Effects Of Fe₃O₄ Nanoparticles On The Formation And Long-Term Stable Operation Of Aerobic Granular Sludge

Traditional activated sludge technology has the disadvantages of low biomass concentration,poor sludge settleability,large footprint and high energy consumption.Compared with activated sludge,aerobic granular sludge（AGS）has better structure and morphology,faster settling velocity,stronger sludge retention capacity and load resistance.In recent years,the rapid development of AGS technology has aroused the attention.Aiming at the problem of the long start-up time of AGS system,this study adopted the method of adding Fe₃O₄ nanoparticles（Fe₃O₄ NPs）to shorten the sludge granulation time.The short-term and long-term effects of different Fe₃O₄ NPs dosages on sludge granulation and pollutant removal rate in AGS system were investigated,and the effects of different Fe₃O₄ NPs dosing frequency on pollutant removal performance,sludge characteristics and microbial community structure succession in AGS system were also studied.The main contents and conclusions were as follows:（1）To study the short-term effect of Fe₃O₄ NPs dosage on AGS granulation,each SBR reaction system（A1,A2,A3,A4 and A5）was successively added with 0,10,20,50 and 100 mg/L Fe₃O₄ NPs every 10 days.The experiment lasted for 90 days,reaction systems all had a good removal effect on COD、PO₄^3--P and NH₄⁺-N,and A4 system had the best removal effect on pollutants.The proportion of granular sludge in each reaction system increased gradually,and the proportion of granular sludge in A4 system was always higher than that in other systems.During the process of sludge granulation,the sludge extracellular polymeric substance（EPS）content in each system increased steadily.The sludge EPS content in A2,A3 and A4 reaction systems was higher than that in A1,and A4 system was the highest,while the sludge EPS content in A5 system was lower than A1.The bacterium with the highest relative abundance in the system was Candidatus＿Competibacter,and Fe₃O₄ NPs had the promoting effect on Candidatus＿Berkiella and the inhibiting effect on Blautia.Paracoccus is a denitrifying phosphorus-accumulating bacterium,which can ensure the effect of systematic nitrogen and phosphorus removal.The relative abundance of Paracoccus gradually increased in the process of sludge granulation,and its relative abundance was the highest in A4system,indicating that the effect of systematic nitrogen and phosphorus removal was gradually enhanced and A4 system had the best effect of nitrogen and phosphorus removal.In general,when the Fe₃O₄ NPs dosage was 50 mg/L,the system pollutant removal efficiency and AGS granulation degree were the highest.（2）Further,the influence of different Fe₃O₄ NPs dosages（consistent with the dosages in（1））on the long-term stable operation of AGS system was studied.The experiment was continued for 60 days on the basis of（1）,and each reaction system had a good removal effect on COD and NH₄⁺-N.Compared with the start-up stage,the removal effect on TN of each system in this stage were all improved.A4 system had the best removal effect on pollutants,and the removal rates of COD,PO₄^3--P and TN are4.32%,17.75%and 4.42%higher than those of A1.In this process,the proportion of granular sludge in A4 system was always higher than that in other systems,and the proportion of granular sludge with particle size greater than 1 mm in A4 system is the highest.The EPS content of sludge in each system increased steadily during the test.The EPS content of sludge in A4 system was always higher than that in other systems.During the experiment,the microbial community richness of each system gradually increased.Fe₃O₄ NPs had the strongest promoting effect on Candidatus＿Berkiella and the strongest inhibiting effect on Blautia.The relative abundance of Defluviicoccus increased gradually during the sludge granulation process,and the relative abundance was the highest in A4 system,indicating that the organic matter removal effect of system was gradually enhanced and A4 system had the best organic matter removal effect.The experimental results showed that adding 50 mg/L Fe₃O₄ NPs had the best effect on long-term stable operation of AGS system.（3）In this chapter,no dosages,50 mg/L Fe₃O₄ NPs dosages every 1,5,10 and 20days（corresponding to A1,A2,A3,A4 and A5 systems,respectively）were added to study the effects of different Fe₃O₄ NPs dosing frequency on AGS granulation.The experiment lasted for 48 days,and each reaction system had a good removal effect on COD,PO₄^3--P and NH₄⁺-N.The removal effect on pollutants of A3 system was the highest and the removal efficiency of COD and PO₄^3--P was 4.84%and 8.92%higher than that of A1,respectively.The COD degradation rate and NH₄⁺-N oxidation rate of A3 system were higher than those of other systems.Compared with A1 system,the time required for completing sludge granulation in A3 system was shortened by more than 18days.The proportion of granular sludge in each system increased gradually,and the proportion of granular sludge in each system increased to 78.2%,69.9%,84%,79.2%and 62.8%on the 45th day of operation,respectively.The proportion of granular sludge in A3 system was always higher than that in other systems.High-throughput assay results showed that the system microbial community richness was highest when 50mg/L Fe₃O₄ NPs was added every 5 days.Thauera had the highest relative abundance in the system,and Fe₃O₄ NPs significantly promoted a variety of bacteria,while Ferruginibacter had the strongest inhibition.OLB8 is an organic-degrading bacterium,and Paracoccus is a denitrifying phosphorus-accumulating bacterium.The relative abundance of these two bacteria was the highest in A3 system,indicating that A3 system had the best organic matter removal and nitrogen and phosphorus removal efficiency.The experimental results showed that adding 50 mg/L Fe₃O₄ NPs every 5 days had the best effect on pollutant removal efficiency and AGS granulation process.