Trickle Tower Filtration - Theory and Practice

by Bryan Bateman
AKCA Certified Koi Health Advisor
Vice President, MPKS

On our recent trip to Japan, we became envious not only of the beautiful Koi there, but of the practice of many of the breeders and dealers there of not using any filtration. They utilize, instead, "flow thru" systems. Bad water out, good water in, using the never ending supply of crystal clear water from the mountains above. This is a luxury that most of us do not have. Instead, we must "re-use" our water. This requires filtration.

There have been numerous articles on many types commonly in use, but the long and the short of it is that we have two tasks to accomplish: removal of solids and removal of dissolved impurities. One of the newer "kids on the block" for many of us (the technology has actually been around a long time) is known as trickle tower filtration. It is the purose of this article to explain what this is, both in terms of what we know about it and what we don't know.

Trickle tower filtration essentially involves the flowing of water over a media where bacteria live, converting ammonia to nitrite and nitrite to the less harmful nitrate. The media is open to the air and is not submerged in the water, thus providing the maximum flow of oxygen to the bacteria while also encouraging the release of harmful gasses such as carbon dioxide.

The normal conversion of ammonia requires a large amount of oxygen, which is abundantly available when the media and splashing water are exposed to the air.

For this reason, trickle filters are especially useful during the summer months when the oxygen requirements of the bacteria as well the fish are at their highest levels. Conversely, due to the high exposure to the air and resulting evaporation and chilling, these filters do not work well in colder weather.

As you may have gathered, trickle tower filtration helps satisfy the second of our filtration tasks - removal of dissolved impurities. For this reason, it is important to use water that has already had solids removed. Thus, the location of this filter would be after some type of mechanical filtration. A simple type of trickle tower filter could be a cylinder of sufficient diameter - say 8 inches minimum - placed above a receiving vessel. The cylinder is filled with most any type of media. Sand or fine gravel would not work so well, as there would not be enough "air space" within that type of media for the oxygen to make contact with the bacteria. Some better types of media would be pvc strapping, plastic bio-balls, large pieces of lava rock, pieces of filter mat, even hair curlers - anything that will remain loosely packed with plenty of air space.

In our system, we use five 12" diameter by 40" long sections of pvc duct pipe left over from a project at work. You could use a garbage can with the bottom cut out. Lots of room for creativity here. In order to distribute the water evenly over the top of the media, we use a 2" diameter pvc pipe coming from a pump. This pipe has ¼" diameter holes drilled every 2 inches. The cylinders sit on a shelf which empties into a 500 gallon tank built from landscape timbers and plywood, lined with rubber. Water exits thru a pipe near the top and flows down a stream back to the pond. This tank, incidentally, also contains the coils for heating our pond water in the winter, so we have a bypass allowing us to turn the trickle towers off during the colder weather.

One question I have been asked is whether this type of filter could be used in a closed system, such as a bead filter. I can see two possibilities here. First, the filter could be placed at the very end of the system immediately prior to the pond return. This is essentially just as we have explained above, and allows the towers to remain open to the air. If, on the other hand, it is desired to keep the trickle tower closed to the air, an air injection system would be required.

This creates two additional problems: how do we remove the air prior to the water moving on thru the closed system, and how to we get enough air into the filter to keep it an "open air" and not a submerged filter. In order to work properly, the water flow would have to be reversed - that is, upflow rather than gravity (down) flow, thus taking advantage of the tendency of bubbles to rise. Some type of pressurized air valve would have to be installed to allow the air to be released from the top of the filter. I leave the rest of this design to some enterprising hydraulic engineer. I understand that our British friends at Evolution Aqua have been working on a closed-system version of the popular Nexus system - a submerged but heavily aerated type of filter that has gained a strong following in the hobby in recent years. If they ever get that figured out, it could perhaps be modified for trickle tower use.

We do know that trickle towers are highly efficient systems for removal of ammonia and nitrites, and for gassing off of harmful pond gasses. The nagging problem of nitrate removal still remains. Although nitrates are not nearly as harmful as ammonia and nitrites, this end product of the nitrogen cycle can be problematic for the serious hobbyist who is concerned with the optimal development of skin and color quality.

We know that certain plants can help keep nitrate levels at bay. We also know that frequent water changes can be effective if done properly. Nitrates can also be removed in an anaerobic system, but this can have dangerous side effects.

There is a fourth possibility - it has been suggested that trickle towers will lower nitrate levels, perhaps even more efficiently than any of the above mentioned. At this point it is only speculative - not proven or explained by controlled experiment. There are three possible ways that this could occur.

First, there could be isolated pockets of anaerobic colonies formed in certain types of media such as lava rock or Bakki house media - a highly porous log-shaped media used in Bakki shower (similar to trickle tower) systems.

Another possibility is that nitrate, in the environment of the aggressive "crashing" of water molecules against the media as it falls, or some such similar phenomena, could convert nitrate to nitrogen and oxygen. In other words, the nitrates could be "gassed off".

A third possibility is that there may be a type of aerobic bacteria which could thrive in a trickle tower environment that actually would consume nitrates as a food source, much the same way that anaerobic bacteria does.

Personal experience has shown that nitrate levels have indeed been lowered when our trickle towers are running. During the summer of 2005, our nitrate levels were hovering in the 50 to 60 ppm level prior to running our towers. After four weeks of running the water thru our five trickle towers at a rate of about 3000 gallons per hour, the nitrate levels in our 10,000 gallon system had been reduced to the range of 10 to 15 ppm.

Was it a result of the trickle tower? Or were there other factors at work here, such as higher levels of algae, increased water changes, etc.? Any way you look at it, trickle towers are an interesting and efficient adjunct to any filter system. We continue to use our original home-made horizontal flow bio-filter, but will also continue to pump water from that into our trickle towers. You just never know.

© Bryan Bateman 2006