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Chapter 15
Potential Uses of Immobilized Bacteria, Fungi, Algae, and Their Aggregates for Treatment of Organic and Inorganic Pollutants in Wastewater Manab Das and Alok Adholeya* The Energy and Resources Institute, I H C, Darbari Seth Block, Lodhi Road, New Delhi 110003 *E-mail:
[email protected].
Bioremediation of wastewater using microorganisms and their aggregates is recognized to be an efficient green treatment (biological origin) mean with a relatively low cost compared to conventional physical and chemical treatment processes. Microorganisms such as bacteria, fungi and often algae are used for removal of targeted pollutants from wastewater. Microorganism can be used in following two ways (1) Direct mixing of free microorganisms with waste water, and there is no separation between microorganisms and treated water (2) Microorganism immobilized in bedding materials or encapsulated within a matrix, and there is a distinct separation between microorganisms and the treated water. However, use of immobilized or encapsulated cells is considered more effective than free cells as it leads to higher biomass loading, easier operation of solid-liquid separation, higher biodegradation rates, better operation stability, greater protection from toxic substances, increased plasmid stability of immobilized cells. Lignocellulosic biomasses, ceramics, polymers from both natural and synthetic origin are commonly used as bedding materials or for entrapment of microorganisms within it. These immobilized cells show immense potential to clean up a wide range of pollutants including phenolic compounds, hydrocarbons, propionitrile, organic and inorganic dyes, N,N-dimethylformamide, pyridine, benzene, toluene and © 2015 American Chemical Society In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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xylene (BTX), heavy metals and also unwanted amount of nutrients such as nitrogen and phosphorus from wastewater streams. Integrated process of assimilation, adsorption and biodegradation is the sole responsible mechanism behind wastewater remediation by immobilized microorganisms. However, much more multifaceted investigations are still required in this field to develop more systematically integrated technologies and increase treatment efficiencies.
Introduction Global populations are expected to exceed 9 billion by 2050 having projected 6.4 billion contributions from urban areas alone and about 1.4 billion from slums. This ever increasing population growth along with urbanization, rapid industrialization, expanding and intensive food production methods are all putting pressure on water resources and on the other hand, inadequate infrastructure and management system increasing the unregulated or illegal discharge of contaminated water within and beyond national borders. Total fresh water on our planet is very limited, only 2.5% of all the water on earth is fresh water. Moreover, only 1% of this fresh water is available for withdrawal and human use. Despite of this fact, about 70-90% of the available fresh water is used in food production and a much of this water is returned to the system having additional nutrients and contaminants load. In addition, joint contributions from human and industrial waste in downstream make this scenario more critical. These wastewaters not only contaminates fresh water resources but also cause disturbances in coastal ecosystem, threatening food security, access to safe drinking and bathing water and providing a major health and environmental management challenge (1). Thus, it is essential to do proper treatment of wastewater prior to disposal into streams, lakes, seas and land surfaces. Most common methods of physical and chemical treatment of wastewater includes advanced oxidation, electro-coagulation and flocculation but they have their own limitations such as high cost, generation of secondary pollutants and also complex operation. On the contrary, biological treatments are associated with low cost and simple in operation. Hence, it has been employed extensively during past few decades for treatment of organic pollutants in wastewater. Suspended or activated sludge process and immobilized microorganism system are the two widely used methods in biological treatment of waste water. The activated sludge processes have several drawbacks including low biomass concentration, easy wash out and hence various novel technologies like acoustic cavitation, ultraviolet irradiation and magnetic field have been developed to increase efficiency of activated sludge processes. But application of these modern technologies becomes limited due to complex construction and high operation costs. On the other hand, higher biomass loading, easier operation of solid-liquid separation, higher biodegradation rates, better operation stability, greater protection from toxic substances, increased plasmid stability make immobilized microorganism systems most popular (over 320 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
activated sludge process) among water researchers worldwide (2, 3). This chapter will review application of immobilized microorganisms for treatment of wastewater more precisely industrial wastewater contaminated with both organic and inorganic pollutants.
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Immobilized Microorganism System An immobilized cell is defined as a cell that by natural or artificial means is prevented from moving independently of its neighbours to all parts of the aqueous phase of the system under study (4). Different techniques (Figure 1) that are extensively used in immobilization of microbial cell includes adsorption, covalent coupling, cross linking of microorganisms, encapsulation into a polymer gel, and entrapment in a matrix (2, 3).
Figure 1. Methods of immobilization of microbial cells.
Adsorption The simplest immobilization method is based on physical interaction between microorganisms and carrier surface. It is a reversible process and can lead to the peeling of adsorbed microorganisms during the operation. 321 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
Covalent Coupling In covalent coupling, microorganisms are immobilized with bonding reaction of reactive groups (e.g. –NH2 or –COOH) at the surface of biological cell, for instance protein. The coupling leads to increase in stability of microorganism but the bioactivity of microorganism decreases rapidly during post-operation process.
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Cross Linking of Microorganisms It is used to link bio-macromolecules each other with covalent bonds by using multifunctional reagents, such as glutaraldehyde, bisdiazobenzidine and hexamethylene diisocyanate. The method is simple but very difficult to control. Encapsulation into a Polymer Gel Many synthetic polymers such as polyacrylamide, polyvinyl alcohol; natural polymers like collagen, agar, agarose, cellulose, alginate, carrageenan etc. are often used to encapsulate microorganisms. Diffusion limitation is one of the inevitable drawbacks associated with encapsulation method. Entrapment in a Matrix Entrapment of the microorganisms in porous polymer carrier is often used to capture the microorganisms from suspended solution and then obtain the immobilized microorganisms. Due to porous structure of polymers, the pollutants and various metabolites produced easily diffuse through the matrix. Factors Responsible for Immobilization of Microorganism Immobilization of microorganism can be further categorized as “passive” (using the natural tendency of microorganism to attach to surfaces-natural or synthetic-and grow on them) and “active” (flocculant agents, chemical attachment, and gel encapsulation (5). Different types of active and passive immobilization method and their characteristics are presented in Table 1. Passive Immobilization Fimbria (pili), capsules (glycocalyx), various holdfast structures, stalks, cell wall component and slimes are variety of structures are responsible for natural attachment of microorganism to a surface. Besides, several other forces are also responsible for passive immobilization: electrostatic interactions, covalent bond formation, hydrophobic interactions (6). Ionic and hydrogen bonding are the most common electrostatic interactions involved in the initial stage of adsorption. Although being individually weak as compared to covalent bonds, they are nevertheless capable of producing relatively firm binding, if number of bonds is sufficiently large (4). 322 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
323
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Table 1. Different Type of Immobilization Methods and Their Characteristics (3, 6) Diffusion restriction
Immobilization Methods
Biomass loading
Toxicity problems
Ease of application
Mechanical stability
Carrier binding Physical adsorption
No
Low
Low
Reversible immobilization
Covalent bonding
No
Low
severe toxicity
High
Simple
Covalent cross linking
No
Moderate
severe toxicity
High
Simple
Micro encapsulation
Relatively low
High
severe toxicity
Low
Complex
Membrane separation/ entrapment
Fouling can cause severe diffusion restrictions
High
No
High
Complex
Entrapment within polymers
Relatively high: varies with the polymer material kind and construction
High
Moderate
Varies greatly with the kind of polymer
Simple
Simple
Examples of carrier/ matrix support Wood, sawdust, polygorskite, montmorilonite, hydromica, porous porcelain and porous glass treated with polycations
Entrapment in a matrix
In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
Alginates, k-carrageenan, agar, agarose, polyacrylamide, polyvinyl alcohol, polyethylene glycol, polycabamoyl sulphonate, microporous membrane filter
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In general, covalent bonds are formed between organic groups located on the bedding materials and microbial ligands (large quantities of different reactive groups present at the outer surface of microbial cells and cell walls) (7). Hydrophobic interactions between the microorganisms and the bedding material are very important as attachment of microorganism and bedding materials takes place in aqueous environment. The water molecule (separator between microorganism and bedding material’s surface) gets removed making possible interaction of hydrophobic group from both microorganism and bedding material. It has been reported that hydrophobic surfaces are more prone to colonization of dental and soil bacteria than are their hydrophilic counterparts (8). Sometime partial covalent bonding between the microbial cells and the bedding material play important role in immobilization. Often, in presence of aqueous solution hydroxides are formed instead of metal oxides incorporated in the bedding material and suitable amino or carboxyl groups on the cell surface can replace those hydroxyl groups (6, 9).
Active Immobilization Methods of active immobilization of microorganisms involve microencapsulation, membrane separation, covalent bonding and crosslinking, and entrapment within polymers. It is a kind of artificial immobilization of microorganism using inherent or induced surface properties of carriers/bedding materials and also of targeted microorganisms.
Carriers Used in Immobilization Both natural and synthetic carriers are used for immobilization of viable microbial cells (Table 2). However, the selection of carriers depends on various factors; it should have good mechanical strength, light weight, flexibility in overall shapes, nontoxic, nonbiodegradable in test conditions as well as cost effective. A comparison between natural and synthetic polymers is presented in Table 3. In general, there are two types of carrier materials used in immobilization of microbial cells :organic and inorganic.
Organic Carriers Mostly, organic carriers have a higher absorptivity compared to inorganic materials. Presences of larger varieties of reaction groups such as carboxyl, amino, hydroxyl on organic carriers are responsible for additional adsorption capacity. Often entrapment of microorganisms is carried out using organic polymers that are more abundant than inorganic materials. Organic polymers can be grouped in two sub categories: natural and synthetic polymer and both of them should be hydrophilic in nature, so that substrates can diffuse into the polymer beads during treatment of wastewater stream (14). 324 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
Table 2. Examples of Different Natural and Synthetic Carriers Used to Immobilize Microorganisms for Use in Biodegradation (2, 10–13) Carrier material
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Agar
Targeted compounds/ reaction
Microorganisms
Inorganic cyanides
Pseudomonas putida
Acrylamide
Pseudomonas sp., Xanthomonas sp.
3-Chloroaniline
Pseudomonas acidovorans CA28
4-Chloro-2-nitrophenol
Mixed culture
Dichloroacetic acid
Xanthobacter autotrophicus
Inorganic cyanides
Pseudomonas putida
p-Cresol
Pesudomonas sp.
Pentachlorophenol
Flavobacterium sp.
Phenol
Candida sp., Pseudomonas sp., Fusarium flocciferum,
Pyridine
Pimelobacter sp.
n-Valeric acid
Alcaligenes denitrificans
Sodium cyanide
Pseudomonas putida
Tertiary treatment of municipal wastewater
Chlorella sorokinian Azospirillum brasilens
Tributylin
Cholorella sp.
Triethyl amine
Arthobactor protoprotophormiae
Diatomaceous earth pellets
Glyphosate
Mixed culture
Diatomaceous earth biocarrier
p-Nitrophenol
Pseudomonas sp.
4-Chlorophenol
Alcaligenes sp. A 7-0, A 7-1, A 7-2
Cyanuric acid
Pseudomonas sp NRRL B-12228
Hydrocarbons
Candida parapsilosis
Granular clay, slag of lava
PAHs
Mixed culture
k-carrageenan
Inorganic cyanides
Pseudomonas putida
Sodium dodecyl sulfate
Pseudomonas C12B
Volatile fatty acids
Alcaligenes denitrificans
Alginate
Granular clay
Polyacrylamide
Continued on next page.
325 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
Table 2. (Continued). Examples of Different Natural and Synthetic Carriers Used to Immobilize Microorganisms for Use in Biodegradation (2, 10–13) Carrier material
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Polyurethane
Targeted compounds/ reaction
Microorganisms
Aliphatic and polycyclic aromatic hydrocarbons
Prototheca zopfii
Chlorophenol
Mixed culture, Rhodococcus sp.
Pentachlorophenol
Flavobacterium sp.
Dechlorination of spent sulphite bleach effluents
Streptomycetes sp.
Table 3. A Comparison between Natural and Synthetic Polymers Used for Immobilization of Microbial Cells (2, 3, 6) Characteristics
Polymers
Desired criteria
Natural
Synthetic
Solubility
Low
High
Low/not
Biodegradability
Low
Medium
Low
Stability
High
Low
High/Medium
Diffusivity
High
High
Medium/Low
Growth
Possible
Good
Good/Moderate
Immobilization procedure
Simple
Simple
Laborious
Toxicity
Non-toxic
Non-toxic
Depends largely on chemical composition and methods of preparation
Cost
Low
Low
Moderate/High
Carrageenan, Ca-alginate, agar, cellulose
Polyvinyl alcohol (PVA), polyethylene glycol (PEG), polycarbomoyl sulphonate (PCS), polyacrylamide (PAM)
Examples
Alginate, chitosan, agar, collagen, agarose and carrageenan are common examples of frequently used natural organic carrier materials for immobilization of microorganisms. The source of these polysaccharides is mainly algae and is prepared by gelation of soluble polymers either by cooling and/or in presence of a solution that contains different ions (6). 326 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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Synthetic polymers can be prepared by gelation of monomers using a wide range of chemical or photochemical reactions. These polymers are not easily biodegradable and have much better mechanical performance compared with natural polymers. However, diffusivity is considered to be lower in synthetic polymers. Synthetic gel such as polyacrylamide (PAM), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polycarbonyl sulphonate (PCS) and synthetic plastics like polyacrylonitrile (PAN), polyethylene (PE), polypropylene (PP), polyurethane (PU), and polyvinyl chloride (PVC) have been explored significantly as carrier materials (3).
Inorganic Carriers Inorganic carries are generally selected to immobilize microorganisms by electrostatic attachment between the cells and the carrier materials. Inorganic carrier materials are not only cost effective but also can resist to microbial degradation and have good thermostability performance. Porous glass, ceramics, clay, activated charcoal, anthracite, zeolite are most frequently used inorganic carriers (3). It has also been found that incorporation of specific metals into the carrier material allows better attachment of microbial cells to carrier materials. For example, incorporation of Fe (II) within a bedding material can increase the binding of yeast cells almost by 50% (9).
Mechanism of Pollutant Removal from Wastewater The main pollutant removal mechanism by microorganisms (e.g. bacteria, fungi, algae) include assimilation, biosorption and biodegradation. Ingestion of unwanted nutrients such as nitrogen, phosphorus, carbon from wastewater by microorganisms and or microbial aggregates and use of the same for its own growth is termed as assimilation. The complex surface structures of (e.g. branched, filamentous, spherical, oval, mushroom sheet) and presence of different functional groups (e.g. bacterial surfaces contains carboxyl, phosphoryl, hydroxyl, and amino functional groups) help microorganisms/microbial aggregates to adsorb heavy metals, organic and inorganic materials. This phenomenon is defined as biosorption and can occur through various processes such as complexation, chelation, co-ordination, ion exchange, flocculation and/or precipitation, reduction. Biodegradation is defined as the decomposition/chemical disbanding of organic materials by microorganisms/microbial aggregates. Often, biodegradation is associated with a complex series of biochemical reactions and targeted compound can be degraded aerobically and/or anaerobically. However, often assimilation, biosorption and biodegradation occur simultaneously during the removal of pollutant (15). 327 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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Application of Immobilized Algae, Bacteria, and Fungi for Wastewater Treatment Immobilization of algae/microalgae for use in variety of biotechnological application started over 40 years ago. To keep living cells within a matrix metabolically active as long as possible and with limited mobility during its functioning are considered as main objectives of immobilization of algal cells. In most cases, entrapped algae get benefited from immobilization. Apart from avoiding grazing by aggressive zooplankton, reduction in competition for nutrients with other microbial species, several improvements in the metabolism, function, and behavior are observed in immobilized algae (5). A few examples of immobilized algae/microalgae used for removal of diverse kind of pollutants from wastewater are presented in Table 4.
Table 4. Examples of Immobilized Algae Used for Removal of Different Kind of Pollutants from Wastewater (5, 11, 16, 17) Immobilizing matrix
Alginate
Targeted pollutants
Algae species
Nitrogen
Chlorella vulgaris, Chlamydomonas reinhardtii
Phosphorus
Dunaliella salina, Nanochloris sp.
Nitrogen and phosphorus
Anabaena doliolum, Scenedesmus obliquus, S. intermedius
Tributylin (TBT)
Cholorella sp.
Phenanthrene, fluoranthene and pyrene
Selenastrum capricornutum
Cadmium
C. vulgaris, Tetraselmis chuii
Cesium
Chlorella salina
Cobalt Copper
Chlorella vulgaris, Tetraselmis chuii
Gold
Chlorella homosphaera
Lead
Chlorella vulgaris, Chlamydomonas reinhardtii
Manganese
Chlorella salina
Nitrogen and phosphorus
Scenedesmus quadricauda, C. Kessler, Spirulina maxima
Cadmium
Chlorella vulgaris, Scenedesmus acutus
Nitrogen and phosphorus
Phormidium laminosum
Carrageenan
Cellulose fibbers
Continued on next page.
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Table 4. (Continued). Examples of Immobilized Algae Used for Removal of Different Kind of Pollutants from Wastewater (5, 11, 16, 17) Immobilizing matrix Loofa sponge
Targeted pollutants
Algae species
Lead
Chlorella sorokiniana
Nickel
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Copper Polyacrylamide
Lead
Chlorella regularis
Zinc Uranium Mixed hydrocarbons
Polyurethane
Polyurethane foam
Prototheca zopfii
n-alkanes Cobalt
Scenedesmus obliquus
Cadmium
Chlorella vulgaris, Scenedesmus acutus
Zinc Polyvinyl alcohol Silica gel
Copper
Sargassum baccularia
Cadmium
Spirulina platensis
Mercury
Chlorella vulgaris
Use of immobilized bacteria is also an important aspect in the field of biological wastewater treatment. There are now plenty of evidences (Table 5) that bacterial cells immobilized onto or into carrier matrix possess severe beneficial properties over suspended free cells, particularly high viability and ability to withstand environmental stresses and increased catalytic activity (31).
Table 5. Example of Immobilized Bacteria Used for Removal of Different Kind of Pollutants from Wastewater Bacteria
Type of pollutants
Immobilizing matrix
Reference
Arthobacter protophormiae
Triethylamine
Alginate
(12)
Mixed culture
Coumaphos, chlorferon, diethylthiophosphate
Alginate
(18)
Pseudomonas putida
Benzene, Toluene, o-xylene
Agave fiber/ polymer
(19)
Continued on next page.
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Table 5. (Continued). Example of Immobilized Bacteria Used for Removal of Different Kind of Pollutants from Wastewater
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Bacteria
Type of pollutants
Immobilizing matrix
Reference
Paracoccus sp. Strain KT 5
Pyridine
Bamboo based activated carbon
(20)
Ochrobacterium sp. DGVK1
Dimethylformamide
PVA-alginate
(21)
Klebsiella oxytoca
Propionitrile
Alginate, cellulose triacetate
(22)
Bacilus cereus
Phenol
Alginate
(23)
Pseudomonas sp.
Azo dyes
Sol-gel silica
(24)
Microbe B350
Cu (II)
Polyurethane
(25)
Bacillus strain CR-7
Pb (II), Al (III), Cr (VI), Cu (II), Fe (III), Zn (II), Ni (II), Cd (II), Co (II), Mn (II)
Alginate
(26)
Cupriavidus sp., Sphingobacterium sp., Alcaligenes sp.
Zn, Cd
Alginate, pectate, synthetically cross linked polymer
(27)
Arsenic oxidizing bacteria (AOB)
As (III)
Polyvinyl alcohol
(28)
Bacillus sp., Pseudomonas sp., Serratia sp.
Hg, Cr, Ni
Alginate, polyacrylamide
(29)
Pseudomonas putida YNS1
Cu, Cd, Phenol
Alginate-silica
(30)
Fungi can produce a large variety of extracellular proteins, organic acids and other metabolites and also capable of adapting severe environmental stresses. All these characteristics recognize fungi as potential microorganism for treatment of industrial wastewater. However, in terms of application ease and effectiveness, a free fungal cell shows severe drawbacks as fungus mycelium are too exposed to environmental stresses. Consequently, a good alternative could be immobilization of fungal biomass on suitable support with the aim to maintain its viability and improve its activity (32). Some examples on use of immobilized fungi for treatment of different kind of pollutants are presented in Table 6. 330 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
Table 6. Examples of Immobilized Fungi Used for Removal of Different Kind of Pollutants from Wastewater
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Fungi
Type of pollutants
Immobilizing matrix
Reference
Yarrowia lipolytica
Oil, COD
Ca - alginate
(33)
Geotrichum candidum
Phenol and COD
Ca - alginate
(34)
Phanerochaete chrysosporium
Phenolic compounds
Wood chips op Italian poplar
(35)
Trametes pubescens, Pleurotus ostreatus
Industrial and model dyes
Polyurethane foam
(36)
Cunninghamella elegans
Synthetic dye
Ca - alginate
(37)
Phanerochaete chrysosporium
Remazol brilliant blue R
Loofa sponge
(38)
Aspergillus niger
Cu (II), Cd (II)
Poly vinyl alcohol
(39)
Trichoderma viride
Cr (VI), Ni (II), Zn (II)
Ca - alginate
(40)
Trochoderma harzianum
U
Ca - alginate
(41)
Polyporus squamosus
Cr, Mn, Fe, Ni, Cu, Pb
Ca - alginate
(42)
Phanerochaete chrysosporium
Pb (II)
Loofa sponge
(43)
Rhizopus cohini
Cr (VI)
Saw dust, polyurethane, alginate
(44)
Suitability of Immobilized Microbial Cells for Use in Bioreactors User’s flexibility to design and manufacture immobilized microbial matrix of different size and shapes with desired cell density make it possible to use immobilized microorganisms as a discrete phase in the bioreactor and also to decouple their hydrodynamic behavior from other existing phases. Thus, a wide range of bioreactor configurations including stirred tank, packed bed, fluidized bed, gas agitated and membrane bioreactor can be used with immobilized microorganisms (Table 7). 331 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
Table 7. Example of Immobilized Microbial Cell Bioreactors
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Design of bioreactor
Type of pollutants
Immobilizing matrix
Microorganism
Reference
Fluidized bed
Phenol
Alginate
Pseudomonas putida
(45)
Membrane
Cr (VI)
Agar-agar
Pseudomonas sp.
(46)
Fluidized bed
Azo dyes
polyvinylalcohol
Aeromonas hydrophila, Comamonas testosterone, Acinetobacter baumannii
(47)
Packed column
Cr (VI)
Sawdust, polyurethane and alginate
Rhizopus cohnii
(44)
Fluidized-bed bioreactor
Aliphatic and PAHs
sawdust, polyvinyl alcohol, polyacrylamide
Rhodococcus sp.
(31)
Stirred tank reactor
Anthraquinone dye drimarene blue K2RL
Scotch-Brite
Aspergillus niger SA1
(48)
Continuous flow column reactor
Propionitrile
Alginate, cellulose tri acetate
Klebsiella oxytoca
(22)
Airlift bioreactor
Dimethylsulfoxide
polyvinylalcohol
Pseudomonas sp. W1
(49)
Continuous packed bed column
Cr (VI), Ni (II) and Zn (II)
Ca - alginate
Trichoderma viride
(40)
Continuous packed bed reactor
N,N-dimethylformamide
Polyvinyl alcoholalginate blend
Ochrobactrum sp. DGVK1
(21)
Batch and fluidized bed column reactor
Cr, Ni, Cu, Cd
Alginate
Yeast, Pseudomonas aeruginosa, Bacillus subtilis, Escherichia coli
(50)
Other advantages of use of immobilized microorganism in bioreactor includes high biomass loading, continuous reactor operation without risk of cell washout, protection of cells from toxic substrates, reaction selectivity, increased biodegradation rate and enhanced operation stability (3, 51). 332 In Water Challenges and Solutions on a Global Scale; Loganathan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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Conclusion Today wastewater is a worldwide concern as most wastewaters are hazardous to living being and environment and must be treated properly before it ends into streams, lakes, seas and land. Biological agents like bacteria, fungi and algae play an important role in wastewater treatment and found very effective in degradation and or attenuation of a wide range of pollutants in wastewater. In this context, immobilization of microscopic biological agents offers several advantages over freely suspended cells in biological treatment system. Easy separation of cells from reaction media, reuse of active cells, minimum wastages of cells, higher volumetric reaction rates because of higher local cell concentration are most important features of biological wastewater treatment with immobilized cells. Additionally, immobilization of viable cells increases the stability of a microbial system, allowing its application under extreme environmental conditions, and development of continuous bioprocesses, avoiding release of contaminants during biodegradation process. However, still we are in very nascent stage of large scale application of immobilized microorganisms for wastewater treatment and more interdisciplinary effort should be given to develop efficient and cost-effective mean for waste water treatment through immobilized cells.
Acknowledgments Authors are very grateful to Dr. R.K. Pachauri, Director General, TERI, for providing the infrastructure and environment for preparation of the manuscript. We apologies to all those researchers whose findings we overlooked or could not include because of accessibility limitations.
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