Nitrification in three-stage and single-stage floating bead biofilters in a laboratory-scale recirculating aquaculture system

Abstract Previous research with biological filters has indicated that nitrification efficiency may be increased by separating the media into multiple stages. This study compared nitrification in two floating-bead biofilter treatments: a three-stage biofilter vs a single-stage or solitary biofilter, having similar total bead volumes for both treatments, and three replicates of each treatment. The filters supported a maximum of 15·6 kg of hybrid tilapia ( Oreochromis niloticus × Oreochromis aureus ) in a 366 liter recirculating system. Generally, the single-stage biofilter had somewhat greater reduction of total ammonia nitrogen (TAN) than the three-stage biofilter, especially when the fish feeding rate was increased to 14·85 kg/day-(m 3 of beads) to challenge the filters. The three-stage treatment usually had differences between stages regarding net reduction of TAN (usually between 5 and 15% reduction of influent concentration in each stage), but there were no consistent trends between stages. Both treatments displayed less than optimal total suspended solids (TSS) filtering capacity and inconsistent agitation during daily flushing of filters, which may have led to variable biofloc retention time and increased variability of TAN removal.