Study On Airlift Oxidation Ditch-Membrane Bioreactorprocessfor Municipal Wastewaternitrogen And Phosphorous Removal And Its Fouling Characteristics

Membrane bioreactors(MBRs)have been widely used in the field of wastewater treatment because of the advantages such as high quality effluent and small footprint.However,relatively high operational energy restricts its further development.Airlift oxidation ditch(AOD)is a new wastewater treatment technology developed in recent years.Compared with the traditional oxidation ditch,it can largely save land area via increasingwater depth.But it still need sludge recirculation device and settling basin.In this study,airlift oxidation ditch-membrane bioreactor(AOD-MBR)was established by placing membrane modules in the up-flow region of AOD.A pilot-scale AOD-MBR was studied in a wastewater treatment plant of Jiangsu Province.Pollutant removal efficiency,mixed liquor properties,and membrane fouling characteristics of the reactor under different operating conditions were investigated.Moreover,the flow characteristics of the reactor were studied using computational fluid dynamics(CFD)toevaluate the possibility of further reducing the air demand and increasing the sludge concentration.The AOD-MBR plant was found to have good performance for the removal of chemical oxygen demand,ammonia nitrogen,total nitrogen,and total phosphorus as the hydraulic retention time ranged from 9 to 5.4h.The effluent total phosphorus concentration reached the secondary standard level of GB18918-2002 in China and the other indicators met the first-A standard.With the reduction of the total aeration rate from 160 to 48 m3/h,the concentrations of both extracellular polymeric substances(e.g.polysaccharides and proteins)and supernatant organics(including polysaccharides,proteins,and humic substances)increased,which may accelerate membrane fouling.Compared with membrane-tank coarse-bubble aeration,bottom-located fine-bubble aeration achieved faster circulation flow causing a stronger shear stress which greatly reduces the membrane fouling potential.CFD simulation showedthat the flow rate and membrane shear stress decrease with decreasing aeration rate(from 160 to 48 m3/h)and increasing sludge concentration(from 8 to 12 g/L).But these ranges were still acceptable according to the experimental pollutant removal and membrane fouling evolution of the AOD-MBR.Therefore,it is promising that the aeration rate may be further decreased to cut the energy consumption,and the sludge concentration be further increased to improve the pollutant removal capacity.