Transport over Membranes

Ecological Processes | Transport over Membranes

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Transport over Membranes A Cano-Odena and I F J Vankelecom, KU Leuven, Leuven, Belgium ª 2008 Elsevier B.V. All rights reserved.

Introduction Single-Process Membrane Separations Hybrid Membrane Processes

Conclusions and Further Directions Further Reading

Introduction

The large pore membranes retain particles with a size larger than the membrane pores. For the more dense membranes, rejection might still depend on the solute size/shape, but its hydrophilicity and interaction with the membrane material becomes increasingly important. For ionic compounds, their charge and the charge of the membrane are also factors that influence the separation. Pervaporation (PV) is the only partial pressure-driven membrane process and the only one involving a phase transition at the membrane, either through a dense membrane or with very small pores. Electrodialysis (ED) is the only process involving an electrical potential, hence necessitating conductive membranes. Membranes are characterized by selectivity and flux. When being used in an actual process, membranes are mounted in modules with certain geometry; flat sheet membranes are turned into spiral wound or plate and frame modules, whereas cylindrical-shaped membranes with decreasing diameter are called tubular, capillary, or hollow fiber (Figure 1). The materials used to prepare membrane forms are organic, inorganic, or a combination of both in hybrid membranes, often referred to as composite, mixed matrix, or organomineral membranes. In general, inorganic membranes are chemically and thermally more stable, but they are more difficult to prepare on a large scale and are more expensive. Membrane processes play an important role in a wide range of applications. Within ecological and biological processes, membranes are mainly found in water treatment, food industry, energy production, medical applications, and fine chemicals production. The membranes can be operated as an isolated separation process, or be integrated with another unit operation in a hybrid process.

In natural systems, membranes play an important role. The cell membrane (also known as plasma membrane) is a semipermeable lipid bilayer that surrounds the cytoplasm, separating the intracellular medium and components from the extracellular environment thus delimiting the cellular space. It is a selective boundary that regulates molecular transport, controls the pass-through and exchange of certain compounds between inner and outer mediums, and maintains the conditions for a correct functioning and life. Membranes are also involved in molecular recognition, because of the presence of certain proteins/substances in the outer surface that act as markers and interact with their respective receptors. The nuclear membrane surrounds the nucleus within a eukaryotic cell and the presence of numerous complex structures (nuclear pores) on this nuclear envelope facilitates and regulates the exchange of materials. The transport mechanisms of materials across biologic membranes can be passive or active, depending on whether input of energy from the cell itself is required. Natural membrane systems have often served as a model for synthetic membranes, which are currently being applied in many industrial processes. The main principles and transport mechanisms can often simply be extrapolated from those natural systems to their synthetic counterparts. In technology, a membrane is defined as a semipermeable barrier between two phases (liquid or gaseous). Components permeate from one side of the membrane to the other under the influence of a driving force, being either a gradient in concentration (or rather activity), (partial vapor) pressure, temperature, or electrical potential. Depending on this driving force, membrane processes can be classified. Dialysis is the only concentration-driven membrane process, while the pressure-driven processes are subdivided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), the latter two often referred to together as hyperfiltration. The line between them is sometimes a blur; generally pressure differences of 0.1–2 bar, 1–10 bar, 10–35 bar, and 15–100 bar refer to MF, UF, NF, and RO, respectively. The applied pressure increases as membrane pore size decreases to end up as dense membranes for the finest separations.

Single-Process Membrane Separations Applications in Water Treatment Water treatment has special relevance as worldwide water demand is constantly under pressure due to the increasing world population and a globally improved standard of living. Such demand aims both at water quantity and quality, the latter relating to lack of sanitation and

3584 Ecological Processes | Transport over Membranes (a)

(b)

Figure 1 Membrane modules. (a) From left to right: plate and frame module schematic; plate and frame module; spiral wound module schematic; and spiral wound module. (b) From left to right: monoliths; tubular membranes; capillary membranes; and hollow fibers. Photographs courtesy of http://www.niroinc.com/ html/filtration/ftechsys.html.

contamination. Moreover, industry focuses more on green technologies and integrated processes, increasing efforts for water reuse, and waste reduction. Membrane technology is useful for recovering clean water from polluted water. Recent technological advances, such as the development of more selective and permeable membranes, increased membrane lifetimes, reduced fouling and cleaning cycles. The development of large-scale modules with lower-energy consumption reduced costs significantly. Especially in the water industry, membrane technology has grown much more than coagulation and ozonation, since membranes require minimal addition of aggressive chemical reagents and produce no byproducts. At present, many membrane-based water reclamation facilities operate worldwide, their number and capacity growing steadily (Ashkelon sea water desalination plant installed in 2004 in Israel with a capacity of 275.000 m3 d 1). Besides technological aspects, economical and political aspects also influence membrane market penetration in water treatment. The changed US legislation forcing the removal of Giardia and Cryptospiridium from drinking water increased UF membrane production and lowered their price, rendering UF more competitive in other separations. For the production of potable water, organic and ionic compounds must be removed. RO can be applied as the final step to obtain potable water by removing basically all ions. NF enables removal of color and larger multivalent ions. UF permits virus removal and recovery of large molecules. MF is used to remove turbidity and larger microorganisms. Water treatment in existing installations uses immersed membrane modules that are simply placed in water tanks where a vacuum at the permeate side drives the collection of purified water. The absence of module housing makes the modules cheaper and their installation inside existing tanks offers a perfect solution when space is limited.

Besides the primary pollutants present in water, there are secondary pollutants less toxic than the primary ones, but whose presence is still limited. They originate from three main sources: energy combustion processes, use of reagents in some cleaning processes, and products originating from the removal of primary pollutants. From the ecological point of view, membrane processes can provide a solution to minimize formation of secondary products during removal of the primary ones. Membrane desalting methods (ED and RO) do not require special sample pretreatments with reagents, and the use of combined methods often permits cost reduction. Applications in the Food Industry Compared to conventional separation processes (distillation or evaporation), membrane technology requires less energy, and also often offers products with better functional properties. However, there is still a need to improve membranes as they might lack robustness, and suffer from fouling problems or cost too high for bulk, low-added-value products. The main applications of membrane operations are in the dairy and beverage industries. In the dairy industry, membranes are used to remove sodium in salty whey (NF), to concentrate skimmed milk (UF), and in fat fractionation to improve butter spreadability or provide texture for low fat cheese (UF and RO). In the beverage industry, membranes are applied to remove particles and bacteria that affect product flavor and appearance. Examples are wine, beer, or fruit juice treatments in clarification, preconcentration, and aroma recovery. Biomedical Applications Since more than 40 years, UF membranes have been a success story in medical applications in hemodialysis for patients with kidney failure where hollow fiber membranes are used to remove toxic low molecular weight organic compounds (urea, creatinine) from the patient’s blood. In tissue engineering, porous collagen-glycosaminoglycan membranes with a silicone elastomer coating have been used as a scaffold for dermal replacement in burn victims. The study of membrane physicochemical features affecting the immunologic response is of special interest. Membranes of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic/oxyphilic polydimethylsiloxane have been designed for immunoencapsulation of pancreatic islets and cells, allowing the diffusion of glucose (cell nutrients) and insulin (produced by cells) but preventing the permeation of inmunoglobulin G (IgG). This has special importance for these scaffolds upon transplantation, as they will not show rejection responses. Moreover, a range of membrane-based devices for controlled drug delivery exist nowadays in different

Ecological Processes | Transport over Membranes Cover Drug molecules Reservoir

Membrane Matrix Removable film

Figure 2 Diagram of a transdermal therapeutic system incorporating a membrane.

configurations, like capsules or transdermal patches (Figure 2), in which the drug release can be modulated by membrane porosity, morphology, and surface modification.

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Several chiral products are currently being produced by means of eMBRs. Examples are the production of a diltiazem intermediate (75 t yr 1) and naproxen using immobilized lipases. ‘Cascade MBRs’ enable to carry out in series reactions with different optimal conditions. This is applied in the production of L-alanine from fumaric acid in two consecutive reactions using two different lyases, each operating at the most optimal conditions of pH and temperature. Both reactors are separated by a membrane that retains the enzymes but allows product exchange. Whole cell membrane bioreactors Water treatment

Hybrid Membrane Processes Membrane Separations Coupled to Reactions Membrane separations can be coupled to a reaction involving biocatalysts that can either be enzymes or whole cells. These membrane bioreactors (MBRs) have been devoted to environmental applications treating wastewater, and to fermentation processes producing food, liquid fuels (ethanol), and plant metabolites, and more complex fine chemicals (pharmaceutical products, fragrances). Most are high-value-added products, making the economics to implement membrane separations more favorable. Biological reactions typically generate complex product mixtures with some of them often inhibitory or toxic for the biocatalyst. Their continuous removal via membrane separations prolongs biocatalyst lifetime and increases the product turnover rate. But most often, the membrane’s major role is in separating in situ the valuable products from the unreacted raw materials and the biocatalysts. The membrane permits to adjust independently the residence times of products, reactants, and biocatalysts in the reactor in order to improve operational flexibility and provide effective process control (e.g., controlled delivery of one of the reagents like in the bubble-free aeration). Finally, the membrane can act as the host for the biocatalyst when immobilized. Enzyme-based membrane reactors

Enzyme-based membrane reactors (eMBRs) find application in the enzymatic hydrolysis of macromolecules (proteins, polysaccharides, oligosaccharides) and in the recycling of cofactors. In corn refining, membranes are combined with the clarification of the cornstarch hydrosylates, concentration, fractionation, and purification of bioproducts. eMBRs can be used to selectively produce di- or monosglycerides by operating the system at steady state at the desired conversion. An example of polysaccharide production is the synthesis of inulin from sucrose using a membraneimmobilized fructosyltransferase.

With the earliest developments in the 1960s, MBRs are state of the art in environmental applications. MBRs offer an important advantage compared to traditional physicochemical approaches; instead of being transferred into a different medium, the complex organic contaminants present in wastewater streams are transformed into simple, harmless, gaseous or water-soluble compounds, together with residual sludge. MF and UF membranes retain high molecular weight solutes, maintain a high biomass concentration in the bioreactor, and enhance the mineralization of organic matter in water effluents. MBRs generally produce less sludge, and require a smaller volume and footprint than conventional biological treatment systems. The membrane also offers an elegant way to control the process via independent adjustment of wastewater residence time in the bioreactor and the product withdrawal rate through the membrane. Moreover, MBRs generally need less pretreatment steps. Wastewater treatment can be achieved either via aerobic or anaerobic conditions. These two differ in the rate of sludge production, hydraulic residence time, biomass concentration, and the type of pollutants remaining to be treated via other processes. Most of the industrial wastewater treatment applications are aerobic and the type of microorganisms used and the process parameters vary depending on the wastewater’s origin. Lower organics removal rates, the potential for odors, and a higher start-up time are main characteristics of the anaerobic processes, although they require less energy to operate. Air bubbling is applied to supply oxygen for the biomass growth and to create turbulent flow conditions near the membrane surface to minimize fouling, the main problem of MBRs. The membranes are mostly immersed in the reactor to save on module costs and to combine aeration with fouling reduction (Figure 3). The possibility to increase the tangential flow may alleviate the fouling problem, but increases the costs and can cause cell lysis via the high mechanical shear. Sometimes, the bubbleless aeration of bioreactors is preferred to improve mass transfer of oxygen to degradative bacteria. High oxygen concentrations are required to keep

3586 Ecological Processes | Transport over Membranes Permeate

Feed water

Vacuum pump

Bleed

Air

Figure 3 Submerged membrane systems for wastewater treatment.

microorganism cells growing in aerobic membrane bioreactors and ensure the performance of photosynthetic processes. In addition, the membrane can also act as a support for biofilm development, employing either gaspermeable dense membranes (silicone) or hydrophobic microporous membranes. In extractive membrane bioreactors, organophilic membranes (like silicone nonporous membranes) can be used to extract organic compounds from the bioreactor into an aqueous part where they are biotransformed. It can solve the problem of accumulating biorefractory pollutants and can prevent biological degradation. Production of biochemicals

One important process is the 40.000 t yr 1 lactic acid production involving the Lactobacillus oxidation of lactose. The MBR productivity is 8 times higher than in a conventional batch reactor with a 19-fold increased biomass concentration. An even 30-fold increased production of ethanol was found upon coupling the Saccharomyces cerevisiae fermentation to a membrane separation. In two-phase transformations, whole cell MBRs use the membrane to separate the two different phases, thus avoiding phase mixing and emulsification. Perfusion MBRs have been introduced for the production of monoclonal antibodies. The mammalian cells that synthesize them are grown in the extracapillary space between the fibers of the module. Nutrients are supplied through the fibers, which also extract the metabolites continuously. The high cell concentrations between the fibers initiate high antibody harvests. These MBRs are being investigated as an alternative concept for bioartificial organs such as liver and pancreas. For ABE (acetone–butanol–ethanol) fermentation, PVcoupled MBRs have been described. The products formed inhibit the biocatalysis, making product removal from the fermentation broth via PV-coupled MBRs an interesting option. Membrane fouling minimization, by selecting the right membrane material and conditions of operation or by introducing an MF step before the PV unit to remove colloidal and macromolecular components, is a challenge.

Waste gas treatment

Another application of membranes in ecological processes is air pollutants removal by using MBRs. Pollutants are transferred through a membrane used as a contactor, for example, in the removal of organics (e.g., propene and chlorinated solvents) or inorganics (SO2, NOx, etc.), by diffusing from the gas phase into a liquid phase where microorganisms degrade them to H2O, CO2, and minerals. These reactors represent an alternative to conventional biofilters, commonly used in these applications, in which the gas is passed through a compost bed or soil that contains degradative microorganisms. The advantage of the MBRs is that a separate water stream maintains humidity of the biomass and allows product elimination. Energy production

The use of hydrogen as fuel can reduce the oil dependency of transportation and power supply with the environmental benefits of using a renewable energy, and thus reduce pollution and toxic emissions. Fuel cells are based on electrochemical devices capable of directly converting any consumable fuel to electrical energy through a chemical reaction. They offer cleaner alternatives to today’s technology; the only by-products are heat, carbon dioxide, and water, which are environmentally safe, with an energy efficiency projected to be twice that of combustion engines. However, significant challenges remain, like cost and durability for both automotive and stationary applications. Polymer electrolyte membrane (PEM) fuel cells – also called proton exchange membrane fuel cells (PEMFCs) – generate most power for a given volume or weight, making them compact and lightweight compared to other fuel cell types. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. The polymer is permeable to protons when it is saturated with water, but it does not conduct electrons. They need only hydrogen (fuel), oxygen from air, and water to operate and do not require corrosive fluids like some other fuel cells. At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. Hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power (Figure 4). PEM fuel cells are used for transportation applications, due to their rapid startup and the low operating temperature (below 100  C). Phosphoric acid fuel cells (PAFCs) use liquid phosphoric acid (contained in a Teflon-bonded silicon carbide matrix) as electrolyte and porous carbon electrodes containing a platinum catalyst. Molten carbonate fuel cells (MCFCs) use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. They operate at high temperatures of 650  C, enabling the use of nonprecious

Ecological Processes | Transport over Membranes Electron flow

H2

O2 H+

H+ Anode

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compartment oxidize the fuel source (glucose or methanol), generating electrons that react at the cathode side reducing oxygen to water. These cells require either the use of electrochemically active bacteria to transfer electrons to the electrode (mediator-less MFC) or a compound (i.e., potassium ferric cyanide, humic acid) that facilitates the electron transfer from microbial cells to the electrode (mediator MFC). In EFC, peroxidases have been used at the cathode, and alcohol dehydrogenase at the anode. The latter contains pyridine nucleotides as coenzymes that help the electronic transfer between the enzyme and the electrode in this mediated bioelectrocatalysis.

Cathode H+

H2O

Membrane (electrolyte) Figure 4 PEM fuel cell scheme.

metals as electrode catalysts, thus reducing costs. They reach efficiencies approaching 60%, and are not affected by carbon monoxide or carbon dioxide ‘poisoning’. Solid oxide fuel cells (SOFCs) use a hard, nonporous solid ceramic compound as the electrolyte, while alkaline fuel cells (AFCs) use a solution of potassium hydroxide in water as the electrolyte and can use a variety of nonprecious metals as catalyst. AFCs are high-performance fuel cells due to the rate at which chemical reactions take place in the cell and reach efficiencies of 60% in space applications. Their disadvantage is the easy poisoning by carbon dioxide. Direct methanol fuel cells are relatively new compared to hydrogen-powered fuel cells. Unitized regenerative fuel cells (URFCs) integrate fuel cell and electrolyzer functions into a single unit. As for the other fuel cells, energy is produced from hydrogen and oxygen and water is a by-product. If they operate in reverse, water is converted into oxygen and hydrogen by electrolysis (solar power). They are interesting as energy source when weight is a concern, as the system is lighter than a separated electrolyzer and generator. Biological fuel cells differ from chemical fuel cells, as they use organic compounds or organic electron donors to produce electrical energy, while the electrode reactions are controlled by biocatalysts. In addition, they work under milder conditions (temperature and pressure) than conventional fuel cells. There are two types of biological fuel cells: microbial fuel cells (MFCs) and enzymatic fuel cells (EFCs). MFCs consist of two compartments (cathodic and anodic) separated by a cation exchange membrane. Microorganisms in the anode

Membrane Separation Coupled to Advanced Oxidation Processes NF plays a crucial role in a continuous process combining wet air oxidation, membrane separation, and biological treatment exemplified in the treatment of PEG-containing wastewaters. In a first brief wet oxidation pretreatment, polymers are degraded to a lower degree of polymerization to increase the rate of the subsequent permeate biodegradation. The integrated process shows much higher treatment efficiency than any of the single optimized processes.

Membrane Separation Coupled to Adsorption Processes Soil erosion, mining industry, and pesticides contribute to the introduction of arsenic in water and soils. The speciation of arsenic is strongly pH dependent and plays a key role in arsenic removal. Conventional processes use adsorption (activated carbon, zeolites, etc.), creating problems of waste disposal and wastewater produced during regeneration of the adsorbent. Coagulation using aluminum or iron salts suffers from the presence of phosphates or silicates in water that may interfere with these agents. Membrane processes like RO, NF, UF, or ED can reach up to 100% removal efficiency. Although MF and UF membranes are efficient in particle and bacteria separation, they cannot separate some organic and inorganic dissolved compounds. With the aim to overcome this, coagulant and adsorbents can be combined with the membranes in hybrid membrane systems. The problem of flux decline due to membrane fouling can be minimized with operation at low fluxes or aeration. Another example of a membrane hybrid process is the combination of activated carbon with a UF membrane, offering the advantages of adsorption together with UF to remove particles and organic compounds from water.

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Conclusions and Further Directions In spite of their advantages and successes, membranecoupled processes still turn out quite expensive, due to costs of membrane and all additional hardware associated with a membrane operation. Membrane separations tend to become more favorable for processes where the selectivity is more important than the conversion, since they replace other purification steps that might lower such selectivity. In MBRs, biocatalyst activity loss and denaturation are potential disadvantages, because of the high mechanical stress the biocatalysts experience at the elevated circulation rates required to maintain a good transmembrane flux, and due to limited mass transfer of nutrients or metabolites when immobilized in small pores or at high cell density. Another major problem is membrane fouling, caused by adsorption on the membrane of metabolites or coagulated proteins from lysed cells, and pore plugging due to normal cell growth. Membrane bioreactors require energy for maintaining biological activity (cell growth) and membrane performance (recirculation, cleaning cycles). Alternative ways to aerate membranes, low-pressure operation, and attempts to reduce membrane fouling should be considered to minimize flow rate decrease, hence energy consumption. New membrane materials and new processes (like hybrid membrane systems) are the directions where current research is focused on. Important goals of current fuel cell research are cost reduction (new membranes with high conductivity at low relative humidity and temperatures up to 120  C) and improvements in mechanical strength and chemical stability.

See also: Transport in Porous Media.

Further Reading Brindle K and Stephenson T (1996) The application of membrane biological reactors for the treatment of wastewaters. Biotechnology and Bioengineering 49(6): 601–610. Cuperus FP (1998) Membrane processes in agro-food. State-of-the-art and new opportunities. Separation and Purification Technology 14(1–3): 233–239. Daufin G, Escudier JP, Carrere H, et al. (2001) Recent and emerging applications of membrane processes in the food and dairy industry. Food and Bioproducts Processing 79(C2): 89–102. Freeman S, Leitner GF, Crook J, and Vernon W (2002) Membrane filtration is gaining acceptance in the water quality field. Water Environment and Technology 14(1): 16–21. Mulder M (1996) Basic Principles of Membrane Technology, 2nd edn. Dordrecht, The Netherlands: Kluwer Academic Publishers. Ng K-S, Ujang Z, and Le-Clech P (2004) Arsenic removal technologies for drinking water treatment. Reviews in Environmental Science and BioTechnology 3(1): 43–53. Paul D and Vienken J (2002) Capillary membranes for medical application. In: Ikada Y (ed.) Recent Research Developments in Biomaterials, pp. 179–220. Trivandrum: Research Signpost. Reij MW, Keurentjes JTF, and Hartmans S (1998) Membrane bioreactors for waste gas treatment. Journal of Biotechnology 59(3): 155–167. Shukla AK, Suresh P, Berchmans S, and Rajendran A (2004) Biological fuel cells and their applications. Current Science 87(4): 25. Stamatialis DF, Rolevink HHM, and Koops G-H (2004) Passive and iontophoretic controlled delivery of salmon calcitonin through artificial membranes. Current Drug Delivery 1(2): 137–143. Turner APF, Karube I, and Wilson GS (1987) Biosensors Fundamentals and Applications. New York: Oxford University Press. Viswanathan B and Scibioh MA (2006) Fuel Cells Principles and Applications. Chennai, India: Universities Press.

Tree Growth B Zeide, University of Arkansas, Monticello, AR, USA ª 2008 Elsevier B.V. All rights reserved.

Problem History of Growth Modeling Growth as an Outcome of Opposing Forces Growth Expansion Growth Decline Adaptation and Two Forms of the Decline Module Solution: Combined Model of Tree Growth

Additive Variants Parameter Interpretation Empirical Verification Is Tree Growth Limited? Strategy of Modeling Further Reading

Problem

variables is a less intrusive and less expensive way to satisfy the rising demand for wood products than altering site quality by artificial fertilization, bedding, drainage, irrigation, etc. The key variable required for optimization is growth (annual increment) of trees. Since the growth is

Yield of forest stands can be substantially increased by optimizing number of trees per unit area at each moment of stand life and the age of harvest. Optimization of these