Denitrification Effect And Microbial Community Analysis Of A Bench-scale Recirculating Aquaculture System Of Sebastes Schlegelii

Nitrate accumulation is one of the important factors affecting the water quality of recirculating aquaculture system.Exploring the denitrification of recirculating aquaculture system is very important to eliminate nitrate accumulation and control the concentration of NH₃-N and NO₂^--N.Aiming at this scientific problem,we constructed a bench-scale recirculating aquaculture simulation system.The recirculating aquaculture simulation system is composed of aquaculture unit,physical filtration device,biological filtration device,disinfection and sterilization device,temperature control system and oxygenation system.After removing solid suspended pollutants,soluble pollutants,disinfection,sterilization and oxygenation,the aquaculture tail water is returned to the aquaculture unit through reflux equipment for recycling.In this water cycle process,biofilter is the core unit of inorganic nitrogen removal.Aiming at the output of mariculture Sebastes schlegelii,a recirculating aquaculture simulation system is constructed.The operation effect of the system,total nitrogen budget of the system,the dynamic research of NH₃-N removal by moving bed biofilm reactor（MBBR）and the characteristics of microbial community characteristics are discussed.The main conclusions are as follows:1.A recirculating aquaculture simulation system was constructed and its treatment effect on inorganic nitrogen was studied.The constructed industrial recirculating aquaculture system includes Sebastes schlegelii aquaculture unit,microfiltration machine（solid waste filtration device）,MBBR,fixed bed biofilm reactor（FBBR）,ultraviolet disinfection pond and ozone disinfection device.Within 100 days of operation of the system,Sebastes schlegeli grew well.During the experiment,the water temperature of the aquaculture unit was22.55±0.46℃,the p H was 7.62±0.08,the dissolved oxygen was 5.88±0.12 mg/L,and the salinity was 35.54±0.41‰.The recirculating aquaculture simulation system has a significant effect on the removal of NH₃-N and NO₂^--N.The concentration of NH₃-N in W₁,W₂,W₃and W₄is finally stable at 0.79±0.42 mg/L,0.68±0.38 mg/L,0.34±0.30mg/L and 0.28±0.28 mg/L,and the removal rate is maintained at 74.05%±13.36%.The concentration of NO₂^--N in W₁,W₂,W₃and W₄finally stabilized at 0.76±0.26 mg/L,0.46±0.27 mg/L,0.27±0.29 mg/L and 0.13±0.13 mg/L,and the removal rate remained at 73.84%±12.61%.The changes of NO₃^--N at each sampling point in the system is basically the same,reaching 42.14 mg/L at W₁on the 100th day of the experiment,and the nitrate accumulation rate is about 0.42 mg/L/d.2.The main ways of total nitrogen budget in the process of aquaculture were explored.In the recirculating aquaculture simulation system,the nitrogen income includes feed input and water exchange,the nitrogen added through feed input is 40.69 g,accounting for about 98.31%of nitrogen input;The nitrogen added through water change is 0.70g,accounting for about 1.69%of the nitrogen input.Feed input is the main source of nitrogen income in the system.Nitrogen output mainly includes nitrogen assimilated by fish,nitrogen in sediment,nitrogen increased by nitrate nitrogen accumulation and nitrogen output by other ways.The nitrogen output from other ways includes the nitrogen assimilated by microorganisms,the nitrogen escaping from the system through denitrification,and the nitrogen that is difficult to collect due to the limitation of experimental conditions.Among them,the nitrogen assimilated by fish is 13.17 g,accounting for 31.82%of the nitrogen output,the nitrogen in sediment is 15.73 g,accounting for 38.00%,the nitrogen increased by nitrate nitrogen accumulation is 9.46 g,accounting for 22.86%,and the nitrogen output by other ways is 3.03 g,accounting for 7.32%.In nitrogen output,nitrogen in sediment>nitrogen assimilated by fish>nitrogen assimilated by nitrate nitrogen accumulation>others.According to the calculation,the nitrogen income is 41.39 g and the nitrogen output is38.36 g.the system N is close to equilibrium and reaches a relatively stable state.3.Based on the Stover-Kincannon model,the kinetic behavior of ammonia nitrogen degradation and removal in MBBR was studied,and the relationship between microbial community characteristics and nitrogen removal was explored.The removal of NH₃-N in MBBR conforms to the Stover-Kincannon kinetic model.The kinetic parameters K_B=2.12 g/m³,U_max=1.96 g/（d·m³）are obtained.The maximum removal rate of ammonia nitrogen obtained by MBBR is 1.78 mg/（L·d）,which is 90.82%of Umax,indicating that MBBR has great NH₃-N degradation performance and still has great nitrogen removal potential.The microbial community richness and diversity of FBBR biofilter were higher.The dominant phylum are Proteobacteria,Actinobacteriota,Bacteroidota,Chloroflexia and Planctomycetota.The dominant class are Alphaproteobacteria,Acidimicrobiia,Bacteroidia,Chloroflexia,Gammaproteobacteria,Anaerolineae.Among the denitrifying functional bacteria,the abundance of Nitrospira have the highest abundance,followed by Nitrosomonas.The similar environment will maintain a relatively stable relationship of microbial community composition.The metabolic function of microorganisms in the system is rich,and the functional composition of all samples is similar.The research results deeply clarify the denitrification effect of the recirculating aquaculture simulation system,and provide a theoretical basis for eliminating the nitrate accumulation in the recirculating aquaculture system in actual production and reducing the concentration of NH₃-N,NO₂^--N and other hazard factors.