Membrane Fouling Mechanism And Sustainable Operation Principle Of MBR

The submerged membrane bioreactor (sMBR) is being increasingly applied for municipal and industrial wastewater treatment. The success and economics of the membrane bioreactor (MBR) is linked to the fouling of the membranes, Which is manifest in a rise in transmembrane pressure (TMP) at a given flux. Fouling in MBRs is complex and influenced by MBR design features, mode of operation and biological factors. Thus, the enhancement of MBR performance, which involves minimizing fouling, will require a range of solutions. This thesis explores membrane fouling mechanisms and various strategies to improve the sustainable operation of membrane bioreactors (MBRs).In membrane fouling study, the rise in TMP is described as a 3-stage process. Stage 1 occurs in a period of a few hours and involves abrupt TMP rise-up due to 'conditioning', presumably by pore blockage and closure. Stage 2 is a prolonged period of slow TMP rise, which we ascribe to accumulation of extracellular polymeric substances (EPS). Stage 3 is a sudden rise in TMP, which rapidly leadsto inoperability of the membranes. This stage could have several causes, which are all driven by the self-accelerating nature of fouling under constant flux operation. In order to understand the complex nature of fouling in submerged MBRs, the MBR fouling 'Roadmap' has been developed that shows the relationship between the many operational and design characteristics of the MBR and the generic 'fouling factors', i.e. the nature of the feed (to the membrane), the membrane properties, and the hydrodynamic environment. MBR 'Fouling mechanism map' which depicts the 3 stages of fouling and the multiple fouling mechanisms that could occur has been also presented.Under the same activated sludge condition, using Millipore series MF and UF membrane, the effect of membrane material and pore size on membrane fouling and SMP rejection have been studied. The initial fouling in TMP "jump" is mainly affected by different material and pore size of MF membrane. At different flux, the TOC rejection by MF membrane in different pore size is stronger and stronger with membrane fouling intensity increassing. Gel Permeation Chromatography (GPC) results show the fouled MF membranes are able to reject EPS, the rejection ability is linked to membrane pore size and fouling intensity. Through the investigation on surface fouling property of fouled membrane, the surface zeta potential of fouled membrane is affected by the activated sludge property. The zeta potential of fouled membrane surface is similar to the activated sludge. The Fourier Transform Infrared Spectroscopy (FTIR) result shows polysaccharide and protein deposit on the membrane surface. Images of Confocal Laser Scanning Microscope (CLSM) and Field Emission Scanning Electron Microscope (FSEM) show the deposit of activated sludge particles is not serious at low flux, while at high flux, EPS and the activated sludge particles will form combined cake layer.Applying Flux-stepping method, through the investigation of flat sheet and hollow fibre MF and UF membrane, the "sustainable flux" determination method and membrane fouling comparison method in MBR are set up. The short-term flux-stepping experiment results are able to predict long-term membrane fouling in MBR despite some errors.Sludge Retention Time (SRT), an important operating parameter of the MBR as it affects the biological characteristics of the sludge and therefore influences membrane fouling. The DGGE profiles suggest a shift in the dominant bacterial community with the prolonged SRT. The total amount of extracellular polymeric substance (EPS) extracted from the floc and the supernatant is approximately constant at the two SRTs under the same organic loading rate. However, The EPS distribution is different. The polysaccharide EPS is dominant in the supernatant EPS, its concentration is decreasing with prolonged SRT. The viscosity of the biomass increases with the prolonged SRT, while the estimated average air induced water velocity is almost same for at the two SRTs. The results of flux stepping tests show that the membrane fouling rate (dTMP/dt) can be decreased and "sustainable flux" can be increased with prolonged SRT. Long-term experimental run at a constant flux of 20 L/m~2.h clearly shows long term fouling is decreased with prolonged SRT.Two parallel lab scale MBRs have been operated for 160 days. Under the same average sludge loading rate, fed with constant concentration synthetic wastewater and regularly variable concentration wastewater to simulate the daily feed fluctuation, the activitated sludge properties are different in stable and unstable state. In 80days unstable state (stage 1), the membrane performance at variable loading is worse than that at constant loading; while in the stable state, the result is reverse. The difference in microbial activities of activitated sludge shows SMP, polysaccharide in supernatant and particle size has been found to impact the fouling and cause the difference on membrane performance in stable state and unstable state.In stable state, the SOUR of sludge at variable loading is higher than that at constant loading, presumably the active bacteria ratio is high in the sludge at variable loading and the "F/M" is decreased, which result in the difference of EPS production by the two kinds of sludge.The polymeric membrane performance enhancer (MPE50) has been applied in two parallel submerged membrane bioreactors to test its effect on membrane fouling. The 'critical flux' determined by the flux stepping test has been increased by 50% after the addition of MPE. Long-term experiments results show, below the "sustainable flux", the membrane performance of MBR with MPE is slightly improved. Above the "sustainable flux", the addition of MPE is found to apparently.mitigate membrane fouling. The possible explanations for these results are the increased in the particle size distribution of the activated sludge as well as the decreased in SMP and EPS (mainly polysaccharides) concentrations in the supernatant due to MPE flocculation. After the termination of the maintenance MPE addition, the particle size doesn't drop too much, while the supernatant EPS and SMP concentration increase dramatically. The membrane performance has been found to be worse than the MBR without MPE, which suggests that SMP and is the dominant fouling factor, membrane fouling will be affected stronger by supernatant polysaccharide than the particle size of sludge.The comparison result shows intermittent permeation is able to control fouling rate and prevent TMP jump. Below sustainable flux, intermittent permeation operation is able to postpone TMP jump. Above sustainable flux, intermittent permeation operation is able to decrease the TMP increase rate at high flux. The main possible reason is the cake layer thickness and density is controlled by intermittent permeation.