by James P. Reilly
Reprinted from Koi USA
The most efficient filtration system today is the trickle or wet/dry filter design. Perhaps the best way to explain this rather bold statement is to study and dissect other systems and identify their weaknesses.
As an approach, I would divide all filtration systems into two groups long term and short term designs. The short term design is one that will work efficiently for a period of time and gradually the performance will begin to fail off. This design is best illustrated by the traditional undergravel in pond filter and the gravel filled chamber filter. In both designs, channeling and anaerobic activity become more and more likely over time. Frequent maintenance can put off real problems, but eventually the low levels of stress created by the fluctuating water quality affect the fish. Low oxygen levels usually go hand-in-hand with this system and it has been demonstrated that only the top few inches of gravel can support aerobic (oxygen loving) bacteria, regardless of the overall media depth.
This short term design can be - and in fact has been improved by the subH stitution of material in the same traditional design configuration. In this evolution of an old idea, gravel is replaced with open cell foam and/or Japanese matting (actually air conditioning filtering material). This simple substitution of material solved 95% of the problems associated with the old systems. Both water distribution and oxygen levels are greatly improved, but over time these submerged materials can become inefficient as well. As bacteria dies and new bacteria "glues" itself to the substrate, a coating or debris builds up. Algae and diatoms also contribute coating to the original surface. Slowly, this once prime growing surface becomes overwhelmed with decaying materials. Oxygen levels still "test" high, but B.O.D. (biological oxygen demand) levels begin to drop.
To prove this point, I would suggest that the hobbyist look at an accelerated version of this scenario. If you use brushes in your prefilter take note of the odor that is emitted from your system when doing normal maintenance. If maintenance is done often, no odor is usually detected. This is due to the likelihood that the bnishes are performing like a biological filter. When a foul odor is detected, it is likely that brushes have been overwhelmed by anaerobic bacteria and decay due to excess debris build tip even though oxygen rich water is rushing by and through the brushes. It certainly would be a disaster if these conditions built up in a biological filter chamber right next door! These same conditions can and do build up - given time in open cell foam and matt filters. Odors during this descent may or may not be detected and oxygen levels may only drop modestly. The evidence, however, will be found in lower pH readings, low B.O.D. tests, periodic ammonia spikes (1-2 hours after feeding). excessively high nitrates, fish flashing in early morning only or midday only and/or frequent cloudy water (bacteria blooms). These all can be classic signs of an undersized filter or an overaged short term system crash.
Other examples of short term designs are reverse flow chamber filters, biobrush systems submerged bioball and canister style systems. It is not my intention to destroy people's confidence in these filter media and designs. Most problems with these systems can be corrected with frequent maintenance and regular water changes. I only want to point out the limitations of these designs and - more importantly - identify these systems as "changing" systems that decline in performance over time. All of these systems. when properly maintained, are fine for planted goldfish and shubunkin ponds seasonal koi ponds and moderately stocked nishikigoi ponds.
The group I referred to as long term filter designs would include any system which can remain debris free and oxygen rich indefinitely and employ in its design superior surface space for aerobic bacteria to grow. The three leading designs currently available are the trickle or wet/dry filter, the bead filter and the fluidized bed filter.
To appreciate these filter designs.. we must first understand the problem - how to maintain a large number of koi in a reatively small body of water.
The facts...
The answer... the trickle fitter! The trickle filter is designed to convert ammonia to nitrate by incorporating an oxygen rich environment and a very large surface area for aerobic bacteria to perform. Water enters the top of a trickle filter through a spray bar or drip plate. No debris can pass through the small openings that distribute water evenly over the plastic media. As the water cascades over the media, two events occur. First, this action allows gases, such as carbon dioxide to escape and this aids in keeping pH high. Secondly. by trickling water over the media contact between bacteria and ammonium is maximized. During this process, fresh air is pumped upward through the media column insuring conH stant high oxygen levels in all areas of the media. As a bonus, the falling water has a secondary cleaning and rinsing effect on the plastic media by washing away any hits of debris and dead bacteria into the sump below. The exit to the tower can be hooked to a foam fractionator to remove the foam that naturally builds up from the agitation created by the rising air and falling water meeting at the base of the tower.
Now that a perfect oxygen rich environment has been created, how much surface is there for bacteria to grow on? The size of the living aerobic bacteria colony will adjust to its food supply and O2 level We do not want to restrict surface area in any way since we have enough oxygen and the koi will certainly supply the food.
Each one of my trickle towers holds 1500 sq. feet of surface space per 150 gallons. I use four towers in all for a total surface space of 6000 sq. feet. If one attempted to create the same surface space in gravel, the filter would measure 100 feet long x 5 feet wide x 4 feet deep. In both cases. however, the true action would be less than the trickle tower, because oxygen levels could never be as high or as uniform as in the tower.
My system has been in place for I year and 4 months at the time of this article. The pond is 4,600 gallons and contains 25 show koi ranging in size from 21" to 28". Water quality is good: oxygen levels of 9.2 and pH of 7.4. 1 believe this system matured about S months ago and has remained solid since then. The towers themselves are located in a basement room and, to my surprise, this location has a slight warming effect on the overall winter pond temperature.
I would highly recommend this long term design to anyone wishing to upgrade their present system and I would further recommend a good foam fractionator as the only other device needed to maintain the highest quality environment for our fishy friends.