In previous articles in this
series -'Filtration Facts and Fiction’ we have established that the nitrifying bacteria are everywhere in the pond.
We have also established that bacteria in general, in an aquatic environment will naturally form biofiIms. More than 80% of
the bacteria found in the pond system are housed within me biofiIms.
In order to increase the surface area available
for bacterial growth we introduce a material into the pond. This material is referred to as the media. The media is the heart
of your recycling system. The success or failure of the whole system is dependant, to a large extent on me media. (size, depth,
type of material etc,).
Progressing along this line of thought we realise that the most convenient way of using this
media to our advantage is to place it in a chamber. This has further benefits. Keeping the media together we can channel or
pipe the pond water to this area. The pond water is then concentrated through and around the media. This has additional benefits
for a simple and important reason. The bacteria that colonise our bioconverters are unique in that they have to be attached
to something. They are not free floating.
This fact dictates that their food and oxygen must be brought to them. They
cannot seek their own nourishment.
This leads us to reach other conclusions. The bacteria that grow on and inhabit
the surface area provided in the bioconverters are microscopic. Therefore, the water laden with the food (ammonia) and the
oxygen should come into contact with the microscopic layer of bacteria by not being too far away.
Another conclusion can
be drawn from the fact that the bacteria adhere to the surface area is that the water passing by should move continuously.
In other words the pump or pumps must run 24 hours a day to bring the necessary nourishment and oxygen into contact with the
Bioconverters compete with the fish in the pond for the oxygen in the pond water to do their job of converting
ammonia to less toxic substances. Bioconverters work better at high levels of oxygen. Koi are healthier at high levels of
oxygen. Therefore, high levels of oxygen should be maintained through out the system.
Following from these facts we
have to concentrate on getting the water molecules into contact with the bacteria as efficiently as possible.
virtually becomes a living entity after it matures. It is home to millions upon millions of bacteria and micro-organisms that
are working in our favour by purifying the passing water. Nitrifying bacteria grow on the surface area of the media in the
bioconverter but so do a host of other organisms, including heterotrophic bacteria that consume organic matter.
studies indicate that the heterotrophic bacteria can, and often do, take up much more space than the nitrifying bacteria.
So provision has to be made for all the life forms supported by the bioconverter.
Media in a bioconverter
is the substance or matter in which and/or on which the organisms that perform bioconversion live and thrive.
I. Media can be used in the bioconverter or in the filter. The term media applies to both utilisations.
Used in a filter, the media is there primarily to provide some sort of barrier to the passing water. This barrier assists
in the trapping of solids.
3. In a bioconverter the media is used to increase the available surface area for the bacteria
that perform the function of nitrification to grow on. Media also traps "food” particles for the organic digesters (heterotrophic
bacteria) to work on.
4. There are a host of bacteria growing in the bioconverter not only nitrifying bacteria but heterotrophs.
All of these need space. Their living space must also be taken into account when designing a bioconverter.
Types of media
A huge variety of materials can be used as bioconverter media. It is critical that the media must be inert and not affect
the water quality in any negative way. Always be sure all materials are non-toxic and will not produce chemicals that will
he harmful to the fish. It's a good idea to thoroughly wash the media with hot water before use in the filter.
gravel, plastic of any description, nylon hair curlers, pot scourers, shade cloth netting, etc, etc, can be used as media.
Clay beads make excellent filter media material. Lava rock has many minute pores and a massive surface area for bacterial
Lava rock in small, golf-ball size pieces makes an excellent bioconverter filter media. Large pieces of lave
rock can also be used but it is difficult to clean without breaking off pieces of the rock. And large pieces of any media
allow unwanted channelling.
A sample of the lava rock to be used must be boiled before use to test if there are no chemicals
such as sulphur in it.
Ceramic by-products that look like coral are emerging, on the market. They are excellent filter
materials but with similar shortcomings to lava rock.
A media with a rough surface makes a more hospitable place for bacterial
colonies to establish themselves and additionally, has a larger surface area. Media with rougher surface areas can withstand
rinsing with less damage to the bacterial biomass but also retains some detritus.
Plastic in various forms is extensively
used as a media. Plastic is convenient and light to work with. One draw back is that it can be very expensive. Another is
that the microscopic surface provided by plastic for nitrifying bacteria is often very smooth. Unless the bioconverter has
an efficient up-stream settlement tank or mechanical filtration of some sort to remove the solids before the bioconverter
media, it must be capable of dealing with the debris load. The hollow spaces in plastic rings make wonderful areas for accumulating
As bacteria are very resilient and biofiIms can form on any surface - any matter can be used as a media.
Occasionally other media such as marble chips, zeolite and oyster shells are used to intentionally increase the hardness
and/or used as a media as well.
I. Ceramic - clay that has been fired at a very high temperature. It is sometimes porous.
Available in various shapes such as spheres, stars and hollow cylinders. By-products of kalians can form pieces of rough stones
with high surface areas.
2. Plastic beads - usually spherical shaped solid balls of plastic material.
3. Plastic shapes
- plastic media is available in a wide variety of shape from extruded and cut shapes to injection moulded shapes. Plastic
matting - random fibers of plastic thermally welded at points of contact.
4. Plastic foam - open cell polyurethane foam.
5. Brushes - usually plastic bristles with stainless steel wound centre wires.
6. Brick chips have been used with
Some thoughts on types of media
Some years ago a koi keeper bought some plastic shavings from a
plastic scrap yard. Placed in vegetable bags this media was placed into the filter chamber. A few days later the koi died.
The reason - on the plastic shaving was machine oil. This water soluble oil dissipated into the pond water and damaged their
Therefore, the point to remember about media it that it must be inert and not affect the water quality at all.
The surface area of bio converter media is important. This is the total surface area available in the bio converter for
bacterial species to grow on. Not only nitrifyes grow in the bioconverter but also a host of heterotrophs. The nitrifyers
convert the ammonia to nitrite to nitrate and the heterotrophs consume the organics that become trapped in the filter media.
Obviously media with a rough surface would be more habitable for bacterial growth when compared to a smooth surface.
Bacteria are very resilient and biofiIms are found on any surface including glass.
The total surface area for bacterial
growth in a bioconverter is increased (even if media with a small surface area is used) by increasing the volume of the media.
The volume can be increased in two ways. Firstly, by increasing the depth of the media. In this case, the flow through
rate will remain the same in the media. Or, increasing the size of the chamber through which the water is passing will spread
the media out (keeping it shallow). The direct consequence of this is that the water flow rate past the bacteria will slow
Media with massive surface areas can mean smaller bioconverters can be built. However, smaller bioconverters
will have very fast water flow rates. Therefore, a compromise must be found between massive surface areas of media (small
bioconverters) and the very fast flow rates that will either scrub the bacteria off some of the media or remove their food
supply before they can utilise it.
Hydraulic loads on media.
Hydraulic load or even flow can be defined as
the even movement, over the whole bioconverter chamber, of water on the interface between the media and the passing water.
Once the water enters the bioconverter chambers it is essential that the water has an even flow through and past all the media.
In other words the water passes evenly through the media with out channelling. Any channelling makes the bioconverter less
In open trickle filters the hydraulic load will be even as the water is sprayed over the top layer of media
and it trickles evenly down as a thin layer of water through the filter media.
Up flow bioconverters must be designed
in such a way as to ensure the water moves evenly upward through the media.
In up-flow bioconverters, a grid to suspend
the media off the floor will ensure the hydraulic load is even. The open chamber underneath the media makes flushing easier.
This is essential for the long term maintenance of the filter chamber.
The flow of water within the chamber is critical
to the success of the system.
Physical Lay-out of the Media.
The way the media is packed inside a bioconverter
chamber has a significant influence on its efficiency. It is critical that little or no channelling takes place in the media.
Any channelling will result in water by-passing a portion of the media and the bioconverter becoming less efficient.
beds with channelling could mean sections are deprived of oxygen rich water. Such areas may die and become anaerobic producing
smelly and toxic hydrogen sulphide. Further, channelling will allow a portion of the pond water returning to the pond with
the impurities it brought into the filter chamber in the first place.
Media placed in bags will have an area inside
the bag which has less flow of water and consequently settlement of solids will take place. Further, vegetable bags tend to
restrict the flow of water - especially after a season or two once algae and bacterial biofiIms develop.
flow rate with different media in different chambers.
Different media will have different flow rate characteristics. When
packed, some media have smaller openings between the pieces. An open media will have less flow restrictions than one with
Chambers with different media can have a greater or lesser tendency to 'back up' or clog due to
their different flow characteristics. Deep media beds will clog over time.
Media with very large spaces between the pieces
will also be less efficient as the water has to come into contact with the bacteria.
Depth of Filter Media.
it is anticipated that deep filter beds will have less and less oxygen in the levels further and further from the inlet, there
is much evidence that this phenomenon is insignificant in most instance.
As ammonia and oxygen rich water passes media
with nitrifying bacterial colonies growing on them, the ammonia will tend to be removed in the presence of the oxygen. Although
this is done immediately on contact the bacteria do have a finite ability in this regard. Thus any residual ammonia in the
water from the first contact simply flows on to the next colony and to the next, etc. Also any ammonia or oxygen not used
by the bacteria simply passes on to the next part of the system.
When the bacteria in the lower areas of the bioconverter
are saturated with ammonia their needs are minimal and the excess ammonia passes to the next portion. As soon as they need
nutrition they begin the oxidation process again. This is an ongoing and dynamic process in the media bed and should not be
viewed as an on - off type of situation.
Maintaining the media
No matter how efficient the mechanical side of
the filtration system is it is a fact of life that organics and solids will build up there. So we have to design the biofilter
and the mechanical filters to be easy to maintain. The media will clog over time; it will have to be flushed regularly. And
will this damage the nitrifying bacteria? Will we waste water?...
But that is all in the next part of,
Facts and Fiction - Maintaining Ponds and Filters'