Biotechnology and Bioinformatics (1+1)

6.2.4.3. Fluidized bed sand filters

Regular sand filters such as the type used for swimming pool filters or potable water filters are virtually worthless as biofilters for aquaculture. The biofilm quickly fills the spaces between the grains and the pressure drop across the filter rises rapidly. Frequent back flushing is required and the active biological film is removed each time.

In contrast, fluidized bed filters have been successfully used for aquaculture applications. A sand filter becomes fluidized when the velocity of the water flowing up through the bed is sufficient to raise the grains of sand up and separate each grain from its neighbors. In hydraulic terms, the drag on each particle is sufficient to overcome the weight of the particle and the particle is suspended in the stream of water. The velocity required to fluidize the particle is a function of the shape, size and density of the particle.

Advantages

· They pack more biologically active surface area into a given volume than any other type of biofilter.

· The best shape for a fluidized bed sand filter is a tall column. Thus they have a small foot print for a given capacity.

· They are self cleaning and relatively tolerant of different nutrient loadings.

Disadvantages

· The fluidized bed sand filter has a relatively high energy requirement because of the high pressure drop necessary to fluidize the sand.

· The pressure required to fluidize the bed varies depending on the amount of biofilm on the sand particle. As the biofilm builds on the sand particle the size of the particle increases while the density of the particle decreases. This means that the depth of the bed will tend to increase as the bed ages. It also means that the bed depth will fluctuate as the loading on the bed varies.

· In order to prevent blowing the sand out of the tank, the tank must be oversized or the flow of water needs to be regulated.

· Another potential problem is the uniformity of the water flow. In order to completely fluidize the bed, the water needs to be evenly distributed across the whole bed. Two things can happen if the flow is not uniform. One possibility is that the water will channel and short circuit through the bed. This means that the treatment capacity will plummet. Another possibility is that the short circuit will happen near the wall of the vessel and the abrasive sand will eat a hole through the wall of the vessel.

Fluidized bed sand filters are limited to the oxygen carried in with the water. This means that the water entering the filter should have a high level of oxygen in order to insure a good level of treatment.