Biotechnology and Bioinformatics (1+1)

6.2.4.4. Bead filters

Bead filters are a relatively new type of biofilter. They are advertised as the complete solution to water quality for recirculating systems.

• They consist of a closed vessel partially filled with small beads of plastic. Usually the vessel is filled with water and the beads float at the top of the vessel.
• Water flows up through the bed of beads. The beads are small enough to trap most large suspended solids.
• In addition, the surface of the beads supports the growth of a biofilm. The small size of the beads means that they have a relatively large surface area per unit volume.
• The more sophisticated systems incorporate a mechanical stirring devices such as a propeller on a shaft.
• Periodically the water flow is shut off and the bed of beads stirred to dislodge the suspended solids.
• The solids are allowed to settle into the bottom of the vessel and then drained off.
• This ability to remove suspended solids and act as a biological filter is the main advantage to bead filters.

The difficulty in successfully operating bead filters lies in striking a balance between the competing functions.

• Too frequent washing to remove solids dislodges the biofilm and disrupts the nitrification process.
• If the beads are not washed enough however, the solids start to plug the bed.
• The other potential problem is the presence of large amounts of carbonaceous solids which tends to encourage the growth of heterotrophs at the expense of N itrobacter sp. and N itrospira sp.
• Another drawback to bead filters is their relatively high energy consumption due to their high pressure drop. Also, the water flow and pressure drop are not constant. As the bed of beads becomes loaded with solids, the pressure drop rises and the water flow decreases. This leads to cyclic rather than constant performance.

Since bead filters are not aerated, they are limited to the oxygen carried in with the water. In gene ral this is not a problem since retention times are low. Bead filter systems are probably suitable for small, lightly loaded systems where labor costs are low. At this time they are not available for large systems except as multiple units.