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Aimed primarily at beginners to the hobby, this series of articles will take you step by step through the process of understanding how a good koi pond works.
Part 2: Improving the biological filter
To recap; in part 1, I said that the only essential elements of a koi pond, apart from the pond itself are a biological filter and a pump to circulate water between the two. Figure 1 shows the example used to illustrate how this could be achieved. As drawn, this basic system has many disadvantages but it will serve as an example of the principles that are involved. Opinions will vary about what is the best media to use in a biological filter. Jap-mat will not be everyone’s favourite but it works well and is an easy media to understand for those who have never really thought about what goes on in a biofilter, so this will be the chosen media for the time being.
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Figure 1: Basic pond with biological filtration (needs much improvement but shows principle)
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What is wrong with the basic biofilter in figure 1?
In a koi pond, there will inevitably be suspended silt and other floating debris, and there is no method yet in our basic set up to remove it before it enters the biofilter. Even when we later develop the filter system to include mechanical filtration to remove as much of this as is possible, there will inevitably still be some left, so consider what will happen to water that contains suspended silt as it travels along a pipe. Ignoring complicated mathematical calculations and sticking to easy approximations, we could say that a 4 inch pipe has a cross sectional area of just over 12 inches. Because the water is confined in such a comparatively narrow space, it will have a fairly high speed within that pipe. Just as a gentle breeze in your garden will only have the strength to blow paper around but a very strong wind can blow your dustbin over, so it is with water flowing in pipes.
As water flows along a pipe, its relatively high speed allows it to carry a lot of silt and heavier debris with it. If water flowing along a 4 inch pipe, (which has a 12 square inch cross section), suddenly enters a chamber which has, for easy numbers, dimensions of 30 inches by 40 inches, it is no longer confined into 12 square inches, it now has 30 inches x 40 inches to flow through, which is 12 hundred square inches. The mathematics doesn’t work out exactly but the general idea should be obvious. It will now travel through the chamber in this example at roughly one hundredth of its speed in the pipe. It has gone from the equivalent of a very strong wind that could move a dustbin to a gentle breeze that could only lift paper. What this means in our simplified example is that, as the water speed drops to one hundredth of what it was, its power to carry silt and debris will also drop to one hundredth of its carrying power in the pipe. Another way to look at this would be to say that it will drop 99% of the silt it was able to carry along the pipe, and this will be dropped all over the floor of the biological filter chamber.
What happens to the silt that doesn’t get dropped?
In the simplified example above, around 99% of what the water carried into the biological chamber would end up being dropped, and would be slowly accumulating on the chamber’s floor. This means that 1% won’t be dropped; it will be carried by the water into the media. Not all of it will pass through and go back to the pond, some will settle inside the media and, over time, will cause it to become clogged. As mentioned in part one, as the water flow through the blocked areas reduces, so will the supply of oxygen it was carrying to the nitrifying bacteria whose job it is to remove the ammonia that is also being carried into the media to be processed into nitrate. As the water flow decreases, the bugs nearer to the edges of the blockage will still be able to take oxygen from the water but those deeper inside the blockage will find that the oxygen supply isn’t sufficient for them to continue with the nitrogen cycle. Since bugs cannot convert ammonia into nitrate without oxygen, the result will be that some of the ammonia won’t be converted, it will simply be returned to the pond untouched. It is also possible that some of the ammonia that had already been converted into nitrate by bugs in well oxygenated areas of the media will be converted back to nitrite again in the blocked areas where there is little dissolved oxygen.
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Science Alert!
This panel is for the more technically minded who might like an explanation of how nitrate can be converted back into nitrite in blocked areas of media. You may safely skip past it to the following section if you prefer.
Some species of bacteria, (facultative anaerobes), are able to live in conditions where there is little or no oxygen. They still need oxygen but it isn’t necessary for there to be oxygen dissolved in the water. Nitrate is NO3 which is a chemist’s way of saying “one atom of nitrogen joined to three atoms of oxygen” and these bacteria are able to obtain their oxygen supply by taking one or more atoms of oxygen from it. If they take one of the atoms of oxygen from NO3 it becomes NO2 again. What this means is that, in well oxygenated areas of the media, the normal nitrogen cycle process will convert ammonia (NH3) into nitrite (NO2) and then into nitrate (NO3) but if this nitrate enters an area of the media where oxygen is very low it will be converted back to nitrite (NO2) again. There will be a dual problem. One part of the problem is that some of the ammonia will be allowed to go back to the pond because normal nitrifying bacteria will be unable to remove it due to lack of oxygen in the clogged areas in the media. This is made worse by the second part of the problem where some of the ammonia that has already been converted to nitrate in better oxygenated areas will be converted back to nitrite again in the poorly oxygenated areas. Under some conditions, it may even be converted back to ammonia.
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Hidden problems
Apart from the slowly worsening water parameters, what is happening underneath the media, as shown in figure 2, will go unnoticed by the pond owner. Debris and silt will slowly accumulate at the bottom of the filter and within the media itself. The accumulation of silt on the floor of the chamber will soon cause problems of its own. Nasty bacteria thrive in accumulated rotting silt. Some add a toxic gas (hydrogen sulphide) into the water which will slowly begin to poison the fish. Other bugs that also breed in silt can infect fish causing bacterial infections and ulcers.
Some means of mechanical filtration will help by removing as much suspended silt as is possible before it gets to the biofilter, and various ways to do this will be discussed later, but some silt will always get past mechanical filtration and a well designed biological filter must be able to deal with this problem.
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Figure 2: The original basic biofilter
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Figure 3: An improved biofilter
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The tendency for silt to accumulate on the floor cannot be avoided. When water slows from a rapid flow to a more gentle flow, it will always drop much of the silt it is carrying. What must be done is to allow what it does drop to be easily flushed away. Figure 3 shows a simple way to achieve this. Instead of water entering from the bottom, it now enters from the side. This allows the bottom to be sloped or benched towards a central pipe with a valve that can be opened to allow a good “whoosh” of water to run to waste. This, in combination with the sloping floor, will wash any accumulation of silt or other debris out of the chamber and down the drain.
How often this should be done depends on each individual system. A light dusting of silt on the floor is quite normal and will cause no problems but, if silt is allowed to build up into a thick bed, it will become anaerobic (no oxygen) and will become a breeding ground for bad bacteria. When our basic system is improved later and has mechanical filtration, water entering the biofilter will be cleaner, meaning that the accumulation of silt will be slower, so a sensible cleaning regime would be to flush the biofilter to waste as soon as the silt is more than just a light dusting and is beginning to look like a thin but continuous layer.
Continuing with Jap-mat as a typical media, another improvement to the basic configuration in figure 2, would be to have the sheets placed vertically in rows and separated by spacers. This is called a Jap-mat cartridge. This has the advantage that water isn’t forced to flow through it. Water flows upwards through the channels and only percolates sideways into the Jap-mat. Since water isn’t forced to flow through the Jap-mat but only percolates slowly into it, it won’t carry any suspended silt in with it. A light dusting may settle on the outside, where it won’t cause harm, but the flow of water into the media will be so slow that very little silt will enter the actual sheets themselves. The biological filter chamber is now nearly as good as it could be but there is another improvement that should be made.
The importance of aeration
As our two colonies of nitrifying bugs work together to convert ammonia into nitrate, they use a lot of oxygen in the process. To take just one single molecule of ammonia and convert it into a molecule of nitrate, they also need to take just over four molecules of oxygen from the water as well. A bacteria colony in a bio-filter uses a lot of oxygen. If the bugs take the necessary oxygen from the water and it isn’t replaced, the water leaving the filter on its way back to the pond will have a much lower level of dissolved oxygen in it than it did when it was drawn out of the pond. In addition to removing ammonia from the pond water, biofilters have the unwanted effect of removing a great deal of oxygen as well which would be to the detriment of the fish. To avoid this situation, biological filters need a good supply of air, which is why figure 3 shows air stones just under the media. This isn’t just the case when using Jap-mat. Air should be added to all types of static media whether it be the chopped-up pipe type of media such as flocor, the porous rock type like alphagrog, or plastic moulded media such as bio balls.
A point to note at this stage, is that different types of biological filters such as Bakki Showers, the Nexus range of filters or aerated media such as aerated K1 don’t need additional aeration because they massively aerate the water by virtue of the way they work. These will be described later in the series, but apart from these types, aeration in the bio-filter chamber is essential, both for the efficiency of the biological process itself and also so as not to return water with a reduced level of dissolved oxygen to the pond.
Figure 4 shows how the improved bio-filter connects to the pond, but there are still improvements that can be made to such a basic filter system. Removing silt and suspended debris by mechanical filtration would, not only give improved water clarity but, more importantly, it would allow the bio-filter to stay cleaner for longer and, as a result, work more efficiently. Different methods of mechanical filtration will be discussed in part three of this series, next month.
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Figure 4: Pond with improved biological filtration
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