Integrated system combined with machinery, bacteria and grass to treat wastewater from a zero-discharge recirculation aquaculture system

Zero-discharge technology is one of keys to recirculating aquaculture to realize sustainable production. An integrated wastewater treatment system combined with machinery, bacteria and grass was used to set up a zero-discharge prototype recirculating aquaculture system where 62 tails Colossoma brachypomum (208.3±28.6 g) were cultured at a density of 4.87 kg/m³. During a period of 25 days, most water quality parameters of the two fish tanks including nitrate-nitrogen (NO^-₃-N), total ammonia nitrogen (TAN), total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), and pH value kept stable (p<0.05), while nitrite-nitrogen (NO^-₂-N) and suspended solids (SS) significantly decreased (p<0.05). In the water treatment system, water qualities of the mechanic filter and the bio-filter were significantly (p<0.01) better than those of the bottom area of the settler. High concentration effluents from the bottom of the settler, 34.3 liters everyday, were transferred to the plant filter to breed 2.55 m² NFT-cultured grass (Festuca arundinacea Schreb.) and were periodically sent back for reuse. 67.5 liters water, significantly (p<0.05) improved by the grass, was returned to the mechanic filter at intervals of four days, and more than 90 percent of NO^-₃-N, NO^-₂-N and SS, more than 85 percent of TN and TP, 67.9 percent of TAN, and 71.6 percent of COD were removed, while electric conductivity (EC) and pH value increased significantly (p<0.05). Fishes grew at 4.55 g per day with a feed conversation ratio of 1.610.