Theory of Bead Filter Operation

Bead filters function both as a solids capture device and a biofilter. They operate very much like a submerged rock bed or undergravel filter except that they are designed to facilitate cleaning. Bead filters dramatically simplify operations by providing two of the most important water reconditioning processes, clarification and biofiltration in a single unit. Clarification is the process of removing suspended solids from water. Suspended solids in a pond consist of small particles of partially digested food, debris, algae, bacteria, clays, and silts, which are small enough to stay suspended in the water column for an extended period of time. Fine suspended solids tend to make the water opaque, interfering with the clarity of the water. The larger organic particles, on the other hand, represent a serious wasteload problem and can consume tremendous amounts of oxygen while adversely impacting the hygienic pond ecology. Bead filters are considered excellent clarification units, capable of maintaining display quality water at high waste loading rates. Bead filters remove suspended solids by four different mechanisms as the recirculated pond water is passed through the plastic bead bed. Physical straining is probably the most dominant mechanism for the larger particles (>80 microns). The bulk of the suspended particles (20-80 microns) are probably removed by interception, a subtle process caused by collisions between the particle and the bead media surface. The finest particles (<20 microns) are removed by bead filtration, but at a slower rate. It is believed that bioabsorption, the capture of particles by the bacterial biofilm, is the dominant process attacking these fine particles.

Biofiltration depends on the development of a filter bed that encourages the growth of beneficial bacteria, which extract dissolved chemicals from the water and convert them to particulate biomass or harmless dissolved compounds. Given the proper encouragement, the bacteria grow in a thin film, coating the surface of each of the 600,000 beads contained in every cubic foot of media. There are literally hundreds of species of bacteria at work in a biofilter. Most of the bacteria fall into the category of "heterotrophic" bacteria, which actively break down organic materials into carbon dioxide and water. The most critical however, are broadly described as nitrifying bacteria, consisting primarily of the genera Nitrosomonas and Nitrobacter. Nitrifying bacteria are responsible for the conversion of the toxic nitrogen forms of ammonia and nitrite to the relatively harmless nitrogen form of nitrate . Success and a wide variety of parameters that influence bead biofiltration have been identified. The pond hobbyist should be aware of these factors. However, the pond itself generally contains a very active population of nitrifying bacteria, so optimization of the filter's operation can be relaxed.