understanding filtration

3. Increase the number and or size of the fish that can be satisfactory maintained.
These objectives are best achieved by the creation and maintance of a biological filtration system.

BIOLOGICAL FILTRATION.
Basic to the successful creation of a biological filtration system is an understanding of how such a system works. Miraculous in its effect, the principle is quite simple once it is understood, and merely harnesses nature’s own way of purifying water as in rivers and streams. The same concept is used in sewerage and septic systems, exploiting the activities of bacteria which consume the relatively non-toxic by-products.
These active and useful micro-organisms are called aerobic bacteria, aerobic because they require a constant supply of oxygen. The purifying process is carried out by groups of these bacteria acting in series. The first group known as heterotrophic bacteria break down fish wastes to produce ammonia, the second group, nitrosomonas S P, break the ammonia down to produce nitrite, and the nitrobacter S P, convert the nitrite to nitrate. Denitrifying bacteria then reduce the nitrate to nitrous oxide (N2O) or to molecular nitrogen.
Another form of bacteria, the anaerobic group, does not depend on a supply of oxygen. The by-product of their activities however is a toxic chemical which will act quickly to the detriment of the fish resulting in sickness and often death. Anaerobic activity occurs for example, when material such as dead leaves, algae, food and fish excrement is allowed to accumulate on the bottom of the pond. These wastes rot and give off a foul odour or gas.
So from this, it can be seen that it is necessary to cultivate aerobic conditions. In order to do so, a continual source of oxygen is needed; otherwise the aerobic bacteria will be destroyed and replaced by the anaerobic group. Aerobic bacteria extract oxygen from the water as do fish, so it is essential to maintain a continual flow of oxygen rich water over the medium on which the bacteria grow.

BIOLOGICAL FILTER MEDIUM (GRAVEL)
What does filter medium mean? Simply, it refers to the material on which aerobic bacteria are able to grow and multiply, build-up their housing settlement in the form of slime called biomass. We have all at some time, picked up a rock from a creek or a river bed and felt the slimy texture of its surface. By doing so we have come into contact with these bacteria and with the type of conditions we are to create in the filters.
However, in order to gain the maximum effectiveness from the biological system, filter beds have been devised comprising of layers of gravel through which a continual supply of oxygen rich water flows. Today you can install filters comprising of Japanese matting, Bio balls, suspended media, beeds etc. These all furnish a large amount of surface area on which the bacteria can grow under aerobic conditions.
The total surface area of a gravel filter should be equivalent to at least one third of the surface area of the pond itself. The minimum rate of water flow through these filter beds should be enough to turn over the total pond volume 8 times daily. The depth of the bed varies with individual ideas, most of the bacteria activity takes place in the first few centimetres from the surface, and most beds average 30 cm in depth, but many set up’s go much deeper. Suspended matter can build up in the filter bed, blocking the flow of water through the gravel. This stops the supply of oxygen to the aerobic bacteria and thus induces anaerobic conditions. In gravel beds, blockages can also occur because of the biomass, which dies and falls off the media on which a new biomass develops. In this self perpetuating process, the dead biomass can accumulate amongst the gravel and block the flow of water. To counter this problem open void systems allow the dead biomass to fall through the bed to a point where it can be discharged outside the system, gravel beds are suspended above the bottom, the open void allows a pipe grid under the filter media to take the clean water back to the pond and an outlet drain to flush the system clean when needed.

BEFORE YOU FILTER
Many Koi enthusiasts have gone to the trouble and expense of setting up a biological or for that matter, any form of filtration system only to find the benefits greatly offset by problems of filter or pump blockage. Despite initially clear water, there occurs a gradual build up of suspended matter in the water that reduces clarity, and can settle and accumulate in corners or depressions in the pond. This build up then produces conditions in which anaerobic bacteria can thrive and produce toxic by-products or pollution in the pond. In addition, by building up in the filter bed, the material blocks the water flow and thus stops the flow of dissolved oxygen to the aerobic bacteria active in the filter. This results in their elimination and subsequent replacement by producing anaerobic bacteria.
With these facts in mind, it becomes apparent that something is necessary to prevent such accumulations before the water from the pond enters the filter area and the answer lies in the use of settling tanks. This feature greatly assists the avoidance of filter and pump blockage, the settling tanks will remove most of the suspended material before it can get to the filter.
The principle of settling tanks is to direct waste water from the lower levels of the pond into a series of separate containers of sufficient dimension and depth to allow suspended matter to settle out as it travels to the next outlet. The settling tanks should be arranged so that water can pressure feed or siphon from the pond into the tanks via pipes, and subsequently from one tank to another. Thus, the flow of water should be decreasing as it spreads out in the settling tank, allowing the settling to take place.
Transfer pipes from one tank to the next should receive water from the cleaner area close to the surface and expel it into the next tank at a lower level. Denser matter that remains, not being of a floating substance will settle rather that rise to the next outlet as the water will do. Transfer pipes may be offset to direct water in at one end of the tank and have it flowing out at the opposite end in order to improve the settling process.
The system can be further improved by having outlets in each tank which can be opened to flush away the settled matter to drain. The last tank contains the pump or pump intake which keeps the system in motion, moving the now cleaner water to the filters and maintaining the siphoning flow of water from the pond and through the settling tanks. At this point it is important to ensure that when designing the system, sufficient allowances are made to enable the siphoning system to be able to keep up with the amount of water the pump is drawing.

BIOLOGICAL START UP
The graph below left represents a typical biological start up. The graph below right is a true reading of my 60,000 lt Main pond. Finished and filled with fresh water in August 1995, a gravel filter and 2 x 400 watt pumps delivering 16,000 lts per hour.
Construction can be seen at "pond construction" link.

The tap water was pH 7.2, Nitrite nil, Nitrate nil, Ammonia 0.4 ppm. August that year the pond water was 13 Deg C on day 18 after water and pond maturing, Clear Pond (active bacteria) was added to the pond to give the filter a boost because of low water temp.
On day 20, five small test koi was introduced on day 30, ten bigger koi went in and on day 35, thirteen large koi went in the pond, as the pond warmed up it took 3 months for the filter and pond to mature fully but has stayed at pH 7.8 ever since.

When starting up a new pond always age your water. Test before introducing fish and test again after.

Article by
Harry Watson.