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Journal of Cleaner Production 141 (2017) 1492e1514
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Review
Applications of fluidized bed reactors in wastewater treatment e A review of the major design and operational parameters Mustapha Mohammed Bello a, Abdul Aziz Abdul Raman a, *, Monash Purushothaman b a b
Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, 50603, Malaysia Department of Chemical Engineering, School of Civil and Chemical Engineering (SCALE), VIT University, Vellore, 632014, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2016 Received in revised form 11 August 2016 Accepted 18 September 2016 Available online 20 September 2016
One of the current challenges of wastewater treatment is the presence of recalcitrant pollutants which are difficult to remove using conventional treatment technologies. This poses a threat to environmental sustainability and hinders the efforts of many industries to adopt cleaner production through zerodischarge and subsequent wastewater reuse. Effective wastewater treatment technologies are therefore needed to address this challenge. Accordingly, the last few years have seen intensified effort to develop more effective wastewater treatment technologies. The use of fluidized bed reactor in wastewater treatment, particularly Advanced Oxidation Processes and biological treatment, represents a unique opportunity for cost-effective treatment of wastewater containing recalcitrant pollutants. Although the application of fluidized bed reactor in biological wastewater treatment is well established with many large-scale plants in existence, its application in advanced oxidation processes is mostly at laboratory-scale. For proper design, upscaling and process improvement, information on the major parameters affecting the processes is important. This paper offers an overview on the applications of fluidized bed reactor in wastewater treatment, with emphasis on the important design and operational parameters affecting its performance. The discussion covers liquid-solid and gas-liquid-solid fluidized bed reactors and their applications in advanced oxidation processes, biological as well as adsorption processes which are effective wastewater treatment technologies. Fluidized bed reactors are excellent contacting devices and have the potential to enhance the effectiveness and energy efficiency of these treatment processes if properly design and utilized. An energy efficient and cost-effective wastewater treatment technology is crucial to industries adopting cleaner production. Important parameters such as reactor geometry, aspect ratio, support materials, reactor internal, superficial fluid velocity and other operational parameters are reviewed. The review concluded with some perspectives on future research interests. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Fluidized bed reactor Fluidized bed Fenton Fluidized bed bioreactor Advanced oxidation processes Adsorption Operational parameters
Contents 1.
2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1493 1.1. Fluidization and fluidized bed reactor (FBR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1494 1.2. Classifications of FBR for wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495 1.2.1. Two-phase versus three-phase FBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495 1.2.2. Upward-fluidization versus downward/inverse-fluidization in FBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495 Applications of FBR in wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495
Abbreviations: AOPs, Advanced oxidation processes; BPA, Bisphenol A; COD, Chemical oxygen demand; DMSO, Dimethyl sulfoxide; DTFBR, Draft tube fluidized bed reactor; FBR, Fluidized bed reactor; FB-Fenton, Fluidized bed Fenton; FB-Photocatalysis, Fluidized bed photocatalysis; FB-Adsorption, Fluidized bed adsorption; FBBR, Fluidized-bed bioreactor; HLR, Hydraulic retention time; IFBBR, Inverse fluidized bed bioreactor; IFBR, Inverse fluidized bed reactor; IFAFB, Integrated flocculation-adsorption fluidized bed; LDPE, Low density polyethylene; MO, Methyl orange; OH, Hydroxyl radicals; OLR, Organic loading rate; OM, Organic matter; PC, Phthalocyanine; PP, Polypropylene; PZC, Point of zero charge; RB, Rhodamine B; RB13, Reactive blue 13; TOC, Total organic carbon; UV, Ultraviolet; VSS, Volatile suspended solids. * Corresponding author. E-mail addresses:
[email protected] (M.M. Bello),
[email protected] (A.A. Abdul Raman),
[email protected] (M. Purushothaman). http://dx.doi.org/10.1016/j.jclepro.2016.09.148 0959-6526/© 2016 Elsevier Ltd. All rights reserved.
M.M. Bello et al. / Journal of Cleaner Production 141 (2017) 1492e1514
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2.1.
3.
4.
5.
FBR-AOPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495 2.1.1. Fluidized bed-Fenton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496 2.1.2. Fluidized bed-photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1497 2.2. Fluidized bed bioreactor (FBBR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1499 2.3. Fluidized bed-adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1501 Effects of design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1501 3.1. Reactor geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1503 3.1.1. Shape and cross-sectional area of the reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1503 3.1.2. Aspect ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1503 3.1.3. Reactor internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505 3.2. Support material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505 3.2.1. Particle size and surface property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505 3.2.2. Particle loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506 3.2.3. Particle density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506 3.3. Superficial fluid velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1507 Effect of operational parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1507 4.1. pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1507 4.2. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1508 4.3. H2O2 concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1508 4.4. Fe2þ concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1508 4.5. Photocatalyst concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1508 4.6. UV intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509 4.7. Hydraulic retention time (HRT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509 4.8. Organic loading rate (OLR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509 Conclusions and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1509 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1510 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1510
1. Introduction There is consensus that more effective wastewater treatment technologies are needed for the removal of recalcitrant pollutants that are increasingly encountered in both domestic and industrial effluents. This is largely due to the need for environmental protection on one hand, and the need to have cost-effective wastewater treatment technologies on the other. Driven by these reasons, industries have intensified efforts to adopt cleaner production using strategies such as zero-discharge (Tabassum et al., 2015), process modifications (Zhang and Wang, 2015) and other appropriate methods. Adopting zero-discharge through wastewater reuse is attractive (Othaman et al., 2014) as it can lower production cost and ensure environmental sustainability. Unfortunately, conventional wastewater treatment technologies are not effective in degrading recalcitrant pollutants which are hydrophobic and generally of high molecular weight (Pouran et al., 2014). Thus, the last few years have seen increased research towards developing treatment technologies that can degrade recalcitrant pollutants while meeting the cost-effectiveness needed by industries. Among the possible technologies for treating recalcitrant wastewater, Advanced oxidation processes (AOPs) have received wide attention. These technologies are based on the generation of powerful oxidants through various processes as put forward by Glaze and coworkers (Glaze et al., 1987). The most common oxidant is the hydroxyl radical (OH), a powerful and non-selective oxidant with a redox potential of 2.8 eV that can effectively degrade organic pollutants. Another attractive technology is biological treatment which is widely used to treat both domestic and industrial wastewaters. While interest on AOPs is due to their effectiveness in degrading recalcitrant pollutants, biological processes are considered inexpensive and eco-friendly. In either case, an effective contacting device is essential for proper application of the technology.
Fluidized bed reactor (FBR) has proven to be an effective reactor in the applications of both AOPs (Tisa et al., 2014) and biological processes (Zou et al., 2016). Some of the excellent features of FBR include low operating cost (Ahmadi et al., 2015), high resistance to system upsets (Brackin et al., 1996), high mass transfer rates and uniform mixing (Andalib et al., 2014). Many researchers have investigated the applications of FBR in wastewater treatment, particularly AOPs and biological processes. Although the application of FBR in AOPs is relatively new, FBR has been extensively used in biological wastewater treatment, with many large-scale fluidized bed bioreactors (FBBRs) in existence. To derive an apposite support for the review, some relevant studies are highlighted here. It is pertinent to note that most of the studies on FBR have been on its applications in areas such as combustion, gasification, catalytic processes and other more established processes. For example Corella et al. (2007) reviewed the application of FBR in biomass gasification, the so called “dual” fluidized bed biomass gasifier. Abdelmotalib et al. (2015) reviewed the heat transfer in gas-solid fluidized bed combustors, discussing the effect of operating parameters on the heat transfer. Similarly, Singh and Kumar (2016) reviewed the current status of oxygenfired fluidized bed combustion. Such literature emphasized gassolid FBR, which is not applicable to wastewater treatment. The earliest studies on FBR applications in wastewater treatment were mainly on aerobic oxidation and denitrification. With increased interest in anaerobic process, studies on the use of FBR in anaerobic wastewater treatment with concomitant methane generation were reported in the early 80s. Heijnen et al. (1989) presented a state of the art review on the application of anaerobic FBBRs in wastewater treatment. The review discussed the basic concept of anaerobic FBBR, process development and the challenges facing the technology. Studies reported around that time were mainly on process performance. Converti et al. (1990) studied the performance of FBBR in anaerobic treatment of wine wastewater containing high COD and proposed a kinetic model for the process. Similarly, Borja
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et al. (1995) evaluated the kinetic reaction of an FBBR treating slaughterhouse waste with concomitant methane generation. Recently, Haribabu and Sivasubramanian (2016) studied the biodegradation of domestic wastewater using FBBR and achieved a COD removal of 96.7% under optimum condition. Wang et al. (2016) conducted anaerobic digestion of primary sludge (PS) and thickened waste activated (TWAS) using FBBR and reported that the system performed better than conventional anaerobic processes. A high-rate autotrophic denitrification using FBBR was reported where complete nitrate removal was achieved at a hydraulic retention time (HRT) of 10 min (Zou et al., 2016). However, the earliest studies on the application of FBR in AOPs were reported in the late 90s (Chou and Huang, 1999). Currently, there is growing interest in the applications of FBR in wastewater treatment, particularly in AOPs where it has shown potential in addressing some of the drawbacks of Fenton oxidation (Chen et al., 2016) and improving the performance of photocatalysis (Shet and Shetty, 2016). Anand et al. (2015) investigated the performance of a fluidized bed solar photo Fenton for the treatment of hospital wastewater and achieved 98% COD removal at HRT of 90 min. The process achieved 92% COD removal at 60 min HRT compared with 67% obtained using conventional solar photo Fenton oxidation. Chen et al. (2015) evaluated the effect of different carriers and operating parameters on the degradation of flax wastewater by fluidized bed Fenton process. SiO2 was reported to be the most appropriate carrier while Fe2þ, H2O2 and bed expansion were significant operating parameters. Matira et al. (2015) studied the degradation of dimethyl sulfoxide (DMSO) using a fluidized bed Fenton process and achieved 95% DMSO degradation and 34% TOC removal after 2 h. The process also showed better performance than conventional Fenton process. Dong et al. (2014) conducted a visible-light photocatalytic degradation of methyl orange over spherical activated carbon-supported and Er3þ:YAlO3doped TiO2 in a fluidized bed reactor. The process achieved an optimum color removal of 65% after 8 h, with a reaction rate constant of 22.17 mgL1 h1. Mailler et al. (2016) studied the removal of emerging pollutants from wastewater treatment plant discharges by micro-grain activated carbon in a fluidized bed as tertiary treatment. The obvious advantages of the process was the continuous injection of fresh dose of adsorbent and non-requirement of additional separation steps. Recent studies have also been directed towards process intensification and energy efficiency through process integration. For example, Apollo and Aoyi (2016) investigated the combined anaerobic digestion and photocatalytic treatment of distillery effluent using FBR. Besides the improved performance of the combined process, the methane generation could provide the necessary power to drive the ultraviolet (UV) lamp. Studies have also been reported on the application of an integrated anaerobic fluidized bed membrane bioreactor for wastewater treatment (Kim et al., 2016). The integrated process results in low energy consumption and reduces membrane fouling. Li et al. (2014) utilized a fluidized bed membrane bioelectrochemical reactor as an energyefficient wastewater treatment process. Besides achieving more than 90% COD and 80% suspended solids removals, the overall energy balance of the process was theoretically neutral. Some reviews have also been presented on FBR applications in wastewater treatment. Burghate and Ingole (2013) presented an overview of FBBR, discussing the basic concepts, advantages and applications in both aerobic and anaerobic treatments. Although the review had highlighted the need for standardizing the design procedure of FBBR, the discussion on the design parameters was limited. In their review for anaerobic biofilm reactors for the treatment of dairy industry wastewater, Karadag et al. (2015) discussed the application of FBBR and highlighted its advantages. Tisa
et al. (2014) attempted to capture the recent applications of FBR in AOPs. However, the review mainly discussed the basic concept of FBR-AOPs and their specific applications to different wastewater streams. Thus, a review that covers the common applications of FBR in wastewater treatment with focus on the major influential parameters is yet to be presented. Despite the wide applications of FBR in wastewater treatment, its designing and operation still pose significant challenges. The lack of proper understanding of the influential parameters can lead to improper design and poor reactor performance. Therefore, a prerequisite knowledge of FBR design and operational parameters is necessary for successful application of the technology. This paper reviews, for the first time, the common applications of FBR in wastewater treatment and the major parameters affecting its performance. The goal is to offer an overview of the recent applications of FBR in wastewater treatment and provide insights on the major design and operational parameters which are prerequisites for successful design and application of the technology. Since fluidization technology has wide applications, large volume of literature on various aspects of the technology exists. Thus, to keep the review within reasonable proportions, the discussion covers only liquid-solid and gas-liquid-solid FBRs and their applications in wastewater treatment. Thus, throughout the paper, FBR refers to either liquid-solid or gas-liquid-solid system used for wastewater treatment. The review is presented as follows: 1. The basic concepts of FBR and its applications in wastewater treatment are first discussed 2. Important parameters related to reactor design and process hydrodynamics are then reviewed 3. The operational parameters are discussed with emphasis on FBFenton, FB-Photocatalysis and FBBR 4. Lastly, the review offered some perspectives on future research interest
1.1. Fluidization and fluidized bed reactor (FBR) The basic concept of fluidization involves passing a fluid through a static bed of solid particles with a superficial velocity enough to suspend the particles and caused them to behave as though they were fluid. When the fluid is introduced into the static bed at a low velocity, it simply passes through the voids of the solid particles and the bed remains fixed. As the velocity increases, the bed expands until the particles become suspended when the buoyancy force balances the drag and gravitational forces. At a particular velocity, the minimum fluidization velocity (Umf), the pressure drop across the bed equals the weight of the particles and the bed becomes completely suspended (Khan et al., 2014). Fluidization impacts excellent features onto the system such as excellent particle mixing, uniform temperature distribution and high mass transfer rate (Tisa et al., 2014). Depending on the fluidization velocity, various flow regimes such as particulate/smooth fluidization, bubbling fluidization, slugging fluidization, turbulent fluidization, and pneumatic conveying regimes can be obtained (Yang, 2003). The principle of fluidization has been extensively utilized, particularly in chemical processes where efficient mixing and mass/heat transfer are essential. FBR is a contacting device that uses the principle of fluidization in its operation. It is similar to the commonly used packed bed reactors in many aspects, except that the packing material is expanded by the upward or downward movement of the fluid (Burghate and Ingole, 2013). The degree of the bed expansion depends on the particle size and density, the up-flow velocity of the fluid and its viscosity. FBR involves multiphase flow system (solid-
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gas, solid-liquid, or solid-liquid-gas) which may include momentum exchange, heat exchange and mass transfer. Because of its excellent features, FBR is one of the most important reactor systems used in chemical and biotechnology applications (Si et al., 2011). Initially, applications of FBR had been limited to catalytic cracking, combustion, coating, granulation, drying and other chemical applications. However, FBR was later deployed for wastewater treatments. Fig. 1 shows the basic concept of FBR in wastewater treatment. The wastewater is introduced into the bed of the reactor at a particular superficial velocity enough to suspend the support media. The purpose of the distributor (spager, if air) is to uniformly distribute the effluent across the reactor bed. Depending on the system design, recycling of the effluent is usually employed. The fluidized media can be an immobilized catalyst in AOPs or microorganisms in biological treatment. 1.2. Classifications of FBR for wastewater treatment In chemical industries, various types of FBRs such as bubbling fluidized bed, circulating fluidized bed, turbulent fluidized bed, floating fluidized bed, twin fluidized bed and many other classifications based on flow regime and reactor design are used (Jordening and Buchholz, 1999). However, most of these have been developed for specific chemical applications, often involving very high superficial fluid velocities typically not needed in wastewater treatment. Thus, this review departs from these conventional classifications and discusses FBR in a way that is more applicable to wastewater treatment (Fig. 2). 1.2.1. Two-phase versus three-phase FBR Two-phase FBR involves a liquid-solid or gas-solid process where fluidization is brought about by the liquid or gas. In wastewater treatment, however, only liquid-solid system is applicable.
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The solid phase could be a variety of support materials or catalysts while the liquid phase is always the wastewater. The three-phase system involves gas-liquid-solid process where aeration or oxygenation is added to a typical liquid-solid FBR. In a two-phase FBR, fluidization is provided by the flow of wastewater through the catalyst or biomass bed. In a three-phase system, fluidization is provided by the concurrent or countercurrent flow of the liquid and gas through the solid bed. The three-phase system was developed to improve, inter alia, the oxygen limitation encountered in twophase FBRs (Choi et al., 2000). However, the three-phase system has some challenges such as particle elutriation with high gas flow rate and increased agitation which may cause reactor failure. Although two phase systems are simpler in design and easier to control, the three-phase systems have seen wider applications (Han et al., 2003). 1.2.2. Upward-fluidization versus downward/inverse-fluidization in FBR FBR can also be classified based on the direction of the fluid flow. Conventional FBR uses solid particles that are denser than the fluid phase and fluidization is achieved by the upward fluid flow from the bottom of the reactor. However, inverse fluidized bed reactor (IFBR) was later introduced to overcome some challenges intrinsic to the conventional FBR, such as uncontrolled growth of biomass which can affect the bioparticles hydrodynamics (Lakshmi et al., 2000). In IFBR, the density of the solid particles is lower than the fluid phase and fluidization is achieved by the downward fluid flow opposite to the net buoyancy of the particles (Nikolov et al., 2000). The IFBR is argued to possess superior hydrodynamic characteristics than conventional FBR. However, the downside of IFBR is that it usually requires higher superficial fluid velocity (Buffiere et al., 1998). 2. Applications of FBR in wastewater treatment Initially developed for gas generation by Fritz Winkler in 1920s, FBR has found various applications in chemical and biochemical industries such as coal gasification, metal refining, catalytic cracking, powder technology, food processing and other numerous applications (Tavoulareas, 1991). However, it was only in the early 70s that FBR was investigated as a possible reactor for biological wastewater treatment. Subsequent years saw a lot of progress, and by 1984, full-scale FBBRs were developed and installed (Heijnen et al., 1989). The application of FBR in AOPs is relatively new, with the earliest literature appearing in the late 90s. The works of Diz and Novak (1998) and Chou and Huang (1999) may have been the earliest reported studies on the application of FBR in AOPs. FBR has attracted interest as it has shown more effectiveness in wastewater treatment compared to other contacting devices such as fixed-bed column and activated sludge (Burghate and Ingole, 2013). Excellent mixing, high mass transfer rates, and low sludge production are some of the features that make FBR an attractive technology (Andalib et al., 2014). Since the solid particles are vigorously agitated by the fluid passing through the bed, an excellent mixing and little or no temperature gradient is obtained (Dora et al., 2013). Since FBR is commonly used in AOPs, biological treatment and adsorption, its applications are discussed under these three areas (Fig. 3). 2.1. FBR-AOPs
Fig. 1. Basic concept of FBR in wastewater treatment application.
The concept of AOPs was put forward by Glaze and coworkers as processes of generating reactive OH radicals with a redox potential of 2.8 eV that are capable of degrading organic pollutants (Glaze et al., 1987). The OH radicals are generated in situ through either
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ClassificaƟon of FBR for wastewater treatment
Based on direcƟon of fluidizaƟon
Based on reactant phase
2-Phase FBR
Upflow (convenƟonal) FBR
3-Phase FBR
Downflow (inverse) FBR
Fig. 2. Classification of FBR applicable to wastewater treatment.
one or a combination of chemical oxidations by using H2O2, ozone, and radiation assisted sources such as ultraviolet (Soon and Hameed, 2011). Typical AOPs include Fenton and Fenton-like processes, photocatalysis, electrochemical oxidation, Ozonation, and ultrasound cavitation. AOPs are very effective in degrading recalcitrant pollutants and have many advantages over conventional wastewater treatment methods. Unlike conventional treatments which either separate the pollutants from the wastewater stream or convert them to some intermediate compounds, AOPs are capable of mineralizing organic pollutants to H2O and CO2 (Ahmadi et al., 2015). This made AOPs very attractive, especially in the treatment of recalcitrant and persistent organic pollutants that have defied the conventional treatment technologies. Since a number of chemical reactions are necessary for the generation of OH, then the choice of a reactor is very important. Because of the excellent features of FBR, many researchers have investigated its potential application in AOPs. Combining FBR technology with AOPs can reduce sludge production (Briones et al., 2012), increase catalyst reusability (Tisa et al., 2014) and improve process performance. The most commonly investigated FBR-AOPs
are fluidized bed Fenton (FB-Fenton) and fluidized bed photocatalysis (FB-Photocatalysis).
2.1.1. Fluidized bed-Fenton Fenton oxidation is considered one of the most effective AOPs because of its rapid formation of OH in acidic medium (Asghar et al., 2015). The process consists of a homogeneous catalytic reaction between ferrous iron (Fe2þ) and hydrogen peroxide (H2O2) to produce OH that can oxidized organic pollutants as shown in Equations (1)e(3) (Alalm et al., 2015). The decomposition of the pollutants using Fenton process occurs in two stages (Lu et al., 1999). The first stage is the rapid reaction of Fe2þ and H2O2 which produces large amount of OH that can rapidly oxidized the pollutants. The second stage involves a reaction between Fe3þ and H2O2 which produces less OH and decomposes the pollutant rather less rapidly. This stage produces hydroperoxyl radicals, which have lower oxidative power. H2O2 þ Fe2þ / Fe3þ þ OH þ OH
Application of FBR in Wastewater Treatment
Advanced Oxidation Processes
FB-Fenton
FBPhotocatalysis
Biological Processes
FBBR
Fig. 3. Applications of FBR in wastewater treatment.
Adsorption
FBAdsorption
(1)
M.M. Bello et al. / Journal of Cleaner Production 141 (2017) 1492e1514
OH þ Organic / Products
(2)
OH þ Fe2þ / Fe3þ þ OH
(3)
When Fe2þ and/or Fe3þ are used as the active sites, the process is referred to as homogenous Fenton and depends on the chemical interactions between the catalysts. Although the homogeneous Fenton process is widely employed because of its effectiveness and ease of operation (Bellotindos et al., 2014), it has inherent disadvantages of excessive sludge production (Anotai et al., 2012a) and limited range of operational pH (Rodríguez et al., 2016). Thus, heterogeneous Fenton oxidation was developed to overcome some of these limitations (Buthiyappan et al., 2016). In heterogeneous Fenton oxidation, the reaction occurs between H2O2 and iron ions existing in multiple forms such as [Fe(OH)2]þ [Fe(H2O)]2þ, Fe2O3, aFeOOH (Soon and Hameed, 2011) or other transition metalsubstituted oxides (Pouran et al., 2014). In addition to the chemical changes, physical adsorption occurs at the surface of the solid catalyst which reduces sludge generation. However, the heterogeneous process is reportedly less effective than homogeneous Fenton oxidation due to mass-transfer limitation. To overcome some of the limitations of Fenton oxidation, researchers have recently explored the advantages of FBR as a possible solution. FBR can combine the effectiveness of homogeneous Fenton and the sludge reduction of heterogeneous Fenton. The solid materials in the reactor provide surfaces for iron crystallization which reduces the sludge generation and increases catalyst reusability (Anotai et al., 2009). A possible reaction mechanism that is thought to occur in FB-Fenton has been proposed by Chou and Huang (1999). Furthermore, because of the excellent features of FBR, the performance of FB-Fenton has been shown to be superior to that of conventional Fenton process (Liu et al., 2014). Lu and coworkers have conducted several works on the application of FB-Fenton in degrading various recalcitrant pollutants such as textile wastewater (Su et al., 2011a, 2011b), dimethyl sulfoxide (Bellotindos et al., 2014), acetaminophen (Luna et al., 2013), monoethanolamine (Su et al., 2013) and phenol (Muangthai et al., 2010). Table 1 summarizes experimental conditions and results from previous studies on FB-Fenton. The reported studies have largely considered synthetic wastewaters under laboratory-scale investigations. The FBRs usually consist of a cylindrical glass column with working volumes ranging between 1 and 2 L. The support materials commonly used include SiO2, Al2O3, and waste Iron oxide (BT4). Anand et al. (2015) studied the performance of a fluidized bed solar Fenton reactor in the removal of COD from hospital wastewater. The reactor was a cylindrical vessel of 1.5 L with silica granules as carriers. Maximum COD removal of 98% was obtained at 90 min HRT. The fluidized bed solar Fenton oxidation performed better than conventional solar Fenton oxidation. Li et al. (2015) studied the oxidation of bisphenol A by PhotoFenton-like process using a waste iron oxide in a three-phase FBR. The system consists of a Pyrex tube with an integrated 15 W UV lamp which removed 90% TOC after 180 min. The use of waste iron oxide, a by-product of tannery wastewater treatment, is attractive as it could increase the cost-effectiveness of the process. The study of Bellotindos et al. (2014) considered the degradation of a synthetic pollutant, Dimethyl Sulfoxide (DMSO), using FBRFenton process with SiO2 as carriers. Up to 98% DMSO degradation was achieved under optimum conditions, with the FB-Fenton process showing superior performance than conventional Fenton process. Similarly, Cheng et al. (2014) studied the degradation of Phthalocyanine (PC) dyes by photo-assisted Fenton process in an up-flow FBR. The system consists of a 1.45 L fiber glass plastic surrounded by 6 UV germicidal lamp and achieved 95% PC
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degradation. However, Liu et al. (2014) investigated the treatment of real wastewater using FB-Fenton. Recalcitrant silicone wastewater was treated using a 3.92 L reactor with different carrier materials. The process achieved 95% COD and 85% TOC removals at HRT of 60 min. Compared to the traditional Fenton process, the COD and TOC removal rates were found to increase by 20% and 15% respectively. Other reported studies are quite similar to the discussed literature. Although most of the studies have been on lab-scale, it can be seen that combining FBR with Fenton process enhances process performance, with the potential to overcome some of the drawbacks of conventional Fenton oxidation. 2.1.2. Fluidized bed-photocatalysis Photocatalytic oxidation is another AOP where the application of FBR has attracted recent interests. In Photocatalysis, a semiconductor metal oxide is used as a photocatalyst to oxide organic pollutants to carbon dioxide and water (Rosa et al., 2015). The process involves illuminating metal oxide in aqueous suspension with irradiation having a photon energy (hv) equal to or greater than the band gap energy of the metal. This generates valence band holes and conduction band electrons which can react with water and the hydroxyl ion to generate OH (Pelaez et al., 2010). Two broad light spectrum, UV (200e400 nm) and visible light (400e700 nm), are commonly used to generate the light photon (Cheng et al., 2016). Although metal oxides such ZnO, NiO, ZnS, Fe2O3 can be used, TiO2 is the most widely used photocatalyst due to its strong photo-activity, high stability, non-toxicity and commercial availability (Ananpattarachai and Kajitvichyanukul, 2016). Once the surface of the TiO2 is photo-activated, OH will be generated and subsequently oxidized organic pollutants (Chong et al., 2015). Details of the fundamentals of photocatalysis have been well documented (Meng et al., 2010). Although powder photocatalyst has large specific surface area which enhances mass transfer during wastewater treatment, it is necessary to have a downstream separation stage which increases the cost of treatment (Pozzo et al., 2000). To overcome this, the powder is usually impregnated onto the surface of other compounds such as Al2O3, SiO2 or perlite which have larger surface areas. This solves some of the challenges of using powder catalyst, including the need for downstream separation and possible particles elutriation. Because of the excellent features of FBR, researchers have investigated its application in photocatalytic oxidation. It is believed that besides its excellent mixing and high mass transfer, FBR can also enhance light penetration and exposure of the interior of the reaction matrix (Nam et al., 2002). Table 2 gives a summary of reported studies on FB-Photocatalysis which have largely been on laboratory-scale using synthetic wastewaters. The FBRs used are mostly three-phase systems where air is supplied to increase the rate of the photocatalytic degradation. Most of the studies have reported an improvement in process performance with the application of FBR. This is largely due to the improved light penetration and possible adsorption of pollutants onto the support materials. Dong et al. (2014) studied the visible-light photocatalytic degradation of Methyl Orange (MO) over AC-supported and Er3þ:YAlO3-doped TiO2 using FBR. Maximum decolorisation of 65.3% was achieved after 8 h. The performance of the system was found to be higher under the UV irradiation. Similarly, Shet and Shetty (2016) compared the performance of a photocatalytic fluidized bed degradation of phenol using Ag@TiO2 under UV and solar irradiations. Around 76% degradation was achieved using UV while only 40% was achieved under solar irradiation. The high band of TiO2 is known to prevent it from utilizing visible irradiation and hence its lower performance compared to UV irradiation.
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Table 1 Applications of FB-Fenton in wastewater treatment. Target pollutant
Reactor and support material properties
Operational conditions
Performance
Reference
DMSO
H: 140 cm D: 5.2 cm Solid: SiO2 Dp: 0.42-0.5 mm PL: 68.97 g/L V: 1.5 L D: 0.053 mm H: 1330 mm Solid: SiO2 Dp: 0.42-0.59 mm PL: 40 g/L Coaxial cylinder V: 1.5 L D: 35 mm H: 300 mm Solid: iron oxide (BT4) Dp: 0.42-0.59 mm PL: 6 g/L Cylindrical V: 1.5 L Solid: Iron oxide (BT4)
pH: 3 Fe2þ: 5 mM H2O2: 32.5 mM HRT: 240 min
DMSO: 95.22 % COD: 34.38 %
(Chen et al., 2016)
pH: 3 Fe2þ: 5 mM H2O2: 50 mM HRT: 90 min Q: :12 L/min
COD: 98 %
(Anand et al., 2015)
pH: 3 T: 25 C H2O2: 25 mg/L UV: 15 W, 365 nm
TOC: 78.9 % Color: 92 %
(Wang et al., 2015)
pH: 3 H2O2: 0.7 mmol/L HRT: 180 min UV: 15 W, 365 nm pH: 3 Fe2þ: 5 mM H2O2: 60 mM HRT: 2 h
TOC: 90 %
(Li et al., 2015)
DMSO: 98 %
(Bellotindos et al., 2014)
pH: 2 e 4 UV light: 254 nm
PC: 95 %
(Cheng et al., 2014)
pH: 3.5 HRT: 60 min H2O2/ Fe2þ 13.6:1 Q: 3 mL/h
COD: 95 % TOC: 85 %
(Liu et al., 2014)
pH: 3 Fe2þ: 0.05e0.1 mM H2O2: 5e25 mM
ACT: 99.6 %
(Luna et al., 2013)
H2O2: 10 mM UV: 254 nm lamp
TFP: 99.65 % Fluoride: 99 %
(Shih et al., 2013)
pH: 3 H2O2: 50 mM MEA/Fe2þ: 3 mM
MEA: 76 % Phosphate: 45 %
(Su et al., 2013)
pH: 3 Fe2þ: 3 mM H2O2: 50 mM
MEA 98.9 % COD: 64.7 % Iron: 67.4 %
(Anotai et al., 2012a)
pH: 2e4 Fe2þ: 0.1e1.0 mM H2O2: 1e17 mM
O-toluidine: 64.2 % (Anotai et al., 2012b) COD: 36.7 % Iron: 100 %
pH: 3.22 Fe2þ: 1e5 mM H2O2: 100 mM
ACT 97.8 %
Hospital wastewater: COD
Orange G
BPA
DMSO
Phthalocyanine (PC)
Organic silicone wastewater
Acetaminophen (ACT)
2,2,3,3-tetrafluoro-1-propanol (TFP)
MEA and phosphate
TFT-LCD wastewater: MEA
O-toluidine
Pharmaceutical wastewater: acetaminophen (ACT)
Cylindrical glass V: 1.3 L D: 5.23 cm H: 133 cm Solid: SiO2 Cylindrical Fiber glass H: 400 mm D: 25 mm Solid: Fe (II)/g-Al2O2 PL: 60 g/L Cylindrical Plexiglass V: 3.92 L D: 8 cm H: 78 cm Solid: quartz / brick/GAC Dp: 0.5 e 0.8 mm Carrier filling rate: 35 % Cylindrical glass reactor V: 1.45 L Solid: SiO2 Dp: 0.5 mm, 2 & 4 mm Cylindrical glass V: 1.5 L D: 7.5 cm H: 50 cm Solid: BT5 iron oxide DP: 0.25e0.5 mm Cylindrical vessel V: 1.45 L Solid: SiO2 Dp: 0.24e0.5 mm PL: 100 g/L 50 % bed expansion Cylindrical glass V: 1.45 L Solid: SiO2 DP: 0.42 e 0.5 mm PL:: 100 g Cylindrical glass V: 1.35 L D:5.23 cm H:133 cm Solid: SiO2 DP: 0.42e0.59 mm PL: 50e300 g Cylindrical glass V: 1.45 L Solid: SiO2 Dp: 0.5 mm PL: 100 g/L
(Briones et al., 2012)
M.M. Bello et al. / Journal of Cleaner Production 141 (2017) 1492e1514
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Table 1 (continued ) Target pollutant
Reactor and support material properties
Operational conditions
Performance
Dyes: RB5, RO16, RB2
Cylindrical glass V: 1.35 L D: 5.23 cm H: 133 cm Solid: SiO2 and Al2O3 PL: 74.07 g/L 50 % bed expansion Cylindrical glass V: 1.35 L D: 5.23 cm H: 133 cm Solid: SiO2 PL: 74.07 g/L 50 % bed expansion Cylindrical glass V: 1.35 L D: 5.2 cm H: 133 cm Solid: SiO2 DP: 0.42 e 0.84 mm Solids: glass beads 2 & 4 mm Cylindrical glass V: 1.35L Solid: Al2O3 DP: 2.5 mm Cylindrical glass V: 1.35 L Solid: SiO2 DP: 0.84-2.00 mm PL: 100 g/L Cylindrical glass V: 1.35 L Solid: Al2O3 DP: 0.8 -2.0 mm Cylindrical Pyrex tube V: 150 m L D: 2.0 cm Solid: SiO2-immobilized iron oxide DP: 0.89 mm D: 2 cm H: 100 cm Solid: g-FeOOH DP: 0.564 mm Density: 1.11 g/cm3 PL: 80 g 50 % bed expansion
pH: 3.22 Fe2þ: 0.06 mM H2O2: 19.87 mM
RB 5 and RO16: 99 (Su et al., 2011a) % RB2: 96 % COD: 34e 49 %
pH: 2e5 [COD]: [Fe2]: [H2O2]¼ 1:0.95:7.94
COD: 86.7 % Color: 97.9 %
(Su et al., 2011b)
pH: 3 Fe2þ: 0.00107 mM H2O2: 58 mM HRT: 60 min
Aniline: 96 %
(Anotai et al., 2010)
pH: 3 H2O2: 2.5 mM Fe2þ:10 mM
100%
(Ratanatamskul et al., 2010)
pH: 3 H2O2: 10 mM Fe2þ:0.25 mM
2,4-DCP: 99 % COD: 55 % Iron: 14 %
(Muangthai et al., 2010)
pH: 2.8 ± 0.2 H2O2: 50 mM Fe2þ: 5 mM
Nitrobenzene: 90 % (Anotai et al., 2009) Iron: 30 e 65 %
pH: 2.8 ± 0.2 H2O2: 500 mg/L T: 30 C Q: 1.5 L/min
TOC: 98 %
(Huang and Huang, 2009)
pH: 2.85e3.74 UL: 0.011 m/s
BA: 95 % Iron: 90 % TOC: 59 %
(Chou and Huang, 1999)
Textile wastewater
Aniline
2, 6, Dimethylaniline
2,4-dichlorophenol
Nitrobenzene and Iron removal
Phenol
Benzoic Acid (BA)
Reference
V: Reactor volume; D: reactor diameter; H: reactor height; DP: diameter of support media: BH: Initial static bed height; Q: liquid flow rate; Qa: air flow rate: HRT: Hydraulic Retention Time; T: Temperature; Umf: Minimum superficial velocity; UL: superficial Liquid velocity; Ug: superficial gas velocity; PL: Solid particle loading; BPA: Bisphenol A; DMSO: Dimethyl Sulfoxide; TFT-LCD: Thin film transistor liquid crystal display; MEA: Monoethanolamine.
Mungmart et al. (2011) investigated the degradation of phenol in a three-phase FBR using three different metal oxides. The performances of reactors utilizing O3, TiO2 deposited on silica beads, metal catalyst (Ni or Co) impregnated on mesoporous carbon beads, or O3 in combination with each catalyst were compared. The reactor with Co impregnated on mesoporous carbon beads gave the highest phenol removal while the reactor with only Ozone gave the lowest phenol removal. The carbon beads provided adsorption sites for the phenol, thereby increasing the removal rate. Kanki et al. (2005) studied the degradation of phenol and bisphenol A in a fluidized bed photocatalytic reactor using TiO2-coated ceramic particles. Two FBRs, one with an internal UV lamp (254 nm, 9 W) and the other with an outside black lamp (365 nm, 15 W) were compared. The reactor with the internal UV lamp degraded the pollutant 4 times faster than the other reactor. Clearly, the intensity and location of the lamp have an effect on the reactor performance.
2.2. Fluidized bed bioreactor (FBBR) FBBR has been widely used for aerobic and anaerobic wastewater treatments. The system consists of microorganisms-coated particles in wastewater which are sufficiently fluidized to keep the phases thoroughly mixed (Vinod and Reddy, 2005). The support materials of FBBR normally have extremely large specific surfaces and achieve treatment levels in shorter time than conventional biological treatment processes (Alfredo et al., 2013). This is because the fluidization maximizes surface contact between microorganisms and the pollutants. It has been argued that FBBR offers the stability and ease of operation of a trickling filter and the high efficiency of activated sludge process (Burghate and Ingole, 2013). For example, it was reported that FBBR operated at lower HRT and gave better performance than a stirred tank reactor in the degradation of phenol (Gonzalez et al., 2001). Fluidization provides a favorable gas-liquid mass transfer, which promotes good pollutant-biomass contact and suitable oxygen transfer rate (Pen and Jose, 2008).
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Table 2 Applications of FB-Photocatalysis in wastewater treatment. Target pollutant
Rector and support material properties
Operational conditions
Performance
Reference
Phenol
Cylindrical column H: 52 cm D: 1.6 cm Catalyst: Ag/TiO2 Loading: 0.25 e 0.75 g Solid: Glass beads Annulus reactor D: 60 mm H: 1 mm Support: Glass beads BH: 33 mm Cylindrical quartz tube V: 235 ml D:31.6 mm H: 300 mm Catalyst: TiO2/Co/Ni Loading: 2.5 e 20 g Quartz glass column H: 600 mm D: 60 mm Catalyst: TiO2 Support: quartz sand Stainless steel reactor with draft tube V: 7 L D: 0.1 m H: 1.0 m Catalyst: immobilized TiO2 Particle size: 130 mm loading: 2.448 g/L Support: SiO2 Stainless steel column with conical bottom catalyst: Titania/Kaolinite loading: 6 g/dm3
pH: 3 HRT 420 min Q: 140 mL/min Qa: 1.5 L/min UV lamps:18 W
Phenol: 84 %
(Shet and shetty, 2016)
LED: 36 W, 455 e 533 nm T: 20 C
MO: 65.3%
(Dong et al., 2014)
Qa: 1 L/min HRT: 10 min UV: 15-W, 254 nm
Phenol: 100 %
(Mungmart et al., 2011)
pH: 7 T: 30 C loading: 0.33 g/L UV: 20 W, 254 nm
Phenol: 99 %
(Zulfakar et al., 2011)
MO
Phenol
Phenol
Sodium lauryl sulfate(SLS)
Congo Red (CR)
Acid dye
Acrylic cylindrical column H: 210 mm D: 90 mm Catalyst: ZnFe2/TiO2-GAC Loading: 20 e 40 g Phenol and Bisphenol A Rectangular column V: 4 dm3 H: 20 cm UV lamps (9 W, 254 nm) Catalyst: Immobilized TiO2 Dp: 0.7 mm Rhodamine B Rectangular acrylic vessel V: 2.8 H: 250 mm Catalyst: TiO2 Loading: 33.8 g/ Microcystin-LR (MLR) Cylindrical column D: 68 cm H: 65 cm Catalyst : TiO2-coated GAC BH: 2.5 cm Rhodamine B V: 24 L Catalyst : TiO2-coated ceramic DP: 1.5 mm Loading: 25 g/L Dyes: Crystal violet & Azure B Cylindrical glass vessel D: 45 mm H: 250 mm Catalyst : ZnO immobilized in alginate gel beads DP: 2.5 e 3.0 mm Loading: 20 g TCE Annular quartz glass tube reactor D: 55 mm H: 600 mm Catalyst: immobilized TiO2 Support: Silica gel DP: 220-417 mm MO Acrylic pipe with draft tube V: 2.5 L D: 10 cm
SLS: 100 % Qa: 0.2 cm/s 30 W UV-A (365 nm) Black Light Blue Lamp 65 W UV-C (254 nm) Germicidal Lamp
(Nam et al., 2009)
pH: 7 Qa: 0.5 dm3/min 11 W UV light (256 nm) Q: 3 L/min HRT: 4 h visible light lamp (150 W)
COD: 80 % Decolorisation: 95.5 %
(Chong et al., 2009)
Acid dye: 60 %
(Wang et al., 2009)
Air-flow rate: 0.5 dm3 HRT: 200 min
Phenol/BPA: 100 %
(Kanki et al., 2005)
Qa: 7.0 L/min 8-W Germicidal lamps
RB: 100 %
(Na et al., 2005)
Q: 150 e 200 cm3/s UV: 4 W, 370 nm T: 20 C
MLR: 95 %
(Lee et al., 2004)
Qa: 1 L/min UV: 20 W HRT: 180 min
87 % degradation
(Na et al., 2004)
Qa: 2 L/min UV: 125 W, 650 nm T: 25 C
Decolorisation: 77-100 % (Couto et al., 2002) TOC: 52-90 %
HRT: 10 min Ug: 5.1 cm/s 6 white lamps (8 W, 365 nm) 6 Germicidal lamps (8 W 254 nm)
TCE: 80 %
(Lim and Kim, 2002)
pH: 3 15 W UV-lamp Qa: 1.5 L/min
MO: 100 %
(Nam et al., 2002)
M.M. Bello et al. / Journal of Cleaner Production 141 (2017) 1492e1514
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Table 2 (continued ) Target pollutant
Rector and support material properties
Operational conditions
Performance
Reference
H: 45 cm Catalyst: Degussa P-25 Particle size: 21 nm Loading: 0.2 g/L V: Reactor volume; D: reactor diameter; H: reactor height; DP: diameter of support media: BH: Initial static bed height; Q: liquid flow rate; Qa: air flow rate: HRT: Hydraulic Retention Time; T: Temperature; Umf: Minimum superficial velocity; UL: Liquid superficial velocity; Ug: Gas superficial velocity; MO: Methyl Orange; TCE: Trichloroethylene.
Obviously fluidization will do away with preferential flow paths, bed clogging and other problems encountered in fixed-bed reactors (Jaafari et al., 2014). Although FBBR is a well-established technology with full-scale plants in existence, lab-scale studies are still being conducted towards process improvement, application of new materials and integration with other technologies. For example, Logan and coworkers have recently combined FBBR with membrane bioreactor as a possible solution to membrane fouling (Kim et al., 2011, 2016). Improved treatment efficiency, stable mixed liquor suspended solid and reduced transmembrane pressure are obtained when FBBR is combined with membrane bioreactor (Shin et al., 2014). Table 3 gives a summary of some reported studies on wastewater treatment using FBBR. Both two- phase and three-phase processes have been used, with some researchers exploring the advantages of an inverse fluidized bed bioreactor (IFBBR). Haribabu and Sivasubramanian (2016) studied the biodegradation of organic matter in domestic wastewater using IFBBR and achieved a maximum COD removal of 96.7%. A three-phase FBR with a working volume of 0.0125 m3 and employing low density biocarries was used. The low density media had a positive effect on the minimum fluidization velocity and increased the efficiency of the process. Lin et al. (2010) investigated the biodegradation of RB13 in a two-stage anaerobic/aerobic FBBR and achieved color and COD removal efficiencies of 86.9% and 90.4% respectively. Rajasimman and Karthikeyan (2007) investigated the effect of HRT on the aerobic digestion of starch wastewater using FBBR with low density biomass support. The COD removal increased with increase in HRT for all initial substrate concentration, with maximum removal (95.6%) at 40 h HRT. Cuenca et al. (2006) studied the anaerobic biodegradation of diesel fuel-contaminated wastewater using FBBR and reported that both diesel and COD removal efficiencies increased with increased in HRT for all the conditions investigated. Mustafa et al. (2014) studied the treatment of municipal wastewater sludge using anaerobic FBBR and reported that the treatment performance decreased with increased in organic loading rate (OLR). A volatile suspended solids (VSS) removal efficiencies of 88%, 79% and 70% were achieved at OLRs of 4.2, 9.5 and 19 kg COD/m3-d respectively. A COD removal efficiency of 68% was obtained at OLR of 19 kg COD/ m3-d. Borja et al. (2004) carried out mesophilic anaerobic digestion of wastewater from the production of protein isolates from chickpea flour using FBBR and reported that the percentage COD removal decreased with increase in OLR. 2.3. Fluidized bed-adsorption Adsorption is an effective and economical method for the removal of recalcitrant pollutants from wastewater, especially when low-cost adsorbent such as grape bagasse (Demiral and € r, 2016) and cow bone (Cechinel et al., 2014) are utilized. Güngo Although treatability studies on adsorption are usually conducted in batch (Abidi et al., 2015) or fixed-bed column rectors (Bello et al., 2013), studies have also been reported on the use of FBR for adsorption process. When FBR is used in adsorption, operational
problems encountered in fixed-bed column adsorption such as clogging, temperature gradient, channeling and dead zones are eliminated. Table 4 shows reported studies on adsorption using FBR. Dora et al. (2013) investigated the adsorption of Arsenic (III) using cashew nut shell in a three-phase FBR and reported a removal efficiency of 92.55% under optimum condition. The adsorption was found to be affected by the gas and liquid velocities, particle size and initial static bed height. Kulkarni et al. (2013) studied the adsorption of phenol from wastewater in an FBR using coconut shell activated carbon. The adsorption was found to depend on the initial phenol concentration, flowrate and bed particle size. Jovanovic et al. (2014) studied the hydrodynamics and sorption studies for the removal of Cu (II) from aqueous solution using FBR packed with Zeolite A beads. The process was optimized, with a maximum sorption capacity of 23.3 mg/g. Zhou et al. (2015) developed an integrated flocculationadsorption fluidized bed (IFAFB) system for the removal of Kaolin clay and phenol from synthetic wastewater. The adsorption capacities of the fluidized regime were found to be 8.77 and 24.70 mg/g greater than those of the fixed bed regime. At shorter HRT (below 50 min), however, the adsorption was higher in the fixed bed regime. At longer HRT and higher superficial velocities, the adsorption performance of the FBR was superior. Under fluidization, the solution forms micro-vortices and create a uniform condition around the adsorbent, resulting in effective utilization of the adsorbent. Under fixed bed mode, however, there are discrepancies between the two sides of the particles. In a similar study, Wang et al. (2011) reported that the adsorption capacity of fixed bed was higher than that of an inverse fluidized bed in their study for aqueous phase adsorption of toluene using hydrophobic aerogels. The breakthrough time was found to be shorter in the inverse FBR than packed bed. This is perhaps because of the short HRT (50 min) where the packed bed normally exhibits a better performance. The use of the inverse FBR might have equally contributed to the lower performance of the process. 3. Effects of design parameters Successful application of FBR requires knowledge of the important design parameters. However, despite the extensive applications of FBR, fluidization is still an empirical science (Yang, 2003) and a single systematic design approach is yet to emerge. Instead, the design is largely application-specific and relies on empirical correlations (Onysko et al., 2002) and experience of the designer (Zhang et al., 2012). For example, Deng et al. (2016) developed an integrated methodology for designing FBBR for the treatment of dyeing effluents. The method was based on theories, experiments and knowledge base. The absence of a robust methodology and the reliance on heuristics may lead to various problems such as insufficient or over-fluidization, poor treatment performance and reactor failure. Although attempts have been made to understand the design parameters, large-scale applications of FBR still pose significant challenge (Reinhold et al., 1996). Despite the wide applications of FBR in wastewater treatment,
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Table 3 Applications of FBBR in wastewater treatment. Target pollutant
Reactor and support material properties
Operational conditions
PS & TWAS
Plexiglass rectangular column V: 16 L H: 3.6 m Support: HDPE DP: 600e850 mm Density: 1554 kg/m3 V: 0.0125 m3 D: 0.1 m H: 1.8 m Support: LDPE Density: 870 kg/m3 BH: 0.6 e 1.0 m Glass column V: 580 ml Support: GAC DP: 0.5 -1 mm
PS, COD: 62%, VSS: 63% (Wang et al., 2016) T: 37 C TWAS, COD: 56%, VSS: 50% HRT: 2.2 e 4 d OLR: 12 -18kg COD/m3 d
Domestic wastewater
Autotrophic denitrification
Aquaculture Effluent: Nitrate removal
V: 2.85 L D: 0.31 m H: 3.9 m BH: 0.9 m Support: Sulfur biofilters DP: 0.3 mm
Aquaculture effluent: denitrification
V: 285 L D: 0.31 m H: 3.9 m Support: sand biofilters DP: 0.11 mm BH: 0.9 m V: 1 L Support: GAC Dp: 0.5 e 1 mm
Denitrification of mining water
Cu, Ni & Zn removal
V: 2.5 L D: 0.08 m H: 1.0 m
Synthetic municipal wastewater: denitrification
Plexiglass column V: 608 ml D: 2.54 cm H: 100 cm Support: zeolite DP: 600 e 850 mm Plexiglass column V: 3.95 L D: 60 mm H: 140 cm BH: 0.6 m Support: PVC DP: 2 mm Plexiglass plate V:7.6 L, Support : GAC Loading: 200 e 300 g Pillar glass reactor V: 486 mL D: 3.6 cm H: 46 cm Support: Gel beads PVC pipe with conical bottom V:2.5 L D: 0.05 m H: 1.0 m Support: LDPE beads DP: 3 mm V: 16 L H: 3.6 m Support: zeolite DP: 425 e 610 mm
Currant wastewater: COD
Domestic wastewater
Formaldehyde
Metal precipitation (Cu, Pb, Cd, Zn)
PS & TWAS
Performance
Reference
COD: 96.7 % HRT: 6.25 e 24 h Q: 10 e 80 mL/min Ug:0.0016 - 0.00318 m/s
(Haribabu and Sivasubramanian, 2016)
N: 100 % pH: 5.8 T: 20 e 30 C Q: 800 mL/min, HRT: 10 min OLR: 500 mg/L h Bed expansion: 25 % N: 49 % Phase I: HRT: 3.23.3 min, Flowrate: 63-65 L/min Phase II: HRT: 3.24.8 min Flowrate: 67-43 L/min 13 e 42% bed expansion HLR: 188 L/min m2 N: 26.9 % HRT: 15 min Q: 13.7 L/min 50 % bed expansion
(Zou et al., 2016)
T: 7 e 22 C HRT: 12 h Q: 800 mL/min 25 % bed expansion HRT: 24 h pH: 7 & 5 OLR: 1 g COD/L. d 30 % bed expansion T: 20 ± 3 C HRT: 0.6 h OLR: 5.9 e 7 kg COD/m3 d Q: 20 ± 2 L/day
Denitrification: 100 %
(Zou et al., 2015)
Cu: 97.5 % Ni: 65.9 % Zn: 97.0 % COD: 61.9 % N2O: 0.53 %
(Janyasuthiwong et al., 2015)
(Christianson et al., 2015)
(Tsukuda et al., 2015)
(Eldyasti et al., 2014)
COD: 96. 9 % T: 35 ± 2 C OLR: 9.4 to 24.2 kg COD/ 3 m Umf: 0.75 m/min 30 % bed expansion
(Jaafari et al., 2014)
T: 15 - 35 C HRT: 6 h
COD: 74.0 ± 3.7%
(Gao et al., 2014)
HRT: 24 h T: 30 & 37 C
Degradation: 98. 37 % at 30 C Degradation: 96.83 % at 37 C
(Qiu et al., 2014)
T: 25 C HRT: 24 h 30 % bed expansion
Removal: 99 %
(Villa-Gomez et al., 2014)
OLR: 8 e 19 kg/m3-d Q: 3.4 L/d
COD: 68 % for PS COD: 55 % for TWAS
(Mustafa et al., 2014)
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Table 3 (continued ) Target pollutant
Reactor and support material properties
Operational conditions
Performance
Reference
Petrochemical wastewater: COD and TPH Glass column with draft tube V: 4.36 L
T: 20 e 25 C HRT: 6 h Qa: 2-5 L/h, OLR:1.61-2.56 kg COD/ m3.d T: 37 C, UL: 1.4 cm/s OLR: 29 ± 1.2 kg COD/ m3d Qa: 0.025 m/s HRT: 26 h 50 % bed expansion
COD: 87 % TPH: 95 %
(Qin et al., 2014)
COD: 88 % TSS: 78 %
(Andalib et al., 2012)
COD: 83.3 % Color: 89.5 %
(Balaji and Poongothai, 2012)
Degradation: 92 %
(Midha et al., 2012)
Thin stillage
H: 3.6 m Support: Zeolite DP: 425 e 610 mm
Dyeing Effluent: color and COD
V: 0.02 m2 D: 0.15 m H: 1.17 m BH: 0.25 m Support: PVC Glass column V: 0.6 L D: 0.045 m H: 0.38 m Support: nylon DP: 2-3 mm Density: 1140 kg/m3 BH: 16 cm V: 300 mL Support: AC DP: 0.5 e 1 mm
Sulfide oxidation
Real acid drainage mine water
T: 30 ± 2 C HRT: 25 e 70 min Uup: 14 e 20 m/h
Sulfate: 90 % T: 35 C COD: 80 % HRT: 12 e 24 h Metal: 99.9 % pH: 2.7 e 7 15 e 20 % bed expansion
(Sahinkaya et al., 2011)
V: Reactor volume; D: reactor diameter; H: reactor height; DP: diameter of support media: BH: Initial static bed height; Q: liquid flow rate; Qa: air flow rate: HRT: Hydraulic Retention Time; T: Temperature; Umf: Minimum superficial velocity; UL: Liquid superficial velocity; Ug: Gas superficial velocity; OLR: Organic Loading Rate; IFBBR: Inverse Fluidized Bed Bioreactor; IAFMBR: Inverse Anaerobic Fluidized Bed Bioreactor; GAC: Granular Activated Carbon; HDPE: High Density Polypropylene; LDPE: Low Density Polypropylene; TPH: Total Petroleum Hydrocarbon; PS: Primary Sludge: TWAS: Thickened Waste Activated Sludge.
the effects of design parameters are not fully established. Also information on the effects of reactor configuration on the mass transfer and the reactor performance are somewhat sparse. Some of the design parameters that may affect the performance of FBR include reactor geometry, aspect ratio, reactor internals, particle size and density, particle loading and fluid superficial velocity.
3.1. Reactor geometry 3.1.1. Shape and cross-sectional area of the reactor Reactor configuration is an important parameter that affects mixing and particle distribution in FBR (Choi and Shin, 1999). Particle mixing plays an important role in the performance of FBR since it affects both heat and mass transfer (Yan et al., 2009). For wastewater treatment, it is necessary to obtain high mass transfer rate and uniform temperature in the reactor through fluid-particle interactions. Although FBRs are conventionally cylindrical, other shapes such as square columns have been used. Dead-zones are encountered more frequently in square columns where the sharp corners of the reactor promote their occurrences. The presence of dead-zones inhibits proper particle mixing in the reactor. In their comparative study on the effect of bed geometry on mixing rate, Gorji-kandi et al. (2015) concluded that mixing rate is greater in a cylindrical bed than a square bed FBR. This was attributed to the presence of dead-zones at the corners of the square reactor which caused slow motion of particles. The presence of slow fluidization on the wall of the reactor had been confirmed earlier (Efstathios and Michaelides, 2013). Therefore, it is necessary to choose appropriate shape of the reactor for effective wastewater treatment. The cross-sectional area of the reactor is another parameter that can affect the hydrodynamics and treatment performance of FBR. Generally, FBR can be divided into a flatbed or a tapered-bed FBR
(Fig. 4). Conventional FBRs are flatbed reactors with uniform crosssectional areas. However, wash-out of particles occurs sometimes due to high superficial velocity. To overcome this challenge, Scott C. D and Hancher C. W. introduced the concept of tapered-bed FBR in 1976 (Parthiban et al., 2007). In Tapered FBR, the cross-sectional area of the reactor is made narrower at the bottom (tapered-in) or both bottom and top (tapered-in tapered-out) (Askaripour and Dehkordi, 2015). This results in a stable feed introduction as well as minimizes eddies and back mixing that could arise in flatbed FBR. However, it is necessary to ensure appropriate taper angle so that turbulent flow due to sudden expansion can be avoided. A comparative study on the performance of a flatbed FBR and a tapered FBR showed that the latter has superior treatment performance and better hydrodynamic characteristics (Huang et al., 2000). Three FBRs having 0 , 2.5 and 5 taper angles were compared in the study. The hydrodynamics characteristics and performance of the three FBRs were in the following decreasing order 5 / 2.5 / 0 . The 5 tapered FBR showed the highest performance while the 0 tapered FBR exhibited the lowest performance. A previous study by Wu and Huang (1996) reported that COD removal efficiency of a tapered FBR was higher than a flatbed FBR when the taper angle does not exceed 5 . Above 5 taper angle, the performance of the flatbed FBR was either higher or lower than the tapered FBR.
3.1.2. Aspect ratio Aspect ratio, defined as the ratio of the static bed height to the reactor diameter, is an important design parameter of FBR. The aspect ratio has an influence on fluid circulation velocity and consequently on the phase mixing in the reactor (Weipeng et al., 2014). Large aspect ratio promotes bubble coalescence and higher solid holdup. This reduces both gas/liquid holdup and the interphase mixing. Conversely, a low aspect ratio promotes higher
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Table 4 Applications of FB-Adsorption in wastewater treatment. Target pollutant
Reactor and support material properties Operational conditions
V: 20 m3 H: 5 m Adsorbent: GAC DP: 100 e 800 mm BH: 1.5 e 2.5 m Kaolin clay and Phenol Cylindrical Plexiglas D: 30 mm H: 570 mm Adsorbent: SiO2/CAC DP: 0.6 e 1.5 mm BH: 35 mm Cu Polycarbonate column D: 2.4 cm H:16.7 cm Adsorbent: Zeolite A DP: 0.71 e 2.2 mm Loading: 10.5 - 12.5 g BH: 4.3 - 10.2 cm Arsenic Perplex column D: 5 cm H: 150 cm Adsorbent: Cashew nut shell DP: 1.1005 e 1.5405 mm BH: 0.06 e 0.14 m Toluene PVC column D: 0.076 m H: 1.47 m Adsorbent: SiO2 DP: 0.7 e 1.2 mm Loading: 50 e 200 g Congo red phosphate and nitrate Conical bottom polyplastic Adsorbent: formulated clay-lime Loading: 0.4 e 1.0 g/L Phenol Cylindrical perplex column V: 70 L D: 0.2199 m H: 1.82 m Adsorbent: PAC Loading: 2 g/L Copper D: 3 cm H: 120 cm Adsorbent: clarifier sludge DP: 0.5 mm Copper D: 3 cm H: 120 cm Adsorbent: Manganese-coated sand DP: 1.0 ± 0.1 mm Phenol Jacket glass column D: 20 mm Adsorbent: GAC DP: 0.937, 1.524 mm Density: 2100 kg/m3 Loading: 12-24 g Emerging micro-pollutants
Performance
Reference
HRT: 10 e 20 min Q: 1400 m3/d
PPHs: 80 % COD: 40 e 45 % OM: 30 e 35 %
(Mailler et al., 2016)
UL: 4 e 8 mm/s HRT: 7 s
Kaolin: 95 % Phenol: 80 %
(Zhou et al., 2015)
HRT: 1.4 - 4.0 s Q: 11.6 - 13.5 cm3/s UL: 2.6 - 3.0 cm/s
adsorptive capacity 23.3 mg/g
(Jovanovic et al., 2014)
Ug: 5 e 20 m/s UL: 0.01 e 0.07 m/s
Arsenic: 93 %
(Dora et al., 2013)
Removed up to 4 % of the adsorbent weight (Wang et al., 2011) HRT: 6.25 e 24 h Ug: 0.0016 - 0.00318 m/s Q: 0.86 e 5.99 L/min
Qa: 1 L/min
Congo red: 99 % Phosphate: 99 % Nitrate: 45 %
(Vimonses et al., 2010)
pH: 3.5 Ug: 0.0219 m/s
Phenol 95 %
(Mohanty et al., 2008)
pH: 4 HRT: 60 min T: 25 C UL: 0.0028 m/s pH: 2 e 8 HRT: 60 min T: 25 C UL: 0.0028 m/s T: 21 e 24 C Q: 0.15e0.35 dm3/min Umf: 0.0085 m/s
Copper: 90 %
(Lee et al., 2006)
Copper: 99 %
(Lee et al., 2004)
Phenol: 62 %
(Wang and Chang, 1999)
V: Reactor volume; D: reactor diameter; H: reactor height; DP: diameter of support media: BH: Initial static bed height; Q: liquid flow rate; Qa: air flow rate; HRT: Hydraulic Retention Time; T: Temperature; Umf: Minimum fluidization velocity; UL: Liquid superficial velocity; Ug: Gas superficial velocity; CAC: Coconut activated carbon; IFAFBR: Integrated Flocculation-Adsorption Fluidized Bed Reactor; IFBR: Inverse Fluidized Bed Reactor; GAC: Granular Activated Carbon; OM: Organic matter: PAC: Powder Activated Carbon.
liquid/gas holdup and encourages interphase mixing. Therefore, low aspect ratio can reduce the fluid flow rate requirement and hence lower the process cost (Ochieng et al., 2003). It is therefore necessary to select the appropriate aspect ratio for proper design and successful application. Typical ranges of aspect ratio for both laboratory/pilot FBRs and technical plants are shown in Table 5 (Jordening and Buchholz, 1999). Laboratory scale FBRs usually have small diameters in relation to the reactor column height and the corresponding static bed height. Since the reactor volume is small, a small diameter and a relatively high static bed height can give the necessary solid loading. In the case of full scale FBR, a long and narrow column may
result in slugging effect (Kunii and Levenspiel, 1991) and hence the diameter is usually made relatively bigger to achieve the necessary degree of fluidization. However, a very large diameter may pose challenges to uniform fluidization. Therefore, a compromise is usually necessary (Jordening and Buchholz, 1999). For example, Ochieng et al., (2002) found an aspect ratio of 10 to be the optimum in their treatment of brewery wastewater using laboratory-scale FBBR. In FBBR, large aspect ratio results in more surfaces for biomass growth. However, higher aspect ratio above the optimum value will increase solid holdup and thereby inhibit proper mixing of the bioparticles and wastewater. The increased amount of solid
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Fig. 4. Flatbed versus tapered-in FBR (Adapted from: Heat and Mass Transfer in Particulate Suspensions In: Springer Briefs in Applied Sciences and Technology, Efstathios and Michaelides, 2013, pp 89e199, with permission from Springer).
Table 5 Typical range of aspect ratio for FBR. FBR type
Aspect ratio
Laboratory scale Technical plant
5e25 2e5
particles due to the high aspect ratio will equally increase the fluid pumping requirement of the system (Sabarunisha and Radha, 2014). 3.1.3. Reactor internals Another parameter that affects FBR performance is the presence of an internal structure in the reactor. Internals, such as tubes and baffles, are sometimes introduced into the reactor to modify the flow structures and improve particles fluidization. Reactor internals promotes uniform mixing which result in effective fluidization (Qin et al., 2014). Although the main purpose of internals is to ensure uniform fluidization, additional benefits such as enhanced phase contact, controlled solid holdup and improved radial mixing are obtained (Jin et al., 2003). The downside of the internals is that they can increase the complexity of system design and operation (Dutta and Suciu, 1992). Internals can be broadly classified into baffles, tubes, packings, inserted bodies, and other configurations developed for various specific applications. Baffles and tubes are the most commonly used internals in wastewater treatments. Examples of baffles include wire mesh, ring, perforated plate etc. While tubes can be draft tubes, horizontal and vertical banks. Many studies have been reported on reactor internals such as vertical internals (Ramamoorthy and Subramanian, 1981), horizontal tubes (Olowson, 1994), perforated baffles (Zhao et al., 1992), ring-type internals (Zhu et al., 1997) and other variations. In general, internals have effects on the bubble behavior, flow distribution and phase mixing. Studies have shown that introducing a draft tube into FBR can enhance the process performance. Wang et al. (2015) reported that an FBR with internal draft tube gave a higher decolorisation and TOC removal compared with conventional FBR in their study for orange G degradation. Similarly, Nam et al. (2009) reported similar findings when they compared the performance of conventional FBR and FBR with an internal draft tube (DTFBR). The DTFBR showed superior performance under all the conditions investigated which was attributed to the more uniform distribution of the catalyst. In another study, Vinod & Reddy (2005) used a draft tube FBBR for the treatment of phenolic wastewater, achieving up to 96%
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removal efficiency. Interestingly, Wei et al. (2000) compared the hydrodynamics of two FBRs, one having a conventional internal draft tube and the other with a convergence-divergence draft tube. Results showed that gas holdup is higher in the FBR with convergence-divergence draft tube than the conventional draft tube FBR. Conversely, liquid circulation velocity was found to be lower in the convergence-divergence draft tube reactor, perhaps due to the decrease in the velocity caused by the divergence/ convergence tube. However, Nam et al. (2002) reported that internal draft tube has a negligible effect on the performance of FBR in the photocatalytic oxidation of methyl orange. The photocatalytic degradation was found to depend largely on the amount of catalyst and lamp power rather than the reactor internals. However, the range of gas flow rates tested in the study may have been too low to make a significant difference in the hydrodynamics of the two different reactors. To buttress that, the authors reported the superiority of the DTFBR over the conventional FBR in a later study (Nam et al., 2009). Thus, it is obvious that internals can affect the reactor hydrodynamics and therefore influences the treatment performance of the FBR. 3.2. Support material The properties of support materials such as particle size, density and surface characteristics can affect the process performance of FBR (Wirsum et al., 2001). The choice of support material would therefore determine, to a great extent, the process engineering (Jordening and Buchholz, 1999). The effects of particle loading, density, size and surface properties on the reactor performance are discussed in this section. 3.2.1. Particle size and surface property Particle size is an important parameter that affects fluidization as well as heat and mass transfer in the reactor. In fact, it has been argued that particle size could be the most important factor that govern mass transfer in a three-phase FBR (Kim and Kang, 1997). Although developed for gas-solid fluidization, Geldart classification of particles can be useful in classifying solids for wastewater applications. Geldart (1973) classified solid particles into four groups (A, B, C and D) based on their mean size and density differences between the particles and the fluidizing medium. Group A are particles with small mean size between 30 and 100 mm, group B ranges between 100 and 800 mm, group C has mean size less than 20 mm while group D has a mean size above 1 mm. For each classification, different flow regimes and bed behavior are observed. Although all these types of solids are used in traditional fluidization applications, only Geldart B and D particles are applicable to wastewater treatment. Both heat and mass transfer in FBR increase with increase in particle size. This is because large particles have the capability to break up and disintegrate large bubbles (Nguyen-tien et al., 1984). For a given liquid velocity, larger particles would result in a better mass transfer and reactor performance than smaller ones (Begum and Radha, 2015). In a study to investigate the effect of zeolite diameter used as a support material in FBBR, it was reported that larger diameter (0.5e0.8 mm) gave slightly higher COD removal ndez et al., 2008). Therefore, than smaller ones (0.2e0.5 mm) (Ferna it is generally believed that mass transfer coefficient in FBR increases with increase in bed particle diameter. However, large particles can increase bed pressure drop (Dora et al., 2012) which will consequently increase the fluidization requirement (Midha et al., 2012). Lakshmi et al. (2000) investigated the effect of particle diameter on the minimum fluidization velocity in a two-phase IFBR using LDPE and propylene particles. They reported that the minimum fluidization velocity increased with
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increase in particle diameter for the two types of particles investigated. This was attributed to the increase in the Archimedes number which increases with increase in particle diameter. Surface characteristic of the solid particle is another important parameter that can affect the performance of FBR. Solid particles can be hydrophobic or hydrophilic. Unlike hydrophobic particles, hydrophilic particles mix excellently with water. This improves the mass transfer coefficient for up-flow fluidization (Kim and Kang, 1997). However, for inverse fluidization, the reverse is the case. Han et al. (2003) compared hydrophobic and hydrophilic particles having the same density and concluded that hydrophobic are better than hydrophilic particles for inverse fluidization. This was attributed to the retardation of rising bubbles near the hydrophobic particles which subsequently increased the gas holdup. A similar concept of hydrophobic/hydrophilic was discussed by Choi & Shin (1999) and observed by Buffiere et al. (1998) during their study of an IFBBR. Kim and Kang (1997) had equally discussed this in terms of the wettability of the particles. This shows the importance of surface properties of FBR support materials in wastewater treatment. Particles with high specific surface areas, good physicochemical and fluidodynamic properties should therefore be used as support materials (Pen and Jose, 2008). Particles with irregular surfaces, sharp angles and crevices are suitable for biomass attachment and development which are important in FBBR (Buffiere et al., 1998). Silica, quartz sand, granular activated carbon, vitreous coke, glass beads, PVC, are some of the common support materials used in FBR (Alfredo et al., 2013). The particle diameter of the support materials is usually less than 1 mm, though larger particles have also been used (Vinod and Reddy, 2005). The specific area of bed materials after fluidization can be calculated using the following formula:
as ¼
6ð1 εÞ dj
(4)
Where. as ¼ specific surface area (m1) ε ¼ expanded bed porosity (dimensionless) d ¼ support particle diameter (mm) j ¼ form factor (dimensionless, equals 1 if considered a pseudospherical particle)
3.2.2. Particle loading For successful application, it is necessary to understand the effect of particle loading/initial bed height on the hydrodynamics of FBR (Delebarre et al., 2004). The initial static bed height is the height of the solid particles in the reactor prior to fluidization. Theoretically, the initial static bed height does not affect the minimum fluidization velocity (Umf) in a conventional FBR (Jena et al., 2009). This is because fluidization is achieved when the upward inertial and drag forces exerted on the particles equal the buoyant weight of the bed. Lakshmi et al. (2000) studied the effect of bed height on Umf in a two-phase FBR and reported that constant velocity is obtained for all bed heights investigated. However, Delebarre et al. (2004) studied the influence of bed inventory on fluidization characteristics of FBR and concluded that the initial static bed height has effect on Umf. An increase in bed inventory led to an increase in Umf. However, there were some inaccuracies in the bed height measurements which might have affected the authors’ conclusion. Previous study by Garcia et al. (1999) had reported a small influence of particle loading on the liquid velocity of the system. In the case of IFBR, however, the fluidization velocity decreases with increase in particle loading
(Han et al., 2003). For a given reactor diameter, particle loading can affect the oxygen mass transfer rate in a three-phase FBR. However, the influence of particle loading on oxygen transfer rate is rather complex. High particle loading promotes bubble coalescence, which in turns reduces the interfacial area of gas-liquid and hence the oxygen mass transfer (Abdel-aziz et al., 2016). Large bubbles will move faster, resulting in shorter residence time and consequent low gas hold up. It was reported that an increased particle loading of 15% caused a 30% drop in oxygen mass transfer in a three-phase FBR for aerobic wastewater treatment (Yu et al., 1999). The initial static bed also affects the pressure drop across the reactor bed. At Umf, the pressure drop is equal to the weight of the particles divided by the cross-sectional area of the bed. The pressure drop in FBR is the sum of the frictional pressure drop and the static pressure drop. However, the static pressure drop is usually negligible and the total pressure drop is then due to the frictional pressure drop only (Askaripour and Dehkordi, 2016). Therefore, the frictional pressure drop required to counterbalance the weight of the bed increases with increase in initial static bed height (Dora et al., 2012). Thus, it is necessary to use appropriate aspect ratio to ensure optimum performance of the system. 3.2.3. Particle density For a given bed height, the density and surface property of the particles would determine the required superficial fluid velocity (Han et al., 2003). Dense particles would require high up-flow velocity to achieve fluidization (Escudero, 2010). Also porous materials result in lower superficial velocity requirement than nonporous materials (Jordening and Buchholz, 1999). Where FBBR is used, polymeric support particles could be the materials of choice because they offer large surface areas for microbial growth (Midha et al., 2012). The use of light particles will result in low fluid pumping requirement and thereby low operational cost. In such case, however, the aspect ratio should be as low as possible in order to achieve bed homogeneity at the low gas/liquid flow rates (Ochieng et al., 2002). Conversely, minimum fluidization velocity decreases with increase in particle density in the case of an IFBR. A study on the minimum fluidization velocity requirement between a low density polyethylene (LDPE) (940 kg/m3) and polypropylene (PP) (840 kg/ m3) revealed that the LDPE particles required lower velocity than the PP particles (Lakshmi et al., 2000). This is because the upward buoyance force increases as the particle density decreases and thus higher liquid velocity is required to achieve fluidization. For a given initial static bed height, particle density affects bed pressure drop. Dense particles increase the weight of the initial static bed height and this increases the pressure drop necessary to counterbalance the weight (Dora et al., 2012). A correlation of gas velocity, phase holdups and pressure drop can give further insights on the influence of the hydrodynamic characteristics on the process performance (Equation (5)).
dp ¼ εs rs þ εL rL þ εg rg g dz
(5)
εs þ εL þ εg ¼ 1
(6)
Where dp is the pressure drop, dz is the bed height; εs, εL and εg are the solid, liquid and gas phase holdups respectively; rs, rL and rg are similarly the densities; g is the acceleration due to gravity. Clearly from Equation (5), the use of dense particles could lead to an increase in pressure drop, which consequently increases power consumption. On the other hand, very low densities could lead to particle wash-out. However, a careful design and reactor internal
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can minimize this problem (Ochieng et al., 2002). 3.3. Superficial fluid velocity Superficial fluid velocity (Uf) refers to the volumetric flow rate of the fluid divided by the cross-sectional area of the reactor. Uf is responsible for the particles fluidization and therefore influences the particles mixing, heat and mass transfer rate in the reactor (Mostoufi and Chaouki, 2001). Therefore, it is necessary to understand how Uf affect FBR performance. Uf is required to be within two extremes, the minimum fluidization velocity (Umf) and the terminal fluidization velocity (Ut). Umf is the lowest fluid velocity necessary to initiate particles fluidization while Ut is the fluid velocity at which particles are carried out with the fluid flow (Jovanovic et al., 2014). Umf is an important parameter which is closely related to the power requirement of the system (Ochieng et al., 2003). A high Umf will result in a high fluidization power requirement. Therefore, it is necessary to control Uf slightly above Umf with as much accuracy and precision as possible (Delebarre et al., 2004). Umf can be calculated using Equation (7) (Alfredo et al., 2013).
Umf ¼ 16:50
d2 ðrs rÞg
m
(7)
Where. Umf ¼ minimum fluidization velocity (m/h) d ¼ solid particle diameter (mm) rs, r ¼ specific weight of the solid and the water, respectively (g/ m3 ) m ¼ dynamic viscosity of water (g/m/h) g ¼ acceleration of gravity (m/h2) Increasing Uf leads to increase in liquid circulation and mixing rate, thus a shorter reaction time. However, this is only true up to the optimum Uf. On the other hand, very high Uf is associated with particle wash-out from the reactor, especially where FBBRs are employed (Jaafari et al., 2014). Therefore, selection of optimum Uf is necessary to ensure successful operation of FBR. Although liquid velocity dominates in a three-phase FBR, the flow regime depends on the ratio of the superficial liquid velocity to the superficial gas velocity. Both velocities have to be properly designed and controlled. It is necessary to have a small liquid velocity to gas velocity ratio in order to have high mass transfer coefficients for counter-current flow FBR (Forster, 1980). However, for a concurrent flow process, a high ratio will give a well dispersed bubbles and hence high oxygen transfer rate whereas a low ratio would result in bubble coalescence and low oxygen transfer rate (Yu et al., 1999). Both superficial liquid and gas velocities will affect solid and liquid holdups. Increasing superficial liquid velocity will cause solid particles to expand faster and hence reduces the solid holdup (Akilamudhan et al., 2014). This will in turn increase the liquid holdup. On the other hand, gas holdup increases with increase in superficial gas velocity (Wei et al., 2000). Nikolov et al. (2000) reported that liquid velocity has a weak effect on the oxygen transfer and gas velocity has a strong effect in a three phase IFBR. In general, the smaller the amount of air supplied, the more economical the process would be which is desirous for industrial applications (Vimonses et al., 2010). However, the capacity of FBBR in aerobic wastewater treatment depends on the oxygen transfer rate of the aeration system (Forster, 1980). Sabarunisha & Radha (2014) studied the hydrodynamic behavior of an inverse FBBR for phenol biodegradation and reported that COD removal increased with an increase in the
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superficial gas velocity. This was attributed to the higher gas holdup, volumetric mass transfer coefficient and oxygen transfer rate due to the increased gas velocity. The highest COD and phenol degradation rates were obtained at a gas velocity of 0.220 m/s. Above this optimum value, however, the degradation efficiencies became lower. At gas velocities above the optimum value, larger bubbles are formed which dominate over the interfacial area, resulting in lower mass transfer. Uf is affected by the properties of bed materials as well as the aspect ratio of the reactor (Zhong et al., 2008). 4. Effect of operational parameters Since FBR is most commonly used in Fenton oxidation, photocatalytic oxidation and aerobic/anaerobic treatments, the discussion on the operational parameters has given emphasis to them. Some of the operational parameters that affect FBR performance include pH, catalyst concentration, amount of H2O2, temperature, UV intensity, HRT and OLR. 4.1. pH pH has a remarkable effect on FB-Fenton as efficient degradation of pollutants is attributed to acidic pH (Li et al., 2015). This is because the production of OH is enhanced at lower pH, usually in the range of 2.5e4.5 (Malik and Saha, 2003). Higher pH favors the formation of ferric and ferric hydroxide complexes which have much lower catalytic capability than ferrous ions. On the other hand, very low pH promotes hydrogen formation which may reduce the number of active sites for generating ferrous ions (Ratanatamskul et al., 2010). In other words, a low pH leads to the formation of Fe2þ:H2O2 complex, which react more slowly with H2O2 and thereby produces OH at a slower rate. A high pH leads to the formation of Fe2þ:OH complexes which reduces the amount of Fe2þ that are responsible for the decomposition of H2O2. Su et al. (2011b) investigated the effect of operational parameters on the decolorisation of textile wastewater by FB-Fenton and reported that the process performance increased as the pH was increased from 2 to 3. However, increasing the pH above 3 led to a decrease in the removal efficiency. The degradation was highest at pH 3. Low pH was associated with production of FeOOH2þ which compete with Fe2þ in reacting with H2O2. At pH above 4, Fe2þ is unstable and easily forms Fe3þ. However, in their investigation of parameters affecting FB-Fenton process for the treatment of recalcitrant organic silicon wastewater, Liu et al. (2014) reported that the pH has an insignificant effect on the COD and TOC removal. Other parameters such H2O2/Fe2þ and particle loading were found to have more impact on the process. Therefore, the effect pH may also depend upon other process parameters such as Fe2þ/H2O2 concentration. From the reviewed literature, the optimum pH in FB-Fenton process ranges between 2 and 4 which is quite similar to the range used in conventional Fenton oxidation (Table 1). In FB-Photocatalysis, pH affects the charge on the catalyst particles, size of catalyst aggregates and the positions of conductance and valence bands (Meng et al., 2010). At low pH, the environment around the photocatalyst would become positively charged and negatively charged at high pH. In order to fully study the impact of pH on photocatalytic oxidation, the concept of point of zero charge (PZC) of TiO2 is important. The PZC is a condition where the surface charge of TiO2 is zero or neutral and lies in the pH range of 4.5e7.0. The implication is that the interaction between the photocatalyst and the contaminants is minimal. Details on the PCZ have been explained elsewhere (Meng et al., 2010). Under acidic conditions, the surface of is TiO2 protonated and deprotonated under alkaline condition (Gaya and Halim, 2008). Thus, TiO2 has higher oxidizing
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power at low pH. Nam et al. (2002) investigated the effect of pH on the photocatalytic degradation of methyl orange in a three-phase FBR and obtained higher performance at lower pH values. At lower pH, reduction by electrons in the conduction band plays an important role in the degradation while at neutral or high pH levels, hydroxyl radicals may be the predominant oxidation species. However, a very low pH may result in excess Hþ which can lower the reaction rate. In FBBR, pH is an important parameter that affects process performance because of its effect on microorganisms. A very high or low pH can affect FBBR performance due to the inhibitory effect of superacidity and superalkalinity on the activity of the intracellular enzyme of bacteria (Jianping et al., 2003). Lin et al. (2010) studied the biodegradation of Reactive blue 13 in a two-stage anaerobic/ aerobic FBBR with a Pseudomonas sp. At pHs between 5 and 9, the color removal efficiency fluctuated between 75.6% and 86.9%, while the total COD removal efficiency varied between 67.7% and 90.4%. The optimum pH for the degradation was between 6 and 7. Zeroual et al. (2007) investigated the effect of pH (3e9) on the decolorisation of different dyes by calcium alginate-immobilized Geotrichum sp. in FBBR. Maximum decolorisation was achieved at pH 5. Above pH 5, the decolorisation rate decreased significantly. Jianping et al. (2003) studied the effect of pH on the denitrification treatment of low C/N ratio nitrate-nitrogen wastewater in a threephase FBR. The optimum pH was found to be between 6.5 and 7.5. Similarly, District et al. (1996) reported an improvement in FBBR performance when pH was increased from 7 to 7.5. 4.2. Temperature Temperature is an important operational parameter, especially in FBBR. Unlike other technologies such as a fixed-bed reactor, temperature fluctuation is uncommon in FBBR because of the excellent phase mixing. Therefore, fluidization will improve the process performance in terms of temperature uniformity. Nevertheless, it is necessary to establish the optimum temperature for a given process. High temperature may cause protein denaturation while low temperature may inhibit the activity of intracellular enzymes, resulting in low metabolism (Jianping et al., 2003). For example, an efficient denitrification was achieved at an optimum temperature of 30 C during autotrophic denitrification using FBBR (Zou et al., 2016) while Jianping et al. (2003) reported an optimum temperature between 20 and 35 C for the removal of nitrate-nitrogen from wastewater using a three-phase FBBR. On the other hand, Zeroual et al. (2007) reported that the decolorisation rate of four different dyes using calcium alginate-immobilized Geotrichum sp. increased when the temperature was increased from 25 to 35 C. However, when the temperature was increased to 45 C, the decolorisation rate decreased drastically. The reduction of decolorisation efficiency at 45 C was attributed to the denaturation of the enzymes involved in the degradation of the azo dyes and the loss of fungal cell viability at high temperature.
H2O2 þ OH / HO2 þ H2O
(8)
HO2 þ OH / H2O þ O2
(9)
However, the effect of H2O2 on the process is closely linked to the amount of Fe2þ. Therefore, the ratio of H2O2 to Fe2þ is equally important. Indeed, some researchers have reported the effect of H2O2 vis- a-vis the concentration of Fe2þ (Su et al., 2011a; 2011b). In a study to establish the optimum condition for degrading 2,4Dichlorophenol using FBR-Fenton process, H2O2 was found to be the major parameter affecting the process (Muangthai et al., 2010). Increasing H2O2 from 1 to 10 mM led to an increased degradation from 70% to 98% at a constant Fe2þ concentration (0.1 mM). Wang et al. (2015) investigated the degradation of Orange G in FB-Fenton and reported that the decolorisation of the dye increases with increase in H2O2 up to 25 mg/L. There is variation in the optimum amount of H2O2 in the FBFenton process reported in the literature (Table 1). Though most of the studies have reported between 50 and 60 mM, others have used concentrations outside this range. Since there are differences in the reported studies either from the pollutant being degraded or other process parameters, these variations were to be expected. However, all the studies have shown that the reaction rate increases with an increase in H2O2 concentration until the optimum amount is reached. High concentration may inhibit the degradation due to the scavenging effect of the H2O2. Also the ratio of H2O2 to Fe2þ was shown to be significant when discussing the effect of H2O2 on the process. Therefore, the optimum dosage of H2O2 is applicationspecific and depends on other parameters such as the Fe2þ concentration. 4.4. Fe2þ concentration Fe2þ acts as a catalyst in decomposing H2O2 to produce OH in FB-Fenton. Therefore, increasing its concentration leads to an increase in the production of OH. However, increasing the concentration above the optimum amount can inhibit the process (Muangthai et al., 2010). This is because excess Fe2þ acts as a scavenger, reacting with OH and decreasing their availability to oxidize the pollutant. Excess Fe2þ may also cause turbidity in the reactor, decreasing light penetration when UV is used (Wang et al., 2015). This underscores the importance of establishing the opti-vis the amount of H2O2. mum dosage of Fe2þ vis-a Previous studies have shown that the concentration of Fe2þ has a positive effect on pollutant degradation in FB-Fenton, with the optimum dosage around 5 mM in most of the reported literature (Table 1). Ratanatamskul et al., 2010 found that the degradation of 2,6-dimethylaniline by FB-Fenton process increased from 83% to 100% when Fe2þ was increased from 1 to 5 mM. The optimum concentration was found to be 2.5 mM. In another study, Briones et al. (2012) studied the degradation of acetaminophen using FBFenton process and found that increasing the iron concentration from 0.01 to 0.1 mmol1 led to increase in ACT degradation. However, the increase was more evident at acidic pH.
4.3. H2O2 concentration 4.5. Photocatalyst concentration In FB-Fenton, the concentration of H2O2 determines the potential OH that can be generated in the reaction. Thus, pollutant degradation increases with increase in H2O2 concentration, until the optimum concentration is reached. Beyond the optimum concentration, process performance usually decreases due to scavenging effect of the OH (Ahmadi et al., 2015). Scavenging effect occurs when the excessive H2O2 acts as radical scavenger and change the more reactive OH to a less reactive hydroperoxyl radicals as shown in Equations (8) and (9) (Li et al., 2015).
FB-Photocatalysis uses metal oxide, usually TiO2, as the photocatalyst. The concentration of TiO2 has a strong effect on the rate of photocatalytic process and hence, the performance of the reactor. When the amount of TiO2 is increased, the photocatalytic activity increases and therefore higher degradation rate is obtained. This is because the surface area available for photocatalytic reaction increases with an increased in the catalyst concentration. However, increasing the concentration above the saturation level may lead to
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turbidity (Meng et al., 2010) which reduces light penetration into the reaction matrix and hinders the photocatalytic performance of the reactor (Zulfakar et al., 2011). Also from an economic viewpoint, excess catalysts should be avoided so as to keep the operating cost as low as possible. Therefore, it is imperative to establish the optimum dosage of the TiO2. Where immobilized TiO2 is used, additional removal is obtained via adsorption of pollutants on the surface of the solid. Youngsoo Na et al. (2005) studied the photocatalytic decolorisation of rhodamine B by immobilized TiO2/UV in an FBR and reported that the decolorisation rate increased with increase in TiO2 dosage. The immobilized TiO2 was responsible for both degradation and adsorption of the dye. However, since UV light was used, excessive amount of the catalyst could hinder light penetration. A dosage of 33.8 g/L was found to be the optimum concentration. Details of typical photocatalyst concentrations reported in the literature can be found in Table 2. 4.6. UV intensity A source of photons, mostly UV lamp, is usually introduced into FB-photocatalysis and FB-Photo-Fenton processes. The rate of photon increases as the number and intensity of UV lamps increase. The degradation efficiency of FB-photocatalysis increases with UV light intensity, and shorter wavelength gives higher degradation (Lim and Kim, 2002). Shorter wavelength light is adsorbed more strongly by TiO2 particles than longer one. Therefore, the penetration distance of photons into TiO2 particles is shorter and electrons and holes are formed closer to the surface of the particles. In their study for the Photooxidation of sodium lauryl sulfate in a three-phase FBR using TiO2/SiO2, Nam et al. (2009) found that the degradation rate increased with an increase in the UV light intensity. The intensity of light determines the potential number of photons that can be generated and hence the number of photons that may eventually reach the catalyst surface. Thus a 65 W lamp gave a better performance than a 30 W lamp. Also the position of the UV lamp in relation to the reaction matrix is also important. The closer the lamp is to the reaction matrix the higher the amount of photons reaching the reactants. Kanki et al. (2005) investigated the influence of UV irradiation on FB-photocatalytic process using TiO2-coated ceramic particles. Two FBRs, one with an internal UV lamp (254 nm, 9 W) and the other with an outside black lamp (365 nm, 15 W) were used. The reactor with the internal UV lamp degraded the pollutant 4 times faster than the other reactor. This signifies the importance of the proximity of the light source to the reaction matrix. 4.7. Hydraulic retention time (HRT) HRT is the average length of time the wastewater stays in the reactor. Generally, long HRT leads to a better performance of FBRAOPs, until the optimum time is reached. Above the optimum HRT, there is usually little or no further degradation of the pollutant. During a treatment of recalcitrant organic silicone wastewater using FB-Fenton, Li et al. (2014) found out that the degradation of COD and TOC increased as the HRT was increased from 15 to 60 min, with no further degradation thereafter. Thus, the optimum HRT was 60 min corresponding to COD and TOC removal efficiency of 90% and 78% respectively. In another study for COD removal from hospital wastewater, 98% removal efficiency was achieved at an HRT of 90 min (Anand et al., 2015). In FBBR, any change in HRT is likely to affect the OLR and hence the performance of the reactor (Haroun and Idris, 2009). The efficiency of pollutant removal in FBBR is a function of the HRT which is concomitant with OLR (Perez et al., 2007). Increasing the HRT
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leads to a decrease in OLR, other conditions being equal. For a constant OLR, increase in HRT will lead to higher performance of the process as microorganisms will have more time to degrade the pollutant. However, above the optimum HRT, the process becomes independent of the HRT (Borja et al., 2001). Although HRT can be controlled through flow rate manipulation, it may pose fluidization challenges for the bioparticles (Christianson et al., 2015). Thus, there is need for a trade-off between HRT and flow rates to obtain the optimum condition for specific applications. For aerobic FBBR, the optimum degradation is usually achieved within a short HRT, depending on the microorganisms, pollutants and other operational parameters (Table 3). Rajasimman and Karthikeyan (2007) investigated the effect of HRT on the treatment of starch wastewater in an aerobic FBBR and reported that the COD reduction increased with increased in HRT for all initial substrate concentrations. The optimum removal efficiency of 93.8 5% was achieved after 24 h. The effect of HRT on anaerobic FBBR follows the same trend as that of aerobic FBBR. Cuenca et al. (2006) investigated the anaerobic biodegradation of diesel fuel-contaminated wastewater in an FBBR and reported that both the diesel and COD removal efficiencies increased with increased in HRT for all the conditions investigated. Lin et al. (2010) studied the effect of HRT on dye degradation using a two-stage anaerobic/aerobic FBR and reported that the overall degradation was enhanced when HRT was increased from 20 h to 70 h. 4.8. Organic loading rate (OLR) OLR is the measure of organic pollutants which is expressed as kilogram (kg) of COD per cubic-meter (m3) per day (d). In FBBR, OLR is normally manipulated through variation of flowrate. When OLR is increased, the performance of the system reduces due to shock and disturbance on the biomass. However, the microorganisms adjust to the new OLR and the process performance usually resumes. Excellent FBBR performance is usually associated with low OLR (Balaji and Poongothai, 2012). On the other hand, when the substrate is limiting in the process, increasing OLR can improve the ndez et al., 2008). In that case, process performance (Ferna increasing OLR provides more food to microorganisms and hence more biomass production. Carbajo et al. (2010) reported a slight decreased in phenol and TOC degradation as the OLR was increased in their treatment of phenol in an anaerobic FBBR. However, the degradation efficiency resumed once the microorganisms acclimatized to the new OLR. Mustafa et al. (2014) studied the anaerobic digestion of municipal wastewater sludge using FBBR and reported that the treatment performance decreased with increased in OLR. A volatile suspended solids (VSS) removal efficiency of 88%, 79% and 70% were achieved at OLRs of 4.2, 9.5 and 19 kg COD/m3-d respectively. A COD removal efficiency of 68% was equally obtained at OLR of 19 kg COD/m3-d. Borja et al. (2004) carried out mesophilic anaerobic digestion of wastewater from the production of protein isolates from chickpea flour in an FBBR and reported that the percentage COD removal decreased with increased in OLR. 5. Conclusions and future perspectives FBRs have been widely used in wastewater treatment, particularly biological processes and AOPs, due to their excellent features such as high mass transfer rate, excellent mixing and low sludge generation. Few studies have also appeared on the use of FBR for adsorptive removal of recalcitrant pollutants. Since the design and operation of FBR depends largely on experience and empirical approach, an understanding of the important design and
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operational parameters is necessary for successful application of the technology. This review discusses the applications of FBR in wastewater treatment, with emphasis on design and operational parameters affecting process performances. Although the review is non-exhaustive, especially with the technology still unfolding, major parameters affecting the process such as reactor geometry, particle size, particle density and loading, superficial fluid velocity, catalyst concentration, pH, HRT, and OLR have been discussed. Reviewed literature shows that using FBR in wastewater treatment increases process performance and could help in addressing some of the drawbacks of the conventional treatment technologies. In the last two years, more than 80% of the reported studies on FBR applications in wastewater treatment have been on FB-Fenton and FBBR. Therefore, these two represent the most important applications of FBR in wastewater treatment. In particular, FBR has shown potential to overcome the problem of excessive sludge generation in conventional Fenton oxidation. Reported studies show that the support materials in FB-Fenton can crystallize about 30e65% iron from the solution and can achieve 15e20% better performance than conventional Fenton oxidation. This indicates that FBR can increase the cost-effectiveness of Fenton oxidation. In biological wastewater treatment, most of the recent studies have been on improving process performance, energy efficiency and fouling prevention in membrane bioreactor. Fluidized bed membrane bioreactor using GAC as support material reduces membrane fouling and consequently lowers energy requirements of the process. In some studies, anaerobic FBBR has been integrated with AOPs such as photocatalysis, where energy generated from methane can provide the power required by the UV. Although previous studies have shown encouraging results on the application of FBR in wastewater, more studies will be needed to address some of the existing gaps in the literature. One of the major challenges is that fluidization is still an empirical science, relying largely on empirical correlations and heuristic approach. As FBR operation is known to be inherently complex, modeling and optimization of wastewater treatment using FBR are challenging due to the additional parameters involved. Fortunately, many models have been put forward for the gas-solid FBR, which can be modified to suit the liquid-solid system. Studies on the influence of the major parameters on treatment performance of FBR are somewhat limited. The existing studies have largely reported on the treatment performance and the effect of some operational parameters. Future studies are therefore needed on process optimization and modeling of FBR in wastewater treatment, taking the process hydrodynamics into consideration. Application of FB-Fenton has shown potential under laboratoryscale investigations. However, more studies are needed towards process improvement and large-scale applications. In this context, optimization of solid carriers used in FB-Fenton is particularly imperative since iron crystallization and removal depend on the solid types, particle size and loading. This is yet to be put forward. It is not clear yet, from the literature, how the superficial velocity affects the treatment performance of FB-Fenton. Since the effect of process parameters depends on the pollutant being degraded, future studies should investigate the efficacy of FB-Fenton in treating a wide range of recalcitrant pollutants. It is also important to consider the effectiveness of FB-Fenton in treating real wastewater as only about 15% of the reported studies have been on real wastewaters. Despite the reported excellent performance of FBFenton under treatability studies, large-scale applications will depend largely on their cost-effectiveness. Although combining FBR with Fenton can increase the cost-effectiveness due to the low sludge production and improved process performance, the cost of the combined technology still needs to be evaluated.
Combining FBBR with other wastewater treatment technologies such as membrane bioreactor is an attractive research area. Some recent studies have shown that combining FBR with membrane bioreactor can improve process performance (Gao et al., 2014) and alleviate membrane fouling (Kim et al., 2016). This has the potential to reduce energy consumption and increase cost-effectiveness of the process. Another approach that is considered attractive is the integration of biological processes and AOPs using FBR. Since AOPs are more cost-effective when applied as tertiary treatment technologies, FBBR can be used as initial treatment to reduce the organic loading, followed by FBR-AOPs to mineralize the recalcitrant pollutants. For example, integrating anaerobic digestion and photocatalytic oxidation using FBR can improve cost-effectiveness as the methane generated from the anaerobic digestion could provide the necessary energy for driving AOPs. It is obvious that the excellent features of FBR can be exploited in wastewater treatment as indicated by the recent growing interest in the topic. Overall, FBR has the potential to improve process performance and increase the cost-effectiveness of various wastewater treatments, particularly AOPs and biological processes. A cost-effective wastewater treatment technology is particularly important to industries as they strive to adopt zero-discharge as a means of cost reduction and environmental sustainability. The outcomes of this study provide a prerequisite knowledge for understanding the important parameters affecting the performance of FBR in wastewater treatment while at the same time highlighting some existing gaps for future studies. Acknowledgment This work was financially supported by the University of Malaya High Impact Research Grant (UM.C/HIR/MOHE/ENG/37) from the Ministry of Higher Education Malaysia and University of Malaya, Kuala Lumpur, Malaysia. References Abdel-aziz, M.H., El-abd, M.Z., Bassyouni, M., 2016. Heat and mass transfer in three phase fluidized bed containing high density particles at high gas velocities. Int. J. Therm. Sci. 102, 145e153. http://dx.doi.org/10.1016/j.ijthermalsci.2015.11.020. Abdelmotalib, H.M., Youssef, M.A.M., Hassan, A.A., Youn, S.B., Im, I.T., 2015. Heat transfer process in gas-solid fluidized bed combustors: a review. Int. J. Heat. Mass Transf. 89, 567e575. http://dx.doi.org/10.1016/ j.ijheatmasstransfer.2015.05.085. Abidi, N., Errais, E., Duplay, J., Berez, A., Jrad, A., Schafer, G., Ghazi, M., Semhi, K., Trabelsi-Ayadi, M., 2015. Treatment of dye-containing effluent by natural clay. J. Clean. Prod. 86, 432e440. http://dx.doi.org/10.1016/j.jclepro.2014.08.043. Ahmadi, M., Ramavandi, B., Sahebi, S., 2015. Efficient degradation of a biorecalcitrant pollutant from wastewater using a fluidized catalyst-bed reactor. Chem. Eng. Commun. 202, 1118e1129. http://dx.doi.org/10.1080/ 00986445.2014.907567. Akilamudhan, P., Sivakumar, P., Mohanraj, R., Senthilkumar, K., 2014. Prediction of solid holdup in an internal loop airlift fluidized bed reactor. Interntional J. ChemTech Res. 6, 4460e4467. Alalm, M.G., Tawfik, A., Ookawara, S., 2015. Comparison of solar TiO2 photocatalysis and solar photo-Fenton for treatment of pesticides industry wastewater: operational conditions, kinetics, and costs. J. Water Process Eng. 8, 55e63. http://dx.doi.org/10.1016/j.jwpe.2015.09.007. pez, J.S., Rodríguez, P.U., 2013. Fluidized Bed Series: Secondary Alfredo, J.B., Lo Treatments [WWW Document]. Technol. Fact Sheets Effl. Treat. Plants Text. Ind. URL. http://www.thermopedia.com/content/46/?tid¼104&sn¼1297. Anand, A.A., Adish Kumar, S., Rajesh Banu, J., Ginni, G., 2015. The performance of fluidized bed solar photo Fenton oxidation in the removal of COD from hospital wastewaters. Desalin. Water Treat. 3994, 1e7. http://dx.doi.org/10.1080/ 19443994.2015.1021843. Ananpattarachai, J., Kajitvichyanukul, P., 2016. Enhancement of chromium removal efficiency on adsorption and photocatalytic reduction using a bio-catalyst, titania-impregnated chitosan/xylan hybrid film. J. Clean. Prod. 130, 126e136. http://dx.doi.org/10.1016/j.jclepro.2015.10.098. Andalib, M., Elbeshbishy, E., Mustafa, N., Hafez, H., Nakhla, G., Zhu, J., 2014. Performance of an anaerobic fluidized bed bioreactor (AnFBR) for digestion of primary municipal wastewater treatment biosolids and bioethanol thin stillage.
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