About Fluidized Bed Filters


Fluidized bed filters are rapidly becoming the primary source of biological filtration in many modern aquaculture systems, replacing the less efficient "trickle" filters now in service. While trickle filters and fluidized beds both rely upon the same species of bacteria for ammonia and nitrite removal, it's how the two filters operate that sets them apart. It's very helpful to remember that in biological filtration, it's actually the bacteria that do the work; the filter itself only provides a suitable "home" for the bacteria to colonize.

Trickle filters can best be described as open containers filled with various forms of solid media. As water enters the top of the filter, it drains down through this media in a random, cascading fashion. Since the media is stationary, it relies upon the changing water currents to keep the bacterial surface area moist.

Fluidized bed filters, on the other hand, are flooded cylinders or tanks; partially filled with a granular media, such as white quartz. Water upflows through this media bed, causing it to expand and fluidize. The large number of bacteria that colonize this expanded media serve to remove the nutrients that are present in the flowing water. What results is a highly efficient filter bed that is no longer solidly packed, but is in dynamic motion. It is this combination that is the key to our QuikSand Filter's success.

While trickle filters may have relatively large amounts of surface area (up to 200 square feet per cubic foot of media), it is virtually impossible for bacteria to colonize all of it. This is because water will take the path of least resistance as it moves down through the media, often leaving dry areas and low flow, anaerobic pockets. Another serious problem with these filters is called "bio-fouling". This occurs as particles of organic material lodge within the filter bed. Once clogged, you have no choice but to tear the system down and clean it. This is not a pleasant job; but if left unattended, your water quality will rapidly degrade.

Fluidized bed filters, by design, do away with the trickle filter's shortcomings. The most obvious benefit is that all of the filter's large surface area is available for bacterial growth. With more than 6,200 square feet of surface area per cubic foot of media, you can quickly understand why fluidized bed filters don't need to be nearly as large as comparable trickle filters. Since the water is evenly distributed as it flows through the cylinder, there are also no anaerobic pockets of bacteria developing. The water that reaches the media is oxygen rich and very conducive to rapid bacterial growth. Also, these filters are almost impossible to clog due to the constant motion of the media. The filter bed is a mildly abrasive environment that is constantly cleaning and renewing itself as it moves quietly around inside the cylinder.