By Tom Graham
Reprinted from Koi USA
For more information on bubble bead filters click here
"Building an effective biomechanical filter is not tough, making one that is easy to maintain was the challenge."
What is the single most important element to a healthy koi pond? Filtration! What often represents the most work in owning a koi pond? Filtration! Where do many pond builders cut comers? Filtration!
It has been said over and over again. The single most important element of koi keeping is water quality, and water quality is a product of good waste treatment. Somehow we must remove the waste products produced in our ponds. I recently had the opportunity to visit Dr. Ron Malone, an Associate Professor in the Department of Civil and Environmental Engineering at Louisiana State University. in Baton Rouge Louisiana. Over the past 12 years, his team of researchers have invested over $750,000 in funding from the Louisiana Sea Grant College Program and the National Coastal Resources Research and Development Institute, studying biological filtration systems.
They have focused on the development of cost effective water treatment approaches for use with high density aquaculH ture production facilities. The result of this effort is a series of head filters ranging from aquarium size to a unit that can handle the largest whale exhibits.
Dr. Malone, who leads the project, spent the day with me and took me step by step through the development and operation of these new filters. He told me that when he began the prqject 12 years ago, they started working with flooded gravel beds, similar to what is used widely in our hobby. As they studied the workings of this type of filter, they saw that the surface area of the media was not efficiently being used, and that the systems were very difficult to clean.
In their research, they studied the entire gamut of filtration media and filter designs. (An interesting story in it's own right). The goal was to find a media that would provide a high specific surface area for biofilm development in a small amount of space (in cubic feet) and to develop a filter design that would be easy to clean and cost effective.
They found that a spherical plastic bead, approximately 1/8in diameter (half the size of a pencil eraser), was the media of choice. The beads they use are made from food grade low density polyethylene plastic and they float. The beads provide a great deal of surface area for bacteria growth - about 400 square feet of surface area for every cubic feet of beads. This compares to around 100 for typical pea gravel, and 125 for bio-balls. And, since they are very durable they never have to be replaced.
They discovered that a floating bead worked particularly well, since the beads would pack into a static bed at the top of a filter chamber, providing the pockets to trap particles and grow bactena, much like an under gravel filter in an aquarium.
Then, when the filter requires cleaning, they turn off the pump and agitate the beads to break free the solids. The solids are then flushed out the bottom of the filter. In their commercial designs, called prop wash systems, they used a large chamber capable of holding 6 to 200 cubic feet of beads. The units are cleaned by a powerful propeller system which intermittently agitates the beads within the filter, shearing off excessive biofloc (loose bacterial colonies) and releasing captured solids. When the propellers are stopped, the beads float to re-form the filtration bed while the solids settle in an internal settling cone forming a thick sludge. The sludge is removed from a drain at the bottom of the cone. Only sludge is removed so the water loss associated with the cleaning process is negligible.
This system has proven to be quite effective in large commercial installations, where very heavy fish loads are being managed. The filters have been tested on systems holding food fish species (such as tilapia, catfish, striped bass, trout) along with a wide variety of specialized applications (including tropical fish, alligators and crayfish).
Once this system was perfected and in use, they switched their efforts to developing smaller, less expensive systems they call bubble bead filters. The new design features an hourglass shaped chamber where air bubbles are used to stir the beads, rather than a motor and prop.
The key element to the bubble bead filter is it's specially designed "washing throat". It is a constriction between the upper and lower chamber, which forces the beads to fluidized (disperse and flow like fluid) as they are gently scrubbed by bubbles which are literally sucked into the filter as the filter is drained. The bubbles move up from the lower chamber, while the water and beads flow down, causing the cleaning turbulence.The cleaning process is designed to remove captured solids without damaging the sensitive biofilms responsible for nitrification, and uses 10 - 15 gallons of water per cubic foot of beads.(A two cubic foot filter will use about 25 gallons).
These smaller systems use from 1 - 3 cubic feet of media, and stand about 4 feet tall. They are constructed entirely out of fiberglass and PVC fittings, with no moving parts whatsoever.
Deciding which system to use is determined by the maximum amount of feed (dry pellets) that is put in the pond on a daily basis. One cubic foot of beads can provide complete solids capture and nitrification for a feeding rate about 1 pound of dry pellets (35 percent protein) per day under production conditions.
For koi ponds, one cubic foot of beads can effectively process one half a pound of feed per day. At a 2 percent (of body weight) feeding rate, a cubic foot of beads will support 25 to 50 fifty pounds of koi food. Commercial food fish production facilities normally support 75 to 100 pounds of fish per cubic foot of beads, but this demands close daily management of the production system. If you compare that to even the most densely populated koi ponds, you can see these systems are extremely powerful.
Bead filters used to clean koi ponds are typically back- washed once or twice a week during the warm summer months and as little as once a month once feeding drops off in the winter. If filters are not washed they slowly clog, gradually shutting off the return flow to the pond. This decline in return flow is usually visually evident, providing a convenient reminder of the need for backwashing.
Flow rates for bead filters are dependent on the total ammonia-nitrogen excretion rates (TAN) and oxygen demand for the biofilters, which are controlled by the feed rate and pounds of fish in the system. A minimum rate of about 5-10 gallons per minute per 100 pounds of fish (or per 2 pounds of feed per day) is normally used to assure proper bio-filter operation. This means the system only requires a very low flow, low pressure pump, however, higher flow rates may be demanded for large ponds with few fish particularly when a UV light is being used for algae control. This does not present a problem for the filter since performance of the filters improves when the flow rates increase.
The bead filters are effective at removing suspended particles. as small as 10 microns, but cannot harvest the small 5-10 micron algae that often infest a pond. If this is a problem, a U.V. light sized to the ponds volume, (turning over the volume of the pond 4 times a day) will produce the desired results.
Since the small bubble bead systems proved to be well suited for ornamental fish ponds. particularly koi ponds, Dr. Malone engaged the assistance of Burt Nichols, of Water Garden Gems, in Marion Texas. Together they have developed a new model designed specifically for backyard koi ponds.. The system uses 2 cubic feet of media and the bubble cleaning design. Burt is now manufacturing and distributing these filters., which are designed to handle up to a 4000 gallon pond packed with koi. The filter can be seen at his facility, and at Koi Unlimited, in Baltimore. Maryland. The larger prop wash filters are manufacturered by Armant Aquaculture (504)265-9216.
Many thanks to Dr. Malone and his associates, particularly Dr. Kelly A Rusch, Assistant Professor-Research, and Doug Drennan. Research Associate. for spending so much of their valuable time with me answering all my questions, and ferrying me all over Baton Rouge to get this story.
For more information on bubble bead filters click here