Identification And Characterization Of Fouling Behavior In Submerged Membrane Bioreactors

In recent years, membrane bioreactors (MBRs) have been widely used in wastewater treatment to achieve higher effluent quality, which is often difficult to be effectively met by conventional activated sludge process. The advantages of MBR are a high mixed liquid suspended solids (MLSS) concentration, producing higher rate of removal of biological oxygen demand (BOD) and chemical oxygen demand (COD), a lower excess sludge production and the treated water can be reused. In addition, the space occupied by MBR systems is greatly reduced due to the absence of settling tanks and the reduction in bioreactor volume made possible by the higher biomass concentration. But a major obstacle for the application of MBRs is the rapid decline of the permeation flux as a result of membrane fouling. The membrane fouling in MBR reduces productivity and increases maintenance and operating costs. Thus, membrane fouling is the crucial problem to be resolved.In this work, fouling mechanisms of submerged MBRs were investigated systematically. The impacts of sludge characteristics and operational conditions on membrane fouling were identified. The micro-mechanism of membrane fouling was also elucidated on the basis of fractal theory and image analysis. This work was performed as follows:(1) In this work, the influence of activated sludge properties such as the MLSS concentration, sludge particle size distribution (PSD), extracellular polymeric substances (EPS), soluble microbial products (SMP), suspended solids in supernatant (SSs), dynamic viscosity (μ), relative hydrophobicity (RH), and zeta potential on membrane fouling was examined. Activated sludge samples taken from different membrane bioreactor processes were used to study their impacts on membrane fouling. The influence of activated sludge properties on membrane permeation was identified using statistical methods. The results showed that MLSS concentration had an exponential relationship with membrane fouling resistance. The sludge particle size (r_p=-0.730) was correlated inversely to the membrane fouling resistance significantly. The total EPS (r_p=0.898) and protein (r_p=0.810) had strong positive effect on membrane fouling resistance, but carbohydrate (r_p=0.626) had a moderate correlation with membrane fouling resistance due to its low amounts. SMP (r_p=0.757), SSs (r_p=0.810), dynamic viscosity (r_p=0.691), RH (r_p=0.837), and zeta potential (r_p=-0.881) also had significant influence on membrane permeability. However, protein (r_p=0.936), SMP (r_p=0.725), SSs (r_p=0.783), dynamic viscosity (r_p=0.633), RH (r_p=0.877), and zeta potential (r_p=-0.953) mainly resulted from the change of EPS concentration. These results suggest that MLSS concentration, PSD and EPS were the predominant factors affecting membrane fouling during membrane filtration of sludge suspension. The membrane fouling resistance can be predicted using a model based on MLSS concentration, PSD and EPS: R_f = 2.250e^{MLss*9×10-5}＋0.111EPS－1.99×10^-2PSD－3.201(2) The influence of filamentous bacteria on membrane fouling process in membrane bioreactor was analyzed in this study. To illustrate the membrane fouling mechanism of filamentous bacteria, the physical and chemical characteristics of floes such as EPS, zeta potential, RH and floc morphology were investigated systematically. The results showed that the absence of filamentous bacteria in sludge floes led to severe membrane pore blocking, the floes existing excessive filamentous bacteria, however, could cause the formation of a non-porous cake layer on the membrane surface. The over growth of filamentous bacteria could result in much more release of EPS, lower zeta potential, higher hydrophobicity of sludge floes, and caused serous harm to membrane permeation. The results also showed that the boundary fractal dimension (D_p) and three-dimensional aspect ratio (AR) of sludge floes increased with increasing filamentous index (FI). In addition, the roundness (R_o) of sludge floes decreased with increasing FI. These results indicate that floes of bulking sludge had a very irregular shape, which would do great harm to membrane filtration process. The sludge floes existing a small quantity of filamentous bacteria had a positive effect on membrane permeation. It is an important kind of microbe to be control in the operation of membrane bioreaetors.(3) In this study, three identical MBRs were operated in parallel with sludge retention time (SRT) of 30 days and organic loading rates (OLRs) of 0.7-0.8 kgCOD/m³d, 1.1-1.4 kgCOD/m³d, 1.7-2.1 kgCOD/m³d, respectively, in order to specify the influence mechanism of OLRs on membrane bioreactor. Although OLRs have no direct impact on membrane permeation, OLRs affect sludge characteristics strongly since the MBR system includes living microorganisms and their metabolites. The results showed that COD removal efficiencies were stable though it decreased slightly as OLR increased, but biomass activity and dissolved oxygen (DO) concentration in sludge suspension decreased as OLR increased. The filamentous bacteria grew easily in the bioreaetor with increasing OLR. The EPS concentration and sludge viscosity became much higher as filamentous bacteria excessively grew. The filamentous bacteria could also induce the formation of large and irregular shaped sludge flocs, and worsen membrane permeation. In addition, the MLSS concentration increased significantly as OLR increased. The results also indicated that sludge viscosity was the predominant factor affecting hydrodynamic conditions of MBR systems. The cross flow velocity of the sludge suspension decreased significantly as the sludge viscosity increased larger than 2.0 mPa s. Under the hydrodynamic conditions of low cross flow velocity, the fouling cake layer formed on the membrane surfaces could not be removed effectively. Moreover, the Re of the sludge suspension in the MBRs was decreased significantly with increasing sludge viscosity.(4) Membrane fouling in three parallel MBRs was studied under different aeration intensities (150 L/h, 400 L/h, and 800 L/h) to have a better understanding of membrane fouling mechanism. The impact of aeration on membrane fouling was interpreted from two aspects: evolution of sludge properties and formation mechanism of fouling cake layer. As the membrane fouling in MBRs mainly results from the formation of a fouling cake layer, scanning electron microscope (SEM) and resistance analysis were performed to characterize and quantify the fouling layer. The results showed that small or large aeration intensity had a negative influence on membrane permeability. The smaller aeration intensity could not remove the fouling cake layer effectively. The larger aeration intensity resulted in a severe breakup of sludge flocs, and promoted the release of colloidal and soluble components from the microbial flocs to the bulk solution due to microbial floc breakage, thus caused a rapid loss in membrane permeability. The colloids and solutes had great contribution to membrane foulants as the MBR operated under high aeration intensity. The results suggested that a dynamic membrane would be formed under the aeration of 400 L/h, which had a positive effect on membrane permeation. Aeration had a positive effect on cake layer removal, but pore blocking became severe as aeration intensity increased to 800 L/h.(5) In this study, the fouling behavior of different MBR technologies, including conventional MBR (CMBR), sequencing batch MBR (SMBR), A/O-MBR and A²/O-MBR, was investigated on the basis of bacth membrane filtration. The biomass characteristics were analyzed in order to elucidate membrane fouling. The membrane foulants formed on the membrane surface were characterized by such techniques as confocal laser scanning microscopy (CLSM), Fourier transform infrared (FTIR) spectroscopy, and X-ray fluorescence (XRF). The formation mechanism of membrane foulants was also analyzed. The results showed that the sludge suspension in A²/O-MBR could result in severe membrane fouling due the higher filamentous concentration and EPS concentration. The sludge suspension in A/O-MBR and CMBR had a moderate membrane fouling tendency. The sludge suspension in SMBR, however, caused a very slight membrane fouling. The filamentous index, bound EPS, soluble EPS and SUV₂₅₄ had strong impacts on membrane fouling. The CLSM and FTIR examination indicated that bacteria clusters, proteins and carbonhydrates were significant contributors to membrane foulants. The XRF suggested that the inorganic foulants resulted from the deposition of Ca, Mg, Si, Al, and Fe. Bridging among bacteria clusters, deposited biopolymers and inorganic materials enhanced the compactness of the fouling layer.(6) The formation of fouling cake layer on the membrane surface is the major problem that hinders the practical application of membrane bioreactor systems. Determination of the cake layer permeability is critical for an accurate analysis and design of membrane filtration. A permeation model, based on fractal theory and Darcy's law, for evaluating cake layer permeability in microfiltration of sludge suspension was developed. The cake layer permeability was derived and found to be a function of the pore-area fractal dimension and micro-structural parameters. The validity of the model was studied systematically. The permeation model was applied to study the effect of MLSS concentration, PSD and EPS on cake layer permeability in a submerged membrane bioreactor. Results showed that the permeation model was a useful tool to study the micro-mechanism of membrane fouling. There was a close correlation between MLSS concentration and cake layer permeability. There were a slight and a distinct decrease of the cake layer permeation as MLSS increased less and larger than 10,000mg/L, respectively. PSD and EPS were two significant factors affecting cake layer permeability in membrane bioreactor. The decrease of PSD or increase of EPS would result in a sharp decrease of cake layer permeability.