Operation Of Iron Enhanced Phosphorus Removal Membrane Bioreactor (EBPR-MBR) And Control Of Membrane Fouling

Membrane bioreactor?MBR?is a promising technology and is widely used for wastewater treatment.However,the high sludge residence time and membrane fouling are the main bottlenecks in the market popularization and development of MBR.In this paper,a bench-scale Anoxic/Oxic MBR system is constructed for treating synthetic wastewater with dosing of ferric chloride?FeCl₃?.Moreover,the reactor's rejection performance and membrane fouling control strategy are systematically investigated.Firstly,the MBR was fed with synthetic wastewater and the operation state of the reactor could be verified through the type of protozoa in the reactor and the three-dimensional fluorescence of the effluent.After 15-day operation,the MBR achieved a quick start.Furthermore the removal efficiency of COD and NH₄⁺-N can reach to a high level,however,the concentration of phosphorus in the effluent cannot reach the emission standard.The further study on the phosphorus adsorption characteristics of the rich iron sludge suggest that the adsorption behavior conforms to the pseudo two order kinetics and fit the Langmuir adsorption isotherm equation.Additionally,the accumulation process of membrane fouling reveled by the visualizations of the fouling layer on the membrane surface and the live and dead distribution on the cake layer section which are achieved by using the scanning electron microscope?SEM?and the fluorescence microscope,respectively.Secondly,the iron is introduced into the reactor for enhancing phosphorus removal.The best dosing ratio of Fe/P?mol/mol?is 1.6 measured by phosphorus removal and sludge specific resistance.Furthermore,the phosphorus removal performance is significantly improved and the average removal efficiency increased nearly by 40%after dosing iron compared to the previous 52%.In addition,the removal efficiencies of COD and NH₄⁺-N maintain around 90%indicating that there is no side effect of iron on COD and NH₄⁺-N removal.The concentration of polysaccharide in EPS secreted by activated sludge decreases after dosing of FeCl₃.The size of the sludge particle size decreases,which is related to the greater hydraulic shear force and the instability of the sludge flocs.By extracting the activated sludge during different periods and analyzing the succession of microbial community,it was found that the iron would reduce the diversity and abundance of some polyphosphate-accumulating organisms.In terms of membrane fouling,which have a side effect on biological phosphorus removal.transmembrane pressure difference?TMP?did not increase significantly,which slowed down membrane fouling.Finally,the cleaning strategies for the fouled membrane module were studied after filtration process.Due to the membrane fouling layer is rich in high concentration of Fe³⁺,two kinds of the reducing agents ascorbic acid and sodium dithionite were used for chemical cleaning.The results showed that ascorbic acid and sodium dithionite have good cleaning effect by the color removal of membrane surface and the SEM-EDS scanning of membrane surface.The optimal concentration of ascorbic acid and sodium dithionite for the cleaning of membrane is 10 mM and 100 mM,respectively.The best pH value of ascorbic acid is 4 while that sodium dithionite have maximum reaction rate at pH 3.In addition,the combination of sodium hypochlorite and ascorbic acid is superior to other combinations in contributing to decreasing membrane resistance and mitigating the increment of TMP.