Biological Filtration

Biological Filtration


Biological Filtration: What Exactly Does That Imply?
By Bert Engbers
April 2002


The aim of biological purification is to keep the water, in which our splendid fish and plants live, at a certain quality in which they feel thenselves pleasantly comfortable. Therefore in other words: to strive for a biological equilibrium in our aquarium. The need of a good biological filter is in my eyes for that reason essential. It ensures that the arisen and produced organic waste products, by plant and animal, are demolished on a justified manner and are converted into harmless inorganic products.

The operative organism in a bio filter (biological filter) are of course bacteria.
For them the organic waste products in the water are the "food" on which they live.
For this reason biological purification is also called bacteriological purification.

We know two types of environments which support the several bacteria that take care of the break down of organic waste products, namely: aerobe and anaerobe environments.

Anaerobe bacteria can demolish organic waste products without the use of oxygen.
These are bacteria which we mainly find in different kinds of aquarium substrates.

Aerobe bacteria are bacteria which demolishes the organic waste products using dissolved oxygen in water. These are the operative bacteria in a biofilter such as we know it. This process is also called aerobe mineralisation or aerobe purification.

Some vital factors for the activity of aerobe bacteria:

  • Temperature (between 10- 45C / 50-113F optimum range: 20- 40C / 68-104F).
  • pH value (between 4.0 - 9.5, optimum between 6.5 - 7.5).
  • Oxygen, dissolved in water.
  • Building material for growth of the bacteria, such as organic and inorganic compounds containing: phosphorus, carbon, sulphfur, nitrogen, calcium, sodium as well as certain trace elements such as potassium, iron, manganese, copper, and so on.
  • absence of toxic substances.

The reproduction rate of bacteria is very high.
The doubling time varies depending on the type of bacterium: from 20 minutes to 30 hours.

Aerobe purification.

This process we can subdivide into several processes, namely: oxidation and nitrification.

In oxidation, using oxidizing bacteria (Pseudomonas, Achromobacter, Arthrobacter, etc.) and oxygen, the organic waste products are demolished and converted into simple inorganic end products, namely: carbon dioxide (CO2), water (H2O) and ammonium (NH4+).

In a new biological filter a culture of oxidizing bacteria arises within a relatively short time, verified by the fact of how quickly ammonium in the water can be measured.

At the nitrification step, using nitrifying bacteria (Nitrosomonas and Nitrobacter) and oxygen, the arisen ammonium is converted into nitrite (NO2-) and later into nitrate (NO3-).

The nitrification reactions are as follows:

2 NH4+ + 3 O2 2 NO2- + 2 H2O + 4 H+. ( nitrite formation by Nitrosomonas)
2 NO2- + O2    2 NO3-.                     
( nitrate formation by Nitrobacter)
2 NH4+ + 4 O2 2 NO3- + 2 H2O + 4 H+.

The production rate of the culture of nitrification bacteria in a bio filter has as a rule a relatively longer duration then the oxidizing bacteria culture, but this is also strongly dependent on temperature. At the temperatures used in our aquaculture, the dividing speed of nitrifying bacteria will be higher than under the normal, lower temperatures.

Essential for a good aerobe purification is therefore, a sufficent level of oxygen in the water, since this oxygen will be consumed in the several processes.
For this reason the presence of plants in the aquarium or a good oxygen absorption by the water is of substantial importance.
Naturally, our fish also needs, of course, to have oxygen available for himself.


The presence of nitrates and the absence of ammonium or nitrites in the out put water of the bio filter indicates a total biological purification, which means all organic waste products are converted. For that reason we speak of "a complete or adult" biological filter.

Tips regarding the arrangment of a biological filter:

As a filter material use filling/material that has a very large surface area with large pore size.
This is because on this filter material the so called biological skin is formed, which consists of humus like substances, in which the bacteria are present.
The larger the surface, the more bacteria that can be present and the better & faster the purification can take place.

The filter material must:

  • Chemical and mechanically be stable, this means, does not pulverize.
  • Be biologically inert for infestation by bacteria, organic waste products and end products.
  • Offer binding possibilities for the biological skin.
  • Divides the fluid flow evenly.
  • It may not be too small of structure, because of the possiblity that the "bio filter bed" would clog up, which would reduce capacity/volume.

A way to prevent filter clogging is to direct the water flow in the filter from top to bottom with the top most filter material being a "finer" or small size and the bigger material being consigned to the lower part of the filter.

Also is it important in a biological filter to use a prefilter to catch the particle pollution in the water before it enters the biological filter area.

The flow speed of the water through the biological filter may not be too fast but also not too slow. An applied rule is that on average the total aquarium contents must pass through the filter within 1 hour.

Make sure that the water, pumped back into the aquarium, can absorb as much oxygen as possible by means of a bubblestone or a sprinkler above the water surface in the aquarium.

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