air pollution prevention and control (bioreactors and bioenergy) || membrane bioreactors

7Membrane Bioreactors

Raquel Lebrero,1 Raul Munoz,1 Amit Kumar2 and Herman Van Langenhove2

1Department of Chemical Engineering and Environmental Technology,Valladolid University, Valladolid, Spain

2Department of Sustainable Organic Chemistry and Technology, Gent University, Gent, Belgium

7.1 Introduction

The increasing public concern about atmospheric pollution has resulted in recent years in more stringentenvironmental regulations to limit the emission of gaseous pollutants such as SOx, NOx, volatile organiccompounds (VOCs), volatile sulfur compounds (VSCs), odors, and so on [1, 2]. In this context, VOCsand VSCs represent a major environmental problem worldwide due to their toxicity, mutagenicity, orcarcinogenicity, their role in tropospheric ozone formation, and their odor nuisance provoked in the nearbypopulation [3, 4]. Therefore, the minimization and control of these gaseous emissions rank nowadaysamong the top priorities of most chemical, petrochemical, or pulp and paper industries, animal farming,and waste treatment facilities, in order to move toward more sustainable production processes and becauseof increasing concerns about their public image.

Among the battery of end-of-pipe treatment technologies available nowadays, biologically basedtechnologies (biofiltration, activated sludge diffusion, biotrickling filtration, bioscrubbing, etc.) exhibitsignificantly lower environmental impacts (in terms of energy and chemicals consumption and CO2footprint) than their physico-chemical counterparts (activated carbon adsorption, chemical scrubbing,incineration, ozonation, etc.) [5, 6]. In addition, despite exhibiting higher initial investment costs, the loweroperational costs of the biotechnologies during waste-gas treatment render them as the most economicoption in a 20–30 year horizon when treating large waste-gas flows containing low concentrations ofpollutants [6]. Only when process robustness is considered do physico-chemical technologies offer abetter treatment performance, although research is rapidly reducing this gap [7]. Among conventionalbiotechnologies, biofiltration and biotrickling filtration are by far the most commonly used for VOC, VSC,and odor abatement, likely due to their ease of operation and accumulated design and operation experience

Air Pollution Prevention and Control: Bioreactors and Bioenergy, First Edition. Edited by Christian Kennes and Marıa C. Veiga.c© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

156 Air Pollution Prevention and Control

[8, 9]. However, both biotechnologies face important limitations when treating poorly water-solublevolatile compounds, and the cost-effective control of biomass overgrowth in both systems still remainsunsolved [8]. In the particular case of biofiltration, filter media acidification, drying and compaction (withthe subsequent formation of preferential pathways), and the accumulation of toxic metabolites significantlyreduce the lifespan and long-term performance of this technology [10]. However, while activated sludgediffusion systems are economically viable exclusively during odor treatment in wastewater treatmentplants provided with aeration via air diffusion, the use of bioscrubbers is limited by their high investmentcosts and limited performance when treating poorly water-soluble compounds [11, 12].

Advanced membrane bioreactors represent a promising alternative to conventional biotechnologies toovercome most of the limitations mentioned here [13]. Membrane bioreactors for waste-gas treatment(MBRWGs) can combine the selective extraction of the target gaseous pollutants and O2 from the contam-inated air emission (circulating through one side of the membrane) with their biodegradation by a microbialcommunity attached on the other side of the membrane (or in suspension) in contact with a discrete aque-ous phase containing the nutrients required for microbial growth [14]. Hence, the membrane acts as aninterphase between the gas and the microbial community, and the gaseous pollutants either diffuse throughthe membrane pores (porous or microporous membranes) or permeate via solution–diffusion mechanisms(dense membranes or composite membranes). The presence of a biofilm or a culture suspension on theother side of the membrane increases the local concentration gradients (due to the rapid consumption of thegaseous pollutants and O2) and therefore the overall mass transfer rates [14]. In addition, this technology isavailable in several bioreactor configurations (flat plate, hollow fiber, and tubular) and provides gas–liquidinterfacial areas as high as 20 000 m2 m−3 [15]. The presence of a discrete water phase in advanced mem-brane bioreactors for waste-gas treatment overcomes the typical operational limitations of biofiltration suchas media acidification, accumulation of inhibitory byproducts, or biofilm drying. The high selectivity ofsome hydrophobic membrane materials such as polydimethylsiloxane or polyolefin can enhance the masstransfer of poorly water-soluble compounds as a result of the increased concentration gradients mediatedby these materials [13, 16]. In addition, the gas and liquid flow rates can be varied independently withoutproblems of flooding or foaming [17]. This technology has been successfully tested for the treatment ofBTEX (benzene, toluene, ethylbenzene, and xylenes), dimethylsulfide (DMS), trichloroethylene (TCEt),NOx, and so on [13].

This chapter reviews the basic principles, merits, and limitations of advanced membrane bioreactors forovercoming some of the key operational limitations of conventional biological systems during waste-gastreatment. Recommendations are made for design and operation of this technology, while the areas needingfurther research and the most recent applications are identified.

7.2 Membrane basics

A membrane can be defined as an interphase (Figure 7.1) between two bulk phases that selectively allowsthe transport of compounds (B) from one phase to the other while other compounds (A) are retained inone phase. The first recorded experiment on gas permeation through a polymeric membrane dates from1829, when Graham used a wet pig bladder inflated to its bursting point to observe gas transport. Sincethen, many different types of membranes have been developed, with the number of industrial applicationsincreasing exponentially during recent decades.

7.2.1 Types of membranes

Membranes can be classified according to different criteria, such as their nature (biological or syntheticmembranes, the latter being subdivided into organic (polymeric or liquid) and inorganic (ceramic or metal)

Membrane Bioreactors 157

Feed

Permeate

Retentate

A

B

Membrane

Figure 7.1 Basic representation of a two-phase system separated by a membrane.

membranes), their structure (symmetric or asymmetric), and their morphology. The morphology of themembrane determines the separation mechanisms and, thus, its application [18]. In addition, symmetricand asymmetric membranes can be subdivided according to their porosity (cylindrical porous, porous, andnonporous or dense) and the characteristics of their layers (porous and composite asymmetric membranes).

The main features of porous, dense, and composite membranes are reviewed in this section, and theirmain characteristics are summarized in Table 7.1.

7.2.1.1 Porous membranes

Porous membranes can be classified according to their pore size into microporous (Dp < 2 nm), meso-porous (Dp = 2 − 50 nm), and macroporous (Dp > 50 nm). They have a porous structure with a porosityof 30–85% [19]. In theory, these membranes present excellent mass transfer characteristics, since thepollutants can cross the membrane by diffusing through the gas-filled pore. For example, a microporousmembrane can be between 10 and 150 times less resistant to mass transfer than a silicone dense membraneof the same thickness [20]. Porous membranes have a high permeability and a poor to no selectivity inpermeation when compared to dense-phase membranes. However, their performance usually decreasesover long-term operation due to the filling of membrane pores with water or the accumulation of deadcells within and on the surface of the membrane (biofouling). Thus, membrane material with hydropho-bic properties (e.g., polypropylene) is usually selected to reduce water penetration at low transmembranepressures. Besides, at high air pressures, transmembrane gas flow may compromise the integrity of themembrane [15, 19, 21].

7.2.1.2 Dense membranes

Dense membranes rely on physico-chemical interactions between the permeating compounds and themembrane material. They have no macroscopic pores; thus, pollutants have to diffuse through the membranematerial, being more selective than porous membranes. They are also more resistant to mechanical abrasion


Table 7.1 Classification and main characteristics of membranes.

Membrane Examples Pore Characteristicstype diameter (A)

Hydrophobicmicroporous

Polypropylene, Teflon 1000–10 000 • High permeability• Transport mechanism: diffusion

Porous Macroporous →Mesoporous →

>50020–500

• Pore structure: static and welldefined

• Transport mechanisms: Knudsendiffusion, viscous, and superficial

Dense Polymeric materials:latex, silicone,polypropylene,polyethylene

– • Based on physico-chemicalinteractions between pollutantand membrane material

• Transport mechanisms: solutiondiffusion (pollutant is absorbed inthe membrane material anddiffuses through the polymer)

Composite Polydimethylsiloxaneand polyvinylidene orpolyacrylamidesupport

– • Combines a dense membrane(prevents overgrowth ofmicroorganisms) with a poroussupport that enhances masstransfer

and chemical compounds, and therefore they can be operated at higher gas pressures [13]. Mass transferthrough the membrane depends on the solubility and diffusivity of the compound in the dense matrix, whichin turn depend on the specific interactions between the compound and the membrane. Therefore, it is ofkey importance to select a membrane material with a high diffusion coefficient for the target pollutants toreduce the mass transfer resistance of the dense phase. At first sight, they seem more suitable for long-termoperation, and, since the pollutant dissolves in the membrane material, the dense-phase material acts as abuffering medium for fluctuating inlet pollutant loads [19].

7.2.1.3 Composite membranes

Composite membranes, consisting of a thin dense top layer (0.1–0.5 µm) on a porous support layer(<50–150 µm), are preferred above self-supporting membranes because of their higher mass transfer,lower biofouling, and better membrane resistance [18]. The dense thin layer is located on the liquid sideto prevent biofouling, while the supporting porous layer is located on the gas side. They combine theadvantages of porous and dense membranes while presenting fewer clogging problems than porous ones.The higher the thickness of the dense layer, the higher the membrane mechanical strength and resistanceto water transfer, but the pollutant transfer resistance across the membrane also increases.

Besides superior pollutant permeation and fouling properties, the mechanical strength of the compos-ites, as well as their role in microbial adhesion and oxygen supply for biofilm development and completesubstrate mineralization, should also be evaluated when composites are considered in gas-to-liquid trans-fer membrane systems. However, composite membranes are also more complex and more expensiveto manufacture.


7.2.2 Membrane materials

Many materials have been used for gas separation in membranes, for example polybutadiene, latex,polyvinyl alcohol, polysulfone, polystyrenesulfone, polyamide, polyethylene, polytetrafluorethene,polypropylene, and polydimethylsiloxane. The selection of the material will depend on the pollutant tobe treated: a balance between mechanical strength, high permeability, and selectivity must be achievedto ensure good and long-term performance of the membrane [13].

In the particular case of dense membranes, which are limited to polymeric materials, a high solubility(affinity) of the polymer for the pollutant is necessary to avoid high mass transfer resistance across themembrane. Among them, polydimethylsiloxane (PDMS) has been extensively used in dense membranesdue to its relatively high permeability and low selectivity toward pollutants [22].

Another important fact to take into account when selecting a polymer is the risk of membrane failuredue to dissolution or swelling of the polymer when the pollutant solubility is extremely high. Increasingthe degree of cross-linking and the molecular weight of the polymer used can lower this effect [19].

7.2.3 Membrane characterization parameters

The characteristics of the membrane determine their use in a specific application, and thus characterizationof the membrane after its preparation using simple techniques is of key relevance. Structural or morpho-logical properties such as the pore size, the pore size distribution, the free volume, or the crystallinitycan give important information on membrane behavior. In porous membranes, the pore size and pore sizedistribution mainly determine the retention of molecules in the membrane, while in dense membranes, themorphology of the material (crystalline, amorphous, glassy, or rubbery) affects the permeability and thusthe mass transfer performance.

7.2.3.1 Membrane thickness

Membrane thickness can vary widely, although most membranes have a thickness in the range of150–800 µm. Thinner membranes are preferable due to the improved mass transfer and lower pressuredrop; however, the mechanical stability of the membrane must also be taken into account. In compositemembranes, the dense layer is usually very thin (<10 µm) to reduce mass resistance, while the thickerporous layer provides the necessary mechanical strength [19].

7.2.3.2 Membrane performance: selectivity and permeance

The two main factors defining the performance of a membrane are the separation factor (selectivity) andthe permeance. The selectivity of a membrane for a compound i over a compound j is expressed in termsof the separation factor αi/j :

αi/j = yi /yj

xi /xj(7.1)

where y and x represent the concentration of a component in the permeate (Figure 7.1) and the feed,respectively, and the suffixes i and j refer to the component.

For membrane characterization, the solubility, diffusivity, and permeability coefficients are commonlyemployed. The solubility (S ) is a dimensionless temperature-dependent coefficient that gives the volumetricratio of a compound’s equilibrium concentration in membrane and air (e.g., S = 902 for toluene in PDMSat 30 ◦C). The diffusivity (Dm ) represents the diffusion coefficient (m2 s−1) of a compound in a polymer,and it also depends on the temperature (e.g., Dm = 2.33 × 10−10 m2 s−1 for toluene in PDMS at 30 ◦C).


The permeability (P ) is given by the product of solubility and diffusivity (m2 s−1). Finally, the flux (J )is the mass of compound flowing through the membrane per unit time (kg s−1).

The selectivity can thus be defined as the ratio of the permeation rate of the single compounds through themembrane under equal driving forces [18, 23]:

αi/j = Ji

Jj= Pi

Pj= Si

Sj× Dm ,i

Dm ,j(7.2)

Equation (7.2) is valid only when the permeability of compound i is not influenced by the presence ofcompound j and vice versa. The overall membrane selectivity is the product of solubility selectivity (Si /Sj )and diffusivity selectivity (Di /Dj ).

The permeance is defined as the feed volume passing through a unit area of membrane at a unit timeand under a unit pressure gradient, and is expressed as:

Pe = V

At�p(7.3)

where Pe is the permeance, V is the volume of gas, A is the area, t is the time, and �p is the pressuredifference [24].

7.2.4 Mass transport through the membrane

The flux of a species i (Ji ) through the membrane, defined as the mass of component transported through themembrane reactor per unit of time, is given by the product of a global coefficient Kov and the driving forceacross the membrane. In membrane bioreactors, a pressure difference is not applied and the driving force isgiven by the concentration difference between the gas and liquid phases:

Ji = Kov × driving force = Kov A

(Cg

H− Cl

)(7.4)

where Ji represents the mass flux through the membrane (g s−1), Kov is the overall mass transfer coefficient(m s−1), A is the membrane surface area (m2), H is the air–water partitioning coefficient for the compoundi (dimensionless), and Cg and Cl are the concentrations of the compound i in the gas and liquid phases,respectively (g m−3). In membrane bioreactors, the concentration of the volatile component in the aqueousphase (Cl ) depends on the microbial activity of the attached biofilm and/or of the cells in suspensionresponsible for its biodegradation. Thus, the higher the biodegradation rate, the higher the concentrationgradient through the membrane and, consequently, the driving force.

The mass transfer of a gaseous pollutant within a membrane bioreactor for waste-gas treatment consistsof a series of mechanisms (Figure 7.2):

1. transport of the pollutant in the gas phase;2. diffusion through the membrane;3. transfer from the membrane, and dissolution and diffusion into the biofilm;4. biodegradation and diffusion in the biofilm; and5. transport of the nondegraded compound and biodegradation products through the liquid phase.

The overall mass transfer resistance for a gaseous pollutant in a membrane bioreactor is a combination ofa series of transfer resistance coefficients: gas phase (kg ), membrane phase (km ), biofilm (kb), and liquidphase (kl ):

1

Kov= 1

kg H+ 1

kmH+ 1

kb+ 1

kl(7.5)


Total Flux = Flux convection + Flux diffusion

Cg

Cl

Gas phase Liquid phase

Membrane

δBoundary layers

Cm

1 2 3/4 5Mass transfermechanisms

Figure 7.2 Schematic representation of mass transport in membrane bioreactors.

The membrane mass transfer coefficient depends on the type of membrane:

• For nonporous or dense membranes, the transfer coefficient is defined as the ratio of the component’spermeability coefficient (P , m2 s−1) and the membrane thickness (δ, m). Since permeation throughrubbery polymers is described by a solution–diffusion model (see Section 7.2.4.2), the mass transfercoefficient can be rewritten as:

km = P

δ= S Dm

δ(7.6)

• For porous membranes, the mass transfer coefficient can be defined as:

km = Dmε

δτ(7.7)

where ε and τ are the membrane porosity and the pore tortuosity, respectively.

• The mass transfer coefficient for composite membranes combines those of the porous and dense layers.Besides, since composite membranes have a clear discontinuity at the boundary of two neighboringbarrier layers, either in the chemical structure or in the morphology of the material of which the barrierlayer is made, an additional interfacial resistance (Ra , s m−1) is considered:

1

km= δ

P= δpτ

Dmε+ 1

Ra+ δd

Pd(7.8)

In Equation (7.8), suffixes p and d refer to the porous and dense layers, respectively.The transport of a molecule or a particle across the membrane depends on the type of membrane.

As mentioned in this chapter, molecules diffuse through the pores of a porous membrane, whereas gasseparation in dense membranes occurs due to the different partitioning of gaseous compounds between airand the membrane.

The main transport models used for each type of membrane are now reviewed.


7.2.4.1 Transport in porous membranes

In porous membranes, the geometry of the pore determines the model that adequately describes thetransport: surface diffusion (Hagen–Poiseuille and Carman–Kozeny) and Knudsen diffusion and vis-cous models.

Parallel cylindrical pores If the pores of the membrane are considered as a number of parallel cylin-drical pores perpendicular or oblique to the membrane surface, and the length of each pore correspondsapproximately to the membrane thickness, the flux may be described by the Hagen–Poiseuille (membranewith a number of parallel pores) or the Carman–Kozeny (closed packed spheres) relations, which indicatethat the solvent flux is proportional to the driving force [18]:

• Hagen–Poiseuille equation

J = εr2�p

8πτδ(7.9)

• Carman–Kozeny equation

J = ε3�p

KηS 2(1 − ε)2δ(7.10)

where r is the pore radius, �p the pressure difference across the membrane, η the membrane viscosity,K the Carman–Kozeny constant, and S the internal surface area. However, very few membranes presentsuch structures in practice.

Asymmetric porous membranes In these, diverse transport mechanisms can be found: Knudsen flowin narrow pores, viscous flow in wide pores, and surface diffusion along the pore wall [18]. The mainparameter that discriminates between the flow mechanisms is the ratio of the pore size relative to the meanfree path of the permeating molecules (λ), given by

λ = RT√2πd2N p

(7.11)

where R is the gas constant, T the temperature, N the Avogadro number, d the collision diameter of thegas molecules, and p the mean pressure across the membrane.

In Knudsen diffusion, the mean free path of the molecules becomes comparable to or larger than thepore size of the membrane (λ/r > 1) since the gas molecules mainly collide with the pore wall instead ofwith other gas molecules. This behavior is observed for low pore sizes (r < 10 nm) or when the pressureof the gas is low [18]:

J = πnr2Dk �p

RT τδwith Dk = 0.66r

√8RT

πMw(7.12)

where n is the number of pores and Mw is the molecular weight of the compound. At larger pore sizes,viscous flow occurs since gas molecules collide entirely with each other. The Hagen–Poiseuille equationdescribes this behavior properly.

7.2.4.2 Transport in homogeneous membranes

Transport through a dense membrane can be described in terms of a solution–diffusion mechanism, whichis based on both solubility and mobility factors. Solution–diffusion mechanisms (absorption or adsorption


at the upstream boundary, solubility through the membrane, and desorption or evaporation on the otherside) are driven by a difference in thermodynamic activity at the upstream and downstream faces of themembrane as well as the interacting forces between the molecules of the membrane material and those ofthe permeate [13]. The activity difference causes a concentration difference that leads to diffusion in thedirection of decreasing activity.

In this model, the permeability is expressed as the product of the solubility and the diffusivity:

P = S × D (7.13)

• Permeability P is a measure of the rate at which a particular component moves through the mem-brane. The conventional (although not standard) unit for expressing permeability is the barrer (1 Ba =10−10 cm3 (STP) cm−1 s−1 cmHg−1).

• Solubility S (with unit of (g m−3)membrane (g m−3)air−1) is a thermodynamic parameter that provides

a measure of the amount of the compound sorbed by the membrane (penetrant) under equilibriumconditions. It is actually the equilibrium partition coefficient between a compound’s concentration in amembrane and its concentration in a second phase (liquid or gaseous) in contact with that membrane.The solubility depends on the vapor pressure, the nature of the penetrant–polymer system, the tem-perature, and the penetrant concentration if it is high. When considering gases at low concentration,solubility becomes independent of the penetrant’s concentration, and it can be described by Henry’slaw [18].

• Diffusivity D is a kinetic parameter that indicates how fast a penetrant is transported through themembrane. It depends on the geometry of the penetrant (a higher molecular size leads to a lowerdiffusion coefficient) and also on the concentration.

Under ideal conditions, both solubility and diffusion coefficients are constant and the system isconcentration-independent (the sorption isotherm is linear), and thus Fick’s law is obeyed:

J = −DdC

dx(7.14)

Integrating over a concentration range from C1 to C2 (concentrations of the pollutant at the low- and high-concentration sides of the membrane, respectively), and assuming steady conditions (a diffusion coefficientindependent of the concentration, typical in rubbery polymers):

J =∫ C2

C1

−DdC

dx= D

(C1 − C2

)δ

(7.15)

As mentioned in this chapter, in the case of gases and vapors, Henry’s law is obeyed. Therefore, C = S × p,and Equation (7.15) can be expressed as:

J = DS(p1 − p2

)δ

= P(p1 − p2

)δ

(7.16)

The solution–diffusion model [25] is usually observed with gases in elastomers. However, with glassypolymers, the sorption isotherms are highly nonlinear, especially at high vapor pressures. The nonidealsorption behavior can be described by free-volume models and Flory–Huggins thermodynamics [18].

7.3 Reactor configurations

The term module refers to the smallest unit into which a membrane area is packed. It is the central part of amembrane installation [18]. There are several types of membrane modules currently available in industry;the most commonly used are shown in Figure 7.3.


Tubular – Plate and frame – Spiral-wound – Capillary – Hollow fiber

Low packing density High packing density

100-400 m2 m−3 up to30000m2 m−3

Figure 7.3 Most commonly used membrane modules in increasing order of packing densities.

While plate and frame and spiral-wound modules include flat-sheet membranes, tubular, capillary, andhollow-fiber modules use membranes prepared in a tubular form. In general, a tubular configuration offers ahigher ratio of surface area to volume, this being the optimal configuration for membrane bioreactors [19].

7.3.1 Flat-sheet membranes

7.3.1.1 Plate and frame modules

This is the first type of module used for industrial membrane applications. It has a simple structure andan easy membrane replacement. The plates of the spacer, the membrane, and the support are stackedalternately. The feed flows in the module inward and outward, enabling the entire membrane surface to becovered by the feed stream (Figure 7.4). The packing density of such modules (membrane surface area permodule volume) is about 100–400 m2 m−3 [18]. The membrane permeate is collected from each supportplate. The space surface is made uneven in order to promote turbulence of the feed of fluid.

7.3.1.2 Spiral-wound modules

This configuration derives from the plate and frame module. A permeate spacer is sandwiched between twomembranes, and three edges of the membranes are sealed with epoxy resin to form a membrane envelope,

Feed

Feed

Permeate

Permeate

Permeate

Permeate

Retentate

Retentate

Spacer

Membrane

Figure 7.4 Basic structure of plate and frame modules.


Collection pipe

Permeate

Feed flow

Membrane

Feed spacer

Membrane feed spacer

PermeateRetentate

Feed

Figure 7.5 Basic structure of spiral-wound modules.

the open end being connected to a central tube with holes (the collection pipe) (Figure 7.5). The membraneleaf obtained is wound spirally around the central tube together with a feed spacer. The feed and thepermeate flow axially through the cylindrical module and radially toward the central pipe, respectively. Thenumber of leaves depends on the module diameter. Polypropylene or polyethylene (0.2–2.0 mm thick) isused for the feed spacer, while polyester cloth (0.2–1.0 mm thick) is used for the permeate spacer. Thepacking density is highly dependent on the channel height, which is a function of the permeate and feed-sidespacer material. It usually varies between 300 and 100 m2 m−3.

7.3.2 Tubular configuration membranes

Depending on their dimensions, tubular configuration membranes are classified as follows [13]:

• hollow-fiber membranes (i.d. <0.5 mm);• capillary membranes (i.d. 0.5–10 mm); and• tubular membranes (i.d. >10 mm).

7.3.2.1 Tubular modules

As opposed to hollow fibers and capillary membranes, tubular modules are too large to be self-supporting.Thus, these membranes are encased inside a porous stainless steel, ceramic, or plastic shell in varyingnumbers (usually 4–18) (Figure 7.6). The feed liquid flows inside the tube, and the permeate flows fromthe inside to the outside of the membrane tube and is collected at the permeate outlet. Tubular modulescan be operated with simple pre-treatment of feed liquid, and membrane contamination can be minimizedby high feed flow rate. Besides, the contaminated membrane surfaces can be easily washed and membranereplacement is easy. However, the ratio of membrane area to module space of these modules is rather low,usually lower than 300 m2 m−3.

Feed

RetentateCollection pipe

Permeate

Figure 7.6 Basic structure of tubular modules.


Feed Retentate

Perm

eate

Fiber bundle Shell tube

Figure 7.7 Basic structure of a capillary membrane module.

7.3.2.2 Capillary membrane modules

This module consists of a large number of membrane capillaries arranged in parallel as a bundle in ashell tube (Figure 7.7). It requires membranes in a self-supporting capillary configuration. This moduleprovides a high packing density (between 600 and 1200 m2 m−3 [18]) while having very low productioncosts. However, the operating pressure must be low due to the limited stability of the capillary membranes(4–6 bar).

7.3.2.3 Hollow-fiber membrane modules

The development of hollow-fiber membranes represented an important advance in membrane manufacturingtechnology. Hollow-fiber modules are very similar to capillary modules, except that larger areas can bepacked into smaller volumes (packing density values can reach 30 000 m2 m−3) [18]. In these modules,the selective layer is on the outside of the fibers, which are installed as a bundle of several thousand fibersin a half-loop with the free ends potted with an epoxy resin on one side. As in capillary modules, the feedsolution can be introduced inside the fiber (inside out) or on the outside (inside in). These modules areused when the feed stream is relatively clean.

7.3.3 Membrane-based bioreactors

In membrane bioreactors, the membrane not only acts as an interphase between two phases but also isthe support where a biofilm develops. The microorganisms forming the biofilm transform the pollutantsof the waste stream into innocuous products, mainly CO2, H2O, and biomass. The following sections ofthis chapter are focused on different aspects of membrane bioreactors, such as the biofilm formation, theiroperation, modeling, and the most common applications.

7.4 Microbiology

Pollutant biodegradation constitutes the final step in the removal of VOCs and VSCs in advanced membranebioreactors and is inherently linked to pollutant transport through the membrane. The microbial communityresponsible for this biodegradation can be attached either onto the membrane (the most common scenario)or in suspension in the nutrient-containing aqueous phase, and its activity largely determines the drivingforce available for mass transfer [20]. Advanced membrane bioreactors, like their biological counterparts,


are based on the enzymatic oxidation of gaseous pollutants, which are finally converted to CO2, H2O,SO4

2−, NO3−, and biomass [26, 27]. The fact that pollutant oxidation occurs at ambient pressure and

temperature without chemical requirements supports the lower environmental impact and operating costsof biological systems. Most VOCs and VSCs are used by microorganisms as carbon and energy sources,while some specific pollutants such as NH3 and H2S are used only as energy sources, with inorganic carbonproviding the carbon source needed for the construction of new cellular material and maintenance [4].

Bacteria and fungi are the two main groups of microorganisms responsible for the destruction of gaseouspollutants in biological systems, even though most of the studies carried out in membrane bioreactors havebeen performed using pure bacterial strains or activated sludge [13]. Bacteria exhibit high growth andbiodegradation rates, high resistance toward pollutant toxicity, and large catabolic potential, although aneutral pH and high moisture content are required for their optimum activity. In this context, bacteriafrom the genus Pseudomonas are among the most commonly found in biofilters and membrane bioreactors[13, 28]. On the other hand, despite fungi possessing a lower catabolic spectrum, they are capable ofwithstanding low pH values (2–5), low humidity, and nutrient limitations [29].

The conditions prevailing in the aqueous phase of membrane bioreactors (high water activity, and con-trolled pH and nutrient supply), together with the hydrophobic and porous nature of most membranematerials, are likely to promote the development of bacterial biofilms adhering to the membrane [20].Biofilms are ubiquitous in nature and possess a complex structure consisting of cell clusters (discrete aggre-gates of microbial cells in an exopolysaccharide matrix) and interstitial voids (open channels connected tothe bulk liquid) [30, 31]. Biofilm structure is very heterogeneous and is influenced by the characteristics ofthe membrane, with surface roughness enhancing biofilm formation [13]. The development of biofilms isgoverned by several mechanisms, such as cell growth, detachment (which itself is a function of the liquidshear forces over the biofilm), substrate mass transport, quorum sensing, cell death, and active dispersal[32]. In recent decades, an intense research effort, especially in the field of wastewater treatment, has beendevoted to the elucidation of biofilm structure by imaging or mathematical modeling [13].

The immobilized microorganisms on the membrane are subjected to substrate, byproduct, and eventemperature diffusion gradients, with subsequent growth rate gradients. Unlike the biofilms in wastewatertreatment bioreactors, where both the pollutants and oxygen that diffuse from the bulk aqueous phase aremainly consumed on the outer layers of the biofilm, the immobilized cells close to the membrane are themost active as a result of the direct uptake of pollutants and oxygen diffusing from the membrane in airtreatment applications [20] (Figure 7.8).

Molecular diffusion, which itself depends on both the water-binding capacity and mobility of the biofilm,is the predominant transport mechanism within the cell aggregates, while in the interstitial voids bothdiffusion and convection take place [30]. The intra-biofilm liquid transport is enhanced by the liquidturbulence on the biofilm–liquid boundary, and although this convective transport significantly determinesthe bioreactor performance in wastewater treatment applications, its impact in off-treatment is limited dueto the fact that both pollutants and O2 reach the biofilm from the membrane side. Recent studies haveshown that the active zone of membrane biofilms is often limited to 0.2 mm (average substrate penetrationdepth), although biofilms of 1 mm can be established at low liquid shear forces [19]. In this context, biofilmovergrowth in membrane bioreactors causes severe operational problems as a result of hindered pollutantdiffusion and reactor clogging. The control of biofilm thickness in advanced membrane bioreactors ismainly achieved by regulating the shear forces of the liquid circulating over the top layers of the biofilm.Thus, the fact that the most active microorganisms in the biofilm are adjacent to the membrane facilitatesthe sloughing of the inactive biomass in contact with the liquid side. The use of intermittent aeration,with the turbulent gas bubbles inducing large shear forces over the top layers of the biofilm, constitutesanother biofilm control strategy commonly applied to avoid membrane clogging in conventional aerobicmembrane wastewater treatment [19].


VOC/VSC

O2

ME

MB

RA

NE

BIO

FIL

M

AQ

UE

OU

SPH

ASE

Water

Nutrient

CO2, byproducts

VO

C /

VSC

/ O

2 co

ncen

trat

ion

GA

S PH

ASE

Figure 7.8 Mass transfer mechanisms underlying VOC and VSC oxidation in gas-phase membrane bioreactors.

7.5 Performance of membrane bioreactors

7.5.1 Membrane-based bioreactors

Since the early days of research on biological membrane-based bioreactors about 30 years ago, a largenumber of systems have been developed and the commercial applications of this technology are continuallyincreasing. The most common application is the treatment of industrial and domestic wastewaters withstrict discharge limits, where water reuse is required or the available space is limited.

However, although wastewater treatment using membrane bioreactors has been widely studied, thereare few studies concerning their application for waste-gas treatment. In MBRWGs, the membrane notonly acts as a barrier for the selective transport of compounds from the gas phase to the liquid phase,but also serves as a support for the microbial population. The driving force in this process is givenby the concentration gradient created between the gas side (feed of pollutants) and the biofilm (wherebiodegradation of pollutants takes place). Gaseous pollutants will be selectively transported through themembrane depending on their permeability [13]. In the typical configuration of membrane bioreactors,the gaseous stream, from which microorganisms receive oxygen, is passed through the dry side of themembrane, while the other side, covered by a biofilm, is submerged into a mineral solution that provideswater and nutrients to the microorganisms (Figure 7.8 and Figure 7.9). The liquid with the nutrients isusually recycled, buffered to maintain a suitable pH, and replaced periodically with fresh solution to replacenutrients and avoid accumulation of byproducts [19, 33]. Until now, MBRWGs have been studied onlyat the laboratory scale, and membrane bioreactors for treatment of VOCs still have to be tested at realscale [34].

The physical separation between the polluted gaseous stream and the biomass allows this treatmentmethod to be used in applications such as inner or spatial air treatment, as well as for specific cases where


Liquid in

Liquid outShell tube

Fiber bundle

Biofilm MembraneGas

out

Gas

in

Liquid in

Liquid out

Gas out

Gas in

Figure 7.9 Basic structure of (a) hollow-fiber and (b) flat gas membrane bioreactors.

the waste stream cannot be in direct contact with the biomass. Besides, conventional biotechnologiesusually fail in efficiently removing the highly hydrophobic VOCs (such as hexane, toluene, BTEX, andso on) from the air, due to mass transfer limitations. In this context, membrane bioreactors offer a largegas–liquid interface and excellent mass transfer properties. Furthermore, the microbial population is alsoprotected from toxic compounds that might be present in the gaseous stream or from fluctuations in thecomposition and/or concentration of the pollutants, since the membrane acts as a buffer zone [20].

7.5.2 Bioreactor operation: Influence of the operating parameters

Membrane-based systems have been evaluated as a method to better control and optimize the biofilmenvironment in bioreactors. Both VOCs and oxygen are transferred to the actively metabolizing biofilmthrough the membrane. These systems have demonstrated favorable results in low organic loading scenarios,but the membrane pores tend to plug with biomass under high loading conditions. If the pores are blockedby biofilm formation, the membrane resistance (km ) will increase, thus reducing the pollutant and O2transfer rate. Many studies have reported a deterioration of the membrane reactor performance due toexcessive biomass growth [33, 35]. Periodic back-flushing with water or air pulses or increasing the shearstress by higher liquid recycling rates may reduce these problems.

Although the liquid recycling velocity seems to have little effect on mass transfer [36], it is an importantparameter affecting the removal performance. High recycling velocities may remove the excess biomassgrowth from the shell side of the membrane module, so that it can be operated without biomass clogging.However, England et al . [37] found better performance of both a single- and a dual-tube MBRWG forthe removal of toluene when the liquid recycling was stopped. This could potentially reduce the energyrequirements and consequently the operating costs of membrane bioreactors. Thus, a balance betweenthe improved performance of the MBRWG due to the removal of the excess of biomass at high liquidvelocities and the costs associated with liquid recycling must be achieved.


In the aerobic treatment of VOCs in conventional bioreactors, oxygen transfer to the microorganismsusually becomes a limiting factor. In liquid-phase bioreactors such as stirred-tank reactors, an air-spargingdevice is used to provide the necessary oxygen. However, the retention time of the air bubbles is notenough for the complete transfer of the oxygen to the aqueous phase, and they may also strip VOCs fromthe liquid before pollutants are biodegraded. In membrane bioreactors, the oxygen is supplied from theair waste stream through the membrane, and their larger surface area increases the oxygen mass transferrate in the bioreactor; thus oxygen limitations are rarely found. Reij et al . [20] showed that most of theoxygen mass transfer resistance in MBRWGs is actually in the aqueous phase, the membrane resistancebeing negligible in a hydrophobic hollow-fiber membrane bioreactor.

Water permeation through composite membranes and the subsequent water condensation in the porousmembrane side (the gas side) may occur if the dense layer of the composite membrane is very thin. Thisphenomenon is also observed in microporous membranes, where the total porosity of the membrane is ofkey relevance to determine the overall transfer of pollutants to the active biofilm, since the membrane isimpermeable to the substrates except at the open pores. Therefore, if water condenses inside the tubes,another resistance should be taken into account. In both cases, mass transfer through the membrane ishindered, resulting in a decrease in reactor performance as reported by some authors [33, 35].

Regarding the membranes, higher and constant removal capacities have been obtained with nonporousmembranes such as PDMS in comparison to hydrophobic microporous membranes. Differences in perme-abilities might be anticipated as the permeability of silicone may change with the filler content and mayalso change with the concentration of the permeating compound. Swelling of rubber can cause an increasein the rate of diffusion not only of the swelling agent itself, but also of any other molecule. Oxidationof rubber generally decreases diffusivity and permeability, not only for oxygen but also for all foreignmolecules [37] (Table 7.2).

Overall, the presence of one component in the membrane will affect the sorption and diffusion proper-ties of the other components through interactions such as plasticizing effects on the membrane, clusteringof one or more penetrants, hydrogen bonding effects, and mutual interactions between the penetrantsand the membrane [41]. Besides, the presence of a secondary pollutant may cause microbial com-petition or inhibition, modifying the performance of the bioreactor. For example, Kumar et al . [42]observed an increase in the removal efficiency of DMS in a flat composite-membrane bioreactor whenmethanol was added at a concentration between 0.6 and 2.3 g m−3, while higher concentrations had anegative effect.

According to a sensitivity analysis conducted on the performance of a hollow-fiber membrane reactor,the biofilm density is in the order of 25–30 gL−1 with small changes in the biofilm density significantlymodifying the removal efficiency of the system. Suspended biomass had an effect at concentrationsgreater than 10 mgL−1 [36]. Similarly, the biofilm diffusion coefficient is of key relevance to thebioreactor performance and a critical parameter to model prediction. However, the biomass yield, theliquid mass transfer coefficient, the aqueous substrate concentration, the inlet gas concentration, andthe number of fibers of the module did not significantly impact pollutant removal when these variablesincreased [36, 43].

7.6 Membrane bioreactor modeling

Gas-phase bioreactor models include terms for the accumulation of contaminants in all phases, dispersioneffects in air, diffusional mass transfer in biofilms, mass transfer in the liquid phase, biological removal,adsorption and desorption of contaminants, biomass growth on the surface, the structure type of supportmaterial depending upon the bioreactor configuration, and operation (mobile phase, counter-, or co-current).


Table 7.2 Permeability, solubility, and diffusivity of different VOCs in membranes.

Compound Type of T Permeability Solubility (cm3 Diffusivity Referencemembrane (◦C) (1010 m2 s−1) (STP) (cm3 (1010 m2

(cmHg)) −1) s−1)

C2H6O(dimethylether)

PDMS 25100150

– 2.951.671.19

1.521.573.59

[38]

DMS PDMS 30 561 ± 25 92.8a 6.05 [23]C7H8 (toluene) PDMS 30 2105 ± 99 902a 2.33 [23]TCEt PDMS 30 1433 ± 53 360a 3.98 [23]

25100150

– 16.541.380.61

1.446.117.25

[38]

TCE PDMS 25100150

– 7.980.430.15

3.237.49

12.90

[38]

CO2 PDMS 30 36.3 ± 1.2 1.43a 25.4 [23]35 22.8 0.015 20 [39]35 28.9 0.017 22 [22]

Glassy polymer 35 0.011 0.051 0.0027 [39]C2H4 PDMS 30 28.8 ± 0.60 2.53a 11.4 [23]

35 26 – – [40]C2H6 PDMS 35 25.1 0.029 11.3 [22]O2 PDMS 30 6.91 + 0.35 – – [23]

35 6.08 0.0024 34 [22]CH4 PDMS 35 7.6 0.005 20 [39]

35 9.12 0.0055 22 [22]Glassy polymer 35 0.008 0.001 0.0004 [39]

CF4 PDMS 35 1.52 0.0025 8.0 [22]

Note: T = temperature; DMS, dimethylsulfide; TCEt, trichloroethylene; TCE, trichloroethane; PDMS, polydimethylsiloxane; – , not reported.aUnits: g m−3 (g m−3)−1.

Recently, several aspects of biofilter modeling have been discussed by Kumar et al . [44]. Numerous modelshave been presented to describe biofilms in conventional biofilters [44–48], hollow-fiber membrane reactors[49, 50], and activities within biofilms [36].

Ergas et al . [36] have used a simplified assumption to solve the differential equations analytically (e.g.,first-order or zero-order growth kinetics with respect to the biomass concentration). Other studies haveincorporated growth, substrate inhibition, decay, and death rates using numerical solutions [51, 52]. Thederived information relies on mass balance principles and on the a priori assumption that all componentsconsidered in the model (chemical substance, microorganisms, etc.) may be treated as on a continuum ratherthan as individual particles [53]. The value of modeling a process is two-fold: first, a well-developed modelpresents insight into the limiting factors in reactor operation; secondly, the model, if verified, may be usedas a predictive tool (for design), which can save significant experimental time. Transport process models anddegradation models can also help determine the rate-limiting processes. However, only a few models haveso far been developed that describe the mass transfer and degradation processes occurring in membrane-attached biofilms, where counter-diffusion of the pollutants in the biofilm is considered [43, 49, 54, 55].


7.7 Applications of membrane bioreactors in biological waste-gas treatment

The first laboratory-scale studies with membrane bioreactors were reported in 1986, but, to date, no full-scale investigation in membrane reactors for waste-gas treatment has yet been performed [15]. Laboratorystudies of MBRWGs have demonstrated a good performance for the biodegradation of a wide range ofVOCs with different hydrophobicity (Table 7.3, adapted from [13]).

In most cases, MBRWGs were seeded either with pure cultures or with mixed consortia enriched fromactivated sludge. Inoculation with activated sludge does not require sterilization, provides a higher adaptivecapacity due to their high microbial diversity, and allows complex mixtures of pollutants to be treated anda continuous process to be operated. Besides, the efficiencies supported by mixed consortia are similar tothose recorded for systems seeded with pure cultures (Table 7.3). However, specialized microorganismsmight be required for the biodegradation of recalcitrant or poorly biodegradable compounds. Short start-upperiods have usually been reported in MBRWGs due to the rapid development of the biofilm attached tothe membrane (a few days). Longer acclimation periods may, however, be necessary in bioreactors treatingpoorly biodegradable VOCs or highly loaded streams.

The typical removal efficiencies observed with nonporous membranes (such as PDMS) are usually higherthan those of hydrophobic microporous membranes. As mentioned in this chapter, water condensation andbiofouling are more frequent in porous membranes, leading to a decrease in the membrane bioreactorperformance [60]. Nowadays, composite membranes are usually preferred due to their improved masstransfer, lower fouling, and higher mechanical strength. In terms of bioreactor configuration, hollow fibersoffer larger specific gas–membrane contact areas, and thus the volumetric elimination capacities (ECv)achieved (grams of compound removed per unit of time and reactor volume) are higher than those sup-ported by flat or tubular membrane reactors. Nevertheless, if the performance is compared in terms ofthe ECm (elimination capacity expressed by unit of specific membrane area a), the efficiencies obtainedwith different configurations are in the same order of magnitude. For example, Fitch et al . [57] treatedsimilar loadings of benzene in a hollow-fiber reactor with a = 34 890 m2 m−3 and in a capillary reactor witha = 368m2 m−3 (Table 7.3). The maximum ECv obtained were 3780 and 997 g m−3 h−1 in the hollowfiber and in the capillary reactor, respectively, while the maximum ECm was higher in the capillary reactor(2.7 g m−2 h−1 compared to 0.1 g m−2 h−1). A review based on 37 articles describing the performance oflab-scale MBRWGs showed that approximately 41% used capillary modules, 24% hollow fiber, 27% flatreactors, and only 5% and 3% employed tubular and spiral-wound membrane reactors, respectively.

7.7.1 Comparison with other technologies

Membrane bioreactors offer all the advantages of gas treatment biotechnologies while overcoming severalof their limitations. First of all, similar VOC removal efficiencies are obtained in MBRWGs comparedto biofilters (BFs) and biotrickling filters (BTFs) at lower gas residence times (average 4.4 s compared to5–15 s in BTFs and 60–90 s in BFs), thus reducing the space requirements and presenting up to five timeshigher ECv [58]. This is due to the membrane-based enhancement of the mass transfer of the VOCs fromthe gas phase to the biodegrading biofilm. For example, Kumar et al . [60] compared the efficiency of acomposite membrane bioreactor for toluene treatment with those reported in BFs and BTFs. Since datarelated to the specific area of the packing material are usually scarce, comparison can be based only onthe ECv. In this aspect, 1.8 and 1.5 times higher elimination capacities were obtained in the MBRWGsthan in the BFs and BTFs, respectively.

The presence of a continuous liquid recycle allows for a better control of the operating parameters suchas pH, temperature, humidity, and moisture content, and the accumulation of acidic compounds and other


toxic metabolites. This control is not possible in biofilters, which usually suffer from media acidificationor drying.

The pressure drops observed in membrane bioreactors are often much lower than those reported in theirbiological counterparts. Jacobs et al . [58] reported pressure drops in the range of 1–11 Pa at gas residencetimes between 1 and 24 s. On the other hand, BFs and BTFs present typical pressure drops of 1000–1500and 400–500 Pa per meter of column height, respectively [6]. Among the different types of membranemodules, hollow-fiber reactors present higher gas pressure drops [57].

Despite the advantages of MBRWGs that are mentioned here, they have not been applied at industrialand full scale. One of the main reasons for this is the lack of studies on the long-term performance of thesesystems and their behavior under nonsteady conditions. Both long-term operation and process robustnessare key parameters when selecting an off-gas treatment technology. In this context, membranes usuallyshow clogging and aging due to the effect of biomass on the membrane material. Most of the studiesconducted evaluated the performance of the membrane over 5–6 months, and very little information wasprovided on the alteration of the membrane characteristics (permeability, porosity, etc.). Physico-chemicaltechnologies for VOC treatment are usually considered more robust than biotechnologies; however, recentstudies have demonstrated that biotechniques are able to rapidly recover from the negative impact of processfluctuations and operational failures [63]. Similarly, stress situations such as increases in temperature orpollutant concentration seem to have no negative effect on the performance of membrane bioreactors [60].

7.8 New applications: CO2 –NOx sequestration

7.8.1 NOx removal

Oxides of nitrogen (NOx) are formed whenever fuel is burned in the air NOx contributes to troposphericozone formation; is harmful to plants, animals, and human health; and is a precursor to acid deposition.Biological treatment of NOx can be carried out under either denitrifying (anoxic) or nitrifying (aerobic)conditions. A number of researchers have investigated NOx removal in conventional gas-phase bioreactorssuch as BFs and BTFs [64–68]. Under anoxic conditions, Apel et al . [64] utilized BFs containing finishedwood compost with molasses as a carbon source. Whereas an average of 90% NOx removal was obtainedwith molasses, a maximum of 20% removal was observed without molasses. The effect of oxygen in theinfluent gas on NOx removal was studied at an empty bed residence time (EBRT) of 2 min. NOx removalefficiencies were over 90% with 500 ppm NOx and <3% O2. However, NOx removals ranged between40 and 45% for the gas stream containing 250 ppm NOx and 5% O2. Lacey et al . [69] investigated NOxremoval efficiencies in an anoxic BF packed with four compost types at 55–60 ◦C, and an EBRT of 13 s.The inlet gas composition was 87% N2, 13% CO2 with a target NOx concentration of 500 ppm. Lactatewas supplied as the carbon source. Over time, the NOx removal rate gradually decreased independently ofcompost type to 40–60%.

Wang et al . [67] investigated a rotating drum biofilter (RDB) for NO removal by denitrification from asynthetic waste gas using glucose as carbon source. The effects of drum rotation speed and EBRT on NOremoval were investigated under anaerobic conditions. At an EBRT of 65 s and a drum rotation speed of0.5 rpm, NO removal efficiency was over 97.9% at an inlet NO concentration of 524 ppm. At increased inletoxygen concentrations, denitrification decreased and chemical oxidation increased. An optimal inlet oxygenconcentration of approximately 5.2% was obtained. In another study, Jun et al . [68] studied a bench-scaleanaerobic RDB for NO removal. The inlet NO concentration fluctuated between approximately 90 and433 mg m−3, while the removal efficiency was maintained at 60–85%, and the average elimination capacitywas 250 g m−3 d−1. However, it is difficult to maintain anoxic conditions for denitrifying processes becauseof combustion gases containing excess oxygen.


Tabl

e7.

3Li

tera

ture

com

pila

tion

ofth

epe

rfor

man

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diffe

rent

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Gs.

Com

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men

talc

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der

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TC

once

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tion

Liqu

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(%)

ECm

ax,v

ECm

ax,m

(s)

(gm

−3)

recy

clin

gra

te(g

m−3

h−1)

(gm

−3(g

m−2

(mL

min

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h−1)

h−1)

Ethy

lace

tate

Flat

reac

tor

Com

posi

tePD

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(0.3

µm)/P

AN

(50

µm)

Pre-

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ted

mix

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bial

cultu

re15

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7560

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31.8

to>

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50.

45[3

3]

Dic

hlor

oeth

ane

Spir

alw

ound

PDM

SX

anth

obac

ter

auto

trop

hicu

sG

J10

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600.

720

0030

79–9

427

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022

[56]

Ben

zene

Tubu

lar

reac

tor

Late

xru

bber

Pre-

adap

ted

activ

ated

slud

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40.

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720

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[57]

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low

fiber

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usPP

Pre-

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ted

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ated

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66.

58

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[57]

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ene

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tor

Com

posi

te:P

DM

S(1

µm)/P

DV

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004

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7532

–483

41–9

939

70.

8[5

8]

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low

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PEPs

eudo

mon

aspu

tida

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–1.

340.

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7–

–[5

9]

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lar:

sing

letu

be,

dual

tube

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mul

tiple

tube

PDM

S

Pre-

adap

ted

mix

edcu

lture

s78 41 23

40

2.5

–3.7

10 15 –

–9 55 –

16.4

78.4

220

2.9

3.0

13.2

[37]

Flat

reac

tor

Com

posi

te:P

DM

S(0

.3µm

)/PA

N(1

85µm

)

Bur

khol

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avi

etna

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nsis

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80.

2–4

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50–1

112

78–9

960

01.

2[1

3]

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reac

tor

Com

posi

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slau

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rhou

sew

aste

wat

ersl

udge

5–2

80.

3–3

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110

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129

–100

609

1.2

[60]


Dim

ethy

lsul

fide

Flat

reac

tor

Com

posi

te:P

DM

S(1

7µm

)/Zrf

(175

µm)

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100.

40.

2[1

5]

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Com

posi

te:P

DM

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0026

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5[4

2]

Flat

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Com

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F(2

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)

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mop

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4370

6484

540.

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and

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e

Tubu

lar

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eudo

mon

aspu

tida

TX1

and

BTE

16

–15

2.3

–9.8

1000

1372

(max

.)71

–99

1225

0.3

[62]

PPm

icro

fiber

s(m

icro

poro

us)

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onas

putid

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dB

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8–1

67.

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5.4

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44(m

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034

200.

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5]

Con

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nsan

dpe

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man

ce:

GR

T,ga

sre

side

nce

time;

LR,

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RE,

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oval

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y;EC

max

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max

imum

elim

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ion

capa

city

base

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me;

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port

ed;


An alternative method for NOx removal is to use the nitrification process, which could be a morecost-effective and stable process than denitrification. Davidova et al . [70] investigated the biofiltration ofNO using nitrification. NO removal efficiencies as high as 70% at inlet concentrations of 60 ppm wereachieved with an EBRT of 12 min. NO removal was also found to have a linear relationship with inlet NOconcentration at 20–140 ppm. The system was reported to be highly dependent on the gas flow rate andwas sensitive to changes in pH. Chou and Lin [65] presented the results of NOx treatment using a pilot-scale BTF packed with slag. A steady removal rate of 80% was attained at an influent NOx concentrationof 892–1237 ppm and an EBRT of 2 min. Over 90% of the eliminated NOx was converted to nitrate bynitrification.

Treatment of NOx in conventional BFs and BTFs has typically required long contact times due to thelow solubility of NOx (Henry’s law constant of 19.8 at 20 ◦C) [71]. A novel bioreactor system for treatmentof low-solubility gases is the hollow-fiber membrane biofilm reactor. The hollow-fiber membranes serveas a support for the microbial population, and provide a large surface area for pollutant and oxygen masstransfer. Compounds in the gas phase are transferred through the membrane pores and partition to themicrobial population, which is surrounded by a circulating nutrient media. In a recent study published byKumar et al . [72], NO gas was treated in a bench-scale hollow-fiber MBR at varying liquid recirculationvelocities of 0.008–0.02 m s−1. NO removal efficiency was observed to be between 68 and 73% at roomtemperature (20 ◦C). The developed model predicted an optimal liquid velocity of 0.015 m s−1 for NOremoval and is in good agreement with the experimental data. Implementation of both conventional andmembrane bioreactors for NO removal will be largely determined by the cooling process necessary tobring down the waste-gas stream to temperatures that fit the operational ranges of the waste-gas treatment.

7.8.2 CO2 sequestration

Biological CO2 sequestration has attracted increased attention as a greenhouse gas mitigation strategybecause it leads to production of biomass energy in the process of CO2 fixation through photosynthesis[73, 74]. Recently, Kumar et al . [74] reviewed biological CO2 fixation for biofuel production. BiologicalCO2 fixation can be carried out by either plants or photosynthetic microorganisms; however, the potentialfor increased CO2 capture by plants is estimated at only 3–6% of fossil fuel emissions, largely due toslow growth rates of conventional terrestrial plants. Microalgae, a group of fast-growing unicellular orsimple multicellular microorganisms, have the ability to fix CO2 with an efficiency 10–50 times greaterthan that of terrestrial plants [75]. Algal biomass can be used as a substrate for biological generationof methane, ethanol, or hydrogen, and some species of microalgae are capable of storing large amountsof oil that can be used as a source of biodiesel or other high-value bioactive compounds [76]. MicroalgalCO2 mitigation can be made more cost-effective and environmentally sustainable when combined withwastewater treatment. The wastewater provides a source of fresh water, nitrogen (N), phosphorus (P), andother nutrients, resulting in decreased nutrient and metal concentrations in the treated effluent [77]. Thepotential for combining CO2 fixation with wastewater treatment by microalgae has been investigated by anumber of researchers using different microalgal strains: Botryococcus braunii was used to remove N and Pfrom secondary treated wastewater in both batch and continuous-flow systems [78], and Chlorelia vulgariswas cultivated using wastewater and flue gas from a steel-making plant [79]. CO2 fixation and NH4

+removal rates were estimated at 620 g of CO2 per cubic meter per day and 22 g of NH3 per cubic meterper day, respectively, when wastewater was supplemented with PO4 without pH control at 15% (v/v)CO2. Scenedesmus obliquus was cultivated outdoors in synthetic wastewater under winter and summertropical conditions of Mazatlan, Mexico [80]. Biomass production rates of 0.009 and 0.016 g m−3 d−1

were observed for winter and summer, respectively, which were equivalent to CO2 fixation rates of 16and 31 g m−3 d−1. Final dissolved N concentrations were 53% of the initial values in winter and 21% in


summer. Phosphorus was removed only during the day, with a total removal efficiency of 45% in winterand 73% in summer.

With the progress made so far on biological CO2 sequestration, many challenges remain. Among these,technologies for supplying adequate amounts of CO2 to the microalgal cells at minimum energy cost andwaste of CO2 to the atmosphere are poorly developed [81]. Bubbling of CO2-enriched air is typicallyperformed using sintered porous diffusers. A drawback of this process is that most of the CO2 is lost tothe atmosphere, leading to increased operational costs. Closed photo-bioreactors offer higher gas transferand photosynthetic efficiencies, lower rates of ammonia volatilization, and smaller space requirements thanopen processes; however, a major limitation of these systems is the toxicity to the microalgae of dissolvedoxygen (DO), which can reach values of 30–40 g m−3 [81].

Hollow-fiber membrane photo-bioreactors (HFMPBs) have the potential to achieve high CO2 masstransfer efficiencies in small reactor volumes with minimal CO2 loss to the atmosphere [82]. In HFMPBs,combustion gases are passed through the lumen of hollow-fiber membranes, which provide a large sur-face area for gas transfer. CO2 is transferred through the membrane pores and partitions into the liquidphase containing the algal cells, which is supplied with wastewater as a nutrient source. Oxygen pro-duced through photosynthesis is transferred out of the bioreactor through the membranes, reducing thetoxicity of the growth media. The use of hollow-fiber membranes also makes it possible to operate atlower gas pressures, as there is no need to counterbalance hydrostatic heads [82]. A number of researchershave used hollow-fiber membrane bioreactors for gas transfer applications including wastewater aeration[14, 83], hydrogenotrophic denitrification [84], and treatment of VOCs [13, 17, 36, 57] and oxides of nitro-gen [85] in polluted air streams. Several researchers have previously reported using membrane-integratedphoto-bioreactors for biological CO2 sequestration, primarily at CO2 concentrations typical of indoor airapplications (0.03–1%) [86–88]. In a recent study by Kumar et al . [55] a bench-scale HFMPB wasinoculated with Spirulina platensis and operated with a 2–15% CO2 supply. A mass transfer modelwas developed and found to be a good tool to estimate CO2 mass transfer coefficients at varying liq-uid velocities. Overall mass transfer coefficients were 1.8 × 10−6, 2.8 × 10−6, and 5.6 × 10−6 m s−1 atReynolds numbers of 38, 63, and 138, respectively. A maximum CO2 removal efficiency of 85% wasobserved at an inlet CO2 concentration of 2% and a gas residence time (membrane lumen) of 8.6 s. Thecorresponding algal biomass concentrations and NO2 removal efficiencies were 2131 mg L−1 and 68%,respectively. The results show that the combination of CO2 sequestration, wastewater treatment, and biofuelproduction in an HFMPB is a promising alternative for greenhouse gas mitigation. Studies should focuson the real composition of combustion gases, wastewater operated at different temperatures, and so on.

7.9 Future needs

There is a huge difference in membrane bioreactor applications in wastewater treatment compared withwaste-gas treatment. Today, to the best of our knowledge, waste-gas MBR applications at full scale oreven at pilot scale have so far not been implemented. Modular MBR systems for wastewater treatmentare commercially available and operational at full scale. In most of these applications, porous membranesare used, which is less advantageous for waste-gas treatment. So it is obvious that scaling-up of MBRWGtechnology is the urgent need to facilitate further development.

From a more fundamental point of view, the methodologies to measure transport properties throughmembranes exist and are readily applied. A more challenging aspect, however, is the microbiologicalpart of the process. Molecular techniques tracing the presence of specific microorganisms and providinginformation on the diversity and composition of microbial flora in biological waste-gas treatment havebeen implemented [89]. Useful information can be extracted from their results, and the application of


these techniques will become standard in the study of operational parameters in biological waste-gastreatment systems in general. However, the link between the biological information and the operationalaspects (e.g., elimination capacity) is still far from being clear and will be an area of intensive study inthe near future. Next to the microbial composition, the structure of the biofilm and its effect on transportand biodegradation characteristics also remain to be studied. A flat membrane bioreactor can be used tomeasure, for example, pH and oxygen gradients as a function of depth of the biofilm, and a few experimentshave been reported in this context. Measuring pollutant profiles in biofilms remains a challenge. Maybemembrane-induced mass spectrometry could be helpful here, but to the best of our knowledge this has notbeen reported until now.

Tools allowing study of the MBRWG operational parameters (gas residence time, elimination capacity,mutual influence of pollutants present in mixtures, etc.) are available and will be used in future to explorefurther the limits of applicability of MBRWGs.

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