Research On Recirculating Aquaculture Water Treatment Technology Based On Multi-layer Fiber Ball Biological Filter

Aquaculture in China has developed rapidly in recent years,with yields ranking first in the world for many years.However,the conventional aquaculture not only wastes a lot of water resources,but also brings serious water pollution,which restricts the sustainable development of aquaculture in China to a certain extent.Industrial recirculating aquaculture system is energy-saving and strongly applicable,which can purify water for aquaculture by means of physical and biological treatment so as to recycle water resources.As the critical issue of recirculating aquaculture technique,water treatment technology determines the operation status of the whole system.A new multi-layer fiber ball biological filter reactor was designed using fiber balls with a large surface area and strong adsorption as a filter material to be applied to the breeding process of tilapia.The multi-layer fiber ball biofilter possessed stepwise-drop design and multi-stage reoxygenation to remove suspended solids?SS?and nitrogen-containing pollutants under zero water exchange conditions,and make effluent contain high-concentration DO at the same time.The results showed that the naturally-formed biofilm made an externally-aerobic and internally-anoxic environment form in the fiber.The reactor could simultaneously accomplish the nitrification and denitrification processes.The effects of different water recirculating rates were discussed.Water recirculating rate with three times a day was optimal for the reactor.The concentration of NH₄⁺-N in effluent could be reduced to 0.2 mg/L with the removal rate of 96.15%.The removal efficiency of NO₃^--N was stable at about 70%.The concentration of NO₂^--N in effluent could be reduced to 0.1 mg/L with the removal rate of 95.82%.The concentration of chemical oxygen demand?COD?in effluent decreased to 4.0 mg/L and the removal rate reached 60%or higher.The SS removal rate was up to 100%and the DO concentration in effluent reached8-9 mg/L.The total DNA of the biofilm on the surface of the filter medium was extracted and the high-throughput sequencing technique was used.The composition,relative abundance and diversity of microbial community on biofilm surface of different filter media were analyzed.The dominant bacteria in each filter layer were also analyzed.The results showed that the dominant bacteria of the multi-layer fiber ball biofilter filter at the phylum level were Proteobacteria,Firmicutes,Bacteroidetes,Actinobacteria,and so on.The dominant bacteria of the multi-layer fiber ball biofilter at the genus level were Acinetobacter,Pseudomonas,Pseudomonas,etc.A dominant strain L4 with high-efficiency aerobic denitrification was screened from the surface of the filter material of the tilapia circulating aquaculture system in the laboratory.The strain was identified as Acinetobacter pittii.The influencing factors and nitrogen removal performance of the strain were discussed.The results showed that sodium succinate was the best carbon source for strain L4,and followed by sodium acetate.The best C/N ratio was 10-15 and the optimal growth temperature was 25-35°C.The optimal p H range was 6-8 and the optimum shaking speed was150-200 r/min.The strain could adapt to a wide range of ammonia nitrogen loads and grow with denitrification at high ammonia nitrogen concentrations?500 and 1000mg/L?.The strain had heterotrophic nitrification and aerobic denitrification performance.The strain could use?NH₄?₂SO₄,Na NO₂and Na NO₃as the sole nitrogen source with the 36-h removal rate of 90.2%,71.3%and 91.1%,respectively.The strain was fixed on the surface of the fiber sphere and added to the sterilized tilapia culture wastewater.The results showed that the removal rates of NH₄⁺-N?NO₂^--N and NO₃^--N reached 51.43%,61.92%and 23.02%after 3 days,respectively.The removal rate was higher than that of the control group without sterilization and the non-sterilized strain,indicating that the strain L4 can be applied to the treatment of aquaculture wastewater.The results of this study provide a theoretical and practical basis for understanding the application and nitrogen removal mechanism of multi-layer fiber ball biofilter in the recirculating aquaculture system.