Comprehensive Evaluation Study On Unitized Technique Of Microfiltration And Ultra Low-pressure Nanofiltration

With the climate changes and rapid development of population due to urbanization and industrialization,the need of water is essential.It is difficult for the conventional activated sludge process to meet the water quality standards which is getting more and more strict.Due to the high biomass,good water quality,small footprint,low production of sludge,membrane bioreactor(MBR)is usually used in sewage treatment process.In order to remove the viruses,inorganics,dissolved organic matters and trace organic compounds of MBR permeate,nanofiltration can be used.Nevertheless,the main drawback of microfiltration and nanofiltration is the membrane fouling.Meanwhile,the residual sludge in MBR and the nanofiltration concentrate in nanofiltration cannot be discharged to the environment.The pilot studies on extracellular polymeric substances(EPS)and soluble microbial products(SMP)of activated sludge in MBR for zero excess activated sludge production process and the pilot study on secondary effluent after ultra low-pressure selective nanofiltration(DF)have been rarely reported.In this study,A~2/O-MBR and DF hybrid system were used to treat the municipal sewage,and the major contents and conclusions were as follows:(1)In contrast to the conventional activated sludge process(CAS system),control A~2/O-MBR(MBR system)operates for zero excess activated sludge production process,analysis the operating characteristics of the system.The removal efficiencies of chemical oxygen demand(COD)and ammonia nitrogen were excellent in the MBR system,which was 92.54 ± 4.86 % and 97.73 ± 5.99 %,respectively;total nitrogen and total phosphorus(TP)removal rate were 75.04 ± 11.47 % and 46.39 ± 16.01 %,respectively.This results implied that the MBR system was more stable and stronger resistance to shock loading.The EPS and SMP of both systems accumulated in the early stage and were gradually degraded by microorganisms.The setting of the sludge mixing liquid and the long sludge retention time were beneficial to the reduction of the total amount of EPS and SMP,thus alleviating the contamination of the microfiltration membranes.The decrease of sludge age and sludge drain would induce the microbial release EPS and SMP to counteract the environmental change.It was found that EPS and SMP were mainly composed of protein-like substances and humic acids-like substances via the EEM analysis.The protein-like substances were higher content,easier to be biodegradable and could be trapped by microfiltration membranes,whose pollution contribute rate was relatively high.By contrast,the humic acids-like substances were relatively stable,which were difficult to degrade and could penetrate the microfiltration membranes,and the pollution contributes rate of that was comparatively low.(2)DF system can run stably with lower operating pressure(<0.32 MPa),which can efficiently intercept pollutants.It suggested that the retention efficiency of multivalent ion was relatively higher,that of the monovalent ion was comparatively lower.The DF membrane flux and desalination rate were significantly correlated with the operational time and the salinity of influent(total dissolved solids and/or conductivity).By increasing the reflux ratio of concentrated water can accelerate the concentration polarization.It could result in the increase of membrane flux decline rate so that the membrane components would be quickly polluted.The analysis of elution liquid after washing indicates that the contamination layer of DF system was mainly composed of calcium,magnesium,aluminum and ferrous,and so on.(3)The MBR-DF system was operated jointly with zero emission of DF concentrated water,in which the removal of COD and ammonia nitrogen in the initial influent was high(88 % and 97 %)in the MBR system.After DF interception,the COD of final effluent was less than 10mg/L,ammonia nitrogen was less than 0.1 mg/L;the phosphorus removal efficiency of the phosphorus enhancement MBR system,which could weaken the interception of phosphate by the DF system,and then alleviate DF membrane fouling.(4)When fluorescence analysis of dissolved organic matter of total influent,it was found that the DOM with the fluorescent properties in municipal sewage was divided into protein substances,and the proportion of humic acids was lower.After the treatment of different systems,the fluorescence intensity of maximum peak in the effluent DOM showed a significant difference,which indicated that there were differences in the ways of degrading organics in each system,and the combination of different systems can compensate for the deficiency so that the removal ability was improved.For the high abundance of endocrine disruptors in the effluent of CAS system—17-β estradiol(E2),the CAS system only removed 70.8 %,while the MBR system and DF system efficiently removed E2(>99.98 %).MBR-DF treatment significantly improved the effluent quality and effectively reduced the health hazards and ecological threats in the process of reuse,realizing the efficient recycling of the source-type sludge reduction in the MBR system and the zero emission of DF concentrated water in the DF system.