Nitrification performance of a bubble-washed bead filter for combined solids removal and biological filtration in a recirculating aquaculture system

Laboratory studies were conducted to evaluate the nitrification performance of a bubble-washed bead filter as affected by backwashing frequency and feed loading rate. The bubble-washed bead filter was used for combined solids capture and biological filtration in a recirculating tilapia (Oreochromis niloticus) culture system. The filter was tested at feed loading rates of 16, 24, and 32 kg day−1 m−3 beads. Backwashing frequencies varied and had to be adjusted at each feed loading rate for satisfactory nitrification, to maintain total ammonia nitrogen (TAN) and nitrite–nitrogen (NO2–N) at concentrations below 1.0 mg l−1. In general, more frequent backwashing improved filter performance, and was necessary at higher feed loading rates. It was essential to provide adequate flow through the bead filter (at least 50 l min−1 kg-feed−1 day−1) to keep the effluent DO greater than 2 mg l−1 just prior to backwashing. At the highest feed loading rate tested of 32 kg day−1 m−3 beads, favorable performance was observed at five backwashes in a 24 h period, achieving average areal TAN and NO2–N conversions of 331 and 451 mg m−2 day−1, equivalent to volumetric conversions of 380 g TAN day−1 m−3 beads and 518 g NO2–N day−1 m−3 beads.