Guo-Ping Sheng

Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: A review

A review concerning the definition, extraction, characterization, production and functions of extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment reactors is given in this paper. EPS are a complex high-molecular-weight mixture of polymers excreted by microorganisms, produced from cell lysis and adsorbed organic matter from wastewater. They are a major component in microbial aggregates for keeping them together in a three-dimensional matrix. Their characteristics (e.g., adsorption abilities, biodegradability and hydrophilicity/hydrophobicity) and the contents of the main components (e.g., carbohydrates, proteins, humic substances and nucleic acids) in EPS are found to crucially affect the properties of microbial aggregates, such as mass transfer, surface characteristics, adsorption ability, stability, the formation of microbial aggregates etc. However, as EPS are very complex, the knowledge regarding EPS is far from complete and much work is still required to fully understand their precise roles in the biological treatment process.

Recent advances in the separators for microbial fuel cells.

Separator plays an important role in microbial fuel cells (MFCs). Despite of the rapid development of separators in recent years, there are remaining barriers such as proton transfer limitation and oxygen leakage, which increase the internal resistance and decrease the MFC performance, and thus limit the practical application of MFCs. In this review, various separator materials, including cation exchange membrane, anion exchange membrane, bipolar membrane, microfiltration membrane, ultrafiltration membranes, porous fabrics, glass fibers, J-Cloth and salt bridge, are systematically compared. In addition, recent progresses in separator configuration, especially the development of separator electrode assemblies, are summarized. The advances in separator materials and configurations have opened up new promises to overcome these limitations, but challenges remain for the practical application. Here, an outlook for future development and scaling-up of MFC separators is presented and some suggestions are highlighted.

Contribution of Extracellular Polymeric Substances (EPS) to the Sludge Aggregation

The contribution of extracellular polymeric substances (EPS), including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), to the aggregation of both aerobic and anaerobic sludge is explored using the extended DLVO theory. It is observed that the aggregation abilities of both sludge samples decrease with the extraction of LB-EPS and TB-EPS, implying the crucial roles of EPS in sludge aggregation. Furthermore, through analyzing the interaction energy curves of sludge before and after the EPS extraction using the extended DLVO theory, it is found that both LB-EPS and TB-EPS have a substantial contribution to the sludge aggregation. The interaction energy of LB-EPS is always negative, suggesting that the LB-EPS always display a positive effect on the sludge aggregation. On the other hand, the interaction energy of TB-EPS is not always negative, depending on the separation distance between sludge cells. These results imply that the LB-EPS and TB-EPS have different contributions to the sludge aggregation.

Fouling of proton exchange membrane (PEM) deteriorates the performance of microbial fuel cell.

The fouling characteristics of proton exchange membrane (PEM) in microbial fuel cell (MFC) and the resulting deterioration of MFC performance were explored in this study. It was observed that the ion exchange capacity, conductivity and diffusion coefficients of cations of PEM were reduced significantly after fouling. Imaging analysis coupled with FTIR analysis indicated that the fouling layer attached on PEM consisted of microorganisms encased in extracellular polymers and inorganic salt precipitations. The results clearly demonstrate that PEM fouling deteriorated the performance of MFCs and led to a decrease in electricity generation. Cation transfer limitation might play an important role in the deterioration of MFC performance because of the membrane fouling. This was attributed to the physical blockage of charge transfer in the MFC resulted from the membrane fouling. With the experimental results, the effect of membrane fouling on the electrical generation of MFCs was evaluated. It was found that the decreased diffusion coefficients of cations and cathodic potential loss after membrane fouling contributed mainly to the deterioration of the MFC performance.

Physicochemical characteristics of microbial granules.

Microbial granules play an important role in the field of biological wastewater treatment due to their advantages over the conventional sludge flocs, such as a denser and stronger aggregate structure, better settleability and ensured solid-effluent separation, higher biomass concentration, and greater ability to withstand shock loadings. A better understanding of microbial granules may help in engineering biological wastewater treatment systems. Recent studies have greatly expanded our vision over the physicochemical characteristics of microbial granules. This paper provides an up-to-date review on recent work in the understanding of physicochemical characteristics of both anaerobic and aerobic granules with regard to settleability, permeability, morphology, mechanical stability, rheology, porosity, surface adsorbability, surface hydrophobicity and thermodynamics, and extracellular polymeric substances. Our growing knowledge on such characteristics might facilitate the engineering and optimization of microbial granulation as one of the most promising techniques in biological wastewater treatment.

Thermodynamic analysis on the binding of heavy metals onto extracellular polymeric substances (EPS) of activated sludge

Metal binding to microbial extracellular polymeric substances (EPS) greatly influences the distribution of heavy metals in microbial aggregates, soil and aquatic systems in nature. In this work, the thermodynamic characteristics of the binding between aqueous metals (with copper ion as an example) and EPS of activated sludge were investigated. Isothermal titration calorimetry was employed to estimate the thermodynamic parameters for the binding of Cu²⁺ onto EPS, while three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy with parallel factor analysis was used for quantifying the complexation of Cu²⁺ with the EPS. The binding mechanisms were further explored by X-ray absorption fine structure (XAFS) and Fourier transform infrared (FTIR) spectroscopy analysis. The results show that the proteins and humic substances in EPS were both strong ligands for Cu²⁺. The binding capacity N, binding constant K, binding enthalpy ΔH were calculated as 5.74 × 10⁻² mmol/g, 2.18 × 10⁵ L/mol, and -11.30 kJ/mol, respectively, implying that such a binding process was exothermic and thermodynamically favorable. The binding process was found to be driven mainly by the entropy change of the reaction. A further investigation shows that Cu²⁺ bound with the oxygen atom in the carboxyl groups in the EPS molecules of activated sludge. This study facilitates a better understanding about the roles of EPS in protecting microbes against heavy metals.

Development of a Novel Bioelectrochemical Membrane Reactor for Wastewater Treatment

A novel bioelectrochemical membrane reactor (BEMR), which takes advantage of a membrane bioreactor (MBR) and microbial fuel cells (MFC), is developed for wastewater treatment and energy recovery. In this system, stainless steel mesh with biofilm formed on it serves as both the cathode and the filtration material. Oxygen reduction reactions are effectively catalyzed by the microorganisms attached on the mesh. The effluent turbidity from the BEMR system was low during most of the operation period, and the chemical oxygen demand and NH(4)(+)-N removal efficiencies averaged 92.4% and 95.6%, respectively. With an increase in hydraulic retention time and a decrease in loading rate, the system performance was enhanced. In this BEMR process, a maximum power density of 4.35 W/m(3) and a current density of 18.32 A/m(3) were obtained at a hydraulic retention time of 150 min and external resister of 100 Ω. The Coulombic efficiency was 8.2%. Though the power density and current density of the BEMR system were not very high, compared with other high-output MFC systems, electricity recovery could be further enhanced through optimizing the operation conditions and BEMR configurations. Results clearly indicate that this innovative system holds great promise for efficient treatment of wastewater and energy recovery.

Characterization of extracellular polymeric substances produced by mixed microorganisms in activated sludge with gel-permeating chromatography, excitation-emission matrix fluorescence spectroscopy measurement and kinetic modeling

In this work the extracellular polymeric substances (EPS) produced by mixed microbial community in activated sludge are characterized using gel-permeating chromatography (GPC), 3-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy measurement and mathematical modeling. Chromatograms of extracted EPS exhibit seven peaks, among which proteins have four peaks and polysaccharides have three peaks. Evolution of the chromatogram area indicates that the quantity of produced EPS increases significantly in the substrate utilization process. With the parallel factor analysis (PARAFAC) approach, two components of the polymer matrix are identified by the EEM analysis, one as EPS proteins at Ex/Em 280/340 nm and one matrix associated as fulvic-acid-like substances at 320/400 nm. The proteins and fulvic-acid-like substances in the EPS increase in the substrate utilization phase, but decrease in the endogenous phase. To have a better insight into EPS production, the kinetic modeling of EPS is performed with regard to their molecular weight distribution and chemical natures identified by GPC and EEM. In this way, the dynamics of these important microbial products are better understood.

Synthesis and characterization of a novel cationic chitosan-based flocculant with a high water-solubility for pulp mill wastewater treatment

In this work, pulp mill wastewater was treated using a novel copolymer flocculant with a high water-solubility, which was synthesized through grafting (2-methacryloyloxyethyl) trimethyl ammonium chloride (DMC) onto chitosan initiated by potassium persulphate. The experimental results demonstrate that the two main problems associated with the utilization of chitosan as a flocculant, i.e., low molecular weight and low water-solubility, were concurrently sorted out. The physicochemical properties of this flocculant were characterized with Fourier-transform infrared spectroscopy, (1)H nuclear magnetic resonance spectroscopy, X-ray powder diffraction and field emission scanning electron microscopy. Reaction parameters influencing the grafting percentage, such as temperature, reaction time, initiator concentration and monomer concentration, were optimized using an orthogonal array design matrix. With an increase in grafting percentage, the water-solubility of the flocculant was improved, and it became thoroughly soluble in water when the grafting percentage reached 236.4% or higher. Its application for the treatment of pulp mill wastewater indicates that it had an excellent flocculation capacity and that its flocculation efficiency was much better than that of polyacrylamide. The optimal conditions for the flocculation treatment of pulp mill wastewater were also obtained.

Nano-structured manganese oxide as a cathodic catalyst for enhanced oxygen reduction in a microbial fuel cell fed with a synthetic wastewater

Microbial fuel cells (MFCs) provide new opportunities for the simultaneous wastewater treatment and electricity generation. Enhanced oxygen reduction capacity of cost-effective metal-based catalysts in an air cathode is essential for the scale-up and commercialization of MFCs in the field of wastewater treatment. We demonstrated that a nano-structured MnO(x) material, prepared by an electrochemically deposition method, could be an effective catalyst for oxygen reduction in an MFC to generate electricity with the maximum power density of 772.8 mW/m(3) and remove organics when the MFC was fed with an acetate-laden synthetic wastewater. The nano-structured MnO(x) with the controllable size and morphology could be readily obtained with the electrochemical deposition method. Both morphology and manganese oxidation state of the nano-scale catalyst were largely dependent on the electrochemical preparation process, and they governed its catalytic activity and the cathodic oxygen reduction performance of the MFC accordingly. Furthermore, cyclic voltammetry (CV) performed on each nano-structured material suggests that the MnO(x) nanorods had an electrochemical activity towards oxygen reduction reaction via a four-electron pathway in a neutral pH solution. This work provides useful information on the facile preparation of cost-effective cathodic catalysts in a controllable way for the single-chamber air-cathode MFC for wastewater treatment.