Design and performance of a zero-discharge tilapia recirculating system

A zero-discharge tilapia recirculating system was evaluated during a 331-day grow-out period in which tilapia were raised from fingerling to market size. In addition to conventional water treatment procedures, the system featured an anoxic treatment stage where sludge was biologically digested and nitrate was reduced to nitrogen gas. Total tilapia biomass production over the grow-out period was 81.1 kg m^-3 with maximum stocking densities of 61.8 kg m^-3. Inorganic nitrogen concentrations were within acceptable levels for tilapia culture. Ammonia production rates corresponded to ammonia removal rates by the nitrifying trickling filter (average ammonia removal: 0.16 g N m^-2 per day) while nitrate removal took place in the anoxic treatment stage, consisting of a sedimentation/digestion basin and a fluidized bed reactor. Nitrate removal was most profound in the sedimentation/digestion basin, specifically in those regions close to where the sludge entered the basin. Around 70% of the phosphorus added with the feed were recovered mainly in the anoxic treatment stage of the system.