Membrane fouling

Abstract: The membrane bioreactor (MBR) can no longer be considered as a novel process. This reliable and efficient technology has become a legitimate alternative to conventional activated sludge processes and an option of choice for many domestic and industrial applications. However, membrane fouling and its consequences in terms of plant maintenance and operating costs limit the widespread application of MBRs. To provide a better understanding of the complex fouling mechanisms and propensities occurring in MBR processes, this review compiles and analyses more than 300 publications. This paper also proposes updated definitions of key parameters such as critical and sustainable flux, along with standard methods to determine and measure the different fractions of the biomass. Although there is no clear consensus on the exact phenomena occurring on the membrane interface during activated sludge filtration, many publications indicate that the extracellular polymeric substances (EPS) play a major role during fouling formation. More precisely, the carbohydrate fraction from the soluble microbial product (also called soluble EPS or biomass supernatant) has been often cited as the main factor affecting MBR fouling, although the role of the protein compounds in the fouling formation is still to be clarified. Strategies to limit fouling include manipulating bioreactor conditions, adjusting hydrodynamics and flux and optimizing module design.

Abstract: Each chapter ends with a section of references. Introduction Definition and classification of membrane separation processes Historical developments Physical chemistry of membrane separations: Chemical potential and osmosis, Vapor pressure, Osmotic pressure and chemical potential Membrane Chemistry Definitions and classification: Depth vs. screen filters, Microporous vs. asymmetric membranes General methods of membrane manufacture: Phase Inversion Process of Membrane Manufacture Polymers used in membrane manufacture: Cellulose Acetate, Polyamide membranes, Polysulfone membranes, Other polymeric materials Composite membranes Inorganic membranes: Properties of inorganic membranes Membrane Properties Pore size: Bubble point and pressure techniques, Direct microscopic observation Predicting flux from pore statistics Passage (challenge) tests: Microfiltration membranes, Ultrafiltration membranes Factors affecting retentivity of membranes: Size of the molecule, Shape of the molecule, Membrane material, Presence of other solutes, Operating parameters, Lot-to-lot variability, Membrane configuration, Fouling and adsorption effects, The microenvironment Performance and Engineering Models The velocity boundary layer The concentration boundary layer Models for predicting flux: the pressure-controlled region Concentration polarization Mass transfer (film theory) model: Determining the mass transfer coefficient, Example The resistance model Osmotic pressure model for limiting flux Factors affecting flux: operating parameters: Feed concentration, Temperature, Flow rate and turbulence Physical properties of liquid streams: Density, Viscosity, Diffusion coefficients Experiment vs. theory: the "flux paradox" Design factors affecting flux Equipment Laboratory scale devices Industrial equipment: Tubular modules, Hollow fibers, Plate units, Spiral-wound Special modules: Rotary modules, Vibrating modules, Dean Vortices Summary Fouling and Cleaning Characteristics of fouling: Water flux Consequences of fouling Mathematical models of fouling Factors affecting fouling: Membrane properties, Solute properties, Process engineering factors affecting fouling Flux enhancement: Turbulence promoters/inserts/baffles, Back-flushing, -pulsing, -shocking and washing, Uniform transmembrane pressure/co-current permeate flow, Permeate back-pressure, Intermittent jets, Pulsatile flow, Electrical methods Summary: Membrane Fouling Cleaning membranes: Important factors during cleaning, Typical foulants and soils, Cleaning chemicals, Sanitizers Process Design Physics of the ultrafiltration process: Example Modes of operation: Discontinuous diafiltration (DD), Continuous diafiltration (CD), Dialysis ultrafiltration Batch vs. continuous operation: Batch operation, Single pass, Feed-and-bleed, Multistage operations, Example, Control methods Minimum process time Fractionation of macromolecules Energy requirements: Example Cost and process economics: Arrays and configurations, System cost Summary Applications Electrocoat paint The dairy industry: Fluid milk and fermented products, Cheese manufacture, Milk microfiltration, Cheese whey ultrafiltration, Microfiltration of whey Water treatment Wastewaters: Oily wastewater, Stillage from bioethanol plants, Caustic and acid recovery, Brine recovery, Printing ink, Laundry wastewater, Micellar-enhanced ultrafiltration Textile industry Latex emulsions Pulp and paper industry Tanning and leather industries Sugar refining Soybean and other vegetable proteins Vegetable oils: Degumming, Deacidification, Bleaching, Removal of metals, Dewaxing, Clarifying Frying Oils Corn and other grains: Dextrose clarification, Protein processing Animal products: Red meat, Gelatin, Egg processing, Fish processing, Poultry industry Biotechnology applications: Separation and harvesting of microbial cells, Enzyme recovery, Affinity ultrafiltration, Membrane bioreactors Fruit juices and extracts Alcoholic beverages: Wine, Beer Appendices List of manufacturers of membrane equipment Conversion factors Books and general Glossary of terms Index More than 350 Tables and Figures Useful reference data is provided in 85 tables. Numerous schematics illustrate membranes, modules, equipment, and processes. Micrographs illustrate membranes and filtration. Here is a small sampling of this supplementary material. * Tables: Characteristics of membrane processes Comparison of energy requirements and costs between evaporation and membrane processes Methods of manufacture of synthetic membranes Materials used for the manufacture of membranes Properties of membrane filters requiring standardization Morphological parameters and bubble points for selected MF membranes-Pore size and surface porosity of ultrafiltration membranes Selected values of gel concentration-Diffusion coefficients-Relationship between channel size and surface area:volume ratio of membrane modules Hollow fibers from various manufacturers-Specifications of spiral-wound membranes from various manufacturers-Typical cleaning reagents and their modes of action-Operating economy of UF plants processing whole milk for cheese manufacture-Water purification process comparison Examples of affinity ultrafiltration Figures: Useful ranges of various separation processes-Classification of filters-Micrograph of multistage depth filter-Schematic representation of ultrastructure of an asymmetric (skinned) membrane-Typical structures of polyamide membranes-Comparison of the performance of commercial cellulose acetate and thin-film composite (polyamide) membranes-Ceramic membrane modules in their housing-Relationship between pore size, molecular weight of ideal solutes, and ratings of ideal and real membranes Permeability of large and small molecules through large and small pore membranes-Schematic representation of the cross section of typical asymmetric UF or MF membrane-Schematic of concentration polarization during UF of colloidal and macromolecular solutes . . .-Schematic of typical plate type membrane module-Multistage filtration sowing several feed-and-bleed systems connected in series-Membrane processing of cheese whey-Selection criteria of separation methods in biopro- cessing

Abstract: Membrane bioreactors (MBRs) have been actively employed for municipal and industrial wastewater treatments. So far, membrane fouling and the high cost of membranes are main obstacles for wider application of MBRs. Over the past few years, considerable investigations have been performed to understand MBR fouling in detail and to develop high-flux or low-cost membranes. This review attempted to address the recent and current developments in MBRs on the basis of reported literature in order to provide more detailed information about MBRs. In this paper, the fouling behaviour, fouling factors and fouling control strategies were discussed. Recent developments in membrane materials including low-cost filters, membrane modification and dynamic membranes were also reviewed. Lastly, the future trends in membrane fouling research and membrane material development in the coming years were addressed.

Abstract: Reverse osmosis (RO) is currently the most important desalination technology and it is experiencing significant growth. The objective of this paper is to review the historical and current development of RO membrane materials which are the key determinants of separation performance and water productivity, and hence to define performance targets for those who are developing new RO membrane materials. The chemistry, synthesis mechanism(s) and desalination performance of various RO membranes are discussed from the point of view of membrane materials science. The review starts with the first generation of asymmetric polymeric membranes and finishes with current proposals for nano-structured membrane materials. The paper provides an overview of RO performance in relation to membrane materials and methods of synthesis. To date polymeric membranes have dominated the RO desalination industry. From the late 1950s to the 1980s the research effort focussed on the search for optimum polymeric membrane materials. In subsequent decades the performance of RO membranes has been optimised via control of membrane formation reactions, and the use of poly-condensation catalysts and additives. The performance of state-of-the-art RO membranes has been highlighted. Nevertheless, the advances in membrane permselectivity in the past decade has been relatively slow, and membrane fouling remains a severe problem. The emergence of nano-technology in membrane materials science could offer an attractive alternative to polymeric materials. Hence nano-structured membranes are discussed in this review including zeolite membranes, thin film nano-composite membranes, carbon nano-tube membranes, and biomimetic membranes. It is proposed that these novel materials represent the most likely opportunities for enhanced RO desalination performance in the future, but that a number of challenges remain with regard to their practical implementation.

Abstract: Although microfiltration is one of the oldest pressure-driven membrane processes, it is probably the least understood when it comes to the filtration of suspensions and macromolecules. Microfiltration is characterized by operation at low pressures, by high permeation fluxes, and by crossflow mode in flat or cylindrical geometries. The major limitation of microfiltration is membrane fouling due to the deposition and intrusion of macromolecules, colloids and particles onto and into the microporous membrane. In this review, we analyze the various components of this problem by focusing on the formation of cakes, the behavior of suspension flows and particle transport in simple geometry ducts, and on the formation and behavior of fouling layers including those resulting from macromolecules, colloids and particles. Some of the work we report on is very recent or is still in progress and needs independent verification. With this understanding, we hope that the reader will be able to use these concepts for analyzing other systems and for investigating new module designs.