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Journal of Environmental Management 146 (2014) 260e275

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Journal of Environmental Management

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Review

Applicability of fluidized bed reactor in recalcitrant compounddegradation through advanced oxidation processes: A review

Farhana Tisa, Abdul Aziz Abdul Raman*, Wan Mohd Ashri Wan DaudDepartment of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia

a r t i c l e i n f o

Article history:Received 24 April 2014Received in revised form22 July 2014Accepted 24 July 2014Available online 2 September 2014

Keywords:Waste water treatmentAdvanced oxidation processesFluidized bed reactorCost estimation

Abbreviations: AOPs, advanced oxidation processeFBR-ozone, fluidized bed reactor with ozonation; FBRwith Fenton oxidation; FBR-photo catalytic, fluidizedalytic oxidation; FBR-homogeneous Fenton, fluidizedneous Fenton oxidation; FBR-heterogeneous Fentonheterogeneous Fenton oxidation; RB5, reactive black 5RO16, reactive orange 16; RB2, reactive blue; 4-CP,aminophen; MEA, mono ethanol amine; TCP, tri chloDCP, di chloro phenol; TFP, tetra fluro propanol; UVcarbon filter; GAC, granular activated carbon; COD, chtotal organic carbon; BOD, Biological oxygen demantime; TSS, total suspended solids; DOC, dissolved orga(iron); FH ratio, Ferrous and hydrogen peroxide ratio* Corresponding author.

E-mail addresses: [email protected], rshazRaman).

http://dx.doi.org/10.1016/j.jenvman.2014.07.0320301-4797/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Treatment of industrial waste water (e.g. textile waste water, phenol waste water, pharmaceutical etc)faces limitation in conventional treatment procedures. Advanced oxidation processes (AOPs) do notsuffer from the limits of conventional treatment processes and consequently degrade toxic pollutantsmore efficiently. Complexity is faced in eradicating the restrictions of AOPs such as sludge formation,toxic intermediates formation and high requirement for oxidants. Increased mass-transfer in AOPs is analternate solution to this problem. AOPs combined with Fluidized bed reactor (FBR) can be a potentialchoice compared to fixed bed or moving bed reactor, as AOP catalysts life-span last for only maximum of5e10 cycles. Hence, FBR-AOPs require lesser operational and maintenance cost by reducing materialresources. The time required for AOP can be minimized using FBR and also treatable working volume canbe increased. FBR-AOP can process from 1 to 10 L of volume which is 10 times more than simple batchreaction. The mass transfer is higher thus the reaction time is lesser. For having increased mass transfersludge production can be successfully avoided. The review study suggests that, optimum particle size,catalyst to reactor volume ratio, catalyst diameter and liquid or gas velocity is required for efficient FBR-AOP systems. However, FBR-AOPs are still under lab-scale investigation and for industrial application coststudy is needed. Cost of FBR-AOPs highly depends on energy density needed and the mechanism ofdegradation of the pollutant. The cost of waste water treatment containing azo dyes was found to be US$50 to US$ 500 per 1000 gallons where, the cost for treating phenol water was US$ 50 to US$ 800 per1000 gallons. The analysis for FBR-AOP costs has been found to depend on the targeted pollutant,degradation mechanism (zero order, 1st order and 2nd order) and energy consumptions by the AOPs.

© 2014 Elsevier Ltd. All rights reserved.

s; FBR, fluidized bed reactor;-Fenton, fluidized bed reactorbed reactor with photo cat-bed reactor with homoge-

, fluidized bed reactor with; RBB, remazol brilliant blue;4 chloro phenol; ACT, acet-ro phenol; BA, benzoic acid;, ultra violet; ACF, activatedemical oxygen demand; TOC,d; HRT, hydraulic residencenic carbon; T-Fe, total ferrous.

[email protected] (A.A. Abdul

1. Introduction

Untreated effluents from industries usually contain a consider-able amount of contaminants and pollutants and are usually highlyrecalcitrant. These toxic effluents can directly affect the health ofthe ecosystem and human life. Therefore, rigid environmentalregulations for the control of effluents are enforced in severalcountries (Ayoub et al., 2011). By the same token, the adverseenvironmental effects of wastewater in recent years have urgedresearchers to find contemporary and competitive wastewatertreatment methods (Mendez-Arriaga et al., 2009). However, tech-nological advancement and more profound research to overcomethe problem are still needed. Textile wastewater, phenol containingwastewater and petroleum wastewater are among the dischargesthat have harsh impacts on our environment (Kalra et al., 2011;Zupanc et al., 2013). Phenols and azo dyes are well known fortheir bio-recalcitrant and acute toxicity (Fan et al., 2009; Zhanget al., 2009; Li and Zhang, 2010). There is a probability that, dye

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F. Tisa et al. / Journal of Environmental Management 146 (2014) 260e275 261

effluent may contain chemicals that are toxic and carcinogenic toliving species in water (Daneshvar et al., 2003; Song et al., 2008).These recalcitrant compounds are continuously added to theaquatic environment through various anthropogenic inputs. In thefollowing sections, characteristics of recalcitrant wastewaters,limitations of different conventional treatment procedures will bediscussed.

Recalcitrant compounds are high molecular species with hy-drophobic nature (Kalra et al., 2011). They are resistant to biodeg-radation and they mainly consist of dyes, alcohols, phenols andnitrogenous and sulfur compounds. A great number of industrialactivities are identified for generating recalcitrant wastewater (DiIaconi et al., 2010; Shukla et al., 2010). Printing, dyeing and textileindustry effluents contain chemicals that exhibit developmentaltoxicity and carcinogenicity (Daneshvar et al., 2003; Oller et al.,2011). However, some chemicals that are released to the environ-ment are resistant to biodegradation. Thus, the environmentalpersistence and toxicity of these compounds are of rising concerns.Wastewater is generally characterized by the biological oxygendemand (BOD), chemical oxygen demand (COD), pH, total organiccarbon (TOC) and color. Table 1 summarizes some characteristics ofrecalcitrant wastewater.

Based on the literature review, it is found that the typical textileindustry wastewater characteristics can be summarized by achemical oxygen demand (COD) range from 150 to 12,000 mg/L,total suspended solids between 2900 and 3100 mg/L, total Kjeldahlnitrogen from 70 to 80 mg/L, and BOD range from 80 to 6000 mg/Lleading to a biodegradability (BOD/COD ratio) of around 0.25,showing that it contains large amounts of non biodegradableorganic matter (Nidheesh et al., 2013) . Pharmaceutical wastewatercontains initial COD range of 300e500 mg/L and petroleum re-finery wastewater contains COD range of 300e2000 mg/L. Con-ventional treatment of these components is difficult becausebiologically resistant organics do not induce oxygen depletion inreceiving water (Guo and Al-Dahhan, 2005). The effluent fromresin, pulp and paper industries carry high COD concentration. Thebiodegradability (BOD/COD) range is very low for these industrialeffluents. According to Malaysia's environmental law, environ-mental quality act, 1974, the Malaysia environmental quality(sewage and industrial effluents) regulations, 1979, 1999, 2000, theCOD limit for wastewater from textile, pulp and paper and petro-leum industry is 50e100 mg/L. Consequently, contemporary

Table 1Typical characteristic of different types of recalcitrant wastewater

Type of wastewater pH COD (mg/L)

Textile waste waterIndustrial 7.2e8.1 e

Industrial 10.66 1476Industrial 6e10 150e10,000Industrial 10 1150Industrial e 111Industrial 6.95 3422Phenolic and petroleum waste waterPetroleum 8.0e8.2 850e1020Petro chemical 7e9 300e600Pharmaceutical waste waterAntibiotics 7.7 ± 0.32Amoxicillin, Ampicilin, Cloxacilin. 520Landfill leachate 8 3840

e 3358.3e8.8 2320e24803.5 7437.25 38,200

Others (pulp and paper, sanitary) 512020,000e50,000

treatment processes are required to treat the wastewater in orderto comply with legal discharge limits. Successful wastewatertreatment is not only required to remove the organic matter but tocompletely convert them into harmless end products and meet theenvironmental regulations (Aparicio et al., 2007).

Biological and chemical processes have failed to convert thecontaminants fully as, biological and chemical processes anddegrade up to 60% of the recalcitrant components and in additionthey require larger operation area and more chemical processes toreduce the sludge. For complete degradation of highly recalcitrantindustrial wastewater, conventional methods are combined withchemical, biological, physical methods and also advanced oxidationprocesses (Slokar and Marechal, 1998; Babuna et al., 1999; Olleret al., 2011). The study of limitations of conventional processes todetermine the necessity of the advanced procedures is necessary toget complete idea of implementing advanced oxidation and reactorsystem. As stated by Martinez et al., (2007) the conventionaltreatment methods suffer from limitations on their application andeffectiveness. Other studies have also showed that industrialwastewater can be toxic even after conventional treatment (Selcuk,2005). Conventional treatment posses some limitations like, in-crease in toxicity level, more power consumption, plugging andclogging (Leiknes, 2009) and less degradation efficiency(Ledakowicz and Gonera, 1999). Table 2 presents some conven-tional treatment procedures used to treat different types of in-dustrial wastewater and their limitations. Because of the scarcity inspace, extremely high land cost and the difficulties in managingchemicals, finding an easy and resourceful treatment process whichrequires minimum chemical consumption and space, for the in-dustrial wastewater seems necessary (Kobya et al., 2003).

Most of the available conventional treatment processes are notefficient and only degrade wastewater partially which producecontaminated intermediates and sludge (Staehelin and Hoigne,1982; Adams et al., 1994). They are also not suitable for highchemical oxygen demand (COD) reduction (Dibble and Raupp,1992). On the other hand, advanced oxidation processes (AOPs)have the potential of completely treating refractory compounds(Chou and Huang, 1999a). It is also proven to be feasible in treatingindustrial effluents (Ayoub et al., 2011). There are four major typesof AOPs e photo catalytic oxidation (which requires a photo activecatalyst and UV light), UV/H2O2, UV/ozone oxidation and Fentonoxidation (Zhang et al., 2008), all of which are covered in this

BOD (mg/L) TOC (mg/L) References

115e730 e (Badani et al., 2005)491 336 (Arslan and Balcioglu, 2001)100e4000 e (Kalra et al., 2011)170 e (Selcuk, 2005)9.3 e (Lin and Chen, 1997)e 900 (Kobya et al., 2003)

570 300e440 (Coelho et al., 2006)150e360 e (Ma et al., 2009)

(Adams et al., 2002)(Elmolla and Chaudhuri, 2009)

1200 e (Chianese et al., 1999)e e (Linde et al., 1995)180e345 (Mohajeri et al., 2010)10 283.6 (Cortez et al., 2010)2200 e (Aygun et al., 2012)4526 (Dias et al., 2005)19,000e25,000 (Gulsen and Turan, 2004)

Table 2Applied techniques on degradation of common recalcitrant pollutant and their limitations

Polluted water Applied System Limitations References

Textile waste waterVat green 01 Biological - Process only inhibits 10% microbial

growth in the textile wastewater(Schrank et al., 2007)

General Reverse osmosis - Highly dependent on solution pH- Difficult to reject undissociatedorganic compounds

- Membrane Cleaning require hightemperature and time

- Reusable only after cleaning

(Ozakia and Lib, 2002)

General Coagulation/flocculation - Higher operation and maintenancecost due to sludge handling andincreased sludge volume

- Increases toxicity level of COD inwastewater

(Golob et al., 2005; Linand Peng, 1996; Selcuk, 2005)

General Electrochemical oxidation - Needs external energy source (Lin and Peng, 1994)General Ultra filtration - Regeneration of adsorbents

- High cost(Ciardelli et al., 2001;Malaeb and Ayoub, 2011)

Phenolic and petroleum waste waterPhenol Electrochemical oxidation - Needs external energy source

- Ineffective treatment(Comninellis and Pulgarin, 1991)

Industrial Biological treatments - Low COD removal- Produce toxic intermediates

(Bianco et al., 2011)

General Coagulation - Secondary phase is generated (Semerjian and Ayoub, 2003)Petro chemical Activated carbon adsorption - Partially degrade the effluent

- Difficult reusability(Ma et al., 2009)

Phenol Wet air oxidation - Total capital and operating costis too high

(Timofeeva et al., 2005)

Pharmaceutical waste waterIbuprofin Biological treatment - These compounds are resistant

to biological degradation- Complete degradation of thecomplex compounds not possible

(Mendez-Arriaga et al., 2009)

General Activated sludge treatment - Incineration of used activatedsludge is costly

- Reusability is not easy

(Sipma et al., 2010)

General Membrane bioreactors - Limited applicability in highlycontaminated pollutants

- High pressures are needed forefficient COD removal whichincrease the operational cost

- Fouling of the membranes and- Requires large amount ofwastewater to produce clean water

(Leiknes, 2009; Zupanc et al., 2013)

F. Tisa et al. / Journal of Environmental Management 146 (2014) 260e275262

review study. However, AOPs are yet not without limitations.Studies reveal that an effective contacting device system can in-crease the potential of advanced oxidation systems. The mostcommon reactors for the treatment of recalcitrant organic com-pounds are fixed bed reactors (Baban et al., 2010), packed bed re-actors (Yamazaki et al., 2000) and fluidized bed reactors (Shih et al.,2013). Bed reactors can carry various multiphase reactions. Thereare twomajor types of bed reactors based on the position of the bedmaterial, which are fixed/packed bed-reactors and fluidized bedreactors. AOPs application with a reactor technology can reducecatalyst damages and also increase the reusability. AOPs can beconducted in lab scale, pilot scale or in large scale bed reactors. Acomparison among the available bed reactors for AOPs applicationhas been studied. The comparison of bed reactors in terms of theneeded hydraulic residence time (HRT) and efficiency is presentedin Table 3. When combined with AOP, packed bed, fixed bed andFBRs are easy to operate and they require less oxidant to degradethe pollutants. Packed/fixed bed reactors have fallen behind insome cases such as being unable to provide greater surface area andgreater mass transfer rate. Tian et al. (2011) came up with similarstatement that, in packed bed photo catalytic reactor, the UV irra-diation is exposed to smaller surface area. Temperature control iseasier in FBR and it provides sufficient contact between the re-actants by overcoming gaseliquid mass transfer limitations.

Fluidized Bed reactor is a technology which is now widelyapplied in many industrial applications. In recent studies it isevident that, fluidized bed reactors can also be an attractive pro-cedure for treating polluted water. Fluidized bed reactors workmore efficiently and can be used in AOPs. AOPs merged with FBRcan be more potential in pollutant abatement. Fluidized bed reactoris widely applied in many industries for various applicationsrecently. It has been found promising to use fluidized bed reactorfor water treatment procedures. When the conventional treatmentprocedures failed to remove recalcitrant compounds in waste wa-ter, advanced oxidation processes came as a foremost choice by theresearchers. Some authors have used fluidized bed reactor withconventional treatment procedures, such as, brown coal (Babanet al., 2010), anaerobic treatment (Maloneya et al., 2002; Sen andDemirer, 2003) and also electrochemical procedures (Zhou et al.,2004). Advanced treatment technologies that involve highlyoxidizing compounds like hydroxyl radicals (OH�) have overcomethe limitations of biological and chemical treatment procedures.

There are a limited number of studies that have used AOPscombined with bed reactors to degrade recalcitrant organic com-pounds. Section 2 has been dedicated to discuss the theory of AOPsand the possibility of enhancement in reactor performance withimplementation of reactor process. Fig. 1 summarizes possible FBR-AOPs treatment methods together with the respective advantages

Table 3General overview and comparison of bed-reactors for pollutant degradation

Reactor system Pollutant degraded Removal Efficiency HRT Reference Highlights

Fixed/packed bed Textile wastewater Reasonable 50 h (Georgiou et al., 2005) - Catalyst reusability innone in case of fixedbed reactor

- Mass and heat transferis low

- initial cost is higher- Catalyst was proved tohave considerably goodstability (consideringsome post treatmentssuch as filtration)

Azo dye reactive dyebath from cotton industry

COD 70e93%Color 99%

14e30 h (Baban et al., 2010)

Phenol COD 100%TOC 77%

120 min (Yang et al., 2008)

Dimethyl sulphoxide Around 25% degradation 240 min (Wu et al., 2006)Trichloroethylene Degraded to intermediate

components4 h (Yamazaki et al., 2000)

Chlorinated phenol Complete mineralizationof phenol, 4-CP, 2.4-DCPand 2.4.5-TCP

150e1140 min (Al-Ekabi and Serpone, 1988)

Textile wastewater COD 80%Color 100%

160 min (Moreira et al., 2005)

Fluidized bed Reactive Black 5, Reactiveorange 16, Reactive blue 2

COD 57e91%Dye 82e100%

100 min (Su et al., 2011a) - Catalyst reusability isvery good (up to 4 or6 cycles)

- Reaction time iscomparatively less

- Very good mass andheat transfer

- Eliminates sludge- Less operating cost- Maintaining cost is low- Large volume of fluidcan be processed

Benzoic acid 100% Depletion of BA andDOC present of 41.3%

4.5 h (Chou and Huang, 1999a)

Benzoic acid 100% Depletion of BA andDOC present of 41.3%

4.5 h (Chou et al., 2001)

TFT-LCDmonoethanolamine(MEA, H2NC2H4OH)

Maximum 98.9%COD 64.7%T-Fe 43.5%TOC 62.0%

2 h (Anotai et al., 2012)

Aniline 60 min (Anotai et al., 2010)Nitrobenzene 30e65% iron

90% oxidation ofnitrobenzene

(Anotai et al., 2009)

Phenol 98% 180 min (Huang and Huang, 2009)


and limitations. Since combining AOP treatment process with re-actors can be more promising in industrial applications, thisresearch area needs to be explored further. Section 3 describes FBRtechnique and in Section 4 fluidized bed reactor applications inpollutant degradation is elaborated to highlight the importantfactors and their effects. A detailed investigation of the effectivedesign and operating parameters of FBR-AOPs is summarized inSection 5. Finally, a brief cost analysis has been done with availableliterature data and general capital cost analysis method to under-stand the cost associated with FBR-AOPs in Section 6.

Waste watertreatment

technologies

Conventional treatment methods ( i.e:

coagulation, flocculation, biological oxidation,

osmosis etc)

-Mostly costly due to huge space requirement-Degradation time needed is higher

Fenton oxidation

Homogeneous Fenton

oxidation

-Degradation rate is higher than heterogeneous -Easier Operation system

FBR-homogeneous

Fenton-Lesser amount of oxidant needed -Efficient mass and heat transfer

HeterogenFenton oxid

-ur-rp

FBR-heterogeneous

Fenton-Minimal catalyst breaking and iron loss-Efficient mass and heat transfer

Fig.1. Overview on treatment of organic

2. Theory of advanced oxidation processes (AOPs)

The advantages such as no remnant toxicity and high applica-bility have made advanced oxidation processes (AOPs) a promisingalternative solution. AOPs are considered environmentally sus-tainable for their lower consumption of process energy (Song et al.,2008). Considerable similarities are found in reaction mechanismsof all the AOPs for the participation of hydroxyl radicals (OH�).Depending on thematrix and on the pollutant, degradation kineticsof AOPs can be zero order, 1st order and 2nd order. First order

Advanced Oxidation Processes

eous ation

Can degrade nder wider ange of pHCatalyst egeneration is ossible

Ozonation

FBR-Ozone

-Alternative of H2O2

-Can degrade gaseous pollutants efficiently as well

-High degradation rate-Works better with catalyst application

Photocatalyst

-Prompt degradati

on reaction

FBR-photo

catalytic-Complete degradation -Quickest degradation rate

pollutants using different FBR-AOPs.


kinetics is achieved for pollutant degradation with respect to con-centration of hydroxyl radicals. It is found from the literature thatnormally pseudo first order kinetic constant is within 1e10�4 s�1

for generation of hydroxyl radicals with AOPs (Chou and Huang,1999a). Continuous “In situ” production of OH� is needed throughphotochemical and chemical reactions due to instability of OH�. Theprinciples of OH� generation is based on various combinations ofstrong oxidants, such as oxygen, ozone, hydrogen peroxide (H2O2),ultra violate (UV), and electron beam (Bach et al., 2010; Garrido-Ramírez et al., 2010). Mostly, a combination of these oxidantswith a catalyst is applied. TiO2 with UV (Kanki et al., 2005), Fentonprocess with iron oxide (Kalra et al., 2011), ozonation with catalyst(Lin and Lai, 1999), Fenton like reaction and etc. are examples ofthese combinations. Theoretically, there are two steps for AOPs,which are (a) Hydroxyl radical generation and (b) Oxidative reac-tion between radicals and molecules. Effective hydroxyl radical isgenerated by the use of UV, UV/H2O2, UV/O3, UV/Fe2þ/H2O2, O3/H2O2, TiO2/H2O2, Fe2þ/H2O2 or O3/H2O2/UV. The mechanisms ofdifferent AOPs are presented in Table 4.

2.1. Fenton oxidation

Fenton's reagent has gained its effectiveness in degradingvarious types of organic contaminants. Fenton oxidation has beenstudied extensively because of its ability to decompose numerousorganic compounds (Lin and Lai, 1999; Qu et al., 2007; Bach et al.,2010). Several reactions can take place in heterogeneous Fentonoxidation with intermediate products, water and carbon dioxide(Bach et al., 2010). Their generally active sites for homogeneousFenton processes are either Fe2þor Fe3þ. Whereas, heterogeneousFenton processes can be activated by the surface of iron ions

Table 4Mechanisms of different advanced oxidation processes

Name of the AOP Types Mechanism Reaction

Fenton oxidation Homogeneous H2O2 þ Fe2þ/Fe3þ þ OH� þ OH�

Heterogeneous Fe2þ þ H2O2/Fe3þ þ OH� þ OH�

Fe3þ þ H2O2/FeHOO2þ þ Hþ

FeHOO2þ þ Hþ/Fe2þ þ HO�

2OH

� þ organics/productsH2O2 þ OH

�/H2Oþ HO

�

2Fe2þ þ OH

�/Fe3þ þ OH

�

Fe2þ þ HO�

2/Fe3þ þ HO�2

Fe3þ þ HO�

2/Fe2þ þ O2 þ Hþ

Ozone O3/H2O2 H2O2 þ 2O3/2OH� þ 3O2

O3/UV O3 þ H2O/2OH� þ O2

UV UV/H2O2 H2O2 ��!l>300 nm2OH�

UV/Fe2þ FeðOHÞ2þ þ UV/Fe2þ þ OH�

Fe2þ þ H2O2/FeðOHÞ2þ þ HO�

Photo catalytic Photo catalysts TiO2 ��!l<400 nme�ðcbÞ þ h�ðvbÞ

e�ðcbÞ þ h�ðvbÞ/heatH2O2 þ h�ðvbÞ/OH

� þ Hþ

OH� þ dyes/colourless

existing in multiple forms of [Fe(OH)2]þ, [Fe(H2O)]2þ, [Fe(H2O)6]3þ,[Fe2(OH)2]4þ, Ferrous polycation, Fe2O3 and a-FeOOH (Soon andHameed, 2011).

The key features of homogenous Fenton system are reagentconditions, i.e. [Fe2þ], [Fe3þ], [H2O2] and the reaction characteris-tics (pH and the concentration of organic and inorganic constitu-ents) (Neyens and Baeyens, 2003). With the application of Fentonprocess, significant reduction of toxicity, improvement of biode-gradability, color, COD, BOD, TSS, oil grease and odor removal isachieved in some stubborn industrial waste water (Mandal et al.,2010; Soon and Hameed, 2011). Some of the limitations of ho-mogeneous Fenton include limited reaction, acidic pH range2.5e3.5, the formation of the sludge because of the post-treatmentprocess, high iron loss to environment, H2O2 scavenger, difficultyof the iron ions recovery and the wastewater cannot be dischargedwith the iron ions if above European Union limits (>2 ppm)(Gogate and Pandit, 2004; Navalon et al., 2010; Soon and Hameed,2011). These have made the development of a heterogeneouscatalyst for the degradation of wastewater pollutants in Fentonsystem imperative. Heterogeneous catalysts are much easier toseparate from liquid products; they are noncorrosive and envi-ronmentally benign. They can diminish the final concentration ofiron ions in the bulk after treatment, thus with the assistance of UVirradiation, the formed Fe3þ complexes can be destroyed, allowingFe3þ ions to participate in the Fenton catalytic cycle (Kasiri et al.,2008).

Several authors (Neyens and Baeyens, 2003; Gogate and Pandit,2004; Bolong et al., 2009; Emami et al., 2010) have well defined thereaction mechanisms, kinetics and stoichiometry in homogenousFenton process. Regardless of the complexity of the reactions, it hasbeen modeled using simple pseudo first order kinetics and pseudo

Highlights Reference

- Degradation of pollutant happensin acidic aqueous mixture

- High efficiency

(Lucas and Peres, 2007)

- Reaction is possible in pH 5e7- Catalyst reusability is possible- Lag phase is observed that happensin activating the catalyst

(Bach et al., 2010;Garrido-Ramírez et al., 2010)

- This combined AOP works betterin higher pH values

- Degrades the pollutants in tosmaller cycle particles

- Inactive in reducing COD- Oxidation rate is higher- Follows direct pathway for pollutantdegradation

(Esplugas et al., 2001)

- UV accelerates ozone molecules andproduces oxidizing radical


- UV irradiation supplies energy to thechemical compounds as radiation

- Reaction molecules reaches theirexcited state absorbing UV andpromotes further reaction


- Effectively can be applied in varietyof pollutant degradation

- high synergy effect between the ozoneand the UV radiation has been noted

(Iurascu et al., 2009)

- Degradation of pollutants takes placeby redox oxidation

- First order kinetics are observed

(Chen and Chou, 1994)


second order kinetics (Arslan et al., 2000; Duran et al., 2011). Inhomogeneous Fenton process, high amount of treated effluent isprecipitated as ferric hydroxide sludge when the reaction solutionwas neutralized in the post-treatment (Bolong et al., 2009).Whereas, in heterogeneous Fenton system, minimal ferric hy-droxide is formed due to leaching of the active components into thebulk solution (Garrido-Ramírez et al., 2010). However, the futureprospect of heterogeneously catalyzed Fenton system and itsapplication seems very intense, which is because of its ease ofseparation, it is still in the laboratory testing stage (Soon andHameed, 2011). Nonetheless, it is found that, the production ofsubstantial amount of Fe (OH)3 precipitate in Fenton can be solvedby the use of iron oxides in a contacting device (Faust and Hoigne,1990). Different contacting devices such as packed bed (Al-Ekabiand Serpone, 1988; Yamazaki et al., 2000; Mesquita et al., 2012)and fluidized bed (Chou et al., 2001; Dong et al., 2007; Anotai et al.,2012) have been used for pollutant degradation with Fentonoxidation.

2.2. Ozonation

During the recent years, ozone has been used for the non-persistent disinfection of water and treated wastewaters, and alsofor industrial wastewater treatment. Ozone is itself a very powerfuloxidant (E0 ¼ 2.07 V vs. SHE) but in certain conditions it candecompose and lead to the formation of hydroxyl radicals (Malaeband Ayoub, 2011). When O3 is applied in aqueous solution it mayreact with various compounds in two possible ways. The first one isthe direct way, where reaction takes place between the molecularozone and the dissolved compounds via cyclo addition or electro-philic reaction. The ozonation takes place by increasing the pH andgenerates hydroxyl radicals, apart from adding H2O2. The kinetics ofthe ozonation of organic chemicals has been extensively studied.Unsaturated aliphatic compounds react faster than saturated hy-drocarbons (Moreira et al., 2005). The second way of ozoneoxidation is indirect way in which ozone is used alone for decom-position of the pollutant and the pollutants are absorbed in thecatalyst particle as the oxidation takes place on the surface of thecatalyst when catalyst and ozone are used together. Hoign�e andBader (1983) established kinetic models for the reaction of ozonewith different organic and inorganic compounds.

By interaction between the ozone and hydrogen peroxide in O3/H2O2 system, hydroxyl radicals are generated by a radical-chainmechanism, a type of combined ozonation (Gulsen and Turan,2004; Georgiou et al., 2005). The efficiency of this process can beimproved by UV irradiation. However, the application of ozonationmight not be feasible from the economic point of view becauseozone is relatively less soluble and stable in water. Its production iscostly and it leads to only partial oxidation of organic compounds.Thus, numerous approaches have been taken to improve theoxidizing efficiency of this procedure, leading to minimization ofthe reaction time and reduce energy cost (Spadaro et al., 1994).Strong adsorbents have recently been applied in ozonation whichalso help oxidation with their catalyzing effects. The application ofadsorbents in ozonation process can reach its peak efficiency whena contacting device is applied. Moussavi et al., (2014) used a FBR forcatechol degradation with ozonation and states that, a fluidized-bed reactor provides proper contact between ozone as a gasphase, wastewater containing catechol as a liquid phase, andcatalyst as a solid phase, thereby increases the mass transfer andrate of catechol degradation reactions. In the same context, appli-cation of irradiation is believed to increase oxidation process andbetter irradiation efficiency can be achieved using reactor. In case ofUV combined with O3, the energy is supplied by UV radiation,which interacts with O3.

2.3. UV oxidation and photo catalytic oxidation

Photolysis of aqueous H2O2 has been investigated in the past bymany authors (Rodriguez Couto et al., 2002). These investigationshave depicted that UV/H2O2 process provides a dominantmeans forentire or partial oxidation of organic pollutants in aqueous me-dium. High-activity (E0 ¼ 2.8 V) hydroxyl radical is generated byusing Fe2þ/H2O2 system in UV Fenton process, (Rodriguez Coutoet al., 2002). UV can photolyze the Fe(OH)2þ to produce the extrahydroxyl radicals in the photo Fenton reaction of an appropriatewavelength (with highest quantum efficiency at 313 nm) (Namet al., 2001; Feng et al., 2003). UV oxidation is expected to workbetter in devices that facilitate increasedmass transfer possibilities.Likewise, ozonation process, UV oxidation requires adsorbents forbetter COD removal and efficiency. Thus, better UV irradiation canbe achieved by applying a contacting device.

The reaction mechanism of photo catalytic oxidation is slightlydifferent compared to heterogeneous catalysis reaction, as the ef-ficiency of photo catalytic oxidation is affected by the photongeneration and mass transfer limitation. The mechanism estab-lished for the photolysis of hydrogen peroxide is the cleavage of themolecule into hydroxyl radicals with a quantum yield of two OH�

radicals formed per quantum of radiation absorbed (Legrini et al.,1993). The interaction involving a semi conductor and the UV ra-diation produces electron hole pairs in the surface of the semiconductor in photo catalysis process (Gaya and Abdullah, 2008).The basic reaction mechanism comprises of illumination of anaqueous TiO2 suspension with irradiation with energy greater thanthe band-gap energy of the semiconductor produced valence-bandholes (hþTiO2

) and conduction-band electrons (e�TiO2) (Mohapatra

et al., 2014). These charged points of the semi-conductor arecapable in reacting with both organic compounds and water(Esplugas et al., 2001). Destruction of the organic compound isachieved by redox reactions whereas in the end hydroxyl radicalsare generated and these radicals react with the organic compound(Gimenez et al., 1997). TiO2 is the most common semiconductorused in photo catalysis. In general, the photo catalytic reactionfollows the LangmuireHinshelwood mechanism. The reaction fol-lows the first order kinetics in diluted solutions. Inactivity of thephoto catalyst can be observed after a while in photo catalyticoxidation. This can be eliminated by applying the oxidation processin a reactor system (Moreira et al., 2005). Many comparisons ofvarious aspects of AOPs have been done by different authors(Esplugas et al., 2001; Mahamuni and Adewuyi, 2010). From theliterature review, it is found that all the discussed AOPs can displayhigher degradation efficiency if a reactor system is applied. It canalso minimize the oxidant amount needed and reduce the reactiontime.

3. Fluidized bed reactor (FBR)

Compared to other types of reactors (e.g. fixed bed reactors),fluidized bed reactors have a number of advantages: In particularfor catalytic reactions, where solid catalyst particles are suspendedby reactant liquid, the advantages include the large liquidesolidinterface area and the nearly isothermal temperature distributioneven for highly exothermal reactions. Moreover, fluidized bedshave excellent particle mixing and liquidesolid contacting. Ingeneral, FBR can be described as a packed bed through which fluidflows at such a high velocity that the bed is loosened and theparticle-fluid mixture behaves like a fluid. The working principle ofa fluidized bed is to fluidize the catalyst with a fluid velocity. Bothgas and liquid flow can be used to fluidize a bed of particles. In thisway, two objectives can be achieved (1) the mass transfer rate isresumed and (2) pH and temperature control are easier. Uniform


temperatures can be maintained in FBR as the fluidized bed takespart in continuous agitation of the solids in contact with the fluid,leading to brilliant contact between the solid and the fluid and thesolid and the reactor wall. FBR is mostly applied in catalyticcracking in the petroleum industry (Graham et al., 2006; Atta et al.,2009). Also, fluidized bed reactors have benefits such as simplicityof construction and operation, low operating cost, and high flexi-bility for liquid and solid phase residence times (Moussavi et al.,2014).

Large volume of fluid can be processed using FBR (Chou et al.,2001; Gulsen and Turan, 2004; Kanki et al., 2005). Fluidizationcan be a two-phase or a three-phase system. The characteristics andbehaviors of a fluidized bed are strongly dependent on solid, liquidand gas properties. Gaseliquidesolid fluidized bed operation is ofconsiderable importance in industries and some processes such aschemical, petro-chemical, bio-chemical, mineral processing in-dustries, incineration of waste and volatile organic compounds andsoftening of drinking water by crystallization process (Oluleye et al.,2012). Nevertheless, FBR process is still unknown and requires awidespread investigation for commercial application. Studies haveshown that combining FBR with AOP oxidation can improvedegradation and decontamination (Anotai et al., 2012). Forinstance, Gulsen and Turan (2004) achieved good COD removal andtreatment of landfill leachate (Gulsen and Turan, 2004), and Senand Demirer (2003) stated that, the HRT for the treatment was 1day for load of 3 kg COD/m3/d and the COD removal efficiency wasat a moderate level. Combination of AOP with FBR leads to higherremoval efficiencies. Besides, the combination of FBR with AOPprocess requires smaller HRT than other treatment processes (Chouet al., 2001). In the study of Lim et al. (2008) it is believed that FBRwill be more advantageous than fixed bed reactor because goodcontacts between catalyst and reactants can be achieved and bettercontact between catalyst and light can be achieved by the use ofphoto catalytic catalysts. Photo catalysts can be easily charged andremoved from the FBR, resulting in easy regeneration andreusability.

4. FBR-AOPs in pollutant degradation

Apart from petro chemical and bio-chemical applications, FBRcan be successfully applied in environmental engineering treat-ment processes such as advanced oxidations, anaerobic treatmentof wastewater, absorption, wet hydrogen peroxide oxidation. Use ofFBR-AOPs in recalcitrant compound degradation is discussed in thissection.

4.1. Textile waste water

Textile wastewater is one of the major concerns in environ-mental protection because of its color, toxicity, and low biode-gradability (Ay et al., 2008; Ho et al., 2010). This industry issubsequently, one of the largest producers of wastewaters (until300 L kg�1 material) (Soares et al., 2014). Textile wastewater con-tains diverse pollutants which include hydrocarbon matter, oil,solid, grease and pigments. And 10e15% of dye pigments of textileindustry are anticipated to be released with the effluent (Yuranovaet al., 2004). Degradation of organic dye molecules are of greatimportance in recalcitrant water treatment. In most cases, dyes arestudied as model compounds for large organic molecules.Advanced oxidation processes (AOPs) are the processes involvingsimultaneous use of more than one oxidation processes, sincesometimes a single oxidation system is not sufficient for the totaldecomposition of dyes (Gupta, 2009). In literature, reactive blue,reactive orange azure B, crystal violet, methyl orange (acidebaseindicator) and real dyeing wastewater have been found to be

degraded with FBR-AOP methods. Criticality of dye molecularstructure governs the degradation rate. Photo catalytic oxidationhas been proven to be more suitable for dyes degradation(Nadtochenko and Kiwi, 1997; Ho et al., 2010). Nam et al. (2001), Hoet al. (2010) and Rodriguez Couto et al. (2002) successfully imple-mented FBR-photo catalytic process for methyl orange (Nam et al.,2001), reactive black b (Ho et al., 2010), crystal violet and azure B(Rodriguez Couto et al., 2002). Degradation happens with hetero-geneous catalysts reactions that occur on the surface of the catalystas well as in the bulk phase in photo catalytic FBR process (Lin andLin, 1993; Lin and Liu, 1994). Chou et al. (2001) treated real-timewastewater from a dyeing mill with initial COD range of185e200 mg/L with FBR-Fenton procedure and reduced COD levelto near 80 mg/L. Ho et al. (2010) examined effect of pH, adsorptionand desorption, UV intensity, initial concentration of RBB anddiffering amounts of H2O2 and B1 catalyst in remazol brilliant blue(RBB) degradation in a FBR-photocatalytic system, and found thisprocess to be quiet efficient. The RBB and TOC removal efficiencywas found to be in range of 95e100% and 35e95%. Analysis on thiswork, shows that, pH effects on the absorbance of UV light oncatalyst and higher pH significantly shift absorbance range as well.

Su et al. (2011b) found FBR-Fenton process to be a possibletechnique for treating textile waste water. This process greatlyenhanced the decolorization of textile wastewater up to 93%. Incase of various dye mixtures, the degradation increases withcontinual increase of H2O2 concentration until a certain limitation;which is dependent on pollutant characteristics. Because of thescavenging effect, extra hydrogen peroxide is consumed at higheramount of peroxide in solution. Similarly, as conducted by Su et al.(2011a), three reactive dyes were treated using a FBR. Dyes withsimple azo bonds (reactive orange 16) showed higher removal ef-ficiency. The removal efficiency of color and COD for RO16 washigher because of the simple structure and azo bond (N]N) (Suet al., 2011a). The N]N bond of dyes are cleaved resulting in co-lor loss (Azbar et al., 2004). Reactive blue 5 was also degraded withphoto catalytic brick grain-supported iron oxide in the presence ofoxalic acid in a FBR and the decolorization experiments were per-formed in the FBR with aeration under UV-A irradiation (Chenget al., 2011). Textile wastewater and organic dyes are normallydegraded in two-phase FBR with Fenton and UV oxidation. Ozoneoxidation can also be useful if additional catalyst is used for CODdegradation. Decolorization of dye wastewaters by ozonation hasbeen accepted bymany investigators (Lin and Lin,1993; Lin and Liu,1994). In the study of Lin and Lai (1999), textile waste water(mixture of four dyes) was degraded by ozone and granular acti-vated carbon in a FBR. Ozone is proved effective in decolorizing thewastewater very quickly but the COD removal is not satisfactoryand for those reason adsorbents (granular activated carbon) wasused.

FBR-AOP degradation is known to increase with increasingoxidant ratio. However, there is still an optimum limit for oxidantamount. In comparison with Fenton, the required amount ofoxidant and catalyst is lesser in FBR-Fenton. Badawy and Ali (2006)showed that 100% color removal in a conventional Fenton processrequires ferrous dosage of 400 mg/L and hydrogen peroxide of550 mg/L (Badawy and Ali, 2006). On the other hand, Su et al.(2011b) mentioned that only 1e5 mg/L of ferrous and 160 mg/Lof hydrogen peroxide was needed in FBR-Fenton process for com-plete degradation of dyes. It clearly shows the mass transfer effi-ciency reduces the amount of catalyst and oxidant in quite largeamount in FBR. Moreover, Su et al. (2011b) explained that FBR-Fenton facilitates more COD removal by producing iron oxide onthe surface of the carriers by crystallization or sedimentation (Suet al., 2011b). Therefore, FBR-Fenton produces less sludge.Another example of comparison can be given in case of combining


ozone with FBR, where only 28.5% of COD removal was achieved insolely O3 oxidation (Azbar et al., 2004) within 30 min of reactiontime. But in the research of Lin and Lai (1999) 60% of COD removalwas achieved using FBR. Likely, increasing decolorization wasobserved with increasing ozone gas flow rate.

4.2. Phenol containing waste water

Chlorinated phenol compounds are considered to be hazardoussubstance and priority pollutant by the U.S. Environmental Pro-tection Agency. It is reported that phenol compounds can causeserious damage to human health at low concentration. Concen-trations above 2000e4000 mg/L, phenol can be recovered fromwastewaters cost-effectively with suitable circumstances. Phenoldestruction is the paramount method for treating phenol contain-ing waste water below these concentrations. It is difficult tocompletely remove chlorinated phenol containing wastewater dueto its biodegradability. Degradation of these phenol compounds byFBR-AOPs has been studied by some researchers (Spadaro et al.,1994; Huang and Huang, 2008). Nonyl-phenol and bis-phenol-Awere chosen as the targeted pollutant by Kanki et al. (2005).These hazardous chemicals were effectively decomposed using.Two types of FBR-photocatalytic have been used by them fordecomposition experiments. FBR-photo catalytic with UV lampinside the reactor is proven to degrade the pollutant more effec-tively. Another example of FBR-photocatalytic in phenol degrada-tion is the work of Huang and Huang (2009). FBR-Ozone wasapplied on phenol containing water by Qu et al., (2007). In thestudy, the degradation was performed in assistance of activatedcarbon adsorption. Phenol was almost completely degraded in FBR-ozone-activated carbon method within 30 min of ozonation (99%for 1e2 g activated carbon fiber).

Combination of FBR with AOP reduced the amount of sludge incase of phenol degradation due to minimization of leached ironspecies (Baban et al., 2010). Application of FBR reduces the requiredreaction time. Zazo et al. (2005) stated that 100 mg/L phenol wasdegraded within almost 250 minwith oxidant amount of 500 mg/Land catalyst of 1 mg/L (Zazo et al., 2005, 2009). However, Huangand Huang (2009) documented that 100% conversion of phenolcould be achieved in less than 120 min for same amount of oxidant.Fluidized bed application also decreases remaining iron content inthe process. Muangthai et al. (2010) showed that degradation of1 mM 2,4, dichlorophenol using the conventional Fenton processachieved 45% of COD removal where FBR-Fenton achieved 51%. Ironremoval percentage in FBR-Fenton (16%) was higher than conven-tional Fenton process (6%).

4.3. Other recalcitrant waste water

Artificial aromatic chemical o-toluidine is used as an interme-diate in textile manufacturing and is for producing rubber, chem-icals and pesticides as a curing agent for epoxy resin systems.Powerful oxidation methods are required because o-toluidine is biorefractory 99.8% of toluidine destruction was achieved by usingFBR-Fenton procedure in the research of Anotai et al. (2012).Nitrobenzene which is widely used in industries (such as dyemanufacturing, pharmaceuticals and rubber) is believed to becarcinogenic. Anotai et al. (2009) concluded that nitrobenzenedegradationwas very fast in Fenton process and even faster for FBR-Fenton process. Another compound, aniline which is widely used inpharmaceutical and pesticide industries was degraded in anotherresearch of Anotai et al. (2010), in which they found that electro-Fenton helped in faster oxidation rate of aniline compared to flu-idized bed-Fenton process. Considering the depletion rate of H2O2,FBR-Fenton is better than electro Fenton. One of the most

commonly detected pharmaceutical compounds, acetaminophen(ACT) is an analgesic and antipyretic substance. ACT was degradedby FBR-Fenton process as stated by De Luna et al. (2013) thedegradation was dependent on Fe2þ and FH ratios (Ferrous andhydrogen peroxide ratio). The degradation path is of the secondorder kinetics.

Fluorinated compound 2, 2, 3, 3-Tetrafluoro-1-propanol is anexample of another recalcitrant organic compounds that isdegraded by FBR-AOP processes. TFP was degraded by three phaseFBR with UV irradiation (Shih et al., 2013), where, UV/H2O2 couldeffectively eliminate all the TOC and fluoride ions from the waste-water by 90%. Benzoic acid, which is an intermediate product formany aromatic compounds, was also oxidized by the use of FBR-Fenton (Chou et al., 2001). Chou and Huang (1999b) appliedbatch fluidization and circulating fluidization in their research.Available literature shows that, FBR-Fenton achieves much higherCOD removal compared to other FBR processes (Muangthai et al.,2010).

Many studies have been done on photo catalytic oxidationcombinedwith two-phase fluidization (Dibble and Raupp,1992). Atthe same time, through studies on oxidation with ozone combinedwith fluidized bed has been done (Spadaro et al., 1994; Lin and Lai,1999). FBR-Fenton that decreases the amount of produced sludgehas been developed as a promising method for wastewater treat-ment as Fenton process has a remarkable disadvantage e the pro-duction of iron sludge and the need to dispose those (Su et al.,2011a). This process follows the following steps: (i) Homogeneouschemical oxidation (H2O2/Fe2þ), (ii) Heterogeneous chemicaloxidation (H2O2/iron oxide) (iii) FBR crystallization, in which ironsludge precipitation on carrier surface occurs and (iv) Reductivedissolution of FeOOH (Muangthai et al., 2010; Su et al., 2011b). Theferric hydrolysis product of Fenton reaction that crystallizes andgrows on the surface of the carriers decreases the precipitation inpuffy ferric hydroxide form (Anotai et al., 2012). Despite itsapparent complexity, De Luna et al. (2013) also modeled the re-actions of FBR-Fenton by using simple pseudo first order andpseudo second order kinetics. The literature available on FBR-AOPshas been presented on Table 5.

5. Parameter affecting FBR-AOPs

FBR has two remarkable features; firstly, maintaining steadystate dissolved iron concentration by providing high mass transferefficiency; and secondly, preventing catalysts from the damage ofmechanical mixing. The catalyst can survive a longer time by usingFBR technique (Chou et al., 2001; Huang and Huang, 2009). How-ever, there are also some challenges associated with FBRs. Sincefluidized beds are a heterogeneous mixture of gas and solids with aliquid-like behavior, the proper description of the phenomenataking place in a fluidized bed is difficult (Rüdisüli et al., 2012).Unlike conventional AOPs, FBR-AOPs can save a big amount ofoxidant cost which indicates FBR-AOP is more feasible and can beeasily commercialized (Muangthai et al., 2010). The significant FBR-AOP parameters should be identified for the implementation of FBRin advanced oxidation systems. Identifying these parameters is alsonecessary for design and scale up. FBR-AOPs is normally two-phasefluidization comprising of gasesolid (Xu et al., 2004; Zhang et al.,2006) and liquidesolid (Su et al., 2011b; Anotai et al., 2012) reac-tion. But in some cases there is three-phase fluidization (3-P) (Linand Lai, 1999; Nam et al., 2001). Close attention should be givento monitor if the fluidization pattern can potentially affect theoxidation process. Fluidization pattern of a FBR is governed byparticle size, character and the flow rate of liquid/gas phase. Someoperational parameters like pH, oxidant concentration, pollutantconcentration, catalyst properties, loading of the carrier or catalyst

Table 5Different parameters on the performance of fluidized bed reactor in degrading organic pollutants

Process Fluidization system Parameters Degradation efficiency Reference

FBR-photo catalytic Solidegas TrichloroethyleneCatalyst: Titanium dioxide supportedUV length: 300e500 nmGas flow rate: 261e361 cm3/min

23e100% (Dibble and Raupp, 1992)

Solidegas Mixed gaseous carbonyl compoundsCatalyst: silica-supported TiO2/SiO2

Size: 2e4 mmLoading: 5 gWorking volume : 100 mlMinimum fluidization velocity: 0.77 cm/sTerminal velocity of particle: 78.93 cm/s

>99% (Zhang et al., 2006)

Solideliquid Phenol and bisphenol A.Catalyst: TiO2 coated SiO2 particlesSize: ~1 mmLoading: 7% volWorking volume: 4 L

>99% (Kanki et al., 2005)

Solidegas Airborne styreneCatalyst:SGP251CCGas flow rate: 0.90 cm/sWorking volume: 0.226 L

80% (Lim and Kim, 2004)

Solideliquidegas Methyl orangeCatalyst: immobilized TiO2

Size: 21 nmLoading: 0.1 g/LWorking volume: 0.848 LLiquid flow rate: 0.5e2 L/min

100% (Nam et al., 2001)

Solideliquid PhenolCatalyst: FeOOH and SiG2

Size: FeOOH: 30e50 mesh Glass beads: 4 and 2 mmLoading: 20e30 g/LWorking volume: 0.15 L

TOC 98% (Huang and Huang, 2009)

Gasesolid Airborne styreneCatalyst: SGP251CCWorking volume: 0.4398 LLiquid flow rate: 0.90 cm/s

80% (Lim et al., 2008)

FBR-Fenton Solideliquid Benzoic acid (BA)Catalyst: Supported FeOOH with Glass BeadsSize: FeOOH: 0.564 mmGlass beads: 4 and 2 mmLoading: 590 gWorking volume: Approximately 4 LLiquid flow rate: 40 m/h

BA 96%COD 59e63%

(Chou et al., 2001)

Solideliquid NitrobenzeneCatalyst: Glass BeadsSize: 0.8 & 2.0 mmLoading: 76.9 gWorking volume: Approximately 2.86 L

30e65% iron removal90%


Solideliquid AnilineCarrier: Glass BeadsSize: 2e4 mmLoading: 100 gWorking volume: Approximately 0.859 L

Ethanolamine 98.9%TOC 18e35%


Solideliquid TFT-LCD monoethanolamine (MEA, H2NC2H4OH)Carrier: Glass BeadsSize: 2e4 mmLoading: 100 gWorking volume: Approximately 0.859 L

Maximum 98.9%COD 64.7%T-Fe 43.5%TOC 62.0%


Solideliquid - Reactive Black 5 (RB5),- Reactive Orange 16 (RO16)- Reactive Blue 2 (RB2)

Carrier: SiO2 or Al2O3

Size: 2e4 mmLoading: 74.07 g carrier/L

Discoloration 99%, 99% and 96%COD e 34%, 47% and 49%

(Su et al., 2011a)

Solideliquid Textile wastewater from dying millCarrier: SiO2

Size: 2e4 mmLoading: 74.07 g/LWorking volume: 1.35 L

Decolorization e 92%Oxidation Efficiency e 49%

(Su et al., 2011b)

Solideliquid 2,4 dichloro phenolCarrier: SiO2

Size: 2e4 mmLoading: 100 gWorking volume: 1.35 L

2,4DCP 99%COD 55%Iron 14%

(Muangthai et al., 2010)


Table 5 (continued )

Process Fluidization system Parameters Degradation efficiency Reference

Solideliquid Landfill leachateCarrier: Quartz sand particlesSize: 0.5 mmLiquid flow rate: 0.229 L/hWorking volume: 13 L

COD 85% (Gulsen and Turan, 2004)

Solideliquid Acetaminophen (ACT)Carrier: SiO2 and Glass BeadsSize: 2e4 mmWorking volume: 1.45 L

60% ACT (De Luna et al., 2013)

FBR-UV Solideliquidegas 2,2,3,3-Tetrafluoro-1-propanolCarrier: BT5 iron oxideSize: 0.25e0.5 mmLoading: 0e20 gWorking volume: 0.6 LLiquid: 40 m/h

99.95% TOC99% fluoride

(Shih et al., 2013)

Solideliquid Reactive Black (RB5)Carrier: Brick-grain supported iron oxideSize:0.25e0.5 mmLoading: 0e20 gWorking volume: 1.73 LLiquid flow rate: 40 m/h

Almost 90% RB5 degraded80% TOC

(Cheng et al., 2011)

FBR-Ozone Solideliquidegas Textile waste waterCatalyst: Granular activated carbonLoading: 100 gWorking volume: 11.3 LGas flow rate of 4 L/min

Color 93%COD >80%

(Lin and Lai, 1999)

Solideliquid Phenolic waste waterCatalyst: activated carbon fiber (ACF)Loading: 0e2 gWorking volume: 0.353 L1.5 L/min ozone airflow rate

Phenol 99%COD 95%

(Qu et al., 2007)


and the flow rate of liquid phase and gas phase (air/ozone) arefound to have major effects on the oxidation process. The effects ofdifferent parameters on the reaction kinetics can be explained withproper knowledge of the chosen catalysis process, working process,hydrodynamics and heat and mass transfer process of a FBR.

5.1. Effect of pH

The dependence of degradation rate on pH of the solutionshould be investigated in order to employ any catalytic degra-dation system in FBR successfully. In some cases, researchershave observed increased decomposition rate with increasing pH(Gulsen and Turan, 2004). Some of the researchers haveconcluded that the catalytic activity increases in a pH controlledenvironment (Lu, 1999; Shih et al., 2013). In the experiments ofHuang et al. (2001), three different iron oxides for degradingphenol were investigated. It was found that, the activation ofcatalyst depended on the solution pH and the surface area of thecatalyst. In the work of Nam et al. (2001), the effect of pH of arange of 3.1e9.9 on degradation of methyl orange in a threephase (3-P) FBR was studied. pH has been found to have a strongeffect on the disappearance rate of methyl orange and highremoval rate of methyl orange was obtained at lower pH values.Reaction which is dependent on pH of the solution (for instanceFenton process) is affected by non-uniform pH in the FBR. The pHdependent reaction can also occur at a little greater pH inanaerobic process rather than aerobic process. The increase insolution pH expedites the complete mineralization of the organiccompounds. So, FBR-Fenton and photo catalytic-FBR normallywork in acidic condition where basic pH is explored by FBR-ozone. It was found that reactor geometry did not affect the re-action under certain pH condition. Degradation of pollutant isalso dependent on the amount and concentration of oxidizingagents in FBR-AOPs.

5.2. Effect of solid particles (catalyst or carrier)

Previous researches have also significantly shown that reactionrate is dependent on the surface property of the catalyst particles(TiO2 in their case). In FBR-AOPs, catalyst particles (photo catalyticand heterogeneous Fenton) have been found to posses much po-tential on affecting the process. Similarly, carriers also affect theprocess. Different studies have highlighted the effect of catalystproperties, type and loading on heat and mass transfer of FBR. Astudy by Su et al. (2011a) revealed better removal efficiency whenSiO2 particles were used in comparison with Al2O3 particles.Furthermore, the type of catalyst can affect the degradation effi-ciency as it actively helps the production of radicals (Lu, 1999;Guimaraes et al., 2008). For example, Martinez et al. (2005)improved the catalysts by embedding crystalline hematite parti-cles into the meso-structured SBA-15 matrix for better degradationof phenol. Some studies have also attained improved catalysts bycreating iron pillared clay catalysts. It is concluded that highlystable catalyst facilitates the application process in a wider range ofpH (Yang et al., 2009; Tian et al., 2011). Particle properties such assize, density, surface area and wettability also affect the FBR-AOPperformance. It was indicated that the efficiency of catalyst todecompose target pollutants depends on the specific characteris-tics of the chosen catalyst (Huang et al., 2001). These propertiesaffect heat and mass transfer in a FBR process. Carriers or particleswith increased surface area exhibit higher removal efficiencies (Suet al., 2011a). Bed particles with large particle diameter, dp increasesheat and mass transfer coefficient. The effect of dp becomes evenmore significant in increased gas flow rates (Kim and Kangt, 1997;Razzak, 2009). Since bubbles are disintegrated by large particles,mass transfer also increases with increasing particle diameter (Kimet al., 1993). On the other hand, viscosity of liquid can reduce theeffect of large particle diameter when high viscous fluids are usedin the reactor (Shih et al., 2013).


The ability of liquid flow to suspend the solid particles withuniform distribution depends on the total amount of catalystweight. Increasing the catalyst loading beyond a certain range re-duces the catalytic reaction rate (Nam et al., 2001). Stoichiometricproportion of catalyst loading with respect to applied concentra-tion of oxidant results in better organic mineralization (Martinezet al., 2005, 2007). Increasing degradation rate of methyl orangewas observed with increasing amount of catalyst in Nam et al.However, the disappearance ratewas not so significant with furtherenhancement of the catalyst amount. It thus supports the fact thatthere exists an optimum amount of catalyst (for photo catalystreaction) (Nam et al., 2001). The semiconductors used in FBR-photocatalytic are of very small particle diameter that they fallinto geldart C classification. There is a need to coat this catalyst withsilica gel of 30e60 mesh that will support and enhance fluidizationquality to avoid poor fluidization by geldart C (Kanki et al., 2005).Catalytic performance is generally evaluated based on aromaticsand total organic carbon (TOC) conversions, whereas the catalyststability is evaluated according to the amount of metal leached intothe aqueous solution.

5.3. Effect of liquid velocity

The pollutants enter the reactor with the velocity equal to liquidvelocity in FBR-AOP processes. The pollutant liquid stream shouldbe recirculated for longer retention time in the reactor. And thefluidization of a solideliquid system is mainly controlled by theliquid flow rate (Costa et al., 1986). The minimum liquid velocityneeded to move the solid particles from the bed is called minimumfluidization velocity, Umf. Bed particles here are the catalyst particleor the carriers. Calculation of Umf, which can be done by Wen andYu equation, is important in analyzing the FBR-AOP process (Limet al., 2008). When liquid velocity, Ul is lower than Umf, the solidparticles inside the bed are fixed and static (Soung, 1977). The bedparticle slowly starts to move as the fluid velocity starts to increasegradually above Umf. If the Ul increases over maximum fluidizationvelocity, the fluid will start to carry the particles out of the reactor(Liang et al., 1996). Thus, in liquidesolid fluidization, the fluidiza-tion velocity of the liquid should be maintained higher than thecalculated Umf and recirculation should be introduced according tothe reaction time required for complete pollutant degradation. Ingaseliquidesolid fluidized bed, liquid phase is introduced to thereactor continuously to fluidize the solid particles and gas is flowedas a dispersed phase. Based on this review work, it can beconcluded that the increasing liquid velocity assists in increasingTOC or COD abatement along with recirculation of the liquidstream.

5.4. Effect of gas velocity

It is apparent that liquid flow rate is the dominant parameterin distributing particles through bed and heat transfer coefficientincreases constantly with increasing liquid flow (Kim and Kangt,1997). The increased liquid velocity causes turbulent flow regimein the reactor which consequently increases the heat transfercoefficient. Kim et al. (1993) studied the hydrodynamics of FBRsand stated that, the effect of liquid velocity on mass transfer co-efficient is less than the effect of gas velocity, Ug and particlediameter (Kim et al., 1993). There are two types of FBR-AOPfluidization, liquid-dominant fluidization (liquid flow is themain cause of particle movements) and gas-dominant fluidization(gas flow is the main cause of particle movements). In gas-dominant fluidization, gas flow is dispersed in the system toimprove the mass transfer efficiency. The three-phase (3-P)fluidization study by Jena et al. (2009) revealed that liquid Umf

increases with increasing particle size at constant Ug while itdecreases with increasing Ug with constant Ul. For low Ul at fixedUg, it has been found that heat transfer decreases with increasingUl (Jena et al., 2009). It is also found that heat transfer coefficient(h) increases with increase in Ug for different velocities andproperties of catalyst and liquid in the bed. It is found from hy-drodynamic studies that, generally, the increase of heat transfercoefficient is initially rapid at low gas flow rates, and it reduces athigher gas flow rates (Kim and Kangt, 1997; Mahmmod, 2008).The increased heat transfer can be attributed to the turbulencegenerated with injection of gas flow into the reactor (Razzak,2009). At low Ug, the bubbles are small and distributed in thereactor uniformly while at higher Ug the turbulence loses itsuniformity (Atta et al., 2009). So, heat transfer coefficient (h) in-creases with increasing Ug until a certain point after which heattransfer co-efficient remains constant.

Kim and Kang (1996) concluded that mass transfer coefficientincreases with increasing Ug due to the increase of gaseliquidinterfacial area. The fluid turbulence also affects mass transfer inFBR. The distribution of turbulence is related to mixing of solidparticles and continuous liquid phase. The gas flow rate is animportant factor for a fluidized photo catalytic reactor. In thegasesolid heterogeneous photo catalysis, the mass transfer resis-tance from the bulk phase to the solid photo catalyst surface wouldreduce the reaction rate. Therefore, it is believed that the masstransfer resistance would decrease with increasing gas flow rate.Zhang et al. (2006) showed that there was an optimum Ug value forall the three components and all the three optimum gas velocitieswere achieved after the calculation of maximum fluidization ve-locity of them in the process. When the gas velocity increased overthe optimum value, the efficiency became lower (Zhang et al.,2006). Nam et al. (2001) used air as the gas phase in their 3-PFBR. It can be seen from their research that, as the air flow rateincreased, the disappearance rate of methyl orange increased. Asthe air flow rate increased, the number of bubbles increased,mixing between catalyst and pollutant methyl orange was thusenhanced and higher mass transfer rate was obtained. However,decrease in degradation rate was observed after a certain air flowrate. At high air flow rates, the fluidized bed was expanded and thevolume fraction of bubbles increased with air flow rate. This hasthus increased bed volume and decreased the photon efficiency inspite of quantum flux profiles with radial direction for FBR. Increasein ozone flow rate is also highly beneficial to the color removal (Linand Lai, 1999).

6. Cost assessment for different FBR-AOPs

Treatment cost of various pollutants using FBR-AOPs was stud-ied. The cost estimation method of Mahamuni and Adewuyi (2010)was followed for this purpose. In the first step, kinetic data werecollected from the literature presented in Table 5. In cases wherethe kinetic data were not available in literature, kinetic rate con-stants were calculated using standard calculations (Fogler, 2010).Table 6 represents the kinetic data collected for these studies;meaning the kinetic rate constant and order of the reaction. Theserate constants were then used to calculate the time required for 90%degradation of the pollutant from its initial concentration. This timewas assumed as the residence time for the reactor for waste watertreatment using the given AOP. The cost estimation was done forthe assumed flow rate of 3 L/min. The reactor capacity was calcu-lated by multiplying the residence time with the design flow rate(3 L/min). From the study of Mahamuni and Adewuyi (2010)average energy consumption data for UV, Fenton, ozone andphoto catalytic oxidation was collected as energy dissipated perunit volume (W/ml). The total amount of energy required to treat

Table 6Rate constants for various FBR-AOPs.

Pollutant Applied process Kinetic rate constant (k) Reference

Reactive Azo Dyes FBR-Photocatalytic 3.85 � 10�4 s�1 (Nam et al., 2001)FBR-Fenton 3.4 � 10�6 ppm�1 s�1 (Su et al., 2011a)FBR-Fenton 0.275 � 10�2 mg�1 s�1 L (Su et al., 2011b)FBR-UV 6.396 � 10�4 s�1 (Cheng et al., 2011)FBR-Ozone 2.158 � 10�4 s�1 (Lin and Lai, 1999)

Phenol FBR-Fenton 3.837 � 10�3 s�1 (Muangthai et al., 2010)FBR-photocatalyst 1.447 � 10�3 s�1 (Huang and Huang, 2009)FBR-photocatalyst 1.5 � 10�4 mg�1 s�1 L (Kanki et al., 2005)FBR-Ozone 0.0388 mg L�1 s�1 (Qu et al., 2007)

Other pollutantsBenzoic Acid FBR-Fenton 2.3167 mM�1 s�1 (Chou et al., 2001)Nitrobenzene FBR-Fenton 5 � 10�4 mM�1 s�1 (Anotai et al., 2009)Aniline FBR-Fenton 4.833 � 10�4 s�1 (Anotai et al., 2010)TFT-LCD FBR-Fenton 2.9167 � 10�4 s�1 (Anotai et al., 2012)Acetaminophen FBR-Fenton 0.41386 M�1 s�1 (De Luna et al., 2013)Tetrafluropropanol FBR-UV 2.157 � 10�4 s�1 (Shih et al., 2013)


the waste water at the designed flow rate for given residence timewas then calculated.

From the literature cost of UV unit and ozone unit was takeninto account. The source of the cost is the quotation taken fromEmperor Aquatics, Inc., Pottstown, PA 19464 USA and SpartanEnvironmental Technologies, L.L.C. Mentor, OH 44060, USA forUV-Lamp and ozone generator respectively. The energy requiredfor these units are also known from the quotation information.Hence the number of such commercial units required for dissi-pating the required energy was calculated. From the number ofcommercial units required, the capital cost of the waste watertreatment unit was calculated (UV/photo catalytic/Ozone/Fentonunit cost). This AOP unit cost was added to the one FBR unit cost(Quotation received from T Nexus Technology Trading, Malaysia)to come up with FBR-Fenton or FBR-ozone or FBR-photo catalyticor FBR-UV cost. This total FBR-AOP cost was used to calculate totalcapital cost with general capital cost chart (Mahamuni andAdewuyi, 2010; Melin, 2000).

The kinetic data presented on Table 6 shows zero order,1st orderand 2nd order reaction rates for different FBR-AOPs depending onthe selected pollutant and treatment procedure. Depending on theorder of the reactions time for 90% pollutant degradation (t90) wascalculated with Eqns. (1)e(3).

For zero order reaction; t90 ¼ 0:9C0k

(1)

Table 7Breakdown of cost estimation procedure for various FBR-AOPs

Process Pollutant K Treatedvolume (L)

P

FBR-Photocatalytic ReactiveAzo dyes

3.85 � 10�4 s�1 0.848 0

FBR-Fenton 3.4 � 10�6 ppm�1 s�1 1.35 0FBR-Fenton 0.275 � 10�2 mg�1 s�1 L 1.35 0FBR-UV 6.396 � 10�4 s�1 1.73 0FBR-Ozone 2.158 � 10�4 s�1 11.3 0FBR-Fenton Phenol 3.837 � 10�3 s�1 1.35 0FBR-photocatalyst 1.447 � 10�3 s�1 0.15 0FBR-photocatalyst 1.5 � 10�4 mg�1 s�1 L 4 0FBR-Ozone 0.0388 mg L�1 s�1 0.353FBR-Fenton BA 2.3167 mM�1 s�1 4 0FBR-Fenton Nitrobenzene 5 � 10�4 mM�1 s�1 1.35 0FBR-Fenton Aniline 4.833 � 10�4 s�1 0.859 0FBR-Fenton TFT-LCD 2.9167 � 10�4 s�1 0.859 0FBR-Fenton ACT 0.41386 M�1 s�1 1.45 0FBR-UV TFP 2.157 � 10�4 s�1 0.6 0

For 1st order reaction; t90 ¼ 2:3025851k

(2)

And for 2nd order reaction; t90 ¼ 9C0k

(3)

Time for treating 90% of the pollutant was multiplied with theassumed flow rate (3 L/min) to calculate the capacity of the reactor,X (L). From literature the average energy,EA (W) required for onecycle treatment using UV, photo catalytic, ozone and Fenton wasfound to be 0.02, 0.45, 0.05 and 0.012 KW. Energy density, (KW/L)was then calculated with Eqn (4).

3¼ EAtreated volume

(4)

Thus the total energy (KW) needed for X (L) volume can becalculated as X 3(KW). Now, if the energy supplied by an AOP unit(UV/photo catalytic (UV þ TiO2)/ozone/Fenton) isEðUV=photocatalytic ðUVþTiO2Þ=ozone=FentonÞ (KW). Therefore, the numberof commercial AOP unit, N can be calculated with Eqn (5).

N ¼ X � 3

EðUV=photocatalytic ðUVþTiO2Þ=ozone=FentonÞ(5)

elec (KW) t90 (min) Energydensity(KW/L)

Volume (L) Vol*density(KW)

Cost (USD)

.45 99.7 0.531 299.1 159 136,042

.012 882.6 0.009 2647.1 23.5 15,686

.012 1.1 0.009 3.27 0.03 19

.02 60 0.012 180.03 2.08 3122

.051 177.9 0.005 533.6 2.41 14,045

.012 10 0.009 30.01 0.27 178

.45 26.5 3 79.578 238.74 218,272

.45 0 0.113 300 33.75 30,8570 0.144 115.98 16.76 97,721

.012 16.6 0.003 49.7 0.15 99

.012 0 0.009 90 0.8 533

.012 79.4 0.014 238.26 3.33 2219

.012 131.6 0.014 394.8 5.52 3677

.012 260.75 0.008 782.25 6.47 4316

.02 177.95 0.033 533.84 17.8 8897

Table 8Capital cost estimation for various FBR-AOPs

Process Totalcost (P)

Piping, valvesand electrical(30%)

Site work(10%)

Subtotal(Q)

Operation andmaintenance(15%)

Subtotal(R)

Engineering(15%)

Subtotal(S)

Contingency(20%)

Subtotal(1.2S)

Amortizationcost for 10 years A ¼ 1:2S�r

1��

11þr

�n

!US$/1000gallon

FBR-Photocatalytic

149,042 44,713 14,904 208,659 31,299 239,958 35,994 275,951 55,190 331,142 231,799 557

FBR-Fenton 28,686 8606 2869 40,161 6024 46,185 6928 53,113 10,623 63,735 44,615 107FBR-Fenton 13,019 3906 1302 18,227 2734 20,961 3144 24,105 4821 28,926 20,249 49FBR-UV 16,122 4837 1612 22,571 3386 25,956 3893 29,850 5970 35,820 25,074 60FBR-Ozone 27,045 8113 2705 37,863 5679 43,542 6531 50,073 10,015 60,088 42,062 101FBR-Fenton 13,178 3953 1318 18,449 2767 21,216 3182 24,399 4880 29,279 20,495 49FBR-Photo

catalytic231,272 69,382 23,127 323,781 48,567 372,348 55,852 428,201 85,640 513,841 359,689 865

FBR-Photocatalytic

43,857 13,157 4386 61,400 9210 70,610 10,592 81,202 16,240 97,442 68,209 164

FBR-Ozone 110,721 33,216 11,072 155,009 23,251 178,260 26,739 204,999 41,000 245,999 172,200 414FBR-Fenton 13,099 3930 1310 18,339 2751 21,090 3164 24,254 4851 29,104 20,373 49FBR-Fenton 13,533 4060 1353 18,947 2842 21,789 3268 25,057 5011 30,068 21,048 51FBR-Fenton 15,219 4566 1522 21,306 3196 24,503 3675 28,178 5636 33,813 23,670 57FBR-Fenton 16,677 5003 1668 23,348 3502 26,850 4027 30,877 6175 37,053 25,937 62FBR-Fenton 17,316 5195 1732 24,242 3636 27,879 4182 32,060 6412 38,472 26,931 65FBR-UV 21,897 6569 2190 30,656 4598 35,255 5288 40,543 8109 48,652 34,056 82


Therefore, multiplying Nwith the cost of each AOP unit, the costfor AOP treatment, A (US$) for X (L) volume of water can becalculated. The results are presented in Table 7. Subsequently, thecost of a fluidized bed reactor was added to the cost of AOP unit,which gives us the total cost, P (US$). Total capital cost (1.2 S) wascalculated considering amortization over a span of 5 years withamortization rate of 70%. Finally the cost is presented in Table 8 asunit of US$/1000 gallons of treated water per a year.

The costs of different FBR-AOPs are expressed in terms ofamount in USD needed to treat 1000 gallons of waste water for 90%degradation. These costs are calculated based on the degradationrate of the particular pollutant abatement. The cost estimated inthis study can be used as a useful guide to estimate the order ofmagnitude of the treatment cost for FBR-AOPs application. Table 8shows cost estimation of various FBR-AOPs for azo dyes, phenol andother recalcitrant compounds. It can be seen that, the cost oftreating azo dyes using FBR-photo catalytic is US$ 550/1000 gal-lons. The cost of FBR-Fenton process is US$ 40 to US$ 100 per1000 gallons of waste water. Whereas, the required cost for usingFBR-ozone in dye degradation is US$ 60 per 1000 gallons of wastewater. Fig. 2 has been presented to show capital cost needed to treat1000 gallons of waste water containing azo dyes. And it can be seenfrom Fig. 2 that, cost for FBR-photo catalytic is higher than otherFBR-AOPs. Also, it can be observed that, lowest cost is needed forFBR-Fenton. Similar trend is found for degradation of phenols and

557

10749 60

101

050

100150200250300350400450500550600

USD

/ 100

0 ga

llon FBR-Photocatalytic

FBR-FentonFBR-UVFBR-Ozone

Fig. 2. Cost estimation of FBR-AOPs for dyes and textile waste water treatment.

other recalcitrant compounds. The cost for phenol degradationusing different FBR-AOPs is plotted in Fig. 3. From this illustration itcan be seen that, FBR-Fenton degradation for phenol compoundsare associated with smaller cost requirements which is US$50 per1000 gallons of waste water. The cost for photo-catalyst has beenfound to be from US$ 160 to US$ 860 per 1000 gallons. And FBR-ozone shows a cost needed of US$ 413 for treating 1000 gallonsof phenol waste water. For other pollutants (e.g. benzoic acid (BA),nitrobenzene, aniline, TFT-LCD, acetaminophen (ACT) and tetrafluro propanol (TFP)) cost estimation for FBR-AOPs is presented inFig. 4. It can be seen from the plot that, for all the pollutants FBR-Fenton showed similar cost estimation of US$ 49 to US$ 65 per1000 gallons of wastewater. FBR-UV showed higher cost require-ment compared with FBR-Fenton.

7. Conclusions and recommendations

The limitations of conventional processes in waste water treat-ment necessitate extensive study on the AOPs. AOPs are able toachieve complete degradation of different pollutants and they caneliminate the need for huge spacewhich is normally required in theconventional processes. However, researchers are still trying toreduce process costs and achieve more stable and noble results.Thus, FBR has been introduced as a pollutant treatment procedure.

49

865

164

414

0100200300400500600700800900

1000

US

$ / 1

000

gallo

n

FBR-Fenton

FBR-photocatalyst

FBR-Ozone

Fig. 3. Cost estimation of FBR-AOPs for degradation of phenol waste water.

49 5157

62 65

82

0102030405060708090

US$

/ 10

00ga

llons

FBR-Fenton (BA)

FBR-Fenton (Nitrobenzene)

FBR-Fenton (Aniline)

FBR-Fenton (TFT-LCD)

FBR-Fenton (Acetaminophen)

FBR-UV (TFP)

Fig. 4. Cost estimation of FBR-AOPs for other recalcitrant treatment.


Hydrodynamic studies of FBR confirm better mass transfer effi-ciency. And literature provides more positive highlights on thisprocedure. The findings from the reviewed literature can beconcluded as below.

1. Implication of FBR can accelerate the efficiency of AOPs. FBR-AOP is cost efficient and it requires less space, produces less/no sludge, and does not need other additional post treatmentprocesses. Parameters such as gas and liquid flow rates, amountand type of catalyst or carriers and pH pose the biggest effect onFBR-AOPs.

2. Studies have shown that AOPs work better when oxidizing gas isintroduced in the process. Therefore, using oxidizing gas such asozone and oxygen as dispersing agent in the FBR-AOPs can berecommended. For instance in an ordinary 3-P FBR-AOP,oxidizing gas can be used in replacement of air.

3. Aerobic condition improves degradation rate in FBR-AOPs atacidic pH. Basic pH can be explored in FBR-AOPs (especially UV/H2O2 or Fenton like or Fenton processes) if nonreactive gases areused for gas dispersion in the FBR-AOP to induce the oxidizingreaction. Anaerobic condition can be followed in FBR-AOPs forbasic pH operation.

4. Geometric effects of FBR on AOP processes are not yet exten-sively studied and this area can be explored further.

5. Cost of FBR-AOPs highly depends on energy density needed andthe mechanism of degradation of the pollutant. The cost ofwaste water treatment containing azo dyes was found to be US$50 to US$ 500 per 1000 gallons. The cost for treating phenolwater was US$ 50 to US$ 800 per 1000 gallons.

6. Commercializing FBR-AOPs requires more in-depth design, costassessment and analytical studies. Economical analysis of thisprocess based onmaterial and energy balances using simulationtools can also be a potential field of research.

Acknowledgments

The Authors would like to thank financial support from Uni-versity of Malaya High Impact Research Grant (UM.C/HIR/MOHE/ENG/37) from the Ministry of Higher Education Malaysia and Uni-versity of Malaya, Kuala Lumpur, Malaysia.

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applicability of fluidized bed reactor in recalcitrant … applicability of ﬂuidized bed reactor...

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