Bioregeneration of spent activated carbon:Review of key factors and recent mathematical models of kinetics

Kwok-Yii Leong ,Siew-Leng Loo ,Mohammed J.K.Bashir ,Wen-Da Oh ,Pasupuleti Visweswara Rao ,Jun-Wei Lim *

1 Centre for Water Research,Department of Civil and Environmental Engineering,National University of Singapore,1 Engineering Dr.2,Singapore 117576,Singapore

2 Singapore Membrane Technology Centre,Nanyang Environment and Water Research Institute,Nanyang Technological University,1 Cleantech Loop,Clean Tech One,#05-05,Singapore 637141,Singapore

3 Department of Environmental Engineering,Faculty of Engineering and Green Technology(FEGT),Universiti Tunku Abdul Rahman,31900 Kampar,Perak,Malaysia

4 Residues&Resource Reclamation Centre,Nanyang Environment and Water Research Institute(NEWRI),Nanyang Technological University,1 Cleantech Loop,CleanTech One,Singapore 637141,Singapore

5 Bio Product Development Program,Faculty of Agro-Based Industry,Universiti Malaysia Kelantan,17600 Jeli,Malaysia

6 Institute of Food Security and Sustainable Agriculture,Universiti Malaysia Kelantan,17600 Jeli,Malaysia

7 Department of Fundamental and Applied Sciences,Universiti Teknologi PETRONAS,32610 Seri Iskandar,Perak Darul Ridzuan,Malaysia

1.Introduction

Water pollution is indeed an elevating issue that needs to be addressed promptly.The intensive spewing of toxic substances without proper control mechanism constitutes a real calamity for humanity.The discharge of some toxic contaminants such as phenolic compounds and heavy metals into the water bodies could cause serious environmental problems and afflict living beings[1,2].

For decades,activated carbon(AC)has been widely used as a versatile and economical adsorbent for removal of various types of pollutants from wastewaters[3-5].Despite its popularity,problems associated with either disposing or regenerating the spent AC pose a serious practical concern.The common approaches to regenerate spent AC can be generally classified into three classes that include physical,chemical and biological regenerations.Among these approaches,biological regeneration or bioregeneration offers an eco-friendly and cost-effective operation to replenish the active sites of AC[6,7].Bioregeneration is a process through the action of microbial activities that biodegrade the adsorbed pollutants from the surface of AC to enable more pollutants to be adsorbed on the renewal adsorptive sites of AC[8].In recent years,numerous studies on bioregeneration have been reported on optimization of the bioregeneration factors in batch and column systems[9-11].The column setup is often preferred over the batch system particularly in large scale wastewater treatment plant due to its simple operation and better utilization of the adsorbent capacity[12].

Generally AC composes of amorphous carbon and graphite layers with a complicated and heterogeneous structure.The pore distribution of AC can be categorized into three main groups,namely micropores(d＜2.0 nm),mesopores(d=2.0-50.0 nm)and macropores(d＞ 50.0 nm).According to Yal?in and Sevin?[13],over 95%of the total surface area of AC is commonly accounted by the presence of micropores distribution.The AC is traditionally fabricated through twostage process such as carbonization and activation.Owing to the exorbitant price of commercial AC,other low-cost materials have been used as the precursor in producing AC adsorbent such as conventional wastes derived from agriculture and wood industry[14-16]and nonconventional wastes derived from municipal and industrial activities[17,18].Despite having cheaper alternatives being proposed,the commercial AC still shows superiority in terms of the adsorption capacity.In order to fully utilize the adsorptive sites of this commercial AC,bioregeneration approach is believed to be a desired strategy.

In order to treat large amount of wastewater(domestic or industrial),continuous system is preferred such as using column setups to packed in more AC.Among the columns that are commonly in used are the fixed-bed column,pulsed-bed column and fluidized bed column.For the fixed-bed column,the operational flow of this column is either down flow or up flow mode.Higher occurrence of clogging is normally found for down flow mode that leads to high pressure drop and foils the extensive use.To address this drawback,the down flow column is devised to have openings at both ends[19].On the other hand,the pulsed-bed column permits the removal of spent adsorbents from the bottom of the column at the regular interval while replacing with the fresh ones at the opening top.The newly added adsorbents must be completely packed in the pulsed-bed column in order to avoid bed expansion amidst the operational period[20].The occurrence of bed expansion will deteriorate the column efficiency due to the carbon mixing that will perturb the zone of adsorption.The pulsed-bed column is normally selected when the feed(influent)does not contain measurable suspended solids[19].Apart from that, fluidized-bed column consists of loosely packed adsorbent beds that resulted from high velocity fluid flow.The shortcoming is when the granular solid beds break up due to the intensive and incessant abrasion during the operation period of fluidized-bed column[21,22].With these concise viewpoints,each column has its own unique properties that can serve the adsorption,desorption and biodegradation processes which are the fundamental requirements for AC bioregeneration.

Lately,many kinetic models have been proposed with the main objectives to describe the bioregeneration process and quantify the rate of bioregeneration.Akta? and ?e?en[8]had unveiled the kinetics of bioregeneration and concluded that the inadequacy studies pertaining to the kinetic aspects of bioregeneration need to be further addressed.The present review aims to impart two major critical assessments on bioregeneration process,viz.,(1)the key factors in enhancing the bioregeneration of spent AC within batch and column setups and(2)the development of mathematical kinetic models in predicting the bioregeneration rate of spent AC.The state-of-the-art perspectives present the pros and cons that affect the bioregeneration process and various operational modes that favor the bioregeneration of spent AC that are described by the neoteric kinetic models.

2.Various Key Factors Influencing Spent AC Bioregeneration

Different perspectives of key factors affecting the bioregeneration of spent AC are discussed in this section.The selection offactors is based on the mechanisms of bioregeneration which involve[23]:

(1)bioregeneration stems from the differences in the concentration gradient.In this case,the adsorbates-cum-organics that are released from AC into the aqueous phase are biodegraded by the action of microorganisms,rendering to lower concentration of organics.This generally promotes further desorption of adsorbed organics resulted from the establishment of concentration gradient across the AC-aqueous phase.

(2)bioregeneration stems from exoenzyme reactions.The exoenzymes excreted by microorganism diffuse into the pores of AC and react with the adsorbed organics.Subsequently,the hydrolytic transformation of organics is induced with the later release of unreacted by-product into the aqueous phase.

From the first mechanism of bioregeneration,to ensure that effective concentration gradient exists between the AC and bulk aqueous phase,the concentration of organics in the bulk aqueous needs to remain low.Therefore,to achieve this objective,a good removal efficiency of organics to produce cleaner effluent is an important criterion which leads to an excellent competence of bioregeneration process.On the other hand,the second mechanism of bioregeneration requires high bioactivity of microorganisms for the continuous excretion of exoenzymes into the pores of spent AC.The illustrations of the two mechanisms could be found in Fig.1.Numerous key factors involve with the operating conditions of biological activated carbon(BAC)column.These key factors significantly contribute to the enhancement of both mechanisms.Despite having some factors that overlap with the batch system,essentially,the selected factors are crucial to be considered and reviewed in ensuring the competency of bioregeneration process of spent AC.Fig.2 demonstrates bridging of operational problems involving BAC column to their respective solutions in sustaining the bioregeneration of spent AC with detailed reviews being outlined hereafter.

2.1.Backwashing and air scouring

Clogging is caused by the extensive growth of biomass in BAC column over time.Clogging contributes to the increase in pressure drop and also leads to the wash out of the biomass from the column system.Generally,clogging can be identified through some indications such as high in turbidity and suspended solids(SS)concentration,low in pH and dissolved oxygen(DO)concentration[24].Upadhyayaet al.[25]reported that the increase of head loss with time was due to the excessive retention of biomass of either newly generated or dead biomasses in the fixed-bed column.The superfluous accumulation of biomass hampers the mass transfer process in which afflicts the organic pollutants removal efficiency and later dampens the bioregeneration process[10,26].The clogged pores also reduce the desorption amount of organic pollutants due to the low concentration gradient establishment between the surface of spent AC and bulk aqueous.

Frequent backwashing is expected to overcome clogging while maintaining an active biofilm layer on AC surface and improving the mass transfer process[27,28].Unfortunately,backwashing alone may not be that effective in removing the clogged debris since it relies primarily upon the dynamic shear force for cleaning[29].In that case,the combination of backwashing and air scouring is necessary to effectively remove the clogged debris firmly lodged in the BAC column.This synergistic pair of backwashing and air scouring can produce high speed movement of upward air bubbles which fortifies the hydrodynamics and medium collision performances of backwashing[29].The synergistic pair is virtually working effectively in an aerobic system,while in an anaerobic system,oxygen-free gases such as nitrogen,helium,etc.are required to suppress the availability of oxygen for the microorganisms which may trigger the growth of aerobic microbes[25].Chenet al.[30]had vindicated that backwashed with both air and water was the most efficient in removing semi-volatile organics.At the optimum flow,time conditions of air(15 m·h-1,3 min)and water(8 m·h-1,5 min),72.4%of di-n-butyl phthalate and 81.8%of bis(2-ethylhexyl)phthalate(two main semi-volatile organics)were successfully removed.Another report by Scholz and Martin[24]showed that the ability to have good removal efficiencies for glycerol,total organic carbon(TOC)and chemical oxygen demand(COD)prolong the lifespan of AC.

2.2.Defouling

Biological fouling on the other hand arises from the excessive proliferations of biomass and by-producton AC surfaces that transpire from the consequential impact of bioregeneration[10].Biological fouling causes the AC pores to be blocked,leading to the deterioration of BAC column performance.The presence of by-products such as soluble microbial products(SMP)reduces the adsorption capacity of AC as con firmed by Zhaoet al.[31]in their study with fluidized-bed column.To overcome the biological fouling,base wash with sodium hydroxide is usually used for digesting and dissolving the biomass and byproducts[31].Another report by Venkatesanet al.[26]reported that the method of defouling and disinfecting was applied for the removal of biological foulant.The results showed that the bioregenerated defouling resins had virtually equal adsorption capacities when compared to the fresh resins in fluidized-bed column.Nevertheless,more extensive works have to be performed to comprehend the influence of biological foulant removal from the active sites of BAC column for further comprehension of quantifying the rate of spent AC bioregeneration.

Fig.1.Mechanisms of bioregeneration process via(a)desorption of the adsorbed substrates into the bulk solution induced by the difference in concentration gradient and(b)exoenzymes reaction.

2.3.Ozonation

The natural organic matter(NOM)is commonly associated to the main source of organic carbon in drinking water resources.In this perspective,ozonation has been broadly used for the removal of NOM derived color,taste and stench in drinking water production.The application of ozonation method transforms the refractory NOM to biodegradable organic carbon(BDOC)and afterward mineralizedviamicrobial activities[32].It is essential to convert the refractory NOM to BDOC in order to extend the lifespan of BAC in column.BDOC can be biodegraded by the attached microorganism,lowering the organic loading rate impact on AC[33].The total removal efficiency of COD was found to be higher in the secondary effluent when ozonation was integrated with BAC process[34].The enhancement of TOC and trihalomethanes removal was materialized by retro fitting ozone treatment into BAC system in replenishing the adsorptive sites of spent AC[35].The presence of ozonation process promotes the biodegradation of refractory compounds and later eases the bioregeneration of spent AC in BAC column[36].A remarkable advantage of incorporating UV into ozone-BAC process(UV/O3-BAC)has been reported by several researchers[37,38].Liet al.[39]showed that the UV/O3-BAC process rendered higher reduction of dissolved organic matter(DOC)when compared with O3-BAC in the secondary effluent treatment.This can be explained by the presence of UV that transforms ozone into secondary radicals such as hydroxyl radical to commence the radical chain reaction,accelerating the degradation of organic matters[40].

2.4.Mixed microbial culture inoculation

Fig.2.Immediate solutions for operational problems of BAC column in sustaining the bioregeneration of spent AC.

The microorganisms are initially inoculated into the BAC column to instigate the microbial activities.The inoculations can be basically divided into two types,namely in-situ and ex-situ inoculations as shown in Fig.3.In-situinoculation takes place when fresh microorganism culture is introduced into the BAC column,permitting the proliferation of biofilm on AC surfaces over time.Conversely,forex-situinoculation,the AC is initially grown with biofilm in the control batch reactor prior to the loading into the BAC column.This can prevent the excessive growth of biofilm in the localized space of BAC column that potentially occludes the flow of aqueous stream as commonly encountered byin-situinoculation column.Besides,the microorganisms used for inoculation in BAC column are also further classified into either pure or mixed culture.Pure culture consists of single strain of microbial species,while mixed culture involves two or more microbial species that form colonies.Comparative studies of exploiting either pure or mixed culture to individually treatvarious xenobiotic compounds found that many pure culture strains were incapable of completely mineralizing these compounds with toxic intermediates being produced during the treatment course[41-43].Mixed culture on another note had been vindicated to be effective in transforming the toxic organics to carbon dioxide and water[44].Kimet al.[45]revealed that the use of mixed culture in the fixed-bed column could simultaneously mineralize three different types of pollutants;manifesting synergistic effect of mixed culture in executing the concurrent biodegradation processes that could be inhibited by one another pollutants.More importantly,in order to enhance the biodegradation rate or removal efficiency of targeted compound,it is crucial to acclimatize the mixed culture of activated sludge[44].Acclimatization is a process performed to develop mixed culture of activated sludge that is resistant towards toxicity of pollutants at predetermined concentration.Pasukphunet al.[46]elucidated that the usage of acclimated activated sludge could curtail the retention time of dye treatment in BAC column as compared with the nonacclimated activated sludge application.In fact,the acclimated activated sludge had been widely used for bioregeneration of spent AC in batch as well as column systems[47-54].Despite a large number of works being reported for both systems in recent years,the modeling study to quantify bioregeneration of spent AC using acclimated activated sludge is far and wide scattered.Therefore,a critical review from systematic compilation is entailed to establish future research routes that can potentiate commercialization.

2.5.Biomass concentration

Fig.3.Microorganism inoculations via in-situ(a)and ex-situ(b)methods in the BAC column.

Biomass concentration is an important parameter that affects the degree and rate of spent AC bioregeneration.Habitually,the increase of biomass concentration in terms of activated sludge mass per unit volume will decrease the adsorption capacity due to the production of more SMP that potentially blocks the active sites of AC.Nevertheless,several findings had showed that despite the continuous growth of biomass on the AC surface,there was no significant effect on the AC adsorption capacity[31,54].Songet al.[54]substantiated that the biofilm formed from biomass attachment did not affect the adsorption capacity of AC,but the biofilm may affect the adsorption kinetics due to the reduction of intraparticle diffusion rate.In linking to spent AC bioregeneration,Vinitnantharatet al.[55]delved the impact of initial biomass concentration on the rate and magnitude of bioregeneration of spent AC loaded with 2,4-dichlorophenol and found that changing the initial biomass concentration would have inconsequential impact on the extent of bioregeneration;but a faster rate of bioregeneration was noticed while using higher initial biomass concentration.This was plausibly due to the shorter time required to achieve the equilibrium concentration.The research outputs were later underpinned by Ohet al.[51]in bioregeneration study with 4-chlorophenol-loaded AC.Their efficiency results revealed insignificant effect of varying biomass concentration on the bioregeneration efficiency provided there was no inhibition effect exerted by increasing concentration of 4-chlorophenol.Abromaitiset al.[56]also discovered that the reactor that had the most attached biomass showed the lowest recovery of adsorption capacity of the AC in the BAC system.Apparently,the dense biofilm does not necessarily improve the AC regeneration and more studies need to be conducted to find out the underlying findings.

2.6.Dissolved oxygen(DO)concentration

The DO concentration will taper off gradually in the BAC column due to the perpetual uptake by microorganisms in the form of biofilm to oxidize the organicsviabiodegradation and inorganics such as in the nitrification process[56,57].Hence,a continuous supply of sufficient DO is vital to ensure the optimum treatment of introduced aqueous stream and bioregeneration of spent AC,and also maintaining the minimum effluent DO concentration of 2 mg·L-1as decreed by the general discharge limit.In bioremediating wastewaterladen with phenolics,the bioactivities of microorganisms showed increasing trends with increasing phenolic loading concentrations that gave rise to a greater amount of DO being consumed[44].On the other hand,Vinod and Reddy[58]reported that the DO concentration maintained at 2 mg·L-1was adequate to completely biodegrade phenol.The major setback of administering uncontrolled excessive DO particularly in treating phenolics is the plausible occurrence of oxidative polymerization[55].As a result,irreversible adsorption happens due to chemisorption process that will afflict the extent of bioregeneration of spent AC.

2.7.Initial pollutant concentration

Bioregeneration of spent AC is also greatly affected by the initial concentration of pollutant presents in the BAC column.Zhaoet al.[31]observed that after operating the BAC column for a year,the adsorption capacities decreased more for higher initial concentration of toluene pollutant.This inhibited the bioactivity of microorganisms since the toxicity effect is usually commensurate with the concentration of pollutant,leading to a lower mass transfer of adsorbed toluene to the surface of AC for biodegradation to occur.Ivancev-Tumbaset al.[59]also studied the effect of different initial concentrations of pollutants towards the quantification of spent AC bioregeneration.It was observed that beyond the threshold phenolic loading,the bioregeneration efficiency of spent AC was found to deteriorate despite having high rate of pollutant removal and well grown of microorganisms on the AC surface.A comparison was also made between the abiotic and biotic BAC columns in treating different initial concentrations of non-ionogenic surfactants(NISS).Stark contrast was observed between the two systems in removing NISS having the latter much outperformed the former[60].Although the high initial pollutant concentration will hinder the microorganism activities in the bioregeneration process,low initial concentration of pollutant will also cause the adsorption to transpire on the high energy binding sites,preponderantly the micropores,which lead to irreversible adsorption process.Furthermore,the low initial pollutant concentration will as well establish insignificant concentration gradients across the solid surface of AC-aqueous phase,imparting negligible diffusion and bioregeneration rate of spent AC.

2.8.Bed height

Bed heightis defined as the distance of the bottom of hydraulic press to the desired working height of BAC column.The time course of pollutant concentrations can be usually monitored through the sampling ports at different bed heights,permitting the mechanism of pollutant biodegradation study that interlocks with the derivation steps of kinetics modeling.Apart from that,the results of microbial population investigated at different bed height showed that dense population was located at the middle of bed height[61].Nevertheless,the variations of microbial population are very much depending on the acclimation process and availability of biodegradable organics as well as nutrients[61].It was also shown that the bioregeneration rate varied with bed heights was attributed by the decrease of DO concentration in BAC column[57,62].In this case,Ipet al.[63]observed that desorption of pollutant was found to be more conspicuous at the bottom of BAC column due to the presence of higher amount of biomass amidst the initial stage of bioregeneration.Thus,sampling at different bed heights permit the reusability estimation of partially bioregenerated spent AC in BAC column which is essential for industrial operations.

2.9.Volumetric flow rate

Volumetric flow rate is generally defined as the volume of fluid which passes through a given surface with time.It is known that at lower flow rate,the residence time of pollutant in the column is longer due to the increasing contact time between AC and pollutant.Lower flow rate also favors the external mass transfer and this is usually used as the controlling step of organic biodegradation in BAC column.Volumetric flow rate can be normally expressed as either hydraulic retention time(HRT)or empty bed contact time(EBCT).The removal of dissolved organic carbon(DOC)was found to increase when the HRT was increased in BAC column[64].Meanwhile,Songet al.[54]assessed various EBCTs to determine the optimum EBCT for the removal of naphthalene-2-sulfonic acid in BAC column.They verified that the EBCT of22 min was sufficient to completely remove the targeted pollutant and better yet,the hassle of incessant AC replacement could be avoided since the self-bioregeneration was sufficient to renew the active sites of spent AC.Later,Seredyńska-Sobeckaet al.[65]agreed that the COD,TOC and phenol concentrations would decrease when EBCT was ratcheted up gradually in BAC column.This could be explained by the lower pollutant concentration in the effluent near the column end,rendering higher driving force for desorption and subsequent biodegradation of organic substances[66],enhancing the bioregeneration efficiency of spent AC.

2.10.Types of AC

Different types of AC are normally produced by different materials and activation methods.Investigations had been carried outon using different types of AC to remove dissolved organic matter(DOC)and promote nitrification process in BAC column[57,62].In order to attain a better performance not only for adsorption but also biodegradation in extending the lifespan of AC,choosing the best type of AC for BAC column operational system is vital.The extent of AC bioregeneration is more effective with chemically activated AC as compared with thermally activated one due to the lower affinity towards oxygen and higher reversibility of chemically produced AC[8].However,Yapsakli and ?e?en[57]found that there was a possibility to fabricate thermally activated AC with high bioregeneration efficiency by altering the non-biodegradable to biodegradable substances occurredviacatalytic reactions under the presence of high oxygen consumption at AC facade.Moreover,the AC with larger pore size will also achieve higher bioregeneration efficiency since the microorganisms preferably grow in the macropore apertures of AC in BAC column.Granular AC with different surface roughness also will have an impact on the extent of regeneration.It was found that net negative effect on bioregeneration of AC was found for AC with surface roughness of 13 μm as compared to smoother surface of 1.6 μm[56].

2.11.Temperature

Variation of temperature can overall affect the performance of microorganisms towards biodegradation of organic pollutants.Melinet al.[67]saw that a difference of 10°C could deteriorate the chlorophenol biodegradation rate by more than seven folds in BAC column.Anderssonet al.[68]also highlighted the adverse impact of increasing temperature on nitrification process operated in BAC column.To cushion the temperature variation threat,incubation of microorganisms at their optimum temperature is initially exercised before inoculating into the BAC column[63,69].Failure infine-tuning the temperature to optimum level will incontrovertibly afflict the bioactivities of microorganisms which will also undermine the bioregeneration efficiency in BAC column.

2.12.pH

The pH also plays a pivotal role in BAC column for both adsorption and bioregeneration.Each microorganism possesses a defined optimum pH range in which its growth and bioactivity are at the peak.Kirisitset al.[70]noticed that the gradual increase of pH from 6.8 to 8.2 would reduce the removal efficiency of bromate in BAC column.Songet al.[54]identified the improvement of bromate removal efficiency when the pH was reduced from 8.0 to 7.0.In both cases,the researchers consented that the optimum pH for bromate removal took place at the pH close to neutral range[54,70].On another aspect,when the extreme pH condition was applied to the microorganism culture,negligible growth and bioactivity were observed by Bhattacharyya and Jha,[71].However,Duanet al.[72]vindicated that,despite at very low pH(＜1)which was harmful to the microorganisms due to the very acidic environment,it somehow provided a favorable condition for biosulphide oxidation.Therefore,it is important to determine the optimum pH to unleash the uppermost performance of targeted microorganismin achieving the highest extent of spent AC bioregeneration.

3.Mathematical Models of Bioregeneration Kinetics

Mathematical models provide new insights for a better understanding of bioregeneration process of spent AC and later for design purposes.In a recent report,Nathet al.[73]provided an up-to-date discussion of the recent bioregeneration kinetic models covering the mass transfer kinetics,microbial growth and solute degradation.In many studies,the kinetic models were used to generate important kinetic parameters that can be used as a representative of bioregeneration rate in predicting an optimum operational condition to achieve the fastest bioregeneration process of spent AC.Since bioregeneration process is susceptible to many operational factors,it is imperative to study the effects of various operational parameters in enhancing the rate of bioregeneration.In this section,a critical review of the recent progresses in mathematical kinetic modeling of bioregeneration is extended to include the effects of various key factors on the rate of bioregeneration in which hitherto has not been addressed.Table 1 provides an overview of the rate constant values of desorption and biodegradation that mainly governs the rate of bioregeneration of AC.

In bioregeneration studies,many kinetic models were built on the basis of any of these three fundamental processes,namely adsorption,desorption and biodegradation.One of the most commonly employed kinetic models is the single step first-order kinetics with respect to the adsorbed substrate[74,75]as follows:

Integrating Eq.(1)gives

whereQ0andQtare the amount of adsorbed substrate at the beginning of bioregeneration and at timet,respectively,andkis the first-order bioregeneration rate constant.

Table 1 Overview of the rate constant values of desorption and biodegradation that governs the rate of bioregeneration of AC

To compensate the lack of experimental data forQt,Akta? and ?e?en[75]calculated theQtvalue from the residual concentration data using Freundlich isotherm model as proposed by Haet al.[76]using Eq.(3).

whereC2(mg·L-1)is the concentration of substrate after reloading the adsorbent with the same constituent of substrate from the initial loading,C2e(mg·L-1)is the equilibrium concentration of the substrate after second equilibrium is achieved,Kandnare the Freundlich constants,V(L)is the total volume of sample andW(g)is the mass of the adsorbent.However,it should be noted that although this method eliminates the need to determine the amount of substrate remaining on the adsorbent,it is only applicable under the equilibrium condition.Therefore,the validity of this model to a non-equilibrium situation such as in bioregeneration process is obscure.Also,it is difficult to compare all the first-order bioregeneration rate constants generated in the literature using this kinetic model since the operational conditions differed among different groups of researchers.Vinitnantharatet al.[55]studied the effect of biomass concentration on the rate of bioregeneration of AC loaded with phenol and 2,4-dichlorophenol using the first-order kinetic model and concluded that faster rate was achieved with the increased in biomass concentration from 100 to 300 mg·L-1.Akta? and ?e?en[75,77]compared the rate of biodegradation with the rate of bioregeneration generated using the first-order model for single and binary systems containing phenolic compounds.For single component system,the rate of biodegradation was slower than the rate of bioregeneration due to the slow diffusion of adsorbed substrate from the pores of AC and the occurrence of irreversible adsorption for the case of thermally produced AC.However,the rate of cometabolic bioregeneration of AC loaded with 2-nitrophenol and 2-chlorophenol in the presence of phenol was faster than their rate of biodegradation.This phenomenon was explained by the fact that phenol was competitively adsorbed by AC to the lower energy sites and therefore,making desorption of 2-nitrophenol and 2-chlorophenol to be much faster than the single component system.The higher rate of desorption coupled with the relatively slow biodegradation of 2-chlorophenol and 2-nitrophenol as analyzed further substantiated the observation.However,it is worth noting that the kinetic formulation of first-order model is generally inadequate to be used even for the fittings of a mono-component bioregeneration system.Many other factors pertinent to the kinetic modeling of bioregeneration such as the possibility of interaction between the adsorbed substrate and adsorbent need to be considered.Therefore,the bioregeneration rate constants generated from the single-step first-order model can be misleading and a better model should be developed.

In a recent study,Nget al.[50]proposed a two-step kinetic model consisting of desorption and biodegradation processes,both of which were assumed to follow first-order kinetics,to describe the bioregeneration of AC loaded with phenolic compounds in sequential adsorption and biodegradation processes.The proposed equation is given as follows:

whereC(mg·g-1)andCi(mg·g-1)are the residual and initial substrate concentrations,respectively,k(h-1)andkd(h-1)are the rate constants for biodegradation and desorption,respectively,m(g)is the mass of adsorbent andq0(mg·g-1)is the initial amount of adsorbed substrate per unit mass of adsorbent.One of the major advantages of the equation is that it allows comparison of the rate constants for desorption and biodegradation.The researchers used the rate constant of desorption as the rate of bioregeneration and their findings leads to the conclusion that the rate of bioregeneration can be enhanced by using non-excess adsorbent condition.However,the kinetic model could not be used as a mean to determine the rate constant of bioregeneration in simultaneous adsorption and biodegradation processes since the kinetic model did not take into account the adsorption process in the bulk solution.

Ohet al.[78]extended the kinetic model to include the adsorption process in obtaining the rate of bioregeneration for simultaneous adsorption and biodegradation of chlorophenols as follows:

wherekaandkdare the second-order adsorption and first-order desorption rate constants for the biotic system,respectively,kmax,KsandKiare the Haldane constants which represent the maximum specific removal rate,the saturation constant and the substrate inhibition coefficient,respectively,andXis the MLSS concentration.The findings of the researchers suggested that the rate of bioregeneration was dependent on the ratio of initial chlorophenol concentration and AC dosage.Therefore,a faster rate of bioregeneration could be achieved by increasing the AC dosage at constant initial chlorophenol concentration.

In another study by Ohetal.[52],the bioregeneration rate equations with first-order and Haldane biodegradation kinetics incorporating the irreversibility of adsorption were used to study the effect of biomass concentration on bioregeneration in simultaneous adsorption and biodegradation processes as follows:

First-order biodegradation

Haldane biodegradation

whereS(mg·L-1)is the residual substrate concentration at timet,qm(mg·g-1)is the maximum adsorption capacity of the adsorbent,ka(L·mg-1·h-1)andkd(h-1)are the second-order adsorption and first-order desorption rate constants,respectively,andk(h-1)is the first-order biodegradation rate constant whereka′(L·mg-1·h-1)andkd′(h-1)are the second-order adsorption and first-order desorption rate constants for the biotic system,respectively,kmax(mg·(mg MLSS)-1·h-1),Ks(mg·L-1)andKi(mg·L-1)are the Haldane constants representing the maximum specific substrate removal rate,the saturation constant and the substrate inhibition coefficient,respectively,andX(mg·L-1)is the MLSS concentration which was treated as a constant during the bioregeneration experiment.The results of their study revealed the superiority of the Haldane biodegradation model over the first-order biodegradation model in describing the bioregeneration of 4-CP-loaded AC in simultaneous adsorption and biodegradation processes.For rate enhancement,the increase in initial biomass concentration offered little effect on the rate of bioregeneration of 4-CP-loaded AC.

It is worthwhile to point out that the rate of bioregeneration is characterized by using the rate constant of desorption by many researchers.It is therefore conceivable that the slow rate of desorption constitutes a major barrier that needs to be overcome to expedite the bioregeneration process of spent AC.The rate of desorption is known to be affected by many extrinsic and intrinsic factors such as type of AC,temperature,pH,etc.as discussed in the above sections.Further studies are needed to overcome the bottleneck of slow desorption process which could be the controlling aspect that hampered the rate of bioregeneration.

4.Future Perspectives

To date,there are still numerous aspects on bioregeneration of AC that can be explored.One key aspect is to recover the active sites of spent AC during bioregeneration process which is mainly caused by adsorption hysteresis.The main factors contributing to the adsorption hysteresis are adsorption irreversibility,formation of biofilm,and pore blockage,which are heavily influenced by the surface chemistry and porosity of AC.For instance,the presence of basic oxygen functional groups(e.g.chromene,pyrone)on the AC surface promotes chemisorption of phenolviaoxidative polymerization reaction while increasing the lactone moiety reduces the degree of phenol adsorption irreversibility[79].Hence,the control of the AC surface moieties can potentially decrease the adsorption irreversibility and increase the bioregeneration efficiency.However,considering the surface chemistry of AC can be very dependent on the AC source which is difficult to control,future research can be directed to the development of new carbon materials(e.g.carbon nanotubes,mesoporous carbon,carbon nanodots)with excellent adsorptive property and tunable surface functionality for the enhancement of bioregeneration efficiency.

Similarly,the formation of biofilm and pore blockage(due to organics,SMP,etc.)reduces the effective active sites for adsorption.The SMP and recalcitrant organics can be adsorbed on the active sites of AC,leading to adsorption hysteresis.Two potential strategies to reduce the biofilm formation and pore blockage can be implemented,namely,(i)by using the immobilized biomass for AC bioregeneration[80-82],and(ii)tuning the surface properties of AC(e.g.controlling the hydrophobicity-hydrophobicity properties,introducing antibacterial group)to prevent the undesirable formation of biofilm.

For the first strategy,biomass immobilization through gelling process(to produce hydrogels,aerogels,cyrogelsetc.)can be used to enhance the separation of the bioregenerated adsorbent and biomass.PVA-hydrogels were initially explored for the immobilization of acclimated biomass for bioregeneration of activated carbon loaded with phenolic compounds[80].However,the results were not promising and these could be caused by several factors such as:(1)preparation of hydrogels using boric acid as the cross linker solution could probably annihilate fraction of microorganisms that are responsible for the assimilation process,(2)the chemical functional groups that are responsible for linking the interconnected walls that form the pores,(3)other type of polymers that are non-toxic and cost effective could be explored as an alternative to the PVA gels and(4)other means of crosslinking and formation of immobilized biomass beads techniques(i.e.entrapment,encapsulation,and adsorption)could be evaluated or created in such a way to enhance better stability and reusability.This leads to the finding of utilizing PVA-immobilized biomass cryogels for bioregeneration of GAC loaded with phenolic compounds[81].Compared to hydrogels,cryogels have higher operational stability with interconnected macropores that permits unhindered solutes diffusion[83].The development of these PVA-immobilized biomass cryogels has partially addressed the underlining factors that were highlighted previously and could be a step up towards finding other factors as well.

For the second strategy,a lot of attention was drawn towards the modification on the surface chemistry of AC for better understanding on the adsorption mechanism[79,84]but the correlation between the surface chemistry of the AC for the prevention of biofilm have yet to be explored.

Besides that,exploring other alternatives that will increase the biodegradability of the adsorbed recalcitrant organics to biodegradable products which could eventually increase the active sites for more organics or contaminants to be adsorbed is of paramount.One common approach is by the incorporation of advanced oxidation processes(AOPs)for wastewater that has low biodegradability(especially industrial wastewater).A combination of either chemical or thermal regeneration with biological regeneration could be of good prospect to increase the extent of regeneration of AC with careful consideration of cost and reusability of the spent AC.

5.Conclusions

Bioregeneration of spent AC should be of immense interest in industrial wastewater treatment plants since it offers cost advantage as compared with other methods.Thus,this review furnishes valuable information on fundamental understanding in BAC column and key factors that will lead to effective spent AC bioregeneration.The review also highlights the development of bioregeneration kinetic models that are found scanted by the degree of desorption.Of this,novel approaches capable of spurring the desorption rate will definitely permit the derivation of comprehensive mathematical kinetic model of spent AC bioregeneration,bringing this research field into a new different epoch.

Acknowledgements

The financial support from the Universiti Teknologi PETRONASviaYUTP-FRG(0153AA-E48)is gratefully acknowledged.

[1]K.Y.Bell,M.J.M.Wells,K.A.Traexler,M.L.Pellegrin,A.Morse,J.Bandy,Emerging pollutants,Water Environ.Res.83(2001)1906-1984.

[2]S.L.Loo,K.Y.Leong,P.E.Lim,Removal and transformation of hexavalent chromium in sequencing batch reactor,Water SA38(2012)9-14.

[3]Y.Li,T.Rao,Z.Liu,Effect of granular activated carbon on the enhancement of cometabolic biodegradation of phenol and 4-chlorophenol,Tsinghua Sci.Technol.15(2010)580-585.

[4]M.F.F.Sze,G.McKay,Enhanced mitigation of para-chlorophenol using stratified activated carbon adsorption columns,Water Res.46(2012)700-710.

[5]K.Y.Leong,S.See,J.W.Lim,M.J.K.Bashir,K.M.Lam,T.L.Chew,Interaction of key process variables acting on the simultaneous adsorption of phenolics in binary solution:Trend and optimization of adsorption,J.Ind.Pollut.Control.33(2017)712-722.

[6]J.Ren,W.Yang,M.Hua,B.Pan,W.Zhang,Bioregeneration of hyper-cross-linked polymeric resin preloaded with phenol,Bioresour.Technol.142(2013)701-705.

[7]W.A.Al-Amrani,P.E.Lim,C.E.Seng,W.S.Wan Ngah,Effects of co-substrate and biomass acclimation concentration on the bioregeneration of azo-dye-loaded mono-amine modified silica,Bioresour.Technol.143(2013)584-591.

[8] ?.Akta?,F.?e?en,Bioregeneration of activated carbon:A review,Int.Biodeterior.Biodegrad.59(2007)257-272.

[9] ?.Akta?,Effect of S0/X0ratio and acclimation of respiratory of activated sludge in the cometabolic biodegradation of phenolic compounds,Bioresour.Technol.111(2012)98-104.

[10]M.Sharbatmaleki,J.R.Batista,Multi-cycle bioregeneration of spent perchlorate containing macroporous selective anion-exchange resin,Water Res.46(2012)21-32.

[11]S.M.Khor,C.E.Seng,P.E.Lim,S.L.Ng,A.N.Ahmad Sujari,Activated rice husk-based adsorbents for chlorophenol removal and their bioregeneration,Desalin.Water Treat.57(2015)10349-10360.

[12]Z.Aksu,?.?.?a?atay,Investigation of biosorption of Germatol Turquise Blue-G reactive dye by dried Rhizopus arrhizus in batch and continuous systems,Sep.Purif.Technol.48(2006)24-45.

[13]N.Yal?in,V.Sevin?,Studies of the surface area and porosity of activated carbons prepared from rice husks,Carbon38(2000)1943-1945.

[14]A.Shehzad,M.J.K.Bashir,S.Sethupathi,J.W.Lim,An insight into the remediation of highly contaminated land fill leachate using sea mango based activated bio-char:Optimization,isothermal and kinetic studies,Desalin.Water Treat.57(2016)22244-22257.

[15]Z.G.Ng,J.W.Lim,H.Daud,S.L.Ng,M.J.K.Bashir,Reassessment of adsorption reduction mechanism of hexavalent chromium in attaining practicable mechanistic kinetic model,Process.Saf.Environ.Prot.102(2016)98-105.

[16]A.Shehzad,M.J.K.Bashir,S.Sethupathi,J.W.Lim,An overview of heavily polluted land fill leachate treatment using food waste as an alternative and renewable source of activated carbon,Process.Saf.Environ.Prot.98(2015)309-318.

[17]P.Chen,Q.Xie,M.Addy,W.Zhou,Y.Liu,Y.Wang,Y.Cheng,K.Li,R.Ruan,Utilization of municipal solid and liquid wastes for bioenergy and bioproducts production,Bioresour.Technol.215(2016)163-172.

[18]E.Iakovleva,P.Maydannik,T.V.Ivanova,M.Sillanpaa,W.Z.Tang,E.Makila,J.Salonen,A.Gubal,A.A.Ganeev,K.Kamwilaisak,S.Wang,Modified and unmodified low-cost iron-containing solid wastes as adsorbents for efficient removal of As(III)and As(V)from mine water,J.Clean.Prod.133(2016)1095-1104.

[19]N.P.Cheremisinoff,Chapter 5-Mass separation equipment,Handbook of Chemical Processing Equipment,Butterworth-Heinemann,New Delhi,2000,pp.244-333.

[20]J.Coca-Prados,G.Gutiérrez-Cervelló,Water Purification and Management,Springer,2011(ISBN:978-90-481-9774-3).

[21]T.Proll,G.Schony,G.Sprachmann,H.Hofbauer,Introduction and evaluation of a double loop staged fluidized bed system for post-combustion CO2capture using solid sorbents in a continuous temperature swing adsorption process,Chem.Eng.Sci.141(2016)166-174.

[22]J.Shabanian,J.Chaouki,Influence of interparticle forces on solids motion in a bubbling gas-solid fluidized bed,Powder Technol.299(2016)98-106.

[23]F.Salvador,N.Martin-Sanchez,R.Sanchez-Hernandez,M.J.Sanchez-Montero,C.Izquierdo,Regeneration of carbonaceous adsorbents.Part II:Chemical,microbiological and vacuum regeneration,Microporous Mesoporous Mater.202(2015)259-276.

[24]M.Scholz,R.J.Martin,Ecological equilibrium on biological activated carbon,Water Res.31(1997)2959-2968.

[25]G.Upadhyaya,J.Jackson,T.M.Clancy,K.V.Snyder,J.Brown,K.F.Hayes,L.Raskin,Effect of air-assisted backwashing on the performance of an anaerobic fixed-bed bioreactor that simultaneously removes nitrate and arsenic from drinking water sources,Water Res.46(2012)1309-1317.

[26]A.K.Venkatesan,M.Sharbatmaleki,J.R.Batista,Bioregeneration of perchlorate-laden gel type anion-exchange resin in a fluidized bed reactor,J.Hazard.Mater.177(2010)730-737.

[27]K.Katsoufidou,S.G.Yiantsios,A.J.Karabelas,An experimental study of UF membrane fouling by humic acid and sodium alginate solutions:The effect of backwashing on flux recovery,Desalination220(2008)214-227.

[28]J.Yang,W.Liu,B.Li,H.Yuan,M.Tong,J.Gao,Application of a novel backwashing process in up flow biological aerated filter,J.Environ.Sci.22(2010)362-366.

[29]M.Gao,Z.Chen,N.Ren,Z.Zhang,A novel application of automatic vacuum membrane bioreactor in wastewater reclamation,Desalination247(2009)583-593.

[30]Y.Chen,X.X.Zhang,B.Wu,B.Liu,L.Xiao,A.Li,S.Cheng,Semivolatile organic compounds removal and health risk reduction in drinking water treatment biofilters applying different backwashing strategies,Int.J.Environ.Sci.Technol.9(2012)661-670.

[31]X.Zhao,R.F.Hickey,T.C.Voice,Long-term evaluation of adsorption capacity in a biological activated carbon fluidized bed reactor system,Water Res.33(1999)2983-2991.

[32]R.Treguer,R.Tatin,A.Couvert,D.Wolbert,A.Tazi-Pain,Ozonation effect on natural organic matter adsorption and biodegradation—Application to a membrane bioreactor containing activated carbon for drinking water production,Water Res.44(2010)781-788.

[33]W.Nishijima,G.E.Speitel,Fate of biodegradable dissolved organic carbon produced by ozonation on biological activated carbon,Chemosphere56(2004)113-119.

[34]X.Wang,L.Wang,Y.Liu,W.Duan,Ozonation pretreatment for ultra filtration of the secondary effluent,J.Membr.Sci.287(2007)187-191.

[35]S.J.Park,T.I.Yoon,J.H.Bae,H.J.Seo,H.J.Park,Biological treatment of wastewater containing dimethyl sulphoxide from the semi-conductor industry,Process Biochem.36(2001)579-589.

[36]L.T.J.Van der Aa,R.J.Kolpa,L.C.Rietveld,J.C.Van Dijk,Improved removal of pesticides in biological granular activated carbon filters by pre-oxidation of natural organic matter,J.Water Supply Res.Technol.61(2012)153-163.

[37]L.Li,W.Zhu,P.Zhang,Q.Zhang,Z.Zhang,TiO2/UV/O3-BAC processes for removing refractory and hazardous pollutants in raw water,J.Hazard.Mater.128(2006)145-149.

[38]D.Wu,X.Cheng,X.Zhai,Y.Wang,The combination process of ozone/ultraviolet/biological activated carbon filter for treatment of Huaihe micro-polluted water,Appl.Mech.Mater.295-298(2013)1384-1388.

[39]L.Li,W.Zhu,P.Zhang,P.Lu,Q.Zhang,Z.Zhang,UV/O3-BAC process for removing organic pollutants in secondary effluents,Desalination207(2007)114-124.

[40]L.Li,P.Zhang,W.Zhu,W.Han,Z.Zhang,Comparison of O3-BAC,UV/O3-BAC and TiO2/UV/O3-BAC processes for removing organic pollutants in secondary effluents,J.Photochem.Photobiol.A Chem.171(2005)145-151.

[41]E.Sahinkaya,F.B.Dilek,Biodegradation kinetics of 2,4-dichlorophenol by acclimated mixed cultures,J.Biotechnol.127(2007)716-726.

[42]G.Briceno,H.Schalchli,C.S.Benimeli,G.Palma,G.R.Tortella,M.C.Diez,Use of pure and mixed culture of diazinon-degrading Streptomyces to remove other organophosphorus pesticides,Int.Biodeterior.Biodegrad.114(2016)193-201.

[43]V.D.Jakovljevic,M.M.Vrvic,Potential of pure and mixed cultures ofCladosporium cladosporioidesandGeotrichum candidumfor application in bioremediation and detergent industry,Saudi J.Biol.Sci.(2016)https://doi.org/10.1016/j.sjbs.2016.01.020.

[44]J.W.Lim,J.Z.Tan,C.E.Seng,Performance of phenol-acclimated activated sludge in the presence of various phenolic compounds,Appl Water Sci3(2013)515-525.

[45]J.H.Kim,Oh.KK,S.T.Lee,S.W.Kim,S.I.Hong,Biodegradation of phenol and chlorophenols with defined mixed culture in shake- flasks and a packed bed reactor,Process Biochem.37(2002)1367-1373.

[46]N.Pasukphun,S.Vinitnantharat,S.Gheewala,Investigation of decolorization of textile wastewater in an anaerobic/aerobic biological activated carbon(A/A BAC),Pak.J.Biol.Sci.13(2010)316-324.

[47]S.See,J.W.Lim,P.E.Lim,C.E.Seng,S.L.Ng,R.Adnan,Enhancement of o-cresol removal using PAC and acclimated biomass immobilized in polyvinyl alcohol hydrogel beads,Desalin.Water Treat.52(2014)7951-7956.

[48]S.See,P.E.Lim,J.W.Lim,C.E.Seng,R.Adnan,Evaluation of o-cresol removal using PVA-cryogel-immobilised biomass enhanced by PAC,Water SA41(2015)55-60.

[49]S.L.Ng,C.E.Seng,P.E.Lim,Quantification of bioregeneration of activated carbon and activated rice husk loaded with phenolic compounds,Chemosphere75(2010)1392-1400.

[50]S.L.Ng,C.E.Seng,P.E.Lim,Bioregeneration of activated carbon and activated rice husk loaded with phenolic compounds:kinetic modeling,Chemosphere78(2010)510-516.

[51]W.D.Oh,P.E.Lim,C.E.Seng,N.Mohamed,R.Adnan,K.Y.Leong,S.Y.Voon,Effects of initial biomass concentration on bioregeneration of 4-chlorophenol-loaded granular activated carbon:Kinetic and efficiency studies,J.Chem.Technol.Biotechnol.88(2013)1157-1163.

[52]Oh.WD,P.E.Lim,K.Y.Leong,S.L.Yong,H.Yin,Bioregeneration of granular activated carbon loaded with binary mixture of phenol and 4-chlorophenol,Desalin.Water Treat.57(43)(2016)20476-20482.

[53]G.Mancini,M.Panzica,D.Fino,S.Capello,M.M.Yakimov,A.Luciano,Feasibility of treating emulsified oily and salty wastewaters through coagulation and bioregenerated GAC filtration,J.Environ.Manag.203(2016)817-824.

[54]Z.Song,S.R.Edwards,R.G.Bums,Treatment of napthalene-2-sulfonic acid from tannery wastewater by a granular activated carbon fixed bed inoculated with bacteria isolatesArthrobacter globiformisandComamonas testosteroni,Water Res.40(2006)495-506.

[55]S.Vinitnantharat,A.Barat,Y.Ishibashi,S.R.Ha,Quantitative bioregeneration of granular activated carbon loaded with phenol and 2,4-dichlorophenol,Environ.Technol.22(2001)339-344.

[56]V.Abromaitis,V.Racys,P.Van der Marel,G.Ni,M.Dopson,A.L.Wolthuizen,R.J.W.Meulepas,Effect of shear stress and carbon surface roughness on bioregeneration and performance of suspended versus attached biomass in metoprolol-loaded biological activated carbon systems,Chem.Eng.J.317(2017)503-511.

[57]K.Yapsakli,F.?e?en,Effect of type of granular activated carbon on DOC biodegradation in biological activated carbon filters,Process Biochem.45(2010)355-362.

[58]A.V.Vinod,G.V.Reddy,Simulation of biodegradation process of phenolic wastewater at higher concentrations in a fluidized-bed bioreactor,Biochem.Eng.J.24(2005)1-10.

[59]I.Ivancev-Tumbas,B.Dalmacijia,Z.Tamas,E.Karlovic,Reuse of biologically regenerated activated carbon for phenol removal,Water Res.32(1998)1085-1094.

[60]A.S.Sirotkin,L.Y.Koshkina,K.G.Ippolitov,The BAC process for treatment of waste water containing non-ionogenic synthetic surfactants,Water Res.35(2001)3265-3271.

[61]C.C.Chien,C.M.Kao,C.W.Chen,C.D.Dong,C.Y.Wu,Application of biofiltration system on AOC removal:Column and field studies,Chemosphere71(2008)1786-1793.

[62]?.Kalkan,K.Yapsakli,B.Mertoglu,D.Tufan,A.Saatci,Evaluation of Biological Activated Carbon(BAC)process in wastewater secondary treatment for reclamation purposes,Desalination265(2011)266-273.

[63]A.W.M.Ip,J.P.Barford,G.A.Mckay,comparative study on the kinetics and mechanisms removal of Reactive Black 5 by adsorption onto activated carbons and bone char,Chem.Eng.J.157(2010)434-442.

[64]A.Imai,K.Onuma,Y.Inamori,R.Sudo,Biodegradation and adsorption in refractory leachate treatment by the biological activated carbon fluidized bed process,Water Res.29(1995)687-694.

[65]B.Seredyńska-Sobecka,M.Tomaszewska,M.Janus,A.W.Morawski,Biological activation of carbon filters,Water Res.40(2006)355-363.

[66]A.R.H.Putz,D.E.Losh,G.E.Speitel Jr.,Removal of nonbiodegradable chemicals from mixtures during granular activated carbon bioregeneration,J.Environ.Eng.131(2005)196-205.

[67]E.S.Melin,K.T.Jarvinen,J.A.Puhakka,Effects of temperature on chlorophenol biodegradation kinetics in fluidized-bed reactors with different biomass carriers,Water Res.32(1998)81-90.

[68]A.Andersson,P.Laurent,A.Kihn,M.Prévost,P.Servais,Impact of temperature on nitrification in biological activated carbon(BAC) filters used for drinking water treatment,Water Res.35(2001)2923-2934.

[69]Y.H.Lin,J.Y.Leu,Kinetics of reactive azo-dye decolorisation byPseudomonas luteolain a biological activated carbon process,Biochem.Eng.J.39(2008)457-467.

[70]M.J.Kirisits,V.L.Snoeyink,H.Inan,J.C.Chee-Stanford,L.Raskin,J.C.Brown,Water quality factors affecting bromated reduction in biologically active carbon filters,Water Res.35(2001)891-900.

[71]P.N.Bhattacharyya,D.K.Jha,Plant growth-promoting rhizobacteria(PGPR):emergence in agriculture,World J.Microbiol.Biotechnol.28(2012)1327-1350.

[72]H.Duan,L.C.C.Koe,R.Yan,X.Chen,Biological treatment of H2S using pellet activated carbon as a carrier of microorganisms in a bio filter,Water Res.40(2006)2629-2636.

[73]K.Nath,M.S.Bhakhar,S.Panchani,Bioregeneration of spent activated carbon:Effect of physico-chemical parameters,J.Sci.Ind.Res.70(2011)487-492.

[74] ?.Akta?,F.?e?en,Effect of activation type on bioregeneration of various activated carbons loaded with phenol,J.Chem.Technol.Biotechnol.81(2006)1081-1092.

[75] ?.Akta?,F.?e?en,Adsorption and cometabolic bioregeneration in activated carbon treatment of 2-nitrophenol,J.Hazard.Mater.177(2010)956-961.

[76]S.R.Ha,S.Vinitnantharat,Y.Ishibashi,A modelling approach to bioregeneration of granular activated carbon loaded with phenol and 2,4-dichlorophenol,J.Environ.Sci.Health36(2001)275-292.

[77] ?.Akta?,F.?e?en,Cometabolic bioregeneration of activated carbons loaded with 2-chlorophenol,Bioresour.Technol.100(2009)4604-4610.

[78]Oh.WD,P.E.Lim,C.E.Seng,A.N.A.Sujari,Kinetic modeling of bioregeneration of chlorophenol-loaded granular activated carbon in simultaneous adsorption and biodegradation processes,Bioresour.Technol.114(2012)179-187.

[79]A.D?browski,P.Podko?cielny,Z.Hubicki,M.Barczak,Adsorption of phenolic compounds by activated carbon—A critical review,Chemosphere58(2005)1049-1070.

[80]R.H.Toh,P.E.Lim,C.E.Seng,R.Adnan,Immobilized acclimated biomass-powdered activated carbon for the bioregeneration of granular activated carbon loaded with phenol ando-cresol,Bioresour.Technol.143(2013)265-274.

[81]K.Y.Leong,R.Adnan,P.E.Lim,S.L.Ng,C.E.Seng,Effect of operational factors on bioregeneration of binary phenol and 4-chlorophenol-loaded granular activated carbon using PVA-immobilized biomass cryogels,Environ.Sci.Pollut.Res.24(26)(2017)20959-20971.

[82]K.Y.Leong,S.See,J.W.Lim,M.J.K.Bashir,C.A.Ng,L.Tham,Effect of process variables interaction on simultaneous adsorption of phenol and 4-chlorophenol:statistical modeling and optimization using RSM,Appl.Water Sci.7(2017)2009-2020.

[83]V.I.Lozinsky,I.Y.Galaev,F.M.Plieva,I.N.Savina,H.Jungvid,B.Mattiasson,Polymeric cryogels as promising materials of biotechnological interest,Trends Biotechnol.21(2003)445-451.

[84]I.Shah,R.Adnan,W.S.W.Ngah,N.Mohamed,Y.H.T.Yap,A new insight to the physical interpretation of activated carbon and iron doped carbon material:Sorption affinity towards organic dye,Bioresour.Technol.160(2014)52-56.