THE FUNCTION AND USE OF GRANULAR FILTRATION IN RECIRCULATING AQUACULTURE SYSTEMS: BASIC PRINCIPLES

Recirculating aquaculture systems (RAS) rely on a water treatment process train to perform both physical and biochemical remediation of the process water to enable reuse and minimize the volume of the effluent stream. The first step in the water treatment process train is the removal of the solid wastes produced in the culture vessel. The efficiency of the solids removal process will significantly influence the intensity and degree to which the subsequent, secondary treatment processes must perform. As such, maximizing the efficiency of the primary, solids treatment will improve the efficiency and reduce the cost required for the downstream treatment processes. Of the many options available to system designers and operators, granular filtration has proven to offer robust and flexible options for meeting the needs of the aquaculture industry.

Granular filtration is largely recognized as an effective means for the capture and removal of solid wastes from process water in a variety of water treatment systems. Granular filtration may also be used as an effective means for biological conversion of dissolved constituents. However, effective granular biological filtration requires an effective solids removal process directly upstream. Properly designed, granular filtration devices may even successfully combine the solids capture and biological filtration processes into a single unit, thus reducing space requirements as well as initial costs. Due to the flexibility in design and dynamic performance characteristics, granular filtration is successfully used in a wide variety of nutrient-loading scenarios and industries. Recent developments in the design and utilization of granular filtration in RAS have made improvements in the concentration of captured solids prior to discharge as a means for reducing the overall volume of the effluent stream.

Proper selection of granular filtration devices requires an understanding of the filter design and function of the media used in the filter. An appropriate characterization of the filter influent will play a large role in the selection of the proper filter type. Particle diameter and capture efficiency, hydraulic headloss, media bed backwash frequency, media characteristics (i.e. shape, material, biofilm retention, etc.), backwashing mechanism, and flow rate/velocity must all be considered in the appropriate selection and application of granular filters.

The purpose of this presentation will be to provide a foundation for understanding the form and function of granular filtration and the application thereof in RAS. Filter design, media effects, and filtration (physical and biological) will be discussed.