Natural biofilter (one of three) on a stream ∧ pond
Biofiltration is simply using Nature's own methods of cleaning water. Building this ability into a garden pond/stream/feature simply enhances and encourages this natural cleansing. In essence, what we are doing is allowing an enhanced Nitrogen cycle to occur, by giving homes to good bacteria which convert "pollutants" (ammonia) into food (nitrates). Placing aquatic plants at the source of this conversion means we get an uptake and removal of nutrients from the water. This gives less opportunity for algae to feed on surplus food and helps keep the water clear. Certain styles of these biofilters can also filter the water mechanically.
In natural ponds, I design horizontal-flow biofilters into the stream beds as bulbous, gravel-filled areas planted with suitable aquatics. Water flows over and through these areas, which provide the home needed by bacteria to extract nutrients, as described opposite.
Formal pond with vertical-flow biofilter
Vertical-flow biofilters are several times more efficient at filtration than horizontal-flow but are more complex to incorporate into a scheme, lending themselves to formal pools and features as in the picture above. Water is delivered into the base of the filter via perforated pipes, which then percolates upwards through several grades of gravel. The gravel makes a home for the types of bacteria we want (Nitrosomona and Nitrobacter) which do the conversion of waste to food.
I beleive that all ponds, including Koi, can be filtered effectively in this way. It does have to be designed with the whole system in mind, and that includes the fish. If you have a soil layer over the top of the membrane as is best for a natural pond, then bottom feeding fish, especially carp should be avoided. Schoaling fish such as golden orfe are always so much more graceful and fun!Do contact me to discuss your water garden and filtration needs.