Any substrate in a recirculation system is prone to colonisation by bacteria. In fact, bacterial clogging is a major factor in reducing flow-through system plumbing. While solids removal is the primary duty of bead filters, they also supply a submerged substrate with a relatively high specific surface area and offer some nitrification. A few systems actually use submerged media as dedicated biofilters. For example, media may be placed over airstones in the fish tank. The rising stream of bubbles moves oxygenated water through the media nurturing the nitrifiers. The drawback to submerging traditional media is that oxygen for oxidation of the ammonia must come from the water and thus the nitrifiers compete with the fish for this often limiting commodity. Also, they require a large volume of media to accomplish nitrification. Therefore, traditional media is almost always used in either RBC's or trickle filters and submerged media biofilters are restricted to the specialized type discussed below.
Suspended Bed Biofilters
Suspended beds make use of the extremely high specific surface area of sand (about 2000) in a way that gets around the problem of clogging that occurs in traditional media when specific surface area exceeds 300 ft3/ft2 (m3/m2). This makes them the only truly compact biofilters. Suspended bed filters contain sand that is kept suspended in a constant state of agitation by upward flowing water. This occurs in a vertical cylinder that is usually clear so that the performance of the filter can be viewed. The constantly tumbling sand grains become coated with a layer of bacteria and, because the water moves around them, there is no clogging. This way the vast surface area of sand is able to be colonized by bacteria and supplied with ammonia and oxygen. Engineering a suspended bed filter that will continually tumble sand without letting the sand settle or blow out the top is difficult, but not impossible. Obviously, flow must be carefully regulated. As the bacteria proliferate on the sand granules, the granules become less dense and float higher and higher in the column of tumbling sand until they blow out of the top and are eventually removed by the solids removal system. In this way suspended bed filters are self cleaning.
When properly set up, suspended beds are compact, efficient biofilters. Their big drawback, of course, is that oxygen for nitrification must come from the water. Because suspended beds are usually employed in sophisticated, high density recirculation systems and because of the inherent limitations of atmospheric aeration, suspended bed biofilters are usually combined with oxygenation.