Comparing trickle towers with Bakki Showers

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Submitted by Syd Mitchell on Sun, 05/06/2018 - 08:50

By: Syd Mitchell

This article delves into the differences and similarities between homes for our nitrifying bugs, and how they work.  The article ends with a quick primer on Anoxic Filtration – there is a full ebook on the subject if you are interested:

A trick question?
The question I posed at the end of part 16 was this; “Although bacteria that remove nitrate must only have a slow water trickle inside a trickle tower, they are perfectly happy under the gushing flow of a Bakki Shower. How can that be?”

The answer is that they can live on media in highly aerated water alongside the normal nitrifying bugs but they won’t remove nitrate for us so although they can live in a Bakki Shower they won’t remove nitrate unless the shower also has a type of media called denitrifying media.  To remove nitrate they have to live inside the media away from the flow of aerated water. That wasn’t so much intended to be a trick question but was designed to be a thought provoking one.

As explained last month, bugs with the complicated description facultative anaerobic bacteria can live in aerated environments where they can take the oxygen straight from the water as it flows past.  In many respects they would be happier with that situation because the easier it is to them to obtain oxygen the better. But these types of bugs are tough, in an environment where there is little or no oxygen they switch lifestyles. They switch to their facultative mode and take oxygen from chemicals such as nitrate in the water.

Showers are similar to trickle towers
Having to switch from being able to take their oxygen directly from the surrounding water to obtaining it from nitrate is exactly what happened to the bugs inside last month’s trickle tower design.  It was a sealed environment so that the only oxygen that could get in was the oxygen dissolved in the water entering it.  Bacteria near the top of the media bed, coloured blue in figure 1, had easy access to this oxygen so they could process ammonia and produce a little more nitrate that slightly added to the pond nitrate level.  But that was only at the very top of the media bed because, in the process, they soon used all the oxygen and, since no more could enter the system, the only bacteria that could live in the mauve area below would be facultative ones that could obtain their oxygen by “sucking it” off of nitrate.

It may seem difficult to believe but this process where bugs are deprived of oxygen is exactly what happens in the highly aerated environment of a Bakki Shower and similar devices.  The bacteria that remove nitrate in the anaerobic conditions inside a trickle tower are the same bacteria that live in a shower but with the slight difference that although they are happy to live on the surface of the media in a shower, they will only remove nitrate if the media is a special type – denitrifying media.

Denitrifying media

Figure 2 shows a simplified representation of how this media works.  It shows how living conditions would vary for bacteria living in a deep narrow hole.  Denitrifying media doesn’t have millions of little holes drilled into it. The various different manufacturing processes, such as sintering, that are used to make these types of media leaves the finished product porous with microscopic holes or cracks in it and the diagram shows what would happen to bugs living in a typical hole.

In figure 2 the red bugs living on the outside of the media and those just inside the hole have easy access to water that is well oxygenated. Deeper inside the hole, the blue bugs will find that much of the oxygen in the water has already been used by the red bugs. Life will be harder but they will be able to survive because there will still be some oxygen left, and they will use the last of it.  Both the ammonia bug and the nitrite bug (nitrosomonas and nitrobacter) can live in the red and blue areas and between them they will convert ammonia into nitrate. Remember that this process requires a lot of oxygen, (over four times as much oxygen as the ammonia they convert), which means that it will get used up very quickly.

Deeper inside the hole, with all the available oxygen having been used up, living conditions for the mauve bugs will be very different.  There will be no oxygen left in the water that reaches them, but it will contain plenty of the nitrate that has been produced by their red and blue neighbours. The only bugs that can live in these conditions are those that can take their oxygen from nitrate.  In other words, by putting denitrifying media in a shower, despite the fact that the water flow over the outside of the media is very aerated, we create a perfect home for facultative anaerobic bugs in the cracks and holes inside it. These bugs will repay us by removing nitrate from our pond water just the same as the bugs did in the trickle tower, but they will be doing it in the anaerobic conditions inside the countless tiny holes in the media rather than in a trickle tower which is really just a big tube full of ordinary media where oxygen is prevented from reaching it.

How shower filters work
The basic layout of a Bakki Shower and that of its many copies is as shown in figure 3.  The principle is simple enough; water is pumped from the pond and over the top of the highest tray of media.

After tumbling over the media it leaves the top tray through a grid or a series of holes drilled in its bottom.  It then cascades in a similar manner over the media in the lower trays and finally collects in a the bottom tray from where it returns to the pond by gravity.

The advantage of this type of filter is that the water becomes highly aerated as it cascades down which helps to gas off some of the ammonia instead of it being converted to nitrite and then to nitrate by the normal nitrifying bugs (nitrosomonas and nitrobacter).  To some extent, this reduces the the amount of nitrate that they would otherwise have produced but the ammonia that isn’t gassed off in this way still ends up as nitrate.

The main way in which nitrate is reduced by these filter only occurs if the media in the trays is of the denitrifying type as described above.

A problem with denitrifying media is that the water fed to it should be very clean. Even tiny particles in the water can block the entrances to the holes in which the bacteria live. If these holes become blocked, not only will the mauve bugs be unable to remove any nitrate, but some of the red bugs, and all of the blue bugs, in figure 2, will also be sealed in by the blockage and they will therefore be unable to function.  After any remaining dissolved oxygen in the water trapped inside the hole has been used, they will initially become dormant and ultimately die.

These are the usual filter bacteria, and if the reduction in the available media space results in the colony sizes of these two types being reduced too much then, in an extreme case, there may not be a sufficient colony to process all the ammonia.  If a system containing denitrifying media is well set up and maintained, the water quality will be impeccable, if not, the water quality could be very poor.

Anoxic filtration
This is a completely new approach to biological filtration and arguably the least understood.  The first misunderstanding to clear up is that anoxic does not mean the same as anaerobic. In an anaerobic region there is a complete absence of oxygen.  In an anoxic situation, oxygen is present but it is at a very low level.  The heart of this system is what is called a biocenosis basket which is nothing more than a planting basket full of cat litter with an aquarium fertiliser called laterite in the centre.  These baskets work better if they also contain an aquatic plant but this is optional.

In a biocenosis basket there is always a low level of oxygen with levels being typically between 0.5 mg/L and 2 mg/L.  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 minimum acceptable oxygen level in a koi pond is 6 mg/L although 7 mg/L is more often recommended as a better minimum to adopt and, in practice, it should ideally be at near saturation level.

Nitrifying bugs in a biofilter are more tolerant of oxygen levels. They need near saturation values for optimum efficiency but can still function to some extent if oxygen levels fall to around 2 mg/L.  Below this value, there isn’t enough oxygen for them to convert ammonia to nitrate.  This is where facultative bugs take over and they perform the same function as they do in a trickle tower or in denitrifying media in a shower but with some added advantages.

Inside a biocenosis basket
The first thing that distinguishes a biocenosis basket from conventional filtration is that in conventional biofilters, water is pumped either through the media or it is pumped up and allowed to flow by gravity over it.  Either way, the bugs take ammonia, (or nitrite or nitrate), from the flow of water that carries it to them.  This doesn’t happen with anoxic filtration.  Water will flood into the basket when it is first submerged but, after that, there is no actual flow of water through it! Everything happens by molecular attraction which is why, although this system is simple to build, it must be done exactly as it was designed by Kevin Novak, the designer of this system.  Any deviation from his design will be doomed to failure because the required molecular attraction and anoxic conditions cannot happen.

The diagram in figure 4 shows how these baskets work and the following description is as simplified as is possible but it is still very complicated and suitable only for the technically minded.  The less technically minded may skip the description of zones A, B and C.

For clarity, the zones have not been drawn to scale.

In practice, zones A and B would only be just a few millimeters thick before any oxygen that percolated into the baskets from the surrounding water had been used by aerobic bacteria that had colonised these outside layers.

The anaerobic zone C, makes up the majority of the volume inside the basket and is where the process of destroying ammonia without leaving nitrate can take place. 

Zone A

Ammonia (NH4+) has a positive charge and, as a result, negative charges in the laterite begin to attract ammonia molecules towards it in the centre of the basket.  As these molecules pass through zone A, nitrifying bacteria (nitrogen cycle bugs), will grab one ammonia molecule and four oxygen molecules and will excrete one nitrate molecule.  (These numbers are approximate and the actual process is far more complicated than that but this is as much detail as we need at the moment in order to understand how the baskets work).

As a result of them using one ammonia molecule and four oxygen molecules to produce one nitrate molecule, the ammonia level in zone A will have dropped a little, the nitrate level will have risen by roughly the same amount but the oxygen level will have dropped considerably, (four times as much). Although there is now far less oxygen, there will still be enough of it for the nitrogen cycle to continue. The remaining ammonia molecules continue to be drawn into zone B.

Zone B
As more ammonia is converted to nitrate, the ammonia level drops even more and the nitrate level rises. So much oxygen has been used in the process that this area can no longer be called truly aerobic, (oxygen rich), but there is still a little oxygen left to sustain some nitrifying bacteria.  A little more ammonia becomes nitrate but the vast majority is drawn into zone C.

Zone C
Ammonia is still being pulled toward the laterite but almost all oxygen in the water has already been used. The nitrogen cycle ceases because it cannot occur if dissolved oxygen levels are below 2 mg/L, and it will be lower than that in zone C.  Only facultative anaerobic bacteria can live in this zone and they will do the real work that these baskets are designed for.  They directly metabolise ammonia. Although the baskets produce a small amount of nitrate in zones A and B it is soon used in zone C.  The rest of the ammonia drawn into the baskets is simply metabolised directly without the end product that results from conventional filtration – nitrate.

A point to emphasise is that there is no actual water flow through the baskets only individual ammonia molecules are pulled in and, since there is no water flow through them, they cannot become silted up and harbour nasty bacteria.  Another advantage of these baskets is that phosphate (PO4+) will also be drawn in and directly metabolised, reducing the level of this pollutant too.

Another point to emphasise is that, although the baskets are simple to build, the description in Kevin Novak’s book on this system must be followed exactly, especially regarding the correct grade of cat litter.  In the UK there are many different types available and choosing the correct one is essential.

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