An Investigation On Nitrification Performance Of Novel Moving Bed Bioreactor In Recirculating Aquaculture Systems Under Different Conditions

Nitrite（NO₂^--N）and ammonia（NH₄⁺-N）are two prevalent nitrogenous compounds in recirculating aquaculture systems（RAS）wastewater,posing a significant health risk to culture aquatic animals.Hence,their removal is necessary to recycle water into the fish rearing tanks.However,keeping acceptable water quality and its reuse in RAS relies on the performance of nitrifying biofilter.Meanwhile,moving bed bioreactor（MBBR）is regarded as a dominant biofilm that removes NO₂^--N and NH₄⁺-N from RAS wastewater through the nitrification process.The core component of the MBBR system is the biocarrier on which the bacteria adhere to form biofilm structures.Therefore,the properties of a biocarrier play a vital role in influencing biological nitrification performance.Sponge biocarrier（SB）is widely considered as an ideal biomass growth biocarrier due to its weight（lightweight）,larger pore spaces for microbial immobilisation,and protection against shock loads.However,the kinetics of SB in MBBR to treat RAS effluent is not well researched.Despite the significant role of biological processes in treating RAS wastewater,information on biofilm composition and chemolithoautotrophic nitrifying bacteria’s activities in SB have remained scarce.Further,the role of novel SB’s in MBBR treating saline RAS wastewater has not been explored.Besides,some inherent challenges in MBBR performance,such as long start-up time and salinity impact on microbial activity,remain unresolved.In view of the above problems,the aim of the present research was to investigate the nitrification performance of novel MBBR in RAS under different conditions.Thus,the research contents and the main outcomes of this work are mentioned below:The first study evaluated the potential application of a novel sponge as a biocarriers in MBBR treating freshwater RAS wastewater.Two lab-scale MBBRs were operated simultaneously for 116 days under various hydraulic retention times（HRTs）.The reactors R1 and R2 were filled with Kaldness（K5）plastic carriers and SB,respectively.From the results,at an optimum HRT of 6 h,ammonia removal efficiency and nitrification rate were86.67±2.4%and 1.43 mg/L.h for the R1,and 91.65±1.3%and 1.52 mg/L.h for the R2,respectively.The microbial community analysis showed that the predominant genera in the nitrifying community were Nitrosomonas（AOB）and Nitrospira（NOB）in co-existence with heterotrophic genera Hyphomicrobium,Mesorhizobium,Zhizhongheella,and Klebsiella spp.Modified Stover-Kincannon model examined the ammonia removal kinetics,and the values of kinetic parameters obtained were U_max:0.909 and 1.111 g/L.d and K_B:0.929 and,1.108g/L.d for the R1 and R2,respectively.The correlation coefficients（R²）of the MBBRs were higher than 0.98,indicating that the model adequately described the experimental data.Achieving quick start-up and microbial biofilm formation remains a significant challenge.Consequently,the second study investigated a novel chitosan-based natural sludge（CS@NGS）seeding strategy for the accelerated start-up of MBBR.The bioreactor was utilised to treat freshwater RAS effluents.Resultantly,the CS@NGS shortened the start-up period from over twenty to seven days due to the enhanced initial microbial adhesion and biofilm formation.The mechanism explorations showed that positively charged amino groups in CS neutralize the hydroxyl and carboxyl groups of extracellular polymeric substances（EPS）,reducing the negative charge on the biofilm surface,thereby enhancing the initial microbial adhesion and biofilm accumulation.The result showed that the ammonium removal approached 99.58?±?0.01%,and low nitrite accumulation was exhibited in the effluents,approximately≤0.03 mg/L.The process performance correlated positively with bacteria from the genera Nakamurella,Hyphomicrobium,Nitrospira,Paenarthrobacter,Rhodococcus,and Stenotrophobacter.The quantitative polymerase chain reaction（q PCR）results demonstrated that the CS@NGS enhanced the expressions of amo A,nxr B,nir K,nir S,nar G,and nap A nitrogen metabolism-related functional genes.The study findings could help in the rapid start-up of aquaculture biofilters for solving high nitrite,and ammonia accumulation in RAS farms recirculated water.In the third study,two proposed modified biofilm carriers’performances in treating RAS wastewater under different salinities（12‰,26‰,and 35‰）for about 92 days were assessed.Three（MBBRs;R1,R2,and R3）were filled with unmodified novel sponge biocarriers（SB）,served as a control,modified novel SB with ferrous oxalate（C₂Fe O₄@SB）,and modified novel SB with combined ferrous oxalate and activated carbon（C₂Fe O₄-AC@SB）,respectively.Under the highest saline condition,a significantly higher ammonia removal efficiency of 98.86±0.7%（p<0.05）was obtained in R3,whereas R2 and R1yielded 95.18±2.8%and 91.66±1.5%,respectively.Microbial analysis showed that Vibrio,Ruegeria,Formosa,Thalassospira,and Denitromonas were predominant genera,strictly halophilic heterotrophic nitrifying bacteria involved in nitrogen removal.Our findings concluded that the synergistic effects of SB,C₂Fe O_4,and AC accelerated biofilm formations and stability,subsequently enhancing nitrogenous compounds removal from the mariculture RAS wastewater by the C₂Fe O₄-AC@SB carriers in R3.Taken together,the outcomes of the present study could improve our understanding and provide a low-cost innovative material with high potential applicability for efficient and cleaner wastewater treatment.