with plant based proteins. For example with peas. This can already be done partially. The issue here are the amino acids Lysine and methionine, which are required for the growth of the fish. These usually are present in required quantities in animal based proteins, but are lacking in plant based proteins. But, as said, a partial replacement is already possible.

I am not sure in how far insects are already used as a protein source. I think this is a very promising approach. Not sure about the economics of insect farming as animal food, though. Also regulations for “food for human food producing animals” are very tight, at least in Germany. I do not know exactly about the current situation. I think using insects is going to be allowed. Maybe it is already.

So yes, agreed, this is a sustainability topic we should have on our radar.
Solutions are possible and we should select the fish food for our systems with this in mind.

Thanks Rolf, great reply.

I guess, if one goes to the basis, turning non-organic matter into organic is the key: any sustainable biological system is based on how producers take minerals to give organic matter to consumers. Obviously, the human intake represents a huge loss within the system that only by good sewage recycling methods may be partially recovered. However, we still have other important leaks within the Aquaponics systems. We have designed a closed nitrogen cycle system, but we need to pursue the completeness of other cycles (phosphorus, sodium and oligoelements are still missing). The more we are depending on external deliveries, the less reliable the system is. Aren’t we missing the soil in our system, with all the living micro-organisms that help to cycle the minerals?

Is there any research on hybrid systems that include aquaponics and soil?

Cheers mate,


Well yes, if we are extracting minerals from the system, we’ll have to supply these at another point. Nitrogen, Oxygen and Carbon are supplied through proteins, air and CO2, but the rest of the nutrients has to enter the system somehow.
You intuition with the soil is correct. Plant roots actually mine these minerals from the soil, subsequently making them easier available for the next generation of plants when the decaying plant matter lands on the soil surface, building humus.
That is the “turning non-organic matter into organic” process you mention.

I am not sure whether we actually have to replicate this in aquaponic systems. The fishes have to be fed anyway and ususally the only nutrient lacking is iron (Fe), which can be supplemented as iron chelate at reasonable cost. There have been suggestions to enhance fish feed with iron, but I don’t see the point of piping the iron through the fish. Why and what for? Adding iron to the plants gives the operator one degree of freedom that would be lost if the iron is formulated at a fixed ratio into the fish feed.

“We have designed a closed nitrogen cycle system”

Actually we don’t. Nitrogen has to be piped into the system in the form of protein and is exiting the system as fish product and in plant matter that is harvested. (Apart from denitrification losses)

I catch and completely understand that you’re idealizing an autonomous circular system, and that you’re scratching the itch of it not being perfect.

The truth is that aquaponic systems are not closed autonomous systems. And they should’nt be. We want to harvest the products. An when there is an output, there has to be in input.
The goal is to minimise losses. the ideal state would be to only having to supply the “raw materials” to replace what is harvested, but this is not entirely possible.

Regarding water use efficiency (WUE) for example we need to supply much more water than what we get back from the harvested fish and plants. And that is not all about the surface evaporation in the tanks, but mainly because of the evapotranspiration of the plants. The plants need to pipe a lot of water from roots to stomata for transporting the nutrients to the growing cells. This can not be optimized away at the plants. In theory it is possible to condensate the humid air in the greenhouse, but in terms of energy consumption this does not make a lot of sense (depending in fresh water availability at your site).
And this aspect should not worry us, since hydroponic WUE is much better than growing in soil, so this is fine.

Regarding nutrient use efficiency (NUE) we have the same situation. Almost no nutrients that are brought into the system leak out of it unused. So while we have to supply them to the system, we only have to supply what we actually harvest.

The NUE of phosphorus can be enhanced by remineralising the solid fish waste. I think roundabout 37% of the phosphorous lands in the aquaculture sludge (not completely sure about that figure, don’t quote me).
Aerobic or anaerobic remineralization makes sense. I think UASB/EGSB reactors are more efficient in theory, but I guess very simple aerobic remineralisation tanks are much easier to operate.

If you wanted to completely close the loop, you’d have to include humans into the equation, since we are consuming the harvest from the system. We’d have to bring them back after digestion. This is not unheard of, and is usually touted “humanure” or “anthroponics”. There is a huge food safety issue attached to this. But if you dig down this rabbit hole, have fun with Henriques melon seeds!

To answer your question about using soil with aquaponics/hydroponics: Yes, there are systems that use conventional pots with earth like subtrates. It is possible. But apart from marketing issues I don’t really see the benefit with respect to NUE. I’d rather veer towards digesterponics, using digestate from anaerobic bio gas plants as an input into the aquaponic system.
AFAIK the digestate has to be treated though, for it to not cause harm to plants and fishes. I think aeration and pH adjustment are recommended.



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Great discussion and information gentlemen. In our experience with Aquaponics, the microbiology that develops is very similar as what happens in soil, the microbial complexity increases over time, and the maturation of the microbial community is one of the main stimulators of aquaponics productivity, making the plants grow at such an enhanced rate v.s. soil grown plants. Much work yet to do on this aspect of aquaponics, which will expand our understanding of this ecology.

As for soil in the system, I guess there may be some benefits: 1. ability to label your produce as “organic” if it is grown in soil, at least that restriction exists in Canada. 2. possibly the ability to transplant your produce into soil e.g. tree seedlings. The addition of soil to an aquaponics system would cause concerns for release into the system and may interfere with filtration. Most media considered for aquaponics are more substantial and uniform than soil to avoid this, as it makes them easier to contain.

We don’t have much experience with anaerobic breakdown of solid waste in aquaponics, but we have used aerobic bioreactors to convert all solids back into liquid nutrients for use in the system. Commercialization of this end of things is not far in coming.


Its not possible. You always get some loss plus you yourself are exporting nutrients by consuming the outputs.

Aquaponics must always be supplemented any one who tells you other wise is a scam artist.

Its far cheaper to use a low cost food and supplement than it is to try and use the overly formulated foods.

While you can certainly make a mostly sustainable system you still do need some outside inputs either mineral salts for ferments but either way your iron, manganese, molybdenum, and other things are depleted over time or oxidized.

Anaerobic sludge processing seems to be the more efficient process with regard of the amount of recovered nutrients in relation to the microbe biomass. The lack of oxygen leads to a microbiota in the reactors that have to be much more ‘creative’ to process inputs for building up their own biomass. Thus the ratio of recovered nutrients to microbe biomass is favourable in anaerobic processing.

But the operation of an anaerobic reactor combination is much more delicate than running an aerobic reactor. For this reason we opted for using an aerobic sludge processing unit in our current project, even though we know that an anaerobic one would recover more nutrients in a smaller volume. In effect you are trading apparatus size and space for maintenance. Since setting up the apparatus is a one time investment and maintenance is an ongoing activity, we think this is a sensible choice. “Keep it simple”!

Please forgive the non-technical comments but i wanted to point out two things…

Back in the 70s or 80’s there was a guy who built a sewage recirculating system to process human waste via an aquaponic system. he had a series of tanks each with a different method of breakdown and take up to various organisms. He had it pretty much perfected for home use where you could control what was being dumped down the drain but he could never get it to the municipal level and it faded from the discussions. I forget his name, sorry.

The second point is regarding insect and protein generation. There is lots of work being done with black soldier flies today, both in commercial feed and home projects. When i was a kid, we used t put our garbage in an outdoor pail and the “Garbageman” would come around and pick it up. I believe he used it to feed his pigs. it would be great to see this type of thing happen today for feeding black soldier flies or fish directly. Likely the pig farmer switched to commercial feed because it was too time consuming to collect garbage… or maybe municipal laws forbade it at some point…?

Achieving a closed loop system is my goal.

Currently, my trade secret is in the culture of natural live foods using fish wastes and bakers’ yeast for fries. Bloodworms, mosquito larvae, maggots and earthworms for the larger fishes. It is still partially closed loop because I still supplement commercial fish feeds and provide bakers’ yeast as new inputs. This practice though reduced my variable costs by 60-80% making it my competitive edge.

A complete closed loop will require an estimated 5x my current space with matching infrastructure dedicated to culturing live foods. Im a small fish farmer afterall.

Btw, fish fed with various live squirmy organisms have superior and clean taste over those fed exclusively with commercial feeds.

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Hi Ignacio,

You can’t have a completely “closed loop” as that is in the same realm of fantasy as perpetual motion machines. However, I am aware of work to recycle food waste to convert it back into something useful, and that’s what I have been attempting on a very small backyard scale. (My perch were too old to learn to take pellets when I got them, so they pretty much only take live feed, so I’ve had to improvise!)

Here are some ideas:

  1. Expired cereals and root vegetable peelings can be fed to morio worms and meal worms.
  2. Other plant waste can be fed to earthworms.
  3. Spoilt meat is used to feed maggots.
  4. Decomposing leaf matter can be fed to things like water lice and mosquito larvae.
    (I have a healthy population of water lice in my second system where I put any dropped leaves or prunings, and I think they would make a good feed for small fish or fry.)
  5. Black soldier flies can be fed waste such as brewers grains.

I’ve encountered a lot of challenges with my own attempts, but they’re mostly due to the small scale I’m doing it on and things being quite labour intensive if you haven’t figured out the right equipment, but if you’re considering attempting any of these then I’d be happy to share my experience.

Whilst it doesn’t “close the loop” it does get close. For all of the fruit and vegetables you harvest from a system it’s relatively easy to replace those nutrients (for free) with stuff that is no longer fit for human consumption but perfectly suitable for invertebrates that in turn can be eaten by fish.

I also “gut load” things like morio worms for a few days before I use them by feeding them on commercial pellets (that my fish refused to eat), so I haven’t completely eliminated wild caught fish protein from my system yet. There are probably more sustainable ways to enrich live feeds like this, but whilst I have a bag of pellets otherwise going to waste I haven’t explored these yet.

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