Study On Removal Mechanism Of Nitrogen Pollution In Recirculating Aquaculture System By Embedded Bacteria

The recirculating aquaculture system（RAS）is an important development direction of the aquaculture industry.It is characterized by the realization of water recycling and free from the constraints of natural resources such as land and water.It is a high-density land-based factory farming method.During the operation of RAS,the water treatment system is the key to determine whether the system can operate stably.And the control of nitrogen pollutants is crucial for the growth of farm animals.The following issues are needed to be addressed:（1）efficient biological removal mechanisms for TAN in recirculating aquaculture systems;（2）NO₃^--N accumulation and efficient bioremoval mechanisms during cycling.To solve these problems,this study designed and built a recirculating aquaculture system.Nitrification of TAN and NO₂^--N by embedded bacteria in water treatment unit was studied.Water quality,purification ability and activity of embedded bacteria and distribution characteristics of microbial community during operation of the system were investigated.Finally,the simulation experiment of denitrification by embedded bacteria was carried out in the laboratory.The prediction model of NO₃^--N removal rate was obtained,and the denitrification scheme was given based on the RAS operating parameters in this study.The research results are as follows:（1）The application of embedded bacteria in nitrification unit can domesticate and effectively remove TAN and NO₂^--N within one week.The environmental factors were in the suitable growth range of fish,and the concentrations of TAN and NO₂^--N were stable in the range of no harm to fish,while NO₃^--N and TN accumulated gradually.（2）The process of removing TAN and NO₂^--N by the embedded bacteria unit can fit the first-order reaction kinetic equation.In the mechanical filtration stage,the reaction rate constants for removal of TAN and NO₂^--N were 0.073 and 0.111,respectively.In the biological filtration stage,the initial ammonia nitrogen concentration in the self-dispensing water is large,and the reaction rate constants for removing TAN and NO₂^--N are 0.186 and 0.106,respectively.The rate of oxygen consumption（gO₂/（L·h））of the ammonia-oxidizing bacteria,nitrite-oxidizing bacteria and heterotrophic bacteria in the mechanical filtration stage were 0.0304,0.1050,and 0.0038,respectively,and the biological filtration stages were0.0284,0.031and 0.014,respectively.（3）At the phylum level,the flora abundance of Planctomycetes,Bacteroidetes and Actinobacteria in BR15 and BR30 was higher than that in B0.At the genus level,Chryseolinea,Pilula,Spartobacteria_genera_incertae_sedis,Tepidispha,Gp6,Phodyliba,Armatimonadetes_gp5 were higher than that in B0.（4）Through the BBD experiment design,a well-fitted model for denitrification removal of nitrate was obtained.It was found that the interaction of temperature and carbon-nitrogen molar ratio was the most obvious,and the rate of removal of nitrate was faster when they both at a higher level.