Bao-Qiang Liao

Anaerobic Membrane Bioreactors: Applications and Research Directions

Membranes provide exceptional suspended solids removal and complete biomass retention that can improve the biological treatment process, but their commercial application to anaerobic treatment has been limited. This review summarizes the state of the art with respect to anaerobic membrane bioreactors (AnMBRs), determines the types of wastewaters for which AnMBRs would be best suited, and identifies the research required to increase implementation. AnMBRs have been tested with synthetic, food processing, industrial, high solids content, and municipal wastewaters at laboratory, pilot, and full scale. Chemical oxygen demand removal ranges from 56% to 99%, while the reported design membrane fluxes range from 10 to 40 L/m2/h. AnMBRs should be immediately applicable to highly concentrated, particulate waste streams like municipal sludges where the membrane can decouple the solids and hydraulic retention times. Opportunity for application to dilute wastewaters also appears strong, while application to highly concentrated soluble wastewaters is likely limited. Greater assessment of vacuum-driven immersed membranes, combining external or immersed membranes with retained biomass reactor designs, control of membrane fouling, and economic feasibility are the key research areas to be addressed.

Sludge properties and their effects on membrane fouling in submerged anaerobic membrane bioreactors (SAnMBRs).

Two submerged anaerobic membrane bioreactors (SAnMBRs) (thermophilic vs. mesophilic) were operated for a period of 3.5 months with kraft evaporator condensate at a feed chemical oxygen demand of 10,000 mg/L. The results show that the filtration behavior of the two systems was significantly different. The filtration resistance in the thermophilic SAnMBR was about 5-10 times higher than that of the mesophilic system when operated under similar hydrodynamic conditions. Comparison of sludge properties and cake layer structure from the two systems was made to elucidate major factors governing the different filtration characteristics. There were more soluble microbial products (SMP) and biopolymer clusters (BPC) produced and a larger portion of fine flocs (<15 microm) in the thermophilic SAnMBR. Analysis of bound extracellular polymeric substances (EPS) showed that the thermophilic sludge had a higher protein/polysaccharide ratio in EPS, as compared to that in the mesophilic sludge. A series of analyses, including Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray spectroscopy (EDX), confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and particle size analyzer showed that the cake layer formed in the thermophilic SAnMBR contained higher levels of both organic and inorganic foulants, smaller particle sizes, and especially, a denser and more compact sludge cake structure. These results indicate that floc size, SMP, BPC, bound EPS as well as cake layer structure are the major factors governing membrane fouling in SAnMBR systems.

Membrane Bioreactors for Industrial Wastewater Treatment: A Critical Review

Membrane bioreactor (MBR) technology has been extensively employed for various industrial wastewater treatments due to its distinct advantages over conventional technologies. To provide present state and development trends of MBR technology used for industrial wastewater treatments, the authors reviewed and analyzed more than 300 scientific publications. They present an overview of the most recent development of MBR technology for treatment of industrial wastewaters (e.g., food processing, pulp and paper, textile, tannery, landfill leachate, pharmaceutical, oily and petrochemical wastewaters). Moreover, they discuss the operational characteristics, fouling characteristics, fouling control strategies, and costs of MBRs in industrial wastewater treatments. Based on the present information on MBR technology, the authors discuss further research aspects of MBRs in industrial wastewater treatments.

Recent advances in membrane technologies for biorefining and bioenergy production

The bioeconomy, and in particular, biorefining and bioenergy production, have received considerable attention in recent years as a shift to renewable bioresources to produce similar energy and chemicals derived from fossil energy sources, represents a more sustainable path. Membrane technologies have been shown to play a key role in process intensification and products recovery and purification in biorefining and bioenergy production processes. Among the various separation technologies used, membrane technologies provide excellent fractionation and separation capabilities, low chemical consumption, and reduced energy requirements. This article presents a state-of-the-art review on membrane technologies related to various processes of biorefining and bioenergy production, including: (i) separation and purification of individual molecules from biomass, (ii) removal of fermentation inhibitors, (iii) enzyme recovery from hydrolysis processes, (iv) membrane bioreactors for bioenergy and chemical production, such as bioethanol, biogas and acetic acid, (v) bioethanol dehydration, (vi) bio-oil and biodiesel production, and (vii) algae harvesting. The advantages and limitations of membrane technologies for these applications are discussed and new membrane-based integrated processes are proposed. Finally, challenges and opportunities of membrane technologies for biorefining and bioenergy production in the coming years are addressed.

Membrane fouling in a membrane bioreactor: High filtration resistance of gel layer and its underlying mechanism

A membrane bioreactor (MBR) was continuously operated to investigate mechanisms of fouling caused by the gel layer in this study. Agar was used as a model foulant for gel layer formation, and filtration resistance of gel layers was systematically assessed. The results showed that gel layer possessed unusually high specific filtration resistance (SFR) and high measured porosity as compared with cake layer. Current knowledge cannot explain the contradiction between high filtration resistance and high porosity of gel layer. A new fouling mechanism based on Flory-Huggins theory was then proposed. Filtration resistance of agar gel layer was found to be independent of pH and ionic strength, but linearly increase with gel thickness. The results are accordant with the mechanism deductions. Simulation of the mechanism model showed that the filtration resistance induced by mixing chemical potential variation was comparable to the experimental data of filtration resistance of agar gel layer, indicating that the proposed mechanism is the predominant mechanism responsible for the high filtration resistance of gel layer. The proposed mechanism was further verified from the bound water viewpoint.

Effects of temperature and temperature shock on the performance and microbial community structure of a submerged anaerobic membrane bioreactor

Effects of temperature and temperature shock on the performance and microbial community structure of a submerged anaerobic membrane bioreactor (SAnMBR) treating thermomechanical pulping pressate were studied for 416 days. The results showed that the SAnMBR system were highly resilient to temperature variations in terms of chemical oxygen demand (COD) removal. The residual COD in treated effluent was slightly higher at 55 °C than that at 37 and 45 °C. There were no significant changes in biogas production rate and biogas composition. However, temperature shocks resulted in an increase in biogas production temporarily. The SAnMBR could tolerate the 5 and 10 °C temperature shocks at 37 °C and the temperature variations from 37 to 45 °C. The temperature shock of 5 and 10 °C at 45 °C led to slight and significant disturbance of the performance, respectively. Temperature affected the richness and diversity of microbial populations.

Performance and fouling characteristics of a submerged anaerobic membrane bioreactor for kraft evaporator condensate treatment

Submerged anaerobic membrane bioreactor (SAnMBR) technology was studied for kraft evaporator condensate treatment at 37 ± 1°C over a period of 9 months. Under tested organic loading rates of 1–24 kg COD/m3/day, a chemical oxygen demand (COD) removal efficiency of 93–99% was achieved with a methane production rate of 0.35 ± 0.05 L methane/g COD removed and a methane content of 80–90% in produced biogas. Bubbling of recycled biogas was effective for in‐situ membrane cleaning, depending on the biogas sparging rate used. The membrane critical flux increased and the membrane fouling rate decreased with an increase in the biogas sparging rate. The scanning electron microscopy images showed membrane pore clogging was not significant and sludge cake formation on the membrane surface was the dominant mechanism of membrane fouling. The results suggest that the SAnMBR is a promising technology for energy recovery from kraft evaporator condensate.

Effects of hydrophilicity/hydrophobicity of membrane on membrane fouling in a submerged membrane bioreactor.

The interfacial interactions between sludge foulants and four different types of membranes were assessed based on a new combined calculation method. Effects of membrane surface hydrophilicity/hydrophobicity on the interfacial interactions were investigated. It was found that, membrane surface hydrophilicity/hydrophobicity was not directly relevant to the interfacial interactions with sludge particles. Increasing membrane surface zeta potential could significantly increase the strength of the electrostatic double layer (EL) interaction and the energy barrier. For membrane with a surface roughness of 300nm, the total interaction was continuously repulsive in the separation distance coverage of 0-4nm in this study. The results suggest that, under conditions in this study, designing membranes with a high zeta potential and certain roughness can significantly mitigate membrane fouling, whereas, the strategy of improving membrane surface hydrophilicity cannot alleviate sludge adhesion in the membrane bioreactor.

A new method for modeling rough membrane surface and calculation of interfacial interactions.

Membrane fouling control necessitates the establishment of an effective method to assess interfacial interactions between foulants and rough surface membrane. This study proposed a new method which includes a rigorous mathematical equation for modeling membrane surface morphology, and combination of surface element integration (SEI) method and the composite Simpsons approach for assessment of interfacial interactions. The new method provides a complete solution to quantitatively calculate interfacial interactions between foulants and rough surface membrane. Application of this method in a membrane bioreactor (MBR) showed that, high calculation accuracy could be achieved by setting high segment number, and moreover, the strength of three energy components and energy barrier was remarkably impaired by the existence of roughness on the membrane surface, indicating that membrane surface morphology exerted profound effects on membrane fouling in the MBR. Good agreement between calculation prediction and fouling phenomena was found, suggesting the feasibility of this method.

Surface Properties of Biofouled Membranes from a Submerged Anaerobic Membrane Bioreactor after Cleaning

The surface structural properties of biofouled membranes from a laboratory-scale submerged anaerobic membrane bioreactor (SAnMBR) treating kraft pulping evaporator condensate after cleaning were studied. A flat sheet polyvinylidene fluoride (PVDF) membrane was used for the study. Three different cleaning methods, physical cleaning (PC), maintenance chemical cleaning (MCC), and recovery cleaning (RC) were applied to the fouled membrane surface, and the treated membranes were subject to flux recovery and surface structural analysis by using spectroscopic methods, zeta potential measurement, attenuated total reflectance-Fourier transform infra red spectroscopy (ATR-FTIR), and advanced correlative microscopic methods, including confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Neither PC, MCC, nor RC methods restored the membrane permeability to initial conditions. Adhesion of a thin extracellular polymeric substance (EPS) layer, consisting of pr...