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This ‘mythconception’ would be amusing if not for the fact that it is responsible for the premature death of so many fish.

The purpose of grow beds in a media-based aquaponics system is to facilitate nitrification and to grow plants……in that order.

There’s a prevailing belief among some aquaponicists that an additional function of grow beds is to trap and process solid wastes.  These solid wastes consist of fish faeces and uneaten food.

This belief ignores the fact that a bio-filter functions most effectively when sedimentary solids are removed…….something that every trainee wastewater treatment operator knows. It also flies in the face of advice from leading aquaponics researchers like Dr Wilson Lennard, Dr James Rakocy and Dr Nick Savidov.

Advocates of retaining the solids argue that the solids need to be mineralised so that the nutrients that are bound up in the solids can be made available to the plants.  They cite the existence of composting worms in their systems as evidence of the fact that mineralisation is occurring.

For the sake of clarity, I agree with the basic proposition that the solid wastes in an aquaponics system should be processed and returned to the system.  Where I differ with advocates of grow bed worm farming is that I don’t believe that this processing should happen in the grow beds.

I’m not just suggesting that solids should not be trapped in the grow beds.  The fact is that there are suspended solids that cannot be trapped by the grow bed media.  .

According to Dr Wilson Lennard, up to 75% of the solids in an aquaponics system may be in solution.  Some of this may precipitate out if the water is allowed to become still for long enough and some of it will only be removed by exposure to nitrifying bacteria.  Incidentally, this information should put paid to the suggestion that the bacteria won’t have enough to ‘eat’ if the solids are removed.

I first became conscious of the impact of suspended solids when I removed the fish from one of my aquaponics units.  Although I continued to recirculate the water, I noticed that (after about four hours) there was a substantial build up of fine sediment on the floor of the fish tank.  I vacuumed it out of the tank.  The next day, there was a further build up of sediment on the tank floor which I also removed.  This continued for four days.

The oft-cited University of Virgin Islands aquaponics unit employs clarifiers to capture sedimentary solids….and mineralisation tanks to trap the suspended solids.  During their brief stay in the mineralisation tanks, the solids yield up the nutrients.  The gunk that remains in the mineralisation tanks is physically removed twice weekly.

It’s also important to understand that, not only are there different types of solids to be dealt with in an aquaponics system, there are also different types of bacteria to be considered.

Let me explain…..there are two main types of aerobic micro-organisms that inhabit aquaponics systems ….autotrophic and heterotrophic.  Autotrophic bacteria convert the ammonia in the water into nitrates.  Heterotrophic bacteria convert fish faeces and uneaten food into ammonia.

Heterotrophic bacteria are fast-growing and most autotrophic bacteria are (comparatively speaking) slow growing.  Heterotrophic bacteria will breed up in much greater numbers if the feedstock is available

……and herein lies a problem.

The purpose of bio-filtration is the conversion of prospectively toxic ammonia and nitrites to less harmful nitrates……but, if heterotrophic bacteria are in disproportionately large numbers, they may be producing more ammonia than the nitrifying bacteria can convert.  It’s a matter of simple deduction, therefore, that vigorous growth of nitrifying bacteria should be encouraged while the conditions that promote the growth of heterotrophic bacteria should be avoided.

But maintaining a predominance of autotrophic bacteria is not the only issue.

The conversion of these solids depletes the system of oxygen.  High levels of dissolved oxygen are in the interests of the fish, plants and bacteria so anything that diminishes dissolved oxygen levels is also to be avoided.

Nitrates, when exposed to low dissolved oxygen levels can be converted by bacteria back to nitrites. At certain concentrations, nitrites (like ammonia) are toxic to fish.

As the solids (or the residue of spent solids) build up, anaerobic patches are created within the grow bed.  This leads to denitrification which, in turn, causes the pH of the system to rise.

With high concentrations of ammonia, diminished oxygen levels and rising pH levels, we have the circumstances for a fish kill.

Toxic grow bed conditions can be avoided in two ways:

  • Regular cleaning of the grow beds coupled with modest stocking and feeding rates.
  • The use of appropriate mechanical and biological filtration devices.

As I said earlier, I’m supportive of the idea that the solid wastes should be mineralised and the nutrients returned to the system.

The best way to handle solids in a small aquaponics system is to remove them before they get into the bio-filters….or grow beds..  This is achieved through the use of sedimentation tanks, clarifiers, swirl filters, screens or similar mechanical filtration devices.

For very small systems, mechanical filtration may be as simple as running the water from the fish tank through a piece of filter foam or even an orphan sock.  Grow bed maintenance can be much less frequent if sedimentary solids are removed.

Mineralisation of the solids can be effected by either aerobic or anaerobic digestion.

Aerobic digestion can be facilitated by putting the solids into drum full of water.  Place an air stone in the drum and leave it for a few days.  Remove the air stone and allow the spent solids to settle out before decanting the nutrient-rich water.

Anaerobic digestion is a bit more complicated but may provide a more useful effluent.  Take an airtight container and attach an airlock device (a home brew fermenter is ideal).  Put the diluted solids into our simple bio-digester and fit the airtight lid.  After a few days, air bubbles will issue from the airlock.  About forty days later (the exact digestion period will depend on the ratio of solids to water and the ambient temperature), the digestion of the solids will be completed.  You can then separate the liquor containing the nutrients from the sludge and use them in your system.

As the arguments in support of using a grow bed to capture and process solids fall prey to science and logic, its advocates often flee to a last bastion called Balance.

The claim of the balance advocates is that a properly designed flood & drain system… stocked and fed at suggested rates… doesn’t need cleaning.

“It’s all about balance,” they cry.  “You have to reconcile your stocking density, feeding rates and plant production with each other.”  They then go on to say that if you are experiencing problems with water quality, it must be that you have too many fish or that you are giving the fish too much food.

While this seems logical enough, it’s not the complete picture.  You see, any consideration of the performance of an aquaculture or aquaponics system has to take account of productivity and sustainability.

Sustainable production of fish requires that you grow a pre-determined amount of fish biomass within a given time while minimising the energy and food costs that it takes to do it.

The electricity that it takes to run an aquaponics system is a fixed cost.  You’ll spend the same amount of money on energy to power the water and air pumps on a system that is producing at optimum levels that you will on one the same system that is under-performing.

If, for example, the accumulation of solids in a system limits your capacity to grow the greatest possible amount of fish in the shortest possible time, then the energy cost per kg of fish or plants may increase to the point where it is no longer economical to grow them.

The other productivity issue arises out of the heightened risk of operating an aquaponics system outside of the optimum water quality parameters.   Producing fish that cost too much is one thing; producing fish that die before you get them to the dinner table is another kettle of (prematurely dead) fish entirely.

To summarise:

  • Bio-filters (including grow beds) function more efficiently when solids are removed.
  • The digestion of large volumes of solids diminishes dissolved oxygen levels.  Fish, plants and nitrifying bacteria all do best when dissolved oxygen levels are optimised.
  • Built up fish wastes (or other sediment) create pockets of anaerobic (without oxygen) activity resulting in denitrification.
  • Denitrification causes the pH of the system to rise.  Ammonia in the presence of high pH levels is toxic to fish.
  • Removal of the solids, and their subsequent mineralisation, is a simple (and far less risky) thing to do.
  • The question of balance in an aquaponics system is more about productivity and sustainability and less about accommodating dogma.

Only by optimising the water quality in an aquaponics system, can you maximise its productivity, and you can only optimise water quality in a small aquaponics system by removing the solids.

You might wonder why people would seek to perpetuate a myth that so obviously defies such logic.

Those most responsible are some of the peddlers of aquaponics products …..and the reason that they do it is because they know that there’s a larger market for their offerings if they create the impression that running an aquaponics system is easy.

Running an aquaponics system doesn’t need to be complicated but it’s not quite as easy as some people would have you believe.

Sounds like the stuff of Aquaponics Mythconception #6 – Aquaponics is Easy!   Stay tuned.

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