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Ultrasonics - Sonochemistry

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A review on the applications of ultrasonic technology in membranebioreactors

Samira Arefi-Oskouia,b, Alireza Khataeeb,c,⁎, Mahdie Safarpourd, Yasin Oroojia,⁎,Vahid Vatanpoure

a College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Chinab Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iranc Department of Materials Science and Nanotechnology Engineering, Faculty of Engineering, Near East University, 99138 Nicosia, Mersin 10, TurkeydDepartment of Chemistry, Faculty of Basic Science, Azarbaijan Shahid Madani University, 83714-161 Tabriz, Irane Department of Applied Chemistry, Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran

A R T I C L E I N F O

Keywords:Membrane bioreactorMembrane foulingCleaning of membranesUltrasonicationHybrid ultrasound

A B S T R A C T

Membrane bioreactors (MBRs) have received increasing attention in the field of wastewater treatment in recentyears. However, membrane fouling is the main problem of MBRs, limiting their widespread and large applica-tions. Membrane cleaning methods can be mainly classified into four types including chemical, physical, phy-sico-chemical and biological clean the fouled membrane. In recent years, ultrasonication has been reported as apromising cleaning technique for the membranes fouled in MBRs. Ultrasonic irradiation can clean the fouledmembrane by creating important physical phenomena including microjets, microstreams and shock waves.Moreover, the ultrasonic method can be combined with other cleaning methods e.g. chemical cleaning andbackwashing in order to improve the cleaning efficiency. It should be noted that the application of ultrasonic inthe MBR system is not limited to the cleaning of membrane. The pretreatment of the wastewater by ultrasonicirradiation or ultrasound coupled with other methods, e.g. ozonation, prior to MBR system, can decrease theorganic loading of the wastewater and subsequently postpone the fouling of the membrane. This paper criticallyreviews the recent advances in the applications of ultrasound in MBR systems. Emerging issues associated withapplication of on-line ultrasound and also hybrid on-line ultrasound for controlling the membrane fouling inMBR systems are critically reviewed. Moreover, application of the ultrasound in ex-situ form for cleaning thefouled membranes and pretreatment of wastewater prior to the MBR system is discussed.

1. Introduction

Membrane bioreactor (MBR) is a wastewater treatment technologywhich is a combination of a biological treatment system like activatedsludge and a membrane filtration process, e.g. microfiltration or ul-trafiltration [1,2]. In recent years, the membrane bioreactors have at-tracted increasing attention in treatment of industrial and municipalwastewater owing to several advantages, e.g. stable and high effluentquality, low footprint, independency from the sludge condition in the

bioreactor and high volumetric loading [3]. However, fouling of themembrane is a challenging problem in MBR systems which limits theirlarge-scale applications of for wastewater treatment [4,5]. Membranefouling in MBR systems results in a decrease in the flux of the permeateor an increase in the trans-membrane pressure required to achieve astable permeate flux during filtration process, which consequently leadsto an increase in operating costs. Based on the nature of the foulant, themembrane fouling in MBR systems can be generally classified into in-organic fouling, organic fouling and biofouling. Inorganic and organic

https://doi.org/10.1016/j.ultsonch.2019.104633Received 14 May 2019; Received in revised form 5 June 2019; Accepted 7 June 2019

Abbreviations: AeMBRs, Aerobic membrane bioreactors; AnMBRs, Anaerobic membrane bioreactors; CBZ, carbamazepine; CUO, catalytic ultrasound oxidation;DCF, diclofenac; EPS, extracellular polymeric substances; FT-IR, fourier transform infrared; HRT, hydraulic retention time; MLSS, mixed liquor suspended solid;MLVSS, mixed liquor vapor suspended solid; MBR, Membrane bioreactor; OLR, organic loading rate; SBAC, sewage sludge based activated carbon; SMP, Solublemicrobial products; SMX, sulfamethoxazole; TOC, total organic carbon; VS, volatile solids; VSS, volatile suspended solids

⁎ Corresponding authors at: Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry,University of Tabriz, 51666-16471 Tabriz, Iran (A. Khataee); College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China (Y.Orooji).

E-mail addresses: a_khataee@tabrizu.ac.ir (A. Khataee), yasin@njfu.edu.cn (Y. Orooji).

Ultrasonics - Sonochemistry 58 (2019) 104633

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foulings are respectively caused by deposition of the inorganic crystalsand organic substances e.g. humic acid, proteins and polysaccharides;while, the biofouling results from precipitation and growth of the mi-croorganisms on the membrane surface. On the other hand, based onthe location of the foulant, the membrane fouling can be classified intointernal fouling and external fouling. Formation of a cake layer on thesurface of membrane is known as dominant fouling in MBR systems.According to the foulant removing mechanism, membrane cleaningmethods can be classified into four main groups including physical,chemical, physico-chemical and biological [6,7]. In a physical cleaningmethod, the foulants can be removed from the surface of the membraneby mechanical treatment. Among the different physical methods, ul-trasonication has got increasing attention in the field of controlling themembrane fouling in MBR in recent years [8–11]. Ultrasonic waves inthe liquid phase induce cavitation phenomenon including nucleation,growth and collapse of bubbles. Bubble collapsing takes place con-tinuously at millions of locations in the liquid media resulting in thegeneration of supercritical conditions such as high temperature(5000 K) and high pressure (~1000 atm) [12–14]. Ultrasonic irradia-tion can clean the fouled membrane through the production of im-portant physical phenomena including microjets, microstreams andshock waves [15]. Indeed, the particles can be released from the fouledmembrane by the aforementioned physical phenomena taking place ina heterogeneous liquid-solid interface. Furthermore, the active hy-droxyl radicals generated in the presence of ultrasonic irradiation canattack the adsorbed foulants and degrade the molecules of foulantswhich consequently result in membrane fouling control. However, themembrane can be damaged through chemical reactions between thegenerated hydroxyl radicals and the membrane [16]. Therefore, theoperational conditions should be optimized in ultrasound-MBR hybridsystems. The ultrasonic can be performed either in-situ (online) or ex-situ (offline) for cleaning the membrane of MBRs. Moreover, pretreat-ment of the wastewater by ultrasonic irradiation or by hybrid ultra-sound methods prior to MBRs can decrease the organic loading of thewastewater and subsequently postpone the fouling of the membrane.This paper reviews the application of the ultrasonic technology in MBRsystems. In this paper, two main applications of the ultrasonic tech-nology in the MBR systems including the cleaning of the fouled mem-brane and pretreatment of MBR feed by ultrasound are discussed indetail. Recent studies performed on the investigation of operationalconditions of the in-situ and ex-situ ultrasound to control the membranefouling and clean the fouled membranes are demonstrated in this paper.

2. Ultrasonication

American National Standards Institute defines the ultrasound as asound with a frequency greater than 20 kHz [17]. Fig. 1 represents thediagram of the ultrasound range. Ultrasound can be classified into threemain categories including power ultrasound (20–100 kHz), high fre-quency ultrasound (100 kHz–1MHz), and diagnostic ultrasound(1–500MHz) according to its frequency [17]. Generally, ultrasoundwith a frequency range of 20 kHz–100 kHz is used in chemical systems[13,18].

By passing the ultrasound through water or many other liquids,regions with high and low pressures are created inside the liquid ac-cording to the periodic expansion and compression of the ultrasound[12,19,20]. This phenomenon in the liquid media induces acoustic

cavitation process including formation, growth and collapse of thebubbles [21–24]. Small bubbles are generated during expansion of theultrasound. In continuation, the generated bubbles grow by absorbingthe energy from the alternating compression and expansion cycles [25].Finally, over a few acoustic cycles, the bubbles reach a critical size andrapidly collapse. It can be assumed that there is almost no heat transferbetween the inside of the bubbles and surrounding media during thefast transient collapse of the bubbles. Therefore, the pressure andtemperature within the core of the bubbles rise to several hundred at-mospheres (~1000 atm) and thousands of Kelvins (5000 K), respec-tively [25,26]. The core region with high pressure and temperature isknown as the “hot spot”. After collapsing, the heat transfers rapidlyfrom the localized high temperature to the surrounding liquid at a highrate of about 109 K/s in the vicinity of the bubbles [26]. In addition,large pressure difference between the inside and outside of the bubblesinduces generating “shock wave” [26,27]. The generated hot spots canaccelerate the pyrolysis of the water molecules to produce hydrogenradicals (H%) and hydroxyl radicals (%OH) according to Eq. (1) [28,29]:

H2O Ultrasonic irradiation→H%+ %OH (1)

Also, some additional consecutive chain reactions can take placeaccording to Eqs. (2)–(4) [21,28,29]:

%OH+H%→H2O (2)

%OH+ %OH→H2O2 (3)

H2O2+H%→H2O+ %OH (4)

Regeneration of the produced radicals causes emission of the light(200–800 nm) for a short period of time (about 100 ps). This phenom-enon is known as sonoluminescence. The shock wave and the radicalsgenerated by the thermal decomposition in or around the bubbles playthe main role in the phenomena occurring in sonochemistry [19,25,26].It should be noted that the operating parameters, i.e. ultrasound fre-quency, dissolved gases, power input and bulk temperature efficientlyinfluence the sonochemical processes [30–32].

The reactors used for sonochemical processes can be classified intotwo main groups of reactors with direct ultrasonic irradiation and re-actors with indirect ultrasonic irradiation. As can be seen in Fig. 2(a), inindirect irradiation, the ultrasound produced by the transducer reachesthe intended solution after transmitting through the mediator fluid (i.e.water). In indirect irradiation, the ultrasound transducer is in directcontact with the reaction vessel. The transducer can be attached to thebottom of the reactor (Fig. 2(b)) or it can be localized inside the reactor(Fig. 2(c)).

3. Membrane bioreactor systems

3.1. Fundamentals of membrane bioreactors

Membrane bioreactor is the combination of a biological treatmentsystem like activated sludge, and a membrane filtration process e.g.microfiltration or ultrafiltration. Indeed, integrating the advantages ofthe membrane processes with biocatalysts, i.e. enzymes, microorgan-isms or plant/animal cells results in the formation of an importanttechnology known as MBR, which is rapidly advancing both in com-mercial applications and research field [34–36]. As shown in Fig. 3, the

Fig. 1. Ultrasound range diagram.

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configuration of the MBRs can be mainly classified into two groups of 1)side-stream and 2) submerged or immersed [3]. The side-stream con-figuration is a pressure-driven process in which the membrane moduleis placed out of the bioreactor. In this kind of MBR, the concentratedsludge is rejected by the membrane and recycled to the bioreactor(Fig. 3(a)). However, in the second kind of MBR, the membrane moduleis submerged within the bioreactor, and a suction pressure is applied toproduce the permeate (Fig. 3(b)). Submerged MBR is more economiccompared with side-stream MBR owning to low energy consumptionand low equipment requirement [3]. The energy consumption of theside-stream MBR is reported to be 10–25 times more than that of thesubmerged one [3]. However, the operating flux in the side-stream MBRis approximately four times more than that of the submerged MBR andit is more robust and flexible [3].

Based on the operating condition, the MBRs can be classified intoaerobic membrane bioreactors (AeMBRs) and anaerobic membranebioreactors (AnMBRs) [38–40]. In AeMBR processes, sparging the air

into the reactor not only supplies the required oxygen to the microbialculture, but also minimizes the membrane fouling by scouring thesurface of the membrane [1]. Furthermore, aeration improves theconditions of mass transfer and air bubbles keep the solids in suspen-sion [1]. However, owning to its dominant advantages such as lowsludge production and low energy consumption compared with AeMBR,AnMBR has been of great interest in recent years. MBR processes havebeen extremely applied for wastewater treatment overall in the world[1]. Applications of MBR technology for treating some special waste-water are summarized in Table 1. It should be noted that there are a lotof papers reporting on wastewater treatment using MBR; however, onlysome sample researches are reported in Table 1. In addition, growinginterest can be seen in recent years in the field of applying enzymaticMBR systems for production of pharmaceuticals, biofuels and food in-gredients [1,41]. Enzymatic MBR process is the hybrid of the biocata-lyst with membrane bioreactor in which the enzymes as a biocatalystare immobilized on/in the membrane through different methods such

Fig. 2. (a) Indirect ultrasonic irradiation, (b) direct ultrasonic irradiation in bath and (c) direct ultrasonic irradiation by horn (Reproduced from Asakura et al. [33]with permission from Elsevier).

Fig. 3. Configurations of membrane bioreactor: (a) side-stream membrane bioreactor, and (b) submerged membrane bioreactor. Reprinted from Robles et al. [37]with permission from Elsevier.

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as covalent binding, physical adsorption, ionic binding or cross-linking[2,42,43].

3.2. Fouling of membrane in membrane bioreactor systems

In MBR processes, fouling of the membranes is known as an un-avoidable problem which consequently results in decreasing the flux ofthe permeate or increasing the trans-membrane pressure needed toachieve a stable permeate flux during filtration process. Indeed, foulingof membranes in MBR processes leads to an increase in operating costs.Based on the location of the foulant, the membrane fouling can beclassified into two main groups including external fouling and internalfouling. The deposition of the particles, macromolecules and colloids onthe surface of the membrane results in external fouling [53–57]. Gen-erally, the external fouling is also known as fouling layer is divided intotwo types including cake layer and gel layer. Cake layer results fromprecipitation of retained solids on the surface of the membrane. While,gel layer is mainly formed due to gelation of the colloidal and dissolvedmatters such as biopolymer clusters and soluble microbial products onthe surface of the membrane [6,58,59]. Internal fouling results fromdeposition or adsorption of solutes or particles in the internal structureof the membrane. This kind of fouling also known as pore blockingresults in narrowing or blocking of the membrane pores [60,61].

On the other hand, based on the nature of the foulant, the mem-brane fouling in MBR systems can be generally classified into colloidalfouling, organic fouling, inorganic scaling and biofouling [62,63]. Itshould be noted that generally a combination of the aforementionedfouling types occurs in the MBRs. Deposition of the organic moleculessuch as carbohydrate, humic acid, proteins, polysaccharides and otherorganic substances onto the membrane is responsible for organicfouling in MBRs. The source of the organic molecules causing organicfouling can be the metabolism of microorganisms and/or feed water.The organic matters such as soluble microbial products (SMP) and ex-tracellular polymeric substances (EPS) produced by microorganisms areknown as the main agents of organic fouling [64–66]. Moreover, inmost cases, organic fouling can be attributed to organic-inorganicpolymers (chelating polymers) produced via interaction between themetal cations and some functional groups of the organic materials [54].Inorganic fouling in MBRs mainly refers to the chemical precipitation ofthe inorganic crystals [4,67]. The chemical reaction of the metal cationssuch as Mg2+, Ca2+, Al3+ and Fe3+ with anions such as OH−, PO4

3−,CO3

2− and SO42− results in producing chemical precipitation, leading

to scaling. Biofouling in MBRs is caused by the attachment and growthof the microorganisms on the surface of the membrane or within themembrane pores which occurs over a long period of operation[5,40,64,68–70]. Gradually, deposited and proliferated microorganismsconsiderably lessen the MBR performance [71]. Henceforth, it is vital toalleviate both the primary attachment of bacteria and the subsequentgrowth on the membrane surface.

Based on the foulant attachment strength to the membrane, thefouling of the membrane in MBR processes can be classified into threemain groups of reversible fouling, irreversible fouling and irrecoverablefouling [4,59,72–75]. Loose attachment of the foulants to the mem-brane results in reversible fouling. Formation of the cake layer on themembrane is known as the main reason causing reversible fouling.Physical cleaning methods e.g. backflush and intermittent filtration(relaxation) can be applied to remove this kind of fouling. Irreversiblefouling is defined as fouling which cannot be removed by physicalcleaning methods. For removing this kind of fouling, chemical cleaningand biological cleaning methods should be used. Strong attachment ofthe foulants on the surface of the membrane and inside the pores resultsin narrowing and blocking of the surface and sub-layer pores of themembrane inducing irreversible fouling. It should be noted that thereversible fouling can be transformed into the irreversible fouling layerduring a long-time and continuous filtration process. The third kind ofthe fouling which cannot be removed through typical chemical orphysical cleaning is termed “irrecoverable fouling”. This kind of foulingoccurs via long-term operation over a number of years [6,76].

3.3. Cleaning of membrane in membrane bioreactor systems

Based on the location of the membrane module during the cleaningprocess, the cleaning methods can be categorized into in-situ and ex-situcleaning. In in-situ cleaning, also termed as online cleaning, the mem-brane module remains in the bioreactor during the cleaning process.However, in ex-situ method, the cleaning of the membrane is performedout of the bioreactor, known as cleaning out of the place or offlinecleaning.

Based on the mechanism used for removing the foulants, cleaningmethods can be categorized into four main groups of physical, che-mical, physico-chemical and biological cleaning [6,7]. Physicalcleaning is attributed to the mechanical treatment to remove the fou-lants from the surface of the membrane. Up till now, different physicalmethods such as backflushing, relaxation, vibration, air sparge and

Table 1Applications of MBR technology for treating some special wastewater.

Type of MBR Type of wastewater Scale Operating condition Influent Effluent Reference

AnMBR Cheese whey (Industrial wastewater) Laboratory scale HRT=1 d/4 dTemp=37 ± 2/37 °CMLVSS=6.4–10 g/L

COD=68.6 ± 3.3 COD=−(98.5%) [44]

AnMBR Petrochemical wastewater (Industrialwastewater)

Laboratory scale HRT=31.5 hTemp=37 °CMLSS >30 g/L

COD=612 COD=19.1 (98%) [45]

AnMBR Textile wastewater (Industrial wastewater) Laboratory scale HRT=24 hTemp=35 °C

COD=730–1100 COD=−(90%) [46]

AnMBR Municipal wastewater Laboratory scale HRT=10 hMLSS=6.4–9.3 g/L

COD=425 ± 47 COD=51 ± 10 (88 ± 2%) [47]

AnMBR Domestic wastewater Laboratory scale HRT=6 hMLSS=14–80 g/LTemp=25 °C

CODt= 540 CODs=65 (88%) [48]

AnMBR Landfill leachate Laboratory scale HRT=7 dMLVSS=3 g/L

COD=41 ± 3.14 COD=3.77 ± 0.34(90.7 ± 1.1%)

[49]

AeMBR Cosmetic industry wastewater Pilot scale HRT=3.5 dTemp=15–29 °C

COD=13.57 COD=−83.73% [50]

AeMBR Acrylonitrile butadiene styrene (ABS) resinwastewater

Laboratory scale HRT=18 h 3380 ± 50 270 ± 20 [51]

AeMBR Textile wastewater Laboratory scale HRT=1.7–4 dTemp=32–34 °C

1380–6033 177–720 [52]

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ultrasonication have been developed for cleaning the membrane[6,7,77]. Among the aforementioned physical methods, backflushingand relaxation have been widely applied in industrial MBR systems forcleaning and regeneration of membranes [78]. Chemical cleaning refersto methods applying chemical agents such as acids (phosphoric acidand nitric acid), alkalines (hydroxides, phosphates and carbonates),oxidants (hydrogen peroxide and hypochlorite) and surface-activeagents (anionic, cationic and nonionic) to remove the foulants [7,77].In order to enhance the effectiveness of the cleaning, physical cleaningcan be performed in the presence of chemical agents known as physico-chemical method [79–81]. Generally, chemical cleaning seems to be aneffective method for removing the foulants. However, using a chemicalagent in the cleaning process results in the formation of chemicalcompounds like organic halogen compounds as the by-product whichare in most cases harmful and toxic [82]. Moreover, chemical agentsused for cleaning can damage the structure of the membrane, resultingin a decrease in the membrane lifetime [83]. Biological cleaningmethods use bioactive agents like enzymes to remove the foulants fromthe membrane [77,84]. This cleaning method can be efficiently used forthe removal of irreversible fouling in the MBR membranes. However,bioactive agents like enzymes are more expensive compared withchemical agents, resulting in an increase in the overall MBR processcosts. Moreover, enzymes represent preferential and selective activitytowards the foulants owning to their intrinsic structure. Therefore, theycannot be used as general cleaning agent for degradation and removalof foulants. In addition, the operational conditions such as temperature,pH, concentration of enzymes and cleaning length of time should becontrolled in the case of using enzymes as the cleaning agent; that isbecause enzymes represent efficient activity in optimum operationalconditions.

Among the documented cleaning methods, ultrasonication as anenvironmental-friendly, sustainable and highly efficient physicalcleaning method has received increasing attention in recent years[16,85–90]. Application of the ultrasonic technique for cleaning thefouled membrane used in MBR systems will be discussed in detail in thefollowing sections.

4. Application of ultrasonic in membrane bioreactors

The application of the ultrasonic technology in the field of MBRsystems can be mainly studied in two fields of cleaning the fouledmembrane using ultrasonic irradiation and pretreatment of the MBRfeed by ultrasound.

Cleaning of the membrane in the MBRs by using ultrasonication canbe performed at either ex-situ or in-situ. Literature review demonstratesthat ultrasonic cleaning is able to efficiently remove the cake layer fromthe surface of the fouled membrane [89,91,92]. Moreover, recent re-searches show that gel layer fouling in the MBR systems can be suc-cessfully controlled using on-line ultrasonic irradiation [85]. Removingthe foulants from the membrane in the presence of ultrasonic irradia-tion can be attributed to the physical phenomena occurring in hetero-geneous solid-liquid systems [10,15]. Formation of microstreaming,acoustic streaming, shock wave and microjets are the fundamentalphysical phenomena causing foulants released during the ultrasoniccleaning [10,15]. Microstreaming occurs in the proximity of the cavi-tation bubbles. Indeed, oscillation in the bubble size, expansion andcontraction of the generated bubbles result in a rapid change in direc-tion and magnitude of the liquid flow. As can be seen in Fig. 4(a), theshrinkage of the generated bubbles during the compression cycle resultsin pulling the molecules of the liquid from the surface of the membrane.On the other hand, the expansion of the cavitation bubbles during theexpansion cycle results in pushing the molecules of the liquids towardsthe membrane surface (Fig. 4(b)). The aforementioned rapid changes indirection and magnitude of liquid flow near the surface of the mem-brane induce shear and dragging force on the membrane surface, re-sulting in the release of the foulant particles [10,11,15]. It should be

noted that the effective range of this physical phenomenon is around1–100 µm (on the order of the diameter of a bubble).

Absorption of the ultrasonic energy by the liquid without cavitationcollapsing forms a fluid flow known as acoustic streaming (Fig. 5)[10,11,15]. In other words, due to the physical force of the acousticwaves, a driving force which results in the displacement of the mole-cules of the liquid is produced to induce the fluid flow of the acousticstreaming [93]. The velocity of the flow generated by acousticstreaming phenomenon is low (~10m/s); this can be increased byenhancing the frequency of the ultrasound and intensity of the power. Itshould be noted that the acoustic streaming is formed only in a fewcentimeters away from the ultrasonic transducer. When the fouledmembrane is exposed to the acoustic streaming, a unidirectional flow isgenerated parallel with the surface of the membrane, which conse-quently results in the removal the foulants from the surface of themembrane [15].

Shock wave is another ultrasonic mechanism that helps the removalof foulants from the surface of the membrane in the presence of ultra-sonic irradiation. Shock waves are formed repeatedly over the com-pression and expansion cycles. At the end of the compression cycle, thecavitation bubbles with a minimum radius come to a sudden halt. Atthis moment, the reflection of the liquid molecules which were movingtowards the bubbles during compression, results in generating shockwaves with high pressure [15,94]. Occurrence of the shock waves in thevicinity of the membrane surface results in detaching the foulants fromthe surface of the membrane.

Microjet generation can be attributed to the asymmetric collapsingof the cavitation bubbles. Generally, the generated cavitation bubbleskeep their spherical geometry as long as the motion of the liquid vici-nity of the bubbles stays uniform and symmetric. However, the motionof the liquid in the vicinity of the solid phase is prevented and results ina pressure gradient around the bubbles. The developed pressure gra-dient results in the deformation of spherical geometry of bubbles.Owning to the pressure gradient and consequently asymmetric geo-metry of the bubble, the collapsing of the portion of the bubble occursfaster than that of the rest of the bubbles and results in the formation ofmicrojet (Fig. 6). Owning to their high velocity, the generated microjetsplay an important role in the cleaning of surface of the fouled mem-brane (about 120–150m/s) [11,94,95]. It should be noted that in ad-dition to the aforementioned physical phenomenon for cleaning thefouled membrane, radicals generated during the ultrasonication canattack the foulants, resulting in degradation of the foulants andcleaning of the membrane [15].

In addition to cleaning of the fouled membrane, ultrasonic tech-nology can be used for pretreatment of wastewater prior to MBR.Generally, pretreatment of the wastewater results in controlling themembrane fouling, and consequently enhances the membrane perfor-mance in the MBR system. Indeed, the growth of the algal and biofilmcan be decreased in the presence of ultrasonic irradiation, resulting in adecrease in the biofouling on the surface of the membrane [96]. Inaddition, pretreatment of the wastewater using ultrasound can convertthe contaminant compounds to biodegradable intermediates which canbe treated biologically by microorganisms in the reactor of the MBR.The application of ultrasonic technology in both cleaning membraneand pretreatment of wastewater in the MBR systems is discussed indetail in the following sections.

4.1. In-situ applications of ultrasound in MBR system

4.1.1. Control of membrane fouling in the MBR using online ultrasoundFouling of the membrane in MBR systems limits their widespread

applications and increases their operational costs. Fouling control of themembrane using online ultrasonication has attracted increasing atten-tion in recent years. Several researchers have reported the applicationof the ultrasound equipment in the reactor of MBR (in-situ) as a mem-brane cleaning technique [85,91,97,98]. In this section, the online

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ultrasound-MBR coupled systems are studied.Xu et al. [85] applied an AnMBR system for the digestion of sludge

under a high volatile solids (VS) at a loading rate of (3.7 gVS/L d) for54 days. The flow diagram of used system is shown in Fig. 7. As can beseen, online ultrasonic equipment was coupled with AnMBR system tocontrol the fouling of the hollow fiber membranes. The results showedthat membrane fouling was successfully controlled through direct ef-fects on the membrane, but not through the modification of digestedbroth. The dominant fouling of the membrane in this system was foundto be a cake layer. Investigating the EPS on the surface of the membranedemonstrated that the loose cake layer was successfully detached byultrasound. However, the compacted cake layer with a high content ofEPS was not affected by ultrasound and remained on the surface of themembrane. In addition, the results showed that the ultrasound had aslight positive influence on controlling the gel layer fouling formed byadsorption of proteins and humic compounds. However, ultrasound wasnot able to significantly affect the gel layer formed by inorganic ma-terials. Amplifying the slight effect of ultrasound on biofouling couldtake place through the remedies presented in the next section.

Yu et al. [97] investigated the effect of the on-line ultrasonicationon controlling the membrane fouling in the AnMBR system. Two MBRsystems including alone MBR and MBR equipped with ultrasound wereoperated more than one year. This research was performed in a 54-dayperiod of time (from 251th day to 304th day) with a volatile solidloading rate (VS) of 3.7 gVS/L d and a hydraulic retention time (HRT)of three days. In order to investigate the mechanism of the fouling, thebacterial communities and characteristics of the extracellular polymericsubstances were studied. The obtained results indicated that on-lineultrasound was able to control the membrane fouling by removing thecake layer from the surface of the membrane. The amount of extra-cellular polymeric substances in cake layer of US-AnMBR was found tobe 25.63 mgTOC/gSS, which was more than that for parallel AnMBRsystem (22.77 mgTOC/gSS), responding to the ultrasonic as an externalforce. Also, in this research, some bacterial populations such as Syn-trophobacterales, Peptococcaceae and Bacteroides were identified tocontribute in the fouling of the membrane. Xu et al. [98] designed ananaerobic membrane reactor coupled with on-line ultrasound equip-ment for long-term digestion of waste activated sludge. The influence ofthe volatile solid loading rate on the performance of the digestion andfouling of membrane was studied in the range of 1.1–3.7 gVS/Ld, andthe optimum value of this parameter was determined to be 2.7 gVS/Ldwhich resulted in volatile solid destruction of about 51.3%. The per-formance of the MBR system with and without ultrasound equipmentwas evaluated by determining the total membrane filtration resistance(Fig. 8). High membrane filtration resistance indicates severe fouling in

Fig. 4. Sketch of microstreaming physical phenomenon (a) during compression cycle, and (b) during expansion cycle.

Fig. 5. Sketch of physical phenomenon of acoustic streaming.

Fig. 6. Sketch of microjet physical phenomenon.

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the membrane. As can be seen in Fig. 8, the membrane fouling waseffectively controlled by ultrasound, which consequently resulted in theimprovement of the operation of the MBR system.

The maximum specific methanogenic activity indicated that theanaerobic microorganisms were not negatively affected by ultrasonicirradiation. In addition to the control of membrane fouling, the diges-tion performance slightly increased in the US-AnMBR hybrid systemcompared with a single AnMBR; this can be attributed to the sludgedisintegration improvement in the presence of ultrasound irradiation.Sui et al. [87] investigated the feasibility of employing on-line ultra-sound for controlling the membrane fouling in the anaerobic membranebioreactor. The total filtration resistance of the membrane in the US-AnMBR system was only 30% of that for a single MBR system, con-firming the efficient performance of the ultrasound in controlling themembrane fouling. The results of this research showed that by in-creasing the concentration of sludge in the reactor, the time of the ul-trasonic irradiation should be enhanced for efficient control of themembrane fouling. The sonication on/off time of ultrasonic with powerintensity of 0.122W/cm2 for efficient control of the membrane foulingwas determined to be 1min/60min and 3min/50min for sludge con-centration of 4 g/L and 6 g/L, respectively in AnMBR with a cross-flowvelocity of 0.75m/s. The ultrasonic irradiation had a slight negative

effect on the activity of the anaerobic bacteria, but the chemical oxygendemand did not significantly decrease. At a sludge loading rate of0.44–1.44 kgCOD/kgVSS d, the COD removal rate was found to be 85%.Ruiz et al. [89] investigated the effect of the ultrasound frequency (20,25, 30 and 40 kHz) on the effluent quality, membrane integrity andperformance of the process. For this purpose, four modules of hollow-fiber polyvinylidene fluoride ultrafiltration membrane of MBR systemwere equipped with four ultrasound transducer with different fre-quencies. To clean the membranes in this research, the fouled mem-branes were irradiated with ultrasound, with power of 15W and dif-ferent frequencies, for 3 s every 3min and then backwashed for 1minwith 5 s of aeration. The results of the experiments showed that theoptimum control of transmembrane pressure was obtained at a fre-quency of 20 kHz without significant negative effect on the structure ofthe membrane. Formation of the cake layer on the surface of themembrane was prevented by the irradiation of ultrasound with a fre-quency of 20 kHz, resulting in controlling the membrane fouling andconsequently controlling the transmembrane pressure. In another re-search, Ruiz et al. [91] investigated the performance of the membranebioreactors with flat-sheet microfiltration membranes. In this study,similar to the aforementioned research, four modules of pilot-scale MBRequipped with on-line ultrasonic transducer were applied. The effect of

Fig. 7. The flow diagram of US-AnMBR hybridsystem: (1) feed tank, (2) feed pump, (3) Anaerobicreactor (continuous stirred tank reactor), (4) hotwater bath, (5) pump for hot water recirculation, (6)gas collector, (7) pump for mixed liquor recircula-tion, (8) ultrasound equipment, (9) Hollow fibermembrane module, (10) suction pump, (11) valve,and (12) manometer. Reprinted from Xu et al. [85]with permission from Elsevier.

Fig. 8. Total membrane filtration resistance for AnMBR and US-An-MBR hybrid systems. Reprinted from Xu et al. [98] with permission from Elsevier.

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the ultrasound frequency (20, 25, 30 and 40 kHz) and ultrasonic power(100, 200 300 and 400W) on the process was studied. Some parameterssuch as color, viscosity, total suspended solids concentration and ef-fluent chemical oxygen demand were not changed significantly underthe ultrasonic irradiation with different frequencies. However, turbidityof the effluent was enhanced from 2 to 20 NTU at high ultrasoundpowers (300 and 400W) and low frequency of 20 kHz, which can beattributed to the deflocculation of the sludge under ultrasonic irradia-tion. The presence of particles larger than the pore size of the mem-brane in influent of ultrasonicated membranes indicted that the ultra-sound (especially with low frequencies) could damage the structure ofthe membranes, which was further proved by SEM images of the surfaceof the membranes. The membrane module sonicated with frequency of20 kHz represented stable transmembrane pressure for 70 days. Li et al.[99] applied ultrasound to clean the fouled membrane in a submergedmembrane bioreactor. Also, on-line ultrasound equipment was appliedto control membrane fouling in the submerged membrane bioreactor. Inorder to evaluate the fouling of the membranes, the transmembranepressure was determined. The higher transmembrane pressure in-dicated more membrane fouling. Moreover, the effect of the ultrasoundon the quality of the effluent and concentration of sludge in the MBRsystem was investigated. In this research, by increasing the trans-membrane pressure of membrane up to 27–33 KPa, the membranemodule was taken out from the reactor of MBR system and submergedin another reactor filled with water and equipped with ultrasonic. Themodule was exposed to ultrasonic irradiation with power of 300W for5min every 15min. Seven sets of frequency combinations including 25,50, 90 kHz; bi-frequency of 25–50 kHz; 50–90 kHz; 25–90 kHz, and tri-frequency of 25–50–90 kHz were studied for cleaning of the mem-branes. After determining the optimum operating frequency of the ul-trasound, the influence of ultrasonic irradiation time, interval time andultrasonic power on the membrane cleaning was explored. During thethree-minute operating conditions for ultrasonic irradiation, frequencyof 50 kHz, or bi-frequency of 25–50 kHz, or tri-frequency of25–50–90 kHz and ultrasonic power of 200W or 300W, relatively lowtransmembrane pressure was obtained, indicating low membranefouling. High COD removal was obtained for both on-line US-MBR andsingle MBR systems proved that the ultrasonic irradiation did not havea negative effect on the quality of the effluent and properties of theactivated sludge. The experimental results showed that the sludgeconcentration was low for a MBR system with on-line ultrasoundcompared with single MBR, resulting in the slowdown of the fouling ofthe membrane.

Pengzhe et al. [88] used online ultrasound to control the membranefouling in an anaerobic membrane bioreactor. The aim of this researchwas to investigate the feasible ultrasonic parameters through short-termrunning experiments. The results showed that with crossflow velocitygreater than 1m/s, the membrane fouling can be efficiently controlledby hydrodynamic methods (without ultrasound). However, the totalfiltration resistance increased gradually when the cross-flow velocitywas lower than 1m/s. The rate of membrane fouling was determined8.33×106 and 3×107 m−1 s−1 for the cross-flow velocity of 0.75 and0.5 m/s, respectively. The results of the experiments revealed that byenhancing the power of the ultrasound, the membrane fouling ratedecreased. The total filtration resistance maintained at about 5× 1011

m−1 for more than one week by using online ultrasound with power of150W (irradiated 2min per 15min) and with a cross-flow rate of0.75m/s. Xu et al. [86] applied an anaerobic membrane bioreactor fordigesting the waste activated sludge. On-line ultrasonic equipment wasemployed to control the fouling of the membrane. The effect of theultrasound irradiation and power intensity of the ultrasonic equipmentwas investigated on the fouling of the membrane. The optimum valuesfor ultrasonic irradiation and power intensity of the ultrasound for ef-ficient control of membrane fouling with no membrane damage wasdetermined as 3min/h and 0.18W/cm2, respectively. The results con-firmed that the ultrasound was able to control the membrane fouling by

controlling the formation of cake layer on the surface of the membraneas the dominant fouling in the AnMBR system studied. However, theultrasonic irradiation did not have significant effect on the control ofinorganic and organic fouling.

Abdurahman and Azhari [8] investigated the treatment of the Palmoil Mill effluent by an ultrasonic-side-stream anaerobic membranebioreactor hybrid system. In this research, several 25 kHz multi-fre-quency ultrasonic transducers were applied to control the membranefouling. The influence of the organic loading rate (OLR) on the per-formance of the ultrasound-AnMBR hybrid system was studied by de-termining the methane gas contents, COD removal efficiency, andparameters of the effluent (e.g. concentration of phenol, stability of pH,suspended solids and volatile fatty acids). At an organic loading rate of0.5 kg/m3.d and hydraulic retention time of 16 days, the highest CODremoval of 98.7% was obtained for the treated wastewater. In addition,the results of the experiments showed that production of volatile fattyacid and methane increased with an increase in the organic loadingrate. The highest content of the volatile fatty acid (1480mg/L) andmaximum gas production (580 L/d) were achieved at an organicloading of 15 kg COD/m3.d. The experimental studies showed that 77%of the produced biogas consisted of methane. This high degree of me-thanization confirmed that a large percent of the organic compoundspresented in the wastewater either in soluble or suspended form couldbe successfully depredated in the reactor of an AnMBR system. Owningto the development of palm oil industries worldwide, different methodsfor treating the wastewater of these industries have been investigatedby different research groups. In another research, Shafie et al. [100]studied the palm oil industry wastewater by the side-stream AnMBRsystem. In this research, on-line ultrasound equipped with the moduleof the membrane was applied for controlling the membrane fouling andincreasing the COD removal efficiency. A hydraulic retention time of11 days was determined as an optimum value for US-AnMBR system.Sonication performed for 2 h resulted in high COD removal efficiency of98.75% and high methane gas production of 32.595mL. In anotherresearch, the effect of the on-line ultrasound irradiation on the foulingof the membrane in an AnMBR system was investigated by Wen et al.[16]. In this research, a side-stream kind of the AnMBR was applied,and the membrane module was equipped with ultrasonic transducer.The results of this research indicated that although the irradiation ofthe ultrasound can control the membrane fouling, damage to membranecan take place under some operational conditions. Chemical reactionsbetween the membrane and generated hydroxyl radicals and also col-lision of micro-particles with the surface of the membrane were knownas the main damaging mechanisms of the membrane under ultrasonicirradiation. It should be noted that the mentioned mechanisms are notonly responsible for membrane damage, but they also play an importantrole in the control of membrane fouling. Therefore, in order to controlthe membrane fouling without damaging the membrane, ultrasoundintensity and ultrasonic irradiation time should be optimized. In addi-tion, the results indicated that maintaining a certain thickness of thecake layer on the membrane surface can efficiently protect the mem-brane from damage under the ultrasonic irradiation. To examine theUS-AnMBR hybrid system with crossflow velocity of 0.75m/s, sludgeconcentration of 8 g/L, and influent COD of 1500mg/L, the optimumvalues for ultrasonic power intensity and ultrasonic irradiation timewere obtained as 0.122W/cm2 and 5min/60min, respectively, re-sulting in an effective membrane fouling control without any mem-brane damage over a one-week operation.

As reported, the on-line ultrasound equipment has been mainly usedto control membrane fouling. However, the effect of the on-line ultra-sonic irradiation on the removal of the organic pollutants in the MBRsystem was investigated by Liu et al. [101]. The results indicated thatthe biological activity was significantly enhanced in the presence ofultrasound equipment with power of 10W. The possible improvementmechanisms of biological activity were proposed as a) the cavitationphenomenon in ultrasonic accelerates the mass transfer by moving the

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particles in a liquid, resulting in acceleration of the entrance of sub-strate in the active part of enzymes and exit of the products; b) in thepresence of slight intensity ultrasound irradiation, the production ofextracellular enzymes can be increased, resulting in the degradationenhancement of organic compounds; c) ultrasound can cause mutationin the microorganisms. Inducing enzymes may efficiently treat thewastewater. The experimental results of the research showed that theinfluence of ultrasound remained for 24 h, so the time interval for ul-trasonic irradiation was set at 24 h.

4.1.2. Control of membrane fouling in MBR using online ultrasound hybridultrasound

Jia et al. [102] introduced a novel wastewater treatment system thatintegrated catalytic ultrasound oxidation (CUO) with membrane bior-eactor. The developed CUO-MBR hybrid system was applied to theadvanced treatment of biologically pretreated coal gasification waste-water as a real wastewater. The results of the research showed thatcoupling of the ultrasonic irradiation with catalytic oxidation resultedin improving the total organic carbon (TOC) elimination and biode-gradability of the wastewater. In this research, effect of different cata-lysts including FeOx/SBAC and MnOx/SBAC on the fouling mitigationof membrane and removal of TOC, BOD5 and COD were investigated inthe CUO-MBR hybrid system (SBAC refers to sewage sludge-based ac-tivated carbon). According to the results obtained, CUO-MBR systemrepresented optimum performance (low membrane fouling and highremoval of TOC, BOD5 and COD) in the presence of 2 g/L of FeOx/SBACas the catalyst with a ultrasonic frequency of 60 kHz;; energy intensityof 1.6W/L; and ultrasonic irradiation time of 20min.

One of the approaches to control biofouling is the employment ofanti-adhesion nanomaterials via blending/coating/grafting [81]. Bac-teriophage treatment of nanocomposite membrane combination miti-gates antibiotic-resistant bacteria and biofouling in MBR systems [103].Hybrid application of sonication on/off time of ultrasonic (~0.1W/cm2; 1min/60min) together with nanocomposite membranes whichare resistant to protein adsorption and bacterial adhesion will con-siderably increase the membrane life [104]. Ultrasonic reflectometrywas also used as a real-time detector of biofilm. The reflected time andamplitude of sound waves were compiled into frequency distributions.Since this data is based on earliest interactions between foulants andmembrane materials [64], the combination of ultrasonic reflectometryand quorum sensing quenching can avert cake sludge and conditioningfilm formation in MBR [105,106].

4.2. Ex-situ applications of ultrasound in the MBR system

4.2.1. Ex-situ cleaning of membrane using ultrasoundPendashteh et al. [92] used an ex-situ ultrasound process for

cleaning the membrane of MBR system, which was applied for treat-ment of synthetic hypersaline oily wastewater. To clean the fouledmembranes after 32 days of operation, the fouled membranes weretaken out from the module and a plastic sheet was used to physicallyremove the cake from the surface of the membrane. Then, the mem-branes were exposed to NaOH solution with pH of 11 for 24 h to desorbthe remaining foulants. Finally, the membranes were exposed to ul-trasound irradiation for 15min in an ultrasonic bath with a frequencyof 45 kHz and temperature of 30 °C. The Fourier transform infrared (FT-IR) analysis indicated that the formation of the fouling layer can beattributed to the deposition of extracellular polymeric substances (in-cluding organic and inorganic compounds such as proteins, poly-saccharides, etc.), hydrocarbon matters and inorganic compounds. Ac-cording to the AFM images of the fouled membranes, the dominantmechanism of the membrane fouling in this research was found to besurface coverage of the membrane by foulants. In addition, the SEMimages of the fouled membrane surface (Fig. 9) indicated that theclusters of the bacteria with the shape of a rode contributed to theformation of the cake layer on the surface of the membrane.

The results of the ICP and EDX analyses indicated that the Na, Fe, K,Al, Mg and Ca were the major metal elements in the cake layer. It wasconcluded that sonication can efficiently remove the cake layer fromthe surface of the membrane resulting in recovering the permeation fluxfor a long time.

Staphylococcus warneri, Pseudomonas vesicularis and Acinetobactercalcoaceticus were removed via ultrasonic treatment after two and a halfminutes whether being dead or alive [107]. Critical power density forloosely and tightly bound extracellular polymeric substance extractionwas 35 and 65W/10mL, respectively [9]. In membrane autopsy, bio-cake on the membrane could be removed by physical cleaning andsubsequent ultrasonic cleaning (35–65 KHz) [108]. As formerly men-tioned, the various ultrasound-assisted mechanisms proposed aimed atmembrane cleaning micro-streamers [106], micro-jet [109], acousticstreaming[110], micro-streaming (Fig. 4), and acoustic energy ab-sorption (fluid flow) [11] which could be used in membrane autopsy aswell [111].

4.2.2. Pretreatment of wastewater using ultrasound prior to MBRIn the MBRs, wastewater treatment using activated sludge results in

the production of considerable amount of sludge, which is one of thechallenging problems of these systems. The produced excess sludgeshould be wasted in a proper way, which increases the operational costsof the process.

Ultrasonic treatment of activated sludge causes cell lysis, floc dis-integration and inactivation (see Fig. 10). Ultrasonic pretreatment isnormally performed at 9–41 kHz (mostly at 20 kHz) for several secondsto 2.5 h (usually< 1 h) with an improved volatile solid removal of9–36% and enhanced CH4/biogas production of 24–138% [112]. It wasreported that ultrasonic pretreatment was only able to increase thedegradation rate of the sludge and had no influence on the sludge de-gradation amount [113].

Yoon et al. [114] used an ex-situ ultrasonic cell to prevent the excessproduction of sludge in the MBR system. Submerged hollow fibermembranes with a surface area of 0.2m2 and a pore size of 0.1 µm wereused in their system. A schematic diagram of the experimental set-up isrepresented in Fig. 11. As shown, two MBR systems were used: MBR-ultrasonic hybrid system and single MBR system. The MBR systemwithout ultrasonic probe acts as a control system to identify the per-formance of the ultrasonic-MBR hybrid system in preventing the pro-duction of excess sludge. In this system, the produced sludge waspumped to a beaker equipped with an ultrasonic probe with power of600W and a frequency of 20 kHz. Then, the ultrasonicated sludge wasreturned to the MBR reactor. The results show that the mixed liquorsuspended solid (MLSS) of the ultrasonic-MBR hybrid system remainednearly constant in the range of 7000–8000mg/L. However, the quantityof the MLSS for a single MBR system increased from 7000mg/L to13700mg/L during the experiment time. These results demonstratethat the applied ultrasonic-MBR system at an organic loading of ap-proximately 0.91 kg BOD5/m3 day was able to successfully prevent theexcess sludge production. Although the production of excess sludge washindered in the ultrasonic-MBR system, the quality of the effluent got alittle worse which can be attributed to the return of disintegrated sludgeto the MBR. Moreover, a reduction in the average particle size of sludge(from 132 to 92 µm) was observed in the presence of ultrasonic irra-diation. In addition, as a drawback of this coupling system, none ofinsoluble inorganic particles can permeate across the membrane. Con-sequently, accumulation of insoluble inorganic materials could be themain problem for zero sludge production.

In another research, the effect of the ultrasonic irradiation on theliquor properties of the activated sludge was studied by Wu et al. [115].The samples of activated sludge for experiments were picked up from afull-scale membrane bioreactor applied for municipal wastewatertreatment. The concentration of volatile suspended solids (VSS) and theconcentration range of mixed liquor suspended solids of the activatedsludge gathered was 6 and 10–12 g/L, respectively. The activated

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sludge samples gathered were exposed to ultrasound in a cylindricalPlexiglas reactor with different power levels (in the range of40–300W). In order to investigate the effect of the ultrasound irra-diation on the activated sludge, operational parameters such as pH,temperature, concentration of released soluble organic matter, particlesize distribution and heterotrophic bacteria activity were determinedperiodically. The results showed that the width of particle size dis-tribution increased by enhancing the ultrasonic intensity up to 2 kJ/mLand then decreased; while, the released soluble organic matter in-creased by enhancing the ultrasonic intensity. Furthermore, the resultsof the experiments showed that less than 10% of the ultrasonic energywas converted to heat, and the pH remained above 7 under the ultra-sonic irradiation with an intensity up to 2.5 kJ/mL. Moreover, thebiodegradability of the activated sludge increased when treating withultrasound. Prado et al. [116] used a combination of ozonation andultrasound (O3-US hybrid) as shown in Fig. 12(a) for the pretreatmentof wastewater prior to the submerged MBR. The experimental set up ofthe submerged MBR used is represented in Fig. 12(b). In order toevaluate the performance of the O3-US hybrid pretreatment system, theremoval of carbamazepine (CBZ), diclofenac (DCF) and sulfamethox-azole (SMX) as the sample of pharmaceutical compounds was in-vestigated. Moreover, the fouling of the membrane in the MBR systemand toxicity potential of the MBR influent was studied. As can be seen inFig. 12(c), pretreatment of the wastewater by O3-US hybrid systemdecreased the membrane fouling which can be mainly ascribed to theeffect of the pretreatment on the products of the microbial metabolism.The decomposition of the organic compounds (feeding of the micro-organisms into the MBR reactor) was increased by being exposed to O3-US hybrid process, resulting in a decrease in the growth of the micro-organisms which are responsible for the production of extracellularpolymeric substance. Therefore, the concentration of the extracellularpolymeric substance showed a reduction of 50% by pretreatment pro-cess. Although the removal of the sample pharmaceutical compoundswas enhanced by pretreatment of the wastewater (Fig. 12(d)), thetoxicity of the wastewater was not decreased. This can be attributed to

the generation of intermediate products during the O3-US processwhich are more toxic than the primary substances. However, the toxi-city potential of the wastewater decreased (about 60%) after treatmentin the MBR system. The results of this research demonstrated thatcoupling of the MBRs with an ultrasound-based pretreatment systemprovides a promising method for the treatment of the wastewater.

Joshi and Parker [117] investigated the pretreatment of waste ac-tivated sludge using ultrasound and hydrogen peroxide before digestingin a submerged anaerobic membrane bioreactor. Their experimentalresults indicated that by employing hydrogen peroxide with a dose of50 g/kg TS and sonication for 60min, solubilization of the chemicaloxygen demand increased by about 40%. In order to investigate theeffect of the chemical oxygen demand solubilization increment on thebiodegradability of the sludge, raw and pretreated waste activatedsludge were fed into the AnMBR system. The obtained results showedthat the biodegradable fraction of the sludge increased by being pre-treated with hydrogen peroxide and ultrasound irradiation, whichconsequently reduced the solids concentration of waste stream andbioreactor.

5. Future perspectives

In spite of the recent advances in the field of application of ultra-sonic technology in MBR systems, there is a significant potential forfuture development. In order to investigate the effect of different op-erational parameters and reduce the cost of experimental tests, sys-tematic studies involving experimental design and modeling of ultra-sound-MBR processes are proposed to be considered in future studies.The current review demonstrates that most of the researches have re-ported the application of ultrasonic technology in MBR systems on thelaboratory scale. Therefore, further research is needed to investigate theultrasound application feasibility on industrial scale. It demonstratesthat nanocomposite membranes in ultrasound-MBR coupled systemshave not been studied in details. Therefore, the effect of the presence ofultrasound-active inorganic nanoparticles in the matrix of the

Fig. 9. Surface SEM images of a) fresh membrane, b) fouled membrane, and c) clusters of bacteria on the surface of the fouled membrane. Reprinted from Pendashtehet al. [92] with permission from Elsevier.

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membrane used in MBR systems can be investigated in future re-searches. Ultrasound-active nanoparticles, i.e. ZnO, TiO2, etc. canproduce hydroxyl radicals in the presence of ultrasonic irradiation.Generated hydroxyl radicals can degrade the foulants adsorbed on thesurface of the membrane or captured in pores of the membrane, whichconsequently results in controlling the membrane fouling. High energyconsumption of the ultrasonic transducers limits the application of

ultrasound-MBR systems on a full scale. Some further researches areneeded on the hybrid methods with low energy consumption for im-proving the application of ultrasonic technology in full-scale MBRsystems.

Fig. 10. Ultrasonic treatment of activated sludge.

Fig. 11. A schematic diagram of MBR system coupled with ultrasound and single MBR system. Reprinted from Yoon et al. [114] with permission from Elsevier.

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6. Conclusion

In this paper, applications of ultrasonic technique in the membranebioreactor systems were reviewed. Fouling of the membrane is knownas the main challenge in both side-stream and submerged MBR systemswhich decreases the flux of the permeate or increases the transmem-brane pressure needed to achieve a stable permeate flux during filtra-tion process, which consequently increases the operating costs. In re-cent years, ultrasonic technique as a promising method for efficientcleaning of the fouled membrane in the MBR systems has received greatattention. Ultrasonic irradiation can clean the fouled membranes byproducing important physical phenomena including microjets, micro-streams and shock waves. Indeed, the particles can be released from thefouled membrane by the aforementioned physical phenomena and/or

by producing hydroxyl radicals in a heterogeneous liquid-solid system.The ultrasonic can be performed either in-situ (online) or ex-situ (off-line) for cleaning the membrane in the MBR systems. Moreover, theultrasonic method can be combined with other cleaning methods, i.e.chemical cleaning and backwashing to improve the cleaning efficiency.Research review indicates that using on-line ultrasonic irradiation canefficiently control the membrane fouling in the MBR systems by con-trolling the generation of the cake layer on the surface of the mem-brane. Moreover, the ultrasound can be applied to the MBR systems toprevent the excess production of the sludge. Results show that pre-treatment of the sludge by ultrasonication can improve the biode-gradability of the sludge and enhance the production of biogas inAnMBR systems. In addition, pretreatment of the wastewater decreasesthe membrane fouling in the MBR systems. Also, effective membrane

Fig. 12. a) Schematic representation of O3/ultrasound hybrid pretreatment experimental set up, b) schematic diagram of the submerged MBR experimental set up, c)trans-membrane pressure over time with and without hybrid pretreatment, and d) removal efficiency of pharmaceutical compounds using different processes.Reprinted from Prado et al. [116] with permission from Elsevier.

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performance was detected in the AnMBR operation. The membranefouling did not grow during the operation period in spite of the highsolids concentrations in the AnMBR. The current review demonstratesthat most of the researches have reported the application of ultrasonictechnology in MBR systems on laboratory scale. Therefore, further re-search is needed to investigate the ultrasound application feasibility onindustrial scale. High energy consumption of ultrasound equipmentlimits their application in a full-scale reactor, so development of eco-nomical and effective hybrid methods with low energy consumptionwould further improve the application of ultrasonic technology in full-scale MBR systems for controlling the membrane fouling and improvingthe wastewater treatment efficiency.

Acknowledgments

We sincerely thank University of Tabriz, Azarbaijan Shahid MadaniUniversity, Nanjing Forestry University (163020115), and KharazmiUniversity for all the supports. We also acknowledge the Iran NationalScience Foundation (INSF) for supporting under grant number of97017831 and Jiangsu Science and Technology Plan Project (Grant No.SBK2019044425) plus National Natural Science Foundation of China(International (Regional) Cooperation and Exchange Program, ResearchFund for International Young Scientists (Grant No. NSFC2191101196)).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2019.104633.

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