Biological Filtration: What Exactly Does That Imply?
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
For them the organic waste products in the water are the "food" on which
For this reason biological purification is also called bacteriological
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
These are bacteria which we mainly find in different kinds of aquarium
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°- 45°C / 50°-113°F optimum range: 20°- 40°C /
- 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.
This process we can subdivide into several processes, namely: oxidation and
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
à 2 NO2-
+ 2 H2O + 4 H+. ( nitrite formation by
2 NO2- + O2 à 2 NO3-.
( nitrate formation by Nitrobacter)
2 NH4+ + 4 O2
à 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
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
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
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
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
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
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.