Control Of Membrane Aerated Biofilm Thickness Based On CFD And Synchronous Removal Of Carbon,Nitrogen And Phosphorus

Membrane Aerated Biofilm Reactor(MABR)process has significant technical advantages in low carbon to nitrogen ratio wastewater treatment due to efficient oxygen transfer and anisotropic mass transfer.However,during the long-term operation of MABR,there is a problem that the growth of the outer denitrification biofilm is too thick,resulting in a decrease in the rate of ammonia nitrogen transfer to the inner nitrifying bacteria biofilm.This paper systematically investigated the relationship between MABR effluent quality and biofilm characteristics under the conditions of natural growth and regular scouring of biofilm.Computational Fluid Dynamics(CFD)was used to simulate the internal flow field characteristics of MABR,optimize the aeration erosion intensity to achieve effective control of biofilm thickness,and preliminarily investigate the synchronous carbon,nitrogen,and phosphorus removal characteristics of MABR.The main conclusions were as follows:The operation results under influent conditions with different carbon to nitrogen ratios show that when the C/N is 5,the removal rates of ammonia nitrogen,total inorganic nitrogen,and COD of MABR all reach 90%.MABR has good simultaneous nitrogen and carbon removal efficiency(ammonia nitrogen,total inorganic nitrogen,and COD removal rates are all higher than 90%)in the thickness range of 140 to 225μ m when the biofilm naturally grows.Using CFD to simulate the flow field characteristics in MABR,the optimal aeration aperture is 2 mm,and the shear force distribution characteristics of biofilms with aeration intensity ranging from 2 to 10 L/min are obtained.Monitor the rheology of biofilms at different stages(the 10 th,15th,20 th,and 25 th cycles of MABR operation),and determine the yield stress as 2.8,4.2,6.7,and 7.4 Pa respectively by evaluating the shear and oscillatory rheology.Comparing yield stress with the average shear stress borne by biofilm,it is concluded that the optimal aeration erosion intensity(i.e.,70% of the shear stress borne by biofilm is greater than its yield stress)for the 10 th,15th,20 th,and 25 th cycles is 3,5,8,9 L/min.Starting from the 10 th cycle,the MABR was operated for 30 cycles every 5 cycles with the optimal scouring intensity for 2 minutes.The biofilm thickness only increased to 270 μ m,which decreased by 80 μ m compared to the thickness without scouring.The ammonia nitrogen removal rate increased by 10 percentage points(70% → 80%),the total nitrogen removal rate increased by 5 percentage points(69%→74%),and the COD removal rate remained unchanged(both above 90%).Adding a non aeration period(4 h anaerobic + 20 h aerobic)in MABR operation,combined with high frequency aeration scouring(30 s per day,aeration intensity 3L/min)for enhanced biological phosphorus removal.The results show that the phosphorus removal rate increases by 50 percentage points(30% → 80%)at the end of 14 cycles,decreases to 10% after doubling the phosphorus load,and stabilizes at45% at the end of 30 cycles.The removal rates of ammonia nitrogen and total nitrogen are both 80%,and the COD removal rate is 90%.High frequency scouring leads to small fluctuations in COD concentration of the effluent;Macrogenomic sequencing revealed that Proteobacteria and Bacteroides,which occupy the dominant position in the biofilm microbial community,are the main functional bacteria of MABR for nitrogen and carbon removal.Among them,Bacteroides contains phosphorus accumulating bacteria and denitrifying phosphorus accumulating bacteria related to biological phosphorus removal,which can effectively degrade organic matter and achieve the effect of simultaneous nitrogen and phosphorus removal.