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Anoxic Filtration   (Anoxic Filtration)


I wrote the following articles for Koi Carp Magazine.
Therefore they own the copyright but the Editor has given permission for them to be republished here.
Thank you, Karen.

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Anoxic Filtration – is it a bog filter?

Part 1

Take a planting basket, fill it with cat litter, then scoop out a depression from the centre and fill that depression with an aquarium plant fertiliser called laterite.  Put a plant into the basket if you wish, but it isn’t necessary.

If it’s that simple and inexpensive to make a biocenosis basket, why are more koi keepers not using the anoxic filtration system?  Possibly the answer lies in the fact that it is rare to find a koi keeper who really understands what anoxic filtration actually is, what it does and how it works.  The reason for this lack of understanding should immediately be obvious.  Already, I have used three words that many koi keepers will find new and mysterious.  What is laterite?  What is a biocenosis basket?  And isn’t anoxic, something to do with anaerobic sludge where harmful bacteria can live?

If it were possible to describe the anoxic system without using such words I would do so, but, whilst it’s easy to build the system, the way it works is extremely complicated. So let us begin by explaining what laterite, biocenosis and anoxic mean.

Laterite: - This is easy enough to understand.  It’s simply a clay based material that is rich in iron.  It is used in tropical fish tanks as a plant fertiliser and can be brought from tropical fish dealers.

Biocenosis: - This is a scientific term for a place where different biological processes take place, each process being of mutual benefit to the other processes.

Anoxic: - Anoxic does not mean the same as anaerobic.  In an anaerobic region there is a complete absence of oxygen.  In an anoxic situation, there is oxygen present but it is at a very low level. In a biocenosis basket there is always a low level of oxygen.  Levels are typically between 0.5 mg/L and 2 mg/L. This is

....it is rare to find a
koi keeper who
really understands
what anoxic
filtration actually is

the key factor that will influence a situation where anoxic filtration can occur. The presence of an extremely low level of oxygen is crucial to the system as will be described later.

In passing, it might be worthwhile to contrast the oxygen level in a biocenosis basket with the oxygen level in the pond itself. The minimum acceptable oxygen level in a koi pond is 6 mg/L, although 7 mg/L is more often recommended as a safer minimum to adopt and, in practice, it should be as near saturation point as is possible.  (Saturation point is the highest level of oxygen that will dissolve into water at any given temperature).

With the first few terms explained, it should now be possible to move on to a better understanding of how the anoxic filter system is quite unlike any filter system that is commonly in use by pond keepers.  There will be more complicated terms as the description unfolds, but each will be explained as we explore the system.  To contrast anoxic filtration with conventional filtration, it is first necessary to understand how biological filters actually work.

Conventional biological filtration
Fish continuously excrete ammonia which is toxic to them, and so it has to be removed from the pond water before it can cause them harm. We all know this.  Any well designed conventional biological filter system will be effective at taking this ammonia and converting it, first into nitrite, and then into nitrate by a process called the “nitrogen cycle”.  This is also well known. Having achieved the conversion of ammonia into nitrate, the task of a conventional biological filter is complete. No further

....the anoxic filter system is quite unlike any filter system that is commonly in use by pond keepers

biological action to remove the nitrate takes place and so the level of nitrate in the water slowly rises.  This is the first disadvantage of conventional biofilter systems.

Nitrate is nowhere near as toxic to koi as either ammonia or nitrite but that doesn’t mean that they are not affected by it.  Hard scientific research on this subject is difficult to come by, but, there is plenty of anecdotal evidence to show that koi kept in low nitrate ponds, show better growth and colour development than those kept in a pond where the nitrate level has been allowed to rise.

Easier to prove, is the effect that nitrate has on algae of all types.  Nitrate is a plant fertiliser.  A rising level will encourage blanket weed and the kind of algae that turns water green.  Water changes will help but, even after a 10% water change, the nitrate level will still be 90% of the original value, and, as more ammonia is converted, the nitrate level will soon begin rising again.

There is a second problem with some conventional biological systems, which is that the biological media can become anaerobic (no oxygen at all).  This will not occur with moving media such as fluidised sand or aerated K1, but where the media is static, water flowing through it carries suspended particulate matter as it passes through.  This will settle within the media and, over time, can cause it to block.

Water will not easily be able to pass through blocked areas and will tend to bypass them.  The water will find it easier to flow through areas that are not blocked (yet!).  Water flowing through media carries oxygen to the bacteria that are living within it.  As the flow through a particular area reduces, the bacteria in it will find that there is less and less oxygen in their environment.  The bacteria that have been oxidising ammonia, firstly to nitrite and then to nitrate, will have been using a great deal of oxygen to do this. As the media becomes clogged, they will find that they no longer have the oxygen available to carry on this process.

There are bacteria that need oxygen as part of their biochemistry but, in some ways, can be thought of as being far more clever than we are.  If we are deprived of oxygen, we soon die.  These bacteria normally take the oxygen that they need directly from the water surrounding them, but if there is no oxygen in that water, there is a way they can get it. They can take oxygen from nitrate. Nitrate (NO3) is the end product of the nitrogen cycle, (as far as koi keepers are concerned). It consists of one atom of nitrogen joined to three atoms of oxygen. These bugs can take away the atoms of oxygen from nitrate. In doing so, they reverse the nitrogen cycle.

What is less well
known is that plants
actually “prefer” ammonia as a
food source

Our two favourite nitrogen cycle bugs, nitrosomonas and nitrobacter, will be as busy as ever converting ammonia to nitrate in the aerobic areas of the biological filter, but bugs that can live in anaerobic conditions will rapidly colonise the anaerobic areas and will become equally busy taking the oxygen that they need from the nitrate that has just been produced by their cousins.  This will result in that nitrate being converted back to ammonia again before it leaves.

This is a completely pointless exercise, yet it is exactly what is happening in many filter systems where the biological media is not clean.  At least part of the good work being done by the bugs in the oxygen-rich areas, is being undone by bugs in areas that are deprived of oxygen.

Understanding anoxic filtration

Figure 1
A typical anoxic pond without plants

The anoxic filtration system was designed and has been developed over many years by Dr Kevin Novak PhD. and it addresses both these problems. The anoxic system doesn’t rely on converting ammonia to nitrite and then into nitrate.  With the anoxic system, ammonia is either converted directly to nitrogen gas, (more correctly called di-nitrogen), by bacterial action in the unplanted baskets, or it is taken up by plant roots in baskets that also contain plants.

It is well known that plants “like” nitrate as one of their nutrients. What is less well known is that plants actually “prefer” ammonia as a nutrient instead, and will take it directly from pond water if it can be presented to their roots in the correct way. The process that causes this to happen will be described in detail in part 2 of this article, but for now, please just accept that ammonia molecules are attracted into the baskets by the negative electrical charges that exist within it.

Another important feature to understand is that it is only the ammonia molecules that are drawn into biocenosis baskets.  Obviously, water floods into them when they are immersed, but after that, water doesn’t actually need to flow through them in order to filter out ammonia. The electrical charges in the centre of the basket only draws in ammonia molecules; it doesn’t draw in water molecules.  This directly addresses the second possible problem that can happen with some of the static types of conventional filter media I mentioned earlier – that suspended particulates can clog media and it will then become anaerobic.

Critics who haven’t taken the trouble to understand how the anoxic system works, often wrongly describe it as a “bog filter, full of nasty anaerobic bacteria”. They warn that the baskets are a breeding ground for parasitic bugs that can then spread to your fish.

In fact the direct opposite is true. Biocenosis baskets cannot clog because, if no water flows through them, there is no way that debris can be carried inside.  On the other hand, if water flowing through conventional media doesn’t have every speck of debris filtered out of it, there will always be the risk that sludge will settle inside and block the media.  So, far from a biocenosis basket being a “bog filter”, it’s more likely that this label could be applied to a conventional system that hasn’t been kept sufficiently clean!

The anoxic filtration system
Figure 1 shows a typical anoxic filtration pond without the plants.  It is simply a shallow pond, around 24 inches deep, with water being pumped into one end and overflowing by gravity back into the main pond at the other. This one contains 22 biocenosis baskets. The small pebbles on top of the baskets prevent water flowing past them from disturbing the cat litter inside and causing it to float away. Figure 2 shows how the entire anoxic filtration system can be “hidden in plain sight”.

The anoxic pond looks like a water garden, not a filter, and yet everything except the pump in the main pond can be clearly seen.  For those who don’t like pump fed systems, the anoxic pond can be gravity fed from a bottom drain. Build the anoxic pond at the same level as a conventional gravity fed system and use a submersible pump or external dry mounted pump to pump water from the anoxic pond back into the main pond. The only limit to how this system can be built or adapted is your ingenuity!

Figure 2
Entire filter system hidden in plain sight.
The top pond is actually the anoxic filter.

There are few hard and fast rules as to how to make biocenosis baskets.  It is important that the planting baskets used should have open lattice type sides to allow ammonia to be drawn in through them, but apart from that, any basket around 30 cm x 30 cm x 20 cm deep will do. The cat litter should be a granular, unscented, “non-clumping” type that should retain its granular structure when wet. Laterite can be obtained from tropical fish outlets or purchased on-line in 1.6 kg packets for less than £25 [GB 2013 prices].

Biocenosis baskets need very little maintenance because the life of the cat litter is indefinite and the laterite is only very slowly depleted by plants.

Photosynthesis is the process that plants use to make energy from sunlight and a green pigment called chlorophyll is essential for this process.  Plants need iron to make chlorophyll, so if there is a plant in the biocenosis basket, the iron in the laterite will become exhausted after about ten years and you will have to add some more laterite or remake the basket.

Part 2 will describe how to build the anoxic filtration system, and how ammonia can be drawn into a basket and destroyed without leaving nitrate to build up in the pond.


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