Design and function of a closed, recirculating seawater system with denitrification for the culture of black tiger shrimp broodstock

A closed, recirculating seawater system with a denitrification process was designed for the culture of black tiger shrimp broadstock. The system comprised a circular rearing tank (9 m(3) volume), a nitrifying biofilter (6 m(3) volume) and denitrification process. The denitrification process comprised a deoxygenation column, a bacterial substrate column (143 L volume) and a re-aeration column connected to the biofilter. The experimental period was 81 weeks, consisting of 3 sequential trials using different substrates. bacterial inoculates and carbon sources: Trial 1- porous plastic balls for substrate, mangrove soil for inoculant and ethanol for the carbon source; Trial 2- crushed oyster shell for substrate, a strain of laboratory cultured bacteria for inoculant and ethanol For the carbon source: acid Trial 3- crushed oyster shell For the substrate, no inoculant and methanol for the carbon source. The nitrifying biofilter controlled ammonium-N and nitrite-N within acceptable ranges ( < 0.5 and < 0.2 mg L-1, respectively). The nitrate-N, however, became elevated gradually during trial 1 ( < 50 mg L-1). During trial 2, modification of bacterial substrate and the inoculation with denitrifying bacteria reduced nitrate-N in the denitrification column and the rearing tank (P < 0.05). Changing the carbon source to methanol and increasing the hydraulic residence time in trial 3 resulted in a significant reduction (P < 0.05) in nitrate-N (from > 160 to < 25 mg l(-1)) without the need for bacterial inoculation. (C) 2001 Elsevier Science B.V. All rights reserved.