Part 5: Biological Filtration

In the last article we discussed chemical and mechanical filtration. In this article I will cover biological filtration.

What is the purpose of a biological filter? It is the removal of toxic compounds by means of living organisms. The typical toxic compound would be ammonia and a living organism would be a bacterium. Doesn’t bacteria grow everywhere in a pond? Then why do we need a filter? The answer is that the pond does not necessarily need a biological filter in addition to the bacteria that is in the pond. In looking at nature, we realize that natural ponds and lakes do not have biological filtration in addition to the bacteria in the natural body of water.

So why are there so many biological filters on the market? Because there is one major difference between a natural body of water and most koi ponds. A natural body of water has very few fish in comparison to most koi ponds. For this reason, a koi pond needs to have a biological filter added to it.

What is a biological filter? It is a device that provides additional area to grow bacteria on. Bacteria will grow on almost any surface. Bacteria are microscopic organisms and therefore a great number of them can live in a very small area.

To understand how the biological process works in a pond with a biological filter we need to understand the nitrification cycle. Ammonia is converted to nitrite by one type of bacteria and then another type of bacteria converts the nitrite to nitrate. Both ammonia and nitrite in small amounts can be harmful to fish. In large quantities, nitrate can be harmful, but usually is somewhat less harmful in small quantities. To complete the process there are two different bacteria involved. The bacteria that converts ammonia to nitrite is very hardy and can easily live in any kind of filter that will support life. The bacteria that converts nitrite to nitrate is easily killed off and takes a quiet environment to survive. Because of this, this bacteria may not be able to live in a lot of filters.

Next we need to take a short course in chemistry. Chemically ammonia is NH4. This is then converted to nitrite which is NO2. This is converted to nitrate which is NO3. By looking at this process we see that hydrogen atoms are released and oxygen atoms are used to convert ammonia to other compounds. Why do we need to know about this chemical process? By understanding how a biological filter works, we can determine how well a filter will work.

Let’s talk about how a biological filter works. The bacteria that is used in this process attaches itself to a solid surface. The bacteria is not free swimming. The food this bacteria lives on must be brought to it in order for it to survive. In an aquatic environment this means that the water must move the ammonia to the bacteria. In our chemistry lessen we learned that for the conversion of ammonia to nitrite and nitrate, oxygen is needed. The oxygen has to come from the water or the air depending upon the filter. This results in a great deal of the oxygen in the pond being used up by the biological process.

Knowing what happens in a biological filter, it is now easy to understand how a biological filter works. Stating it simply, what is needed to make a good biological filter is a media that has a great deal of surface area for the bacteria to grow on and water circulation to the entire surface area of the media. Of course if this was all that was required it would be easy to make the perfect filter. Unfortunately it is not quite that simple. The real problem stems from a topic I discussed in an earlier article. Water always follows the path of least resistance. Because a lot of surface area is needed to grow bacteria, quite a few filters try to send the water through the media to get more surface area. The problem is that when the media starts to grow the bacteria needed, it tends to clog up and the water goes around the media. Even if the surface of the media looks smooth, if we were to look at it under a microscope we would see that it is actually rough. This provides the bacteria a lot more surface area to grow on. These tend to clog up very quickly, thus losing a large share of the surface area. These are just two reasons that biological filters start to break down.

Not too long ago the Japanese believed that a biological filter had to be 1/3 the size of the pond. The English thought that it had to be 15% of the pond volume. In reality there are not too many ponds in this country where the biological filter is 1/3 the volume of the pond. Explaining this further, if the pond volume is 3000 gallons, then the filter would be 1000 gallons. That sounds awfully large. So why did the Japanese want a filter that large? They determined number by trial and error and found this formula worked the best. At the time they were using stone as their filter media. Stone has a very small surface area for the volume that it takes up. It also tends to clog up easily. For both these reasons a large filter was necessary and it worked wonderfully. At the time it was believed that the water going through a filter had to stay in the filter for 20 minutes. Based upon this, the entire volume of the pond would need to go through the filter every hour.

Why did a filter this simple work this well? First it is now known the water going through the filter doesn’t have to stay in contact with the bacteria. In fact, bacteria grab the toxic compounds as soon as they come in contact with each other. By keeping the turnover rate through the filter ton once per hour it kept the ammonia and nitrite levels extremely low in the pond. Because the flow going through the filter was moving at a slow speed, it provided an environment conducive to the bacteria’s conversion of nitrites to nitrates. A filter with faster moving water tends to kill the bacteria.

Finally this filter was so oversized that when parts of it clogged up there was still plenty of filter left to handle the load. This allows bacteria and enzymes the necessary time to eat up the clog, thus enabling the formerly clogged portion of the filter to begin functioning again. Thus this filter becomes self cleaning.

Now before you all start drawing up plans to make a filter like those of the Japanese, there are drawbacks. This type of filter has the potential to become a breeding ground for all types of toxic compounds and nasty critters.

Now that we have some basic knowledge of biological filtration, in the next issue we will take a look at various types of filters on the market. We will review their good and bad points.

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