Anoxic Filtration – is it a bog filter?
Part 1 By: Syd Mitchell of the UK
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 is not necessary.
If it is 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 actually 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 is 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 is 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 ppm (mg/L) and 2 ppm. This 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 absolute minimum
acceptable oxygen level in a Koi pond is 6 ppm, although 7 ppm is more often
recommended as a safer minimum to adopt and, in practice, it should be at
saturation level (as high as is possible 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 fish, 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 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 bio-filter
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.
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.
The anoxic
filtration system was designed and has been developed over many years by Dr
Kevin Novak Ph.D. and it addresses both these problems. The anoxic system does
not 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” to feed on nitrate. What is less well known is that plants
actually “prefer” ammonia as a food source, 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 Laterite
that we put into the centre.
Another
important feature to understand is that it is only the ammonia molecules that
are drawn into the biocenosis baskets.
Obviously, water floods into them when they are immersed, but after
that, water does not actually need to flow through them in order to filter out
ammonia. The Laterite 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 that
have not 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 does not 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 is more likely that this label could
be applied to a conventional system that has not 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 do not 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!
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 (GB) £25.
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 five
to 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.
Photo #1 Old milk crates to diffuse the water. |
Photo #2 This pond also has an electric fence to keep the Raccoons out. |
(PART-2)
https://itunes.apple.com/us/book/ano...04698627?mt=11
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