John L. Cleasby

A Review of Hydraulics in Fluidized-bed Biological Filters

Fluidized sand-beds are efficient and cost-effective methods for biological treatment within recirculating aquaculture systems, and have recently been the topic of considerable research. However, their hydraulic design has been largely undescribed in the aquaculture literature, and at this point, hydraulic criteria are typically obtained from preliminary laboratory fluidization studies on media samples obtained from distributors. This article summarizes and applies techniques developed in other disciplines for calculating fluidization hydraulics, based solely upon characteristics of the granular medium, for the purpose of designing biological fluidized-bed units for use in recycle aquaculture. These calculations require knowledge of five characteristics: (1) and (2) the medium’s size gradation and effective diameter, estimated from sieve analysis provided by the supplier, (3) the medium’s density, obtained from a property of materials table, and (4) and (5) the medium’s sphericity and the bed porosity, both of which can be reasonably assumed, particularly with sands. Solutions to fluidization calculations are presented as simplified curves useful for sensitivity analysis for optimizing unit design. This article also discusses the (1) characteristics which make a medium suitable for use in nitrifying fluidized beds in aquaculture, (2) design criteria for proper distribution of water at the bottom of the fluidized bed, and (3) special issues involving fluidization hydraulics and the application of fluidized beds to aquaculture.