Construction and evaluation of a tailwater treatment system for intensive aquaculture—a case study on bullfrog (Rana catesbeiana) farming

Abstract

Abstract Treatment of tailwater in intensive aquaculture has been a major issue that affects the utilization efficiency and pollution of water resources. In this study, the intensive farming of bullfrogs (Rana catesbeiana) was taken as an example to construct a tailwater treatment system consisting of a flotation tank, a biochemical tank and wetlands. The operating conditions of the biochemical tank were first optimized. The results showed that at an influent flow rate of 250 m3·h− 1, a hydraulic retention time of 6 h, and an aeration intensity of 2000 m3 h− 1, the levels of CODCr, ammoniacal nitrogen and total phosphorus in the effluent of the biochemical tank were reduced by 70%, 43% and 42%, respectively. Under these operating conditions, the efficiencies of the flotation tank, biochemical tank and constructed wetlands were 8.3%, 67.3% and 5.6% for CODCr removal, 4.2%, 40.4% and 70.5% for ammoniacal nitrogen removal, and 16.8%, 42.1% and 59.4% for total phosphorus removal, respectively. After one month of continuous operation, the following treatment results were obtained: 71.7% and 90% removal for CODCr and BOD5, 83% and 92% removal for ammoniacal nitrogen and nitrite nitrogen, 86.7% and 76.5% removal for reactive phosphorus and total phosphorus, and 95.1% removal of suspended solids. The dissolved oxygen level increased from 1.2 mg·L− 1 (influent) to 3.5 mg·L− 1 (effluent), indicating significantly improved water quality that met the Grade 1 national wastewater discharge standard and the criteria of recycled water utilization in bullfrog farming. This system thus realized the regeneration and utilization of tailwater in intensive aquaculture.