A novel zero discharge intensive seawater recirculating system for the culture of marine fish

Results are presented of a zero-discharge marine recirculating system used for the culture of gilthead seabream Sparus aurata. Operation of the system without any discharge of water and sludge was enabled by recirculation of effluent water through two separate treatment loops, an aerobic trickling filter and a predominantly anoxic sedimentation basin, followed by a fluidized bed reactor. The fish basin was stocked for the first 6 mo with red tilapia Oreochromis niloticus x O. aureus at an initial density of 16 kg/m³. During this period salinity was raised from 0 to 20 parts per thousand. Then, gilthead seabream, stocked at an initial density of 21 kg/m³, replaced tilapia at day 167 and were cultured for an additional 225 d. Non steady-state inorganic nitrogen transformations occurred as a result of these salinity changes. After day 210, the system operated at all times with those water quality parameters considered critical for successful operation of mariculture systems, within acceptable limits. Thus ammonia, nitrite, and nitrate concentrations did not exceed 1.0-mg total ammonia-N/L, 0.5-mg NO₂-N/L and 50-mg NO₃-N/L, respectively. Sulfide levels in the fish basin were below detection limits and oxygen > 6 mg/L after the oxygen generator was added at day 315. Ammonia, produced in the fish basin and to a lesser extent in the sedimentation basin, was converted to nitrate in the aerobic trickling filter. Nitrate removal took place in the sedimentation basin and to a lesser extent in the fluidized bed reactor. Sludge, remaining in the sedimentation basin at the end of the experimental period, accounted for 9.2% of the total feed dry matter addition to the system. The system was disease-free for the entire year and fish at harvest were of good quality. Water consumption for production of 1 kg of tilapia was 93 L and 214 L for production of 1 kg of gilthead seabream. Additional growth performance data of gilthead seabream cultured in a similar but larger system are presented. During 164 d of operation of the latter system, maximum stocking densities reached 50 kg/m³ and fish biomass production was 27.7 kg/m³. Relatively poor fish survival and growth resulted from occasional technical failures of this pilot system.