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Introducing Kaldnes Biomedia In Hozelock Trinamic Pond Filter Design & Operation

The primary job of a biofilter is to convert ammonia (or ammonium) to nitrites and then to nitrates using specific bacteria types. In practical terms this requires that ammonia, which is dissolved in the circulating water, must make contact with bacteria at an interface and in conjunction with oxygen in such a way that a chemical reaction can take place.

Take a look at our diagram again.

workings of a pond bio-filter

Once the chemical reaction has taken place then the product of the reaction which is nitrite or nitrate must be able to be transferred back into the body of the circulating water and removed from the immediate presence of the bacteria to make way for the next bit of ammonia.

If you go back to our coffee examples what do you think would happen if there was

  1. only a small area for contact with the bacteria or

  2. the rate of flow through the filter was slow or

  3. if the temperature was say only 5 degrees centigrade or

  4. if there was insufficient oxygen in the water.

All of these things happen to be bad for biofiltration and all can be explained by an understanding of the principles of mass transfer.

Therefore a well-designed biofilter MUST have:

  • A large surface area on which bacteria can grow and to which a maximum amount of pond water can be exposed

  • Biofilters work best at higher flow rates/lower residence times - ie turbulent flows are best.

  • The presence of large amounts of oxygen at the interface also creates the concentration differences required for good mass transfer. This is why top class koi keepers have vortex filters and blow massive amounts of air around the biomedia that is generally Japanese matting.

  • The Hozelock Trinamic products (by far the best so get as much information as you can from Bradshaws) and before this most types of pressurized pond filters (and especially the FishMate series which contain Alfagrog biomedia) are excellent examples of good use of mass transfer principles

  • On a much larger scale and used extensively in large koi ponds the biomedia Kaldnes (as used in the Trinamic) allows small footprint biofilters to replace massive underground vortex filter installations. The reason is the dramatic increase in turbulence and surface area contact made possible by the properties of Kaldnes. Before Kaldnes came along all biofilter media were static and relied upon turbulent flow passing a static concentration of bacteria. With Kaldnes and the Trinamic the bacteria are in a continuous state of agitation and they love it.


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