Efficiency And Mechanism Of Municipal Wastewater Treatment In Membrane Aerated Bacteria-algae Biofilm Reactor

Membrane aerated biofilm reactor（MABR）is an emerging biofilm technology,which has the advantages of highly efficient oxidation and no blow-off.It is widely used in wastewater treatment process containing refractory and volatile substances.However,during operation,nutrients removal process is still severely affected by organic matter.The Algae-bacteria symbiosis（ABS）technology is also a hot spot of current research and it has a strong capabality to remove nutrients.However,in actual operation,additional aeration is still needed,and the ordinary aeration process will cause CO₂ blowing-off.Besides,excessively high dissolved oxygen（DO）resulting from aeration will limit photosynthesis.This paper will combine the MABR and the ABS based photobioreactor into a membrane aerated bacteria-algae biofilm reactor（MABAR）to solve the problem of ABS oxygen supply and MABR poor removal of nutrients.In MABAR,a light source is added and the biofilm will be replaced by a bacteria-algae biofilm.Under the premise of controlling the same DO concentration,the three ABS systems were subjected to bubble aeration of different intensities（20 m L/min,50m L/min and 100 m L/min）,and the operation for 45 days was observed.CO₂ stripping,dissolved inorganic carbon（DIC）residual,and algal accumulation were monitored.Results show that with the increase of aeration intensity,the amount of CO₂ stripping increases,and the DIC in the effluent decreases.The stripping amount of CO₂ can reach 306.29 m L/d at an intensity of 100 m L/min,which is 4 times that of aeration intensity of 20 m L/min.In terms of microalgae accumulation,the algae accumulation biomass and algae/bacteria in the aeration intensity of 20 m L/min were 3 times and2.6 times higher than those at 100 m L/min,respectively.The carbon and oxygen imbalance caused by aeration is the main cause of the imbalance of bacterial and algal growth.Next,the negative effects of aeration on the growth balance of bacteria and algae were reduced by aerating only in the dark stage.The results show that under the same aeration conditions,compared to continuous aeration,changes in influent contaminants do not negatively affect intermittent aeration systems.In NA-ABS,the aeration rate of 20 m L/min is the best.The removal rates of NH₄⁺-N,TN,PO₄^3--P and TOC can reach 99.18%,90.30%,89.16%,and 96.59%,respectively.Besides,the algae accumulation concentration can reach to 5893.19μg/L,and the ratio of algae to bacteria increased to 2689.32μg/L.According to the conclusion aboved,the bubble-free aeration method of membrane aeration was used to supply oxygen to the ABS system and to compare to bubble aeration（HABAR）.The positive effect of membrane aeration on algae proliferation under different influent COD was evaluated.Results show that membrane aeration can effectively enhance the accumulation of algae.In addition,the strong oxidizing ability and no stripping ability of membrane aeration retained more DIC for algae,so that DIC was no longer a limiting factor for the growth balance of bacteria and algae.Under the condition of 400 mg/L organic carbon,the accumulation of algae in MABAR can reach 20.95 mg/cm²,which is 7 times that without the addition of organic carbon,and 2 times that in HABAR under the same operating conditions.In turn,investigating whether the MABR under the algae supporting can reduce the impact of COD on TN removal and expand its wider adaptation to wastewater with different C/N ratios is meaningful.By establishing two bioreactors,including MABR and MABAR,and changing the COD/N ratio to 2,5and 8 to invesgate the difference in TN removal.Regardless of the COD/N ratio,the TN removal efficiency in MABAR is significantly improved,and the phenomenon that denitrification was limted with low COD and nitrification was restricted in high COD concentration disappeared.Interestingly,with the increase of COD,the highest removal rate of TN can reach 87.63%.Besides,compared to MABR,when the COD/N ratio was 2,5,and 8,the effluent TN concentration decreased by 14.34 mg/L,0.50 mg/L,and 12.10 mg/L,respectively.Nitrification inhibition test showed that algae assimilation has a significant contribution to NH₄⁺-N removal in MABAR（at least 18.18 mg/L）.In addition,Redundancy Analysis（RDA）showed that MABAR was negatively correlated with nitrifying bacteria,but positively correlated with NH₄⁺-N removal load.These results are consistent with the kinetic results that it is algae assimilation,rather than nitrification-denitrification,dominated the nitrogen removal in MABAR.By monitoring the differences between MABAR,HABAR and MABR for pollutant removal and shock resistance under different influent quality,the performance of the coupled system on pollutant removal efficiency and stability was evaluated.The results show that the removal efficiency of NH₄⁺-N,TN,phosphate and COD in MABAR is far superior to the other two types of reactors in each stage,and the highest can reach:99.34%（stage 1）,86.43%（stage 3）,78.63%（Stage 5）,and97.48%（Stage 5）.Even in the low nutrient stage,the effluent quality can reach the Class A standard.Under low nutrient conditions（stage 1）,MABAR reduces the phenomenon of stagnant TN removal due to limited denitrification in the system through algal assimilation.In addition,the MABAR system relies on the strong oxidizing power of membrane aeration to maintain stable COD effluent concentration under high pollutant impact（Stage 5）.At the same time,the absorption of nitrogen by microalgae makes up for the insufficient removal of NH₄⁺-N caused by the sharp decrease in the abundance of nitrifying bacteria.A large number of genera beneficial to algal reproduction and accumulation were detected in MABAR,including Acidovorax,Rhodobacter and Acinetobacter,and their abundance was much higher than that in HABAR.In addition,the growth of Cyanobacteria,which is harmful to the water body,is inhibited by the proliferating Actinobacteria,which reduces the possibility of secondary pollution caused by the introduction of algae.The abundance of Cyanobacteria in all five stages was less than 7%,only one third of that in HABAR.This paper aims to solve the imbalance of bacteria and algae accumulation and single sludge caused by carbon and oxygen imbalance under aeration by coupling two high-efficiency oxygen supply reactors,MABR and ABS reactor.The problem that the removal of TN in the system is limited by the concentration of organic matter reveals the internal mechanism of the bacteria and algae accumulation balance in MABAR,and provides a reference for the reduction of energy consumption in the aerobic unit of the wastewater treatment plant.