Perhaps the most discussed, argued, analyzed, and misunderstood topic is the pond filter system. I know that almost anything I state here will he disagreed with by some, but that is the nature of this topic. I will attempt to keep the technical complexity to a minimum, which also means that some statements will be made without complete substantiation. First of all, let me limit the range of discussion for this article to the portion of the system dealing just with the biological nitrification process. Nitrification is a two-step process in which Ammonia/Ammonium waste from the pond inhabitants is converted to Nitrite and then the Nitrite is converted to Nitrate. The Ammonium-to-Nitrite step of this process is accomplished by autotrophic bacteria of the genus Nitrosomonas. In the presence of Oxygen, Nitrosomonas are capable of oxidizing Ammonium to Nitrite, and the bacteria use the energy released by the reaction as their energy source. In the second step of the nitrification process, Nitrobacter bacteria convert the Nitrite to Nitrate and utilize the energy produced to drive their processes. The first step of the process produces almost four times more energy than the second, which makes the Nitrosomonas bacteria considerably hardier than the Nitrobacter. Both steps require substantial amounts of Oxygen.
Webster defines a filter as: "A porous article or mass (as of paper or sand) through which a gas or liquid is passed to separate out matter in suspension." The biologic activity within the pond "filter" does not trap the matter in suspension but acts on dissolved components that could not be separated regardless of how fine the filter pores. Although this device may perform a dual role as a mechanical filter, to emphasize the processes of interest, you will see that I will refer to it as the biologic converter or bio-converter or just converter, not as a filter. I consider it very important to have a mechanical filter as the first component of the system. This removes as much particulate matter as possible prior to the bio-converter and allows the convener to handle theNitrification process only.
There are two primary requirements for an effective biologic converter. The first is to provide a media with sufficient internal surface area for the growth of the Nitrosomonas and Nitrobacter bacteria. The second is to provide a unifonn flow of Oxygen rich water containing the molecular "food" for the bacteria through the media and across these internal surface areas. This flow rate is of importance. If it is too fast, the bacteria do not have sufficient time to carry out their conversion processes. If it is too fast, the bacteria can be washed completely out of the converter. Although it is not a particular problem if the flow rate is very slow, it would only mean the converter is larger than is actually required.
Traditionally, biologic converters have been put into two classifications, up flow and down flow, based on the direction of water flow through the media. The side view of a simple up flow convener configuration is shown in Figure 1 and a down flow in Figure 2.
Often the inlet pipe of the up flow or the outlet pipe of the down flow is placed the container (passing through the media) as shown in Figure 3. It doesn't make it work any better, it is just easier to build, A variety of the down flow called a "trickle" or "wet/dry" converter is shown in Figure 4.
In this configuration, the media is not submerged in water but is wet with continuous (or intermittent) flow through the media and thus over the internal surface.
From mechanical viewpoints, each their own particular advantages and disadvantages. A down flow tends to plug up (and can overflow the container) when there is insufficient mechanical filtration, but it provides better entrapment of sediment in the bottom. The up flow tends to open channels through the media, causing some of the media to be bypassed with a corresponding reduction in the effectiveness of the converter. An up flow also has a higher tendency to carry sediment particles out of the converter and back to the pond. The design of a trickle flow is more complex to ensure a uniform distribution of the inlet water over the entire external and internal surface areas of the converter media. If flow is lost, the bacteria in the trickle converter starts dying off in about two hours, about half the time of the submerged media converters. I am not going into the so called "bubble bead" or fluidized bed converters at this time since few of these are "home made," They also have their own inherent advantages and disadvantages, So, which is best? There is no valid answer to this question since it depends on the application and other design considerations, The selection is most often based on the topography of the pond location, From the Nitrification process aspect, it must be emphasized that there is essentially no difference between the effectiveness of any of these. The bacteria don't care, All they want is plenty of food and oxygen and sufficient space to live and multiply, Sounds just like us.
The next area of major discussion and disagreement is the physical size requirements of the converter. Without getting too technical and providing lots of formulas and calculations to back up the statements, let me provide you with a set of four "Rules of Thumb" for the average pond and biologic converter system. I know that examples functioning systems deviating widely from these values can be cited, but the values provided below have been demonstrated over many years to be a conservativ,yet effective approach to the biologic converter design and operation. Using these Rules of Thumb to design an effective bio-converter, the most complex calculation required is that of area.
To start, we need to know the amount of water that is going to be processed. The first Rule of Thumb will help determine this value as the number of gallons per hour.
First Rule of Thumb The total water in the system should be cycled once every one to two hours. This also provides a guideline for pump selection. A larger pond (5000 gallons or more) can edge toward the two hour mark, whereas a smaller pond (500 gallons or less) should be closer to the one hour target. Note that this is the total water in the system and includes not only the water in the pond but also that in the mechanical filter, the bio-converter, and even the pipes. If a long cycle-time is used, the water may not pass through the converter often enough to keep the waste under control. A very short cycle-time probably means the electric company is getting paid too much. Assuming a 2500 gallon system, a pump rated at about 2500 gallons per hour would be a good choice. We know that due to flow friction and head losses that the actual cycle time will probably be a little more than once an hour.
Second Rule of Thumb The flow rate applied to the media in the biologic converter should be approximately 150 gallons per hour per square foot of media flow surface area. This, combined with the Third and Fourth Rules of Thumb, is used to determine the physical size of the bio-converter. The 150 gallon per hour value is fairly broad ranged. If it is exceeded, significantly, our friendly bacteria within the bio-converter will have insufficient time to do their job. If the flow rate is considerably less, the converter is probably physically larger (and therefore more expensive) than is necessary. The flow rate thus sets the cross sectional external surface area of the media, Based on our earlier example of a flow rate of 2500 gallons per hour, we would like our converter to have 16 to 17(2500 divided by 150) square feet of flow area. Now we know how big the top of the bio-converter container must be. A four foot square box would give us 16 square feet. A three feet by six feet rectangular box would give us 18 square feet. A round container that has a diameter of four and one-half feet would provide about 17 square feet. The possibilities are endless.
The Third Rule of Thumb is a table which will help select the size of inlet and outlet pipes required for a given gravitational flow rate. Combined with the Fourth Rule of Thumb, we can then
| Gravity Flow Rate (gal/hour) | 350 | 750 | 1500 | 3000 |
| PVC Pipe size (inches) | 1 1/2" | 2" | 3" | 4" |
Multiple pipes are often used to provide proper flow. For instance, two 1 1/2 inch pipes provide about the same flow as a single two inch pipe. For a flow rate of 6000 gallons per hour, two 4 inch pipes would be used on both the inlet and outlet. For our example of 2500 gallons per hour, we would probably select a four inch pipe. Since we need three times the pipe diameter both above and below the converter media, this sets the depth of our converter container to the thickness of the converter media plus 24 inches. If the inlet water is not provided by gravity flow from a mechanical pre-filter but under pressure from the pump, the inlet pipe can he smaller.
The Fourth Rule of Thumb is probably the most controversial since it deals with the type of media. First of all almost anything that does not react with the water or does not emit anything harmful into the water can probably be used as the media. (Be extremely careful of your media selection source. I once purchased some open cell foam from a foam vendor which rapidly killed off most of my fish before I found it bad been impregnated with an arsenic-based fungicide.) Other than a non-interactive requirement for the media, most other considerations are strictly mechanical. A Koi pond in Tahiti uses coconut shells as the converter media. They have to be replaced annually as they deteriorate, but the pond keeper has lots of coconut shells. Bacteria need as much internal surface area as possible for the bacteria to grow on. If the internal open spaces around these surface areas are too small, they will plug up rapidly. If they are too large, too much of the nutrient rich water bypasses the bacteria colonies. Crushed or smooth river rock of various sizes down to sand and volcanic rock of various sizes are widely used. Rock is relatively inexpensive, readily available, but heavy. The volcanic rock is normally lighter than the other rock and is touted as having the additional internal surface area due to the porosity. These tiny pores tend to rapidly plug up and soon the volcanic rock has essentially the same effectiveness as crushed rock of the same general size. Some material sold today as "volcanic rock" is man made out of cement. Many plastic-based products are available in the form of balls, rings, strands, webbing,cell foam and mats. These are normally more expensive than rock but provide an effective, light weight, non degradable media. Some of these will float and a top grating must be used to keep them in their proper place. It is very important that whatever media is chosen it is uniformly distributed in the media area. Any gaps or channels within the media can significantly decrease the converter's effectiveness. Mats and open cell foam, when properly installed provide a structure that prevents chanels from developing. The Fourth Rule of Thumb is actually closely related to the Second Rule of in that the two of them combine to determine how long the water is in contact with the internal surface areas of the biologic converter media. Although the interaction between the two is quite complex, the Rules of Thumb were selected to provide a system that will meet the needs of almost all ponds when the most commonly available materials and components are used.
| Open Cell Foam | 8" | 1/10" to 1/8" cell size |
| Matting | 10" | 1/10" to 1/8" cell size |
| Stranded Plastic | 12" | unifrom packing & position control |
| Balls, Cylinders | 16" | uniform packing required |
| Sand | 20" | may plug quickly |
| Rock 3/8"-1/2" | 22" | crushed smooth or volcanic |
| Rock 1"-2" | 24" | crushed, smooth or volcanic |
Wait a minute, why does it have to be up or down? The bacteria don't care and probably have no knowledge which way is up. What would happen if the flow was horizontal. Figure 5 shows a top view of a horizontal flow . Let's check out some of the advantages and disadvantages of this layout. All the design considerations from Fourth Rule of Thumb Media Thickness Notes above apply, except the inlet/outlet areasonly have to be about the same width as the pipe size, not three times the pipe size. This cuts down our container size in one dimension by 4 times the pipe size. The plugging considerations are less than that of a down flow but still a little higher the an up flow. If the height of the media is a little less than the top of the container, it cannot overflow the container although the water may flow over the top of the media (an automatic indicator that it needs cleaning). The inlet and outlet sediment areas are the full depth of the converter container, which is great. However the complexity has gone way up. Although we don't need a bottom support since the media goes all the way from top to the bottom, we have to provide support for the media on both sides and the supporting framework has to be strong enough to hold the media in place when there is no water in the container. It would be very difficult to use sand as a media in this arrangement. The channeling risk is much less than an up flow, and, depending upon the media support design, probably also less than the down. But, we have access to both the inlet and outlet sides(from the top) without removing the media. This last one overrides everything else for me; I'm going horizontal flow.
Note that converting a standard 55 gallon drum from up flow or down flow to a horizontal flow configuration will increase the media flow area. The drums are normally about
22" in diameter and 36" high. Using any media that requires
less than 16 inches of thickness in a horizontal flow mode will provide about 5.5 square feet of media surface area (22x36/144). This would still provide reasonable inlet and outlet settling areas and handle about 800 gallons per hour. When used in an up flow or down flow mode, the same barrel can only provide a media surface area of 2.6 square feet. less than half the down flow.
Now how else can we arrange this horizontal flow converter concept to further minimize the physical size of the container without decreasing the amount of converter action provided? Often it is simpler to build two or more smaller converters and connect them in parallel. i.e. equal portions of the water flow though each one. (There should never be a reason to connect biologic converters in series. (i.e. end to end) Figure 6 shows the top view of two horizontal flow converters connected in parallel. This leads to the idea of putting two converters in the same container to save additional space. Then, since the thickness of the media also controls the size, we will
shift our selection to open cell foam which only needs 8" of media thickness and put it in slightly diagonally to further save space. Figure 7 shows the top view of such a dual,
horizontal flow bio-converter that requires a container that is 3 feet deep, 3 feet long, and 2 feet wide which provides 18 square feet of media (two pieces 3 feet by 3 feet). It can easily handle a 2700 gallons per hour flow rate. Compare that with a 18 square feet up flow or down flow using the same media that would require a container almost 3 feet deep, and 3 feet wide by 6 feet. Notice that the inlets and outlet can interchanged if that is more convenient for a given installation. If two inlets are used, each should be equally supplied with half the water. I actually bring in each of the inlets about 6 inches off the bottom and take the single outlet from the top as a waterfall back into the pond.
I have two of these dual horizontal flow biologic converters in my yard. The first has been running for about five years. It is in a 2 1/2 feet by 2 feet container that is 3 feet deep. The foam is self supporting when cut about 1" longer than the container and weged into place. No framework was required. This provides 12 square feet of media flow area and is used on a 1200 gallon pond with a flow rate of about 1200 gallons per hour. The second has been in use for 2 1/2 years and is contained in a cell 2 1/2 feet wide by 4 feet long by 5 1/2 deep. The open cell foam media, each peice is about 4 feet by 5 1/2 feet, is also self supporting, although I did build a lightwieght framework out of 1/2" pvc pipe for it. This provides 42 square feet of media flow area supports an 8500 gallon pond with a flow rate around 6000 gallons per hour. Not that the latter configuration has a "footprint" feet by 2 1/2 feet and that a comparable up flow or down flow would have to be over 4 feet by 10 feet.
Because of the direction of flow and the deep settling areas on the inlet and outlet sides, the media seems to be almost self- cleaning with the most sediment build up occurring in the outlet area. If a deep cleaning of the media is required, only one side needs be cleaned at a time, thus keeping a live converter on line at all times. Open cell foam media may have to be cut into smaller pieces (about 2 feet by 2 feet) for ease in handling and then stacked like bricks in the media support structure. It does get heavy when filled with detritus, I have not yet needed to deep clean any of my converter media. Periodic cleaning of the sediment areas is quite easy since the media does not have to be removed. It can be cleaned out by either simply draining and flushing the container or using a wet/dry vacuum from the top.
Is a horizontal flow biologic converter the best? Again, no answer to that question is possible. I believe it to be the best selection for my applications. Even if it turns out not to be the best choice for yours, it is another alternative to consider. Quite often an existing up flow/down flow can be easily converted to horizontal flow or even dual horizontal flow and provide substantially more biologic conversion capability in the same container. No matter what the choice or what the application, the basic Four Rules of Thumb provided still apply and can be used to provide your Koi with an effective bio converter system.