| Membrane bioreactor (MBR), which integrates membrane separation with biological treatment, is a new technology for wastewater treatment. As compared with conventional activated sludge (CAS) system, MBR technology owns significant advantages, including highly-improved effluent quality, reduced footprint, easy auto-control, etc., and has gained increasing popularity for wastewater treatment and reclaim. However, the universal appeal of MBR for wastewater treatment is limited by membrane fouling, which reduces productivity and increases operation consumption of MBR. Thus, it will be of great significance to explore the mechanism and critical influence factors of membrane fouling, which will be favorable to the control of membrane fouling and the promoted application of MBR technology.Based on a continuously operated MBR, the mechanism and key influential factors of membrane fouling were discussed deeply. The effects of EPS, pH, ionic strength and membrane roughness were investigated. The internal mechanisms of above effects were then elucidated comprehensively and thoroughly on the basis of osmotic pressure mechanism and XDLVO theory. The main results are presented as follows:(1) A new membrane fouling mechanism:osmotic pressure was existed during the cake layer filtration in MBR system. An osmotic pressure model based on chemical potential difference was then proposed. Simulation of the proposed model showed that osmotic pressure accounted for the major fraction of total filtration resistance of cake layer, and pH, applied pressure and ionic strength were the key determining factors for osmotic effect.(2) The structure and composition analysis of cake layer showed that the cake layer formed on membrane surface was elastic and highly hydrated, and the chemical potential of interstitial water in cake layer would vary along with cake depth. The counter-ions intercepted by the negatively charged functional groups were the original of osmotic pressure effect in the cake layer. Furthermore, SMP and BPS in supernatant highly affected osmotic pressure mechanism, and thus caused high cake filtration.(3) The resistance of virgin membrane was slightly increased with pH increased. Pore clogging resistance was minor and not dramatically affected by pH changed. And the adhesion force of sludge particles was weakened with the advancing pH level. According to XDLVO theory analysis, a repulsive energy barrier was found when the flocs got close to membrane surface. This energy barrier was minished with the decrease of pH, which will lead to the enhanced attachment of sludge particles. The resistance of cake layer observably increased with pH owing to the effect of osmotic pressure mechanism.(4) In the range of 0.005-0.05 mol/L, ionic strength had no apparent influences on the pore clogging and cake layer resistance. In the process of sludge foulants getting close to membrane surface, there existed a secondary energy minimum and repulsive energy barrier. This energy barrier would significantly reduce with the ionic strength increased, implying existence of a critical ionic strength above which the energy barrier would disappear. In addition, cake resistance was highly depended on SMP in supernatant, which could be deciphered by the osmotic pressure mechanism.(5) As compared with smooth membrane, sludge flocs could more easily attach on and detached from rough membrane surface. Generally, lower asperity radius would favor adhesion of a sludge floc, and there existed a "critical" asperity radius above which the total interaction energy in the certain separation distance coverage would be continuously repulsive. The effects of asperity height on membrane fouling were highly dependent on the hydrodynamic conditions of MBR. Under profitable hydrodynamic conditions, higher asperity height would be good for membrane fouling control. The influences of floc size on membrane fouling were so complicated that comprehensive evaluation should be implemented to verify the impact of floc size on membrane fouling in MBR. |
PDF Full Text Request