Fluidized Sand Biofilters - Tech Talk 112
The purpose of a biological filter is to convert ammonia to nitrate. Fish excrete ammonia in proportion to the amount of food they eat. In the AES catalog, you will see a "Feed per Day" rating based on a 40% protein content, 10% moisture content feed, with typical digestibility.
Fluidized bed biofilters using sand media are extremely compact and very inexpensive compared to other biofilters because the media is sand. Water flows upward through the sand causing the sand grains to float or "fluidize." When the water flow is too low to fluidize the bed it is called a "collapsed bed." When the proper amount of water is flowing, the sand expands upward, a condition referred to as an "expanded bed." If too much water is flowing, the bed will over expand and the smaller grains will be carried out of the biofilter in the outflow water.
There are three important aspects of fluidized sand biofilters. The first is the water inflow diffuser at the bottom. The diffuser creates a uniform, low-turbulence sand flow pattern. Excessive turbulence can erode the biofilter vessel and scour nitrifying bacteria from the sand grains. The second aspect is water flow velocity. A narrow water flow range must be maintained to keep the sand properly fluidized. Flow variations caused by pre-filters can result in collapsed or "blown-out" sand beds. The third aspect is refluidization. At start-up, a little extra pump pressure and an effective water diffuser design are required for initial fluidization.