Biological Filtration Ponds

Biological filtration relies on the activity of specific bacteria cultured inside the filter system. These bacteria are required for the breakdown of toxic waste products and turn them into non harmful substances. These bacteria occur naturally and are used in the domestic sewage industry to break down the sewage in special treatment plants. Water treated in these plants becomes so pure it is often recycled for domestic usage, when looking at this your biological pond filter can be refered to as a mini treatment plant as its job is exactly the same, exploiting the natural sequence of biochemical reactions that occurs in the nitrogen cycle.
There are two stages in the breakdown of ammonia in the biological filter system, each stage involving different types of bacteria to power the detoxification process. In the first stage, ammonia is broken down to nitrite by a number of different nitrifying bacteria, The most important of which are Nitrosomonas. A second group of nitrifying bacteria, principally Nitrobacter, converts the nitrite to nitrate ( These stages can be seen in the expandable diagram on the right). Both these groups of bacteria are aerobic which means they need oxygen to thrive and thus purify the water. The nitrifying bacteria require an oxygen level of at least 1mg per litre, and this can be ensured by adding one of your air pump lines and stones at the start of your filter. This oxygen supply needs to be constant, you cannot turn the air supply off as the bacteria will soon use up the oxygen and start to die off, which is the same for your pond fish as explained in the water quality oxygen content section.
The process of fitration will inevitably produce a build up of sediment at the bottom of the filter, this build up can deplete the oxygen content in the water and causes the growth of anaerobic bacteria, ( the bacteria that thrives with the absence of oxygen), rendering the filter inefective, it is therefore also vital to keep sediment to an absolute minimum by flushing the system with pond water. Most lager koi filters are fitted with drains to allow this flushing process, simply leave the pond water flowing into the filter and open the bottom taps or valves. The pond can then be topped up as you would normally.

Biological filtration starts in the second chamber of the filter system, the one after the settlement chamber or vortex, The second chamber will be fitted with a medium that will provide a large surface area for the nitrifying bacteria to grow on. There are hundreds of varieties of filter menium as explained in the mechanical filtration section, some claim to be more effective than others and generally this is true and their costs reflects this, it is the surface area that is important, not the name or brand, for example draw a circle and then draw a hexagon inside a 5 cm square box, measure the circumference of the circle and then measure each line of the hexagon, you will find although both are the same scale, the hexagon has more lenght if made into a straight line, this is the extra surface area.
It really does not matter what medium you use as long as it has enough surface area for the bacteria to thrive meaning the toxic substances can be broken down into harmless ones, thus providing your pond with the pure water it needs. Fish stocks, planting levels etc will all need to be taken into account and when buying your first filter always make sure it is bigger than you really need, buying to small a filter will always cost you more in the long term.

The biological aspect of filtration takes time to develop, it is not simply a fit that day and it will work, there are many chemicals that can be added to the filter to encourage the growth of bacteria, however these are costly and only really work for aquariums as a small volume of water is being filtered. A pond needs weeks and months to run before the biological filtration can really kick in, this is why with a new pond you really must wait a good few months before adding any stock. New stock will add a strain to the filter and the balance needs to be prepared for the influx of additional waste from the fish, it is said a filter and pond takes a good 6 to 12 months to really be fully functioning.
When introducing fish you may find the ammonia levels increase dramatically, sometimes to dangerous levels. These high levels may only last a matter of days to subside in the summer but upto several weeks in the winter months, this is because the biological activity is significantly reduced by external parameters such as the weather, ph, temperature and the number and size of the fish in the pond. It is important to remember that the growth and reproduction of bacteria are inhibited by the cold, both types will cease with temperatures below 5 degrees celcius ( 41 degrees F ).
During the spring any filter will benefit from a boost of freeze dried or live cultures of nitrifying bacteria ( add them during the late evening when the water is at its warmest )  if you have a local stockiest of these then it is worth keeping in mind however the cost also has to be brought into mind.