A review on hydrodynamic parameters and biofilm characteristics of inverse fluidized bed bioreactors for treating industrial wastewater
Introduction
The present problem of water scarcity enhanced by the discharge of untreated industrial effluents into the freshwater system may further arise the issues related to the production of food grains, expansion of industries, conflicts of risk, environment, and human health. Because of this, the treatment of industrial effluents has become a significant concern around the globe [1]. Preliminary, primary, secondary, and tertiary treatments are generally applied processes for treating the industrial wastewaters by incorporating the different physical unit operations and chemical & biological unit processes while treating [2]. Since, the secondary treatment (biological) process is effective in eliminating the suspended solids, fine particulates, and soluble colloidal organics from the wastewater effluents through the application of bacteria and microorganisms, and it has gained the significant attention [3] and become a successful treatment process amongst the existing different treatment process like physical, chemical, and biological types in the recent past [4]. Aerobic or anaerobic microorganisms are generally used in the biological treatment process of industrial effluents for reducing the organic content. In the anaerobic treatment process, complex organic molecules will be converted into simpler end products, and the oxygen plays the role of electron acceptor in this process, whereas in the aerobic treatment process, complete degradation of organic matter takes place. The energy needed for the growth of microorganisms is liberated from the metabolic reactions, and the organic matter is degraded by these microbial mass [5].
At lower concentration of pollutants, feeble amounts of energy is released in the contaminant degradation during the microorganism's metabolic reaction, and toxicity associated with the contaminants are the major challenges faced during the biological treatment of organic and inorganic environmental contaminants. However, process such as Fluidized-bed bioreactor (FBBR) has overcome the majority of these challenges and has proven efficient for a wide range of applications [6]. Currently, the utilization of fluidized bed bioreactors has created more interest in the biological wastewater treatment sector, and its importance in many applications of environmental engineering has increased tremendously since from 1990 onwards [6]. Fluidized bed bioreactors are traditionally operated in up-flow type, for either two-phase systems (gas-solid, liquid-solid), or three-phase systems (gas-liquid-solid phases) [7]. Excellent mass transfer is achieved in gas-liquid-solid fluidization systems, because of the intimate contact between the solid, liquid, and gas
phases of the systems, attained in either co-current or counter-current mixing of particles in the system [8]. The solid biomass carriers are suspended in fluidized bed bioreactors (FBBRs) at high fluid or gas flow rates allowing fluidization of the carrier particles [6]. Winkler F. has developed the Fluidized bed reactors (FBR) initially, for the generation of gas in the 1920s, and afterwards, FBR has expanded its application into various other fields like gasification of coal, refining of metal, catalytic cracking, powder technology, processing of food, etc. [9].
Due to the latest developments in the areas of implementation, mainly biochemical processing and wastewater treatment, fluid fluidization techniques have received more attention [10] and proven to be an effective reactor in biological processes [11]. Now it has gained importance in biotechnology also, where bacteria or enzymes are entrapped within porous particles or immobilized on the surface of the inert solids [12]. However, FBBR was accepted as a feasible reactor for the treatment of wastewaters biologically, only in the early 70 s. Much progress has been made in the following years, and full-scale FBBRs have been created by 1984 [13]. A remarkable breakable breakthrough was achieved in wastewater treatment by the implementation of fluidized bed bioreactor method. When compared to conventional treatment processes, fluidized bed media provided the higher surface area and made it as one of the successful processes in biological wastewater treatment [14]. As a replacement for standard suspended and fixed-film wastewater treatment procedures, the fluidized bed biological reactor has attracted significant attention due to its better performance and improved efficiency [15].
Fluidized bed bioreactor has some excellent features such as low operating cost [16], high tolerance to system upsets [17], preferred in many industries because of simple design and construction, high mass transfer rates as a result of good mixing [12]. Due to the large specific area of support particles available for biomass retention, this technology offers several advantages in high-strength effluent treatment by using reduced spaces and shorter hydraulic retention time [18]. A fluidized bed biological reactor (FBBR) has attracted considerable interest as an alternative to the conventional suspended growth and fixed-film wastewater treatment processes due to its high-efficiency performance [15]. The superior performance of the FBR systems from the very high-biomass concentration (30–40 kg m−3) that can be achieved due to the immobilization of cells onto and/or into the solid particles [14]. Due to the increased interest in the wastewater treatment by the FBR, the excellent features of the FBR are explored well, and it has shown better results in terms of improving the cost-effectiveness and higher cost-effectiveness for different wastewater treatment applications, especially with reference to the biological treatment approaches. For implementing the zero-discharge method for minimizing the cost of the process and also to achieve environmental sustainability, a cost-effective methodology for wastewater treatment is essential, particularly for the industrial effluents. The studies on the FBR were mainly focused on evaluating the performance of the process in conjunction with hydrodynamic parameters, and some of them include the application of FBR for the treatment of wastewater effluents [11].
Narayana, in 2019, [19], has reviewed the design features and performance characteristics of semifluidized bed bioreactors by considering the three-phase (gas–liquid–solid) and two–phase (liquid–solid) operations. This review also included the survey of hydrodynamic parameters (semifluidization velocity, bed expansion ratio, the height of packed section formed, fractional fluid holdups in both fluidized and packed sections) of biofilm reactors and immobilized enzyme bioreactors. Wang et al., in 2019, [20], has summarized the progress in the past several decades concerning Liquid-Solid Circulating Bed Reactor (LSCFBs) and Gas-Liquid-Solid Circulating Bed Reactor (GLSCFBs), with more focus on the reported applications, and they restricted their discussion on experimental models that are still in laboratory-scale. Swain et al., in 2018, [21], reported that the hydrodynamic parameters which affect the FBR performance for the treatment of industrial effluents are commonly phase holdup, pressure drop, phase flow rates, minimum fluidization velocity, friction factor, bubble size and rise velocity; mass & heat transfer coefficients; residence time distribution; and settled bed to bioreactor volume ratio. It has also been reported that the major hydrodynamic and process parameters which affect the performance of AIFBBRs are hydraulic retention time, bio-carriers, aspect ratio, superficial gas velocity, superficial liquid velocity, bed expansion, mass transfer characteristics [8], biomass thickness on the carrier particles, pollutant concentration in the influent, nutrients and type of biomass used for the treatment, operation time, temperature, and pH conditions [21]. In the recent past, majority of the FBR studies in wastewater treatment reported the application FBR-Fenton and FBBR, which is giving an essential indication of the FBR application in the wastewater treatment process [11].
The present study aims to explore the IFBBR's potential and to identify the uncovered areas where the future research work can be focused further. Hence, the discussion is specifically concentrated on the design as well as process aspects of IFBBR, and the same are furnished in the respective sections. The discussion in the following sections includes the basic concept related to AIFBBR, development of the process, and also the technological constraints that are currently faced by the AIFBBR. It also provides an overview of the application of IFBR technology for wastewater treatment in the recent past, and the prerequisites such as design and operational parameters that are required for the successful application of the IFBBR process are provided here within. Since the application of fluidization has a wide reception and huge amount of literature available on various aspects of the process, thus the discussion was kept within the limited reasonable proportions of the process. The review was focused entirely on gas-liquid-solid FBRs/IFBBR/AIFBBR/TIFBBR and their respective applications in the treatment process of wastewater. Hence, throughout the paper, FBR means the gas-liquid-solid process applied to the treatment of wastewater. The complete review of the present study include the following aspects:
- i
Basics aspects related to IFBBR, different types, and their advantages.
- ii
Major design parameters related to the reactor process and their hydrodynamics.
- iii
Overview of the recent application/trends of IFBBR in COD removal of different industrial wastewater
- iv
Information related biofilm formation, the effect of detachment forces on biofilm, measurement of biofilm thickness, biomass concentration, mean biofilm thickness, dry biofilm density, thickness of oxygen penetration, and detachment rate coefficient.
- v
The aspects related to future research were included at the end of the discussion.
Section snippets
Classification of fluidized bed bioreactors
Solid particles in the two or three-phase fluidized systems are fluidized in the opposite direction of gravity by either liquid or gas stream flow [22]. Based on the direction of fluidization, FBRs (Fig. 1.) are classified as up-flow (conventional/classical) FBBRs and downflow (inverse) FBBRs [11]. If the density of the solid bed media particles used for the fluidization is more than that of the fluid density, then it is termed as a classical or conventional type of fluidization. Whereas, in
Design strategy for FBBR
The design strategy of an FBBR has been described in Fig. 4. The carriers were first considered in their original state and then managed the controllable variables to correct the influence of biofilm growth over the hydrodynamic behaviour of the carrier material [32], as explained in Fig. 4.
Importance of biofilm carrier materials in FBR for treating the wastewater
The aim of the selection and designing of bio carrier material is to achieve more surface area per unit volume on which microorganisms can grow well for reducing the pollutant concentration effectively [63]. Bio carrier materials are defined as the solid particles utilized for the biofilm formation on their surface. Haribabu and Sivasubramanian, 2014 [29], tried a new bio carrier material with a density of 870 kg m−3 and achieved a maximum COD removal of 97.5 % with an initial bed height of
Recent trends in the removal of COD from wastewater using inverse fluidized bed bioreactor (IFBBR)
Inverse fluidized bed bioreactor has achieved the wider implementation in the wastewater treatment process because of the following advantages like consumption of low energy, lesser pressure drops, efficient gas holdup, mass and heat transfer at a higher rate and also the most recent developments in the IFBBRs have provided enough encouragement for implementing the IFBBR in different industrial applications such as biotechnological, environmental and chemical [5]. Even though FBBR is a
Importance of Biofilm thickness, formation, and effect of detachment forces for treating wastewaters in IFBBR system
Particles that are having irregular shapes and surfaces, sharp angles, and crevices are favourable for the growth of biomass and its growth, and they will play an important role in the FBBR process. Hence, particles that are having more specific areas, better physicochemical, and fluid dynamic characteristics are utilized as supporting bed media particles. Generally, materials like Silica, quartz sand, granular activated carbon, vitreous coke, glass beads, and PVC are used as supporting bed
Biomass concentration (g AVS/g solid)
For the calculation of suspended biomass, the dry weight method is use and total suspended biomass expressed as g L−1, but in the case of attached biomass concentration, the rise in the attached dry volatile solids over the polystyrene bed media particle is measured. The dry volatile solids (attached over the bed media) are calculated by collecting the 40–50 number of bioparticles from the reactor top portion that are covered with the biofilm, afterwards, they were air-dried for 2 h at 110 ℃,
Disadvantages of fluidized bed bioreactor
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The biofilm thickness restricts fluidized bed reactor applications in wastewater. Due to the increased biofilm thickness, during the multiplication process of microorganisms causes problems such as restriction of oxygen diffusion and/or the organic substances to the internal layers of the biofilm, subsequently results in microorganism starvation at the base of the biofilm and removal of the biofilm particles from biofilm base and causes the failure of bioreactor process [44].
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A maximum of
Conclusions and future perspectives
In the current discussion, aspects of the design parameters like Hydraulic retention time, bio carriers, reactor aspect ratio, superficial gas velocity, superficial liquid velocity, bed expansion, and mass transfer coefficient and the biofilm characteristics like biomass concentration, mean biofilm thickness (Lf), dry biofilm density (Xf), biofilm thickness (δ), the thickness of oxygen penetration (Zo) and the detachment rate coefficient (bs) are furnished and thoroughly discussed. Both design
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are thankful to the School of Infrastructure, Indian Institute of Technology Bhubaneswar, India, for providing facilities to carry out the research work.
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