Start-up Study On MBBR With Medical Stone Powder Modified Filler For Treating Marine Aquaculture Wastewate

With the rapid development of aquaculture in the recent years,the discharge of aquaculture wastewater causes a tremendous impact on surrounding environment,which leads to the continued deterioration of aquatic environment,while ecological balance and biodiversity are destroyed.Recirculating aquaculture system（RAS）,especially seawater RAS,has been developed rapidly worldwide because of its stability,excellent fish production performance and high environmental controllability.However,the effluent from seawater RAS aquaculture ponds contains a high level of ammonia nitrogen which is toxic to fish,so it is necessary to overcome the salinity conditions to achieve rapid and efficient nitrification for recycling.Moving bed biofilm reactor（MBBR）,as a new biofilm treatment process,has become one of the main treatment methods for a variety of wastewater and has been widely used in seawater RAS.However,the start-up problem of MBBR in seawater has not been well solved due to salinity stress and harsh inoculation conditions（sludge inoculation cannot be used for aquatic food chain and biosafety concerns）.Fillers are important factors to determine the start-up and treatment efficiency of MBBR.Aiming at the poor hydrophilicity of polyethylene（PE）,a common filler of MBBR,this study adopted the liquid phase oxidation modification method,combined with the characteristics of porous surface,high roughness,strong adsorption and alkalinity adjustment of rice flour,and adopted the liquid phase oxidation-wheat flour surface coating modification method to modify PE.The hydrophilic properties,surface morphology and chemical structure of common polyethylene filler（PE）,liquid phase oxidized modified filler（LO-PE）and liquid phase oxidized modified filler（LO-SCPE）coated with rice flour were compared and analyzed.In order to explore the application effect of the above-mentioned filler MBBR in the treatment of seawater aquaculture wastewater,the study took LO-SCPE and LO-PE as the experimental group and PE as the control group,and investigated the membrane hanging speed,water treatment effect and microbial community structure of the MBBR nitrification reactor under different modified conditions.After successful start-up,the study took hydraulic residence time（HRT）,ammonia impact load and operating temperature as the influencing factors,and compared the system stability of the MBBR reactor with the above-mentioned packing under the influence of single factors by observing the concentration of ammonia nitrogen and nitrous nitrogen in the effluent.The main results of this study are as follows:（1）The contact Angle test results showed that the LO-PE contact Angle was45.5°after the optimal liquid phase oxidation modification conditions（60℃,K₂Cr₂O₇:H₂SO₄:H₂0=1:15:15,t=1.5 h）,which was 49.8%lower than that before modification.（2）The results of scanning electron microscopy（SEM）showed that the surface of PE was irregularly corroded after liquid phase oxidation,and the surface of PE was covered by irregularly outcrops after further coating of maifstone powder,indicating that the modification improved the surface area and roughness of the filler.Energy spectrum analysis（EDS）results showed that the two modification methods increased the content of oxygen elements on the surface of the filler,and the LO-PE surface O/C value increased by 10.93%,while the LO-SCPE surface O/C value increased by177.7%.The results of infrared spectroscopy（FTIR）showed that liquid phase oxidation-surface coating modification introduced oxygen-containing functional groups C-O and C=O into the surface of the filler,and the hydrophilicity of the filler surface was enhanced.The results show that the hydrophilicity and roughness of polyethylene filler can be improved by liquid phase oxidation and surface coating of wheat flour.（3）The start-up effect of MBBR on the treatment of seawater aquaculture wastewater（HRT=24 h）showed that the LO-SCPE reactor（127 days）shortened the start-up period of membrane hanging compared with the LO-PE reactor（131 days）and PE reactor（137 days）.In addition,LO-SCPE（95.8%）showed better ammonia nitrogen removal effect than PE（91.7%）and LO-PE（92.8%）at the initial stage of successful film hanging.（4）Alpha diversity analysis showed that（sampling of biofilm microorganisms:day 160）,the Chao value and Observed＿species value of microorganisms in the LO-SCPE reactor were higher,indicating a higher microbial diversity.The analysis of community composition at phylum,class and genus level showed that LO-SCPE reactor had more diverse and abundant AOB and NOB.The results of quantitative qPCR showed that the amoA and nxrB copy values in the LO-SCPE reactor were higher than those in the PE and LO-PE reactors.These results indicate that the modified LO-SCPE reactor has higher biocompatibility and more species and quantity of attached microorganisms.（5）Based on the successful start-up of the reactor,the treatment efficiency of three groups of MBBR reactors with different parameters,including the effects of different HRT,temperature and ammonia nitrogen impact load on the MBBR system,was studied and compared.The removal rate of NH₄⁺-N in PE,LO-PE and LO-SCPE groups was higher（>97%）at 24 h and 12 h HRT.However,the accumulation of NO₂^--N at 12 h was higher than that at 24 h（NO₂^--N<0.3 mg/L）（0.53 mg/L,0.5mg/L and 0.46 mg/L,respectively）.When HRT was further reduced to 6 h,NH₄⁺-N removal rate of MBBR in the three groups decreased significantly（93%,92.8%and93.8%）,while NO₂^--N accumulation increased further（0.65 mg/L,0.64 mg/L and0.54 mg/L,respectively）.The results showed that the ammonia oxidation and nitrification processes were not completely carried out when the HRT was low.However,under the ammoniac-nitrogen shock loading of 50 mg/L and 100 mg/L,the three groups of MBBR reached a higher level of NH₄⁺-N removal efficiency（>99%）after a short adaptation period.Due to the short duration of ammonia impact loading in this study（12 h,24 h）,the inhibition of nitrate bacillus and other nitrite oxidizing bacteria was weak,and no long-term accumulation of NO₂^--N was caused.In fact,ammonia-nitrogen impact loading with high concentration and low water content can stimulate the growth and reproduction of ammonia-oxidizing bacteria,which is conducive to improving the efficiency of ammonia nitrogen removal.When the operating temperature was reduced to 20±1℃,the NH₄⁺-N removal rate（>99%）of MBBR in the three groups was similar to that at 25±1℃and the NO₂^--N accumulation was lower（0.4 mg/L）after a short adaptation period.After further lowering the temperature（15±1℃）,the NH₄⁺-N removal rate of MBBR in the three groups remained at a high level（>99%）,but caused a certain amount of NO₂^--N accumulation（0.5 mg/L）.In general,the MBBR reactor modified LO-SCPE showed higher NH₄⁺-N removal rate,less NO₂^--N accumulation and shorter adaptation period than the other two fillers in the single-factor study,with stronger system stability.