Cavitation: an auxiliary technique in wastewater treatment...

Ž .Advances in Environmental Research 6 2002 335�358

Cavitation: an auxiliary technique in wastewatertreatment schemes

Parag R. Gogate�

Chemical Engineering Section, U.D.C.T., Matunga, Mumbai-400 019, India

Abstract

New techniques are being added to wastewater treatment schemes for meeting the high standards of environmen-tal regulations. The present work highlights the use of one such technique, cavitation, for wastewater treatmentapplications. Two types of cavitation phenomena depending on the type of generation have been discussed and theoptimum operating and geometric parameters have been presented for maximum efficiency. Experimental resultshave been given for sonochemical reactors for the degradation of formic acid to supplement the discussion aboutoperating parameters. Available literature for acoustic and hydrodynamic cavitation have been critically assessed andrecommendations have been made for enhanced energy efficient applications of cavitation based technologies.Overall it appears that a lot of work is required in designing the cavitational reactors for large scale operations and afew recommendations for theoretical and experimental studies have been made. � 2002 Elsevier Science Ltd. Allrights reserved.

Keywords: Wastewater treatment; Acoustic cavitation; Hydrodynamic cavitation; Optimum operating parameters; Energy efficiency

1. Introduction

In the past, wastewater treatment objectives wereconcerned with the removal of suspended solids andfloatable materials; the treatment of biodegradable or-ganics and the elimination of pathogenic organisms.With increased understanding of the environmentaleffects and other adverse long-term effects caused bywastewater discharges, the objectives were based onthe aesthetic and environmental concerns along withhigher concern for earlier objectives. In recent times,because of increased scientific knowledge and an ex-panded information base, wastewater treatment hasbegun to focus on the health concern related to toxic

� Fax: 91-22-4145614.Ž .E-mail address: [email protected] P.R. Gogate .

and potentially toxic chemicals released into the envi-ronment. As a consequence of the shift in the treat-ment objectives, the required degree of treatment hasincreased significantly and new methods of treatmentare being developed to cater to the new objectives andgoals.

Any wastewater treatment scheme, until recent years,Žwas comprised of the conventional physical screening,

mixing, flocculation, sedimentation, floatation, filtra-. Žtion, etc. , chemical precipitation, adsorption, disinfec-

.tion, oxidation using chemicals, etc. and biologicalŽremoval of contaminants brought about by biological

.activity methods. These conventional methods, how-ever, failed to meet the demands of the newer environ-mental regulations and hence new methods such as wet

Žair oxidation Wilhelm and Knoop, 1979; Baillod et al.,. Ž1985 , oxidation technologies using cavitation Cheung

et al., 1991; Kotronarou et al., 1991, 1992; Petrier et al.,

1093-0191�02�$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.Ž .PII: S 1 0 9 3 - 0 1 9 1 0 1 0 0 0 6 7 - 3

( )P.R. Gogate � Ad�ances in En�ironmental Research 6 2002 335�358336

1992a; Bhatnagar and Cheung, 1994; Serpone et al.,1994; Pandit et al., 2000; Sivakumar and Pandit, 2000a;

.Sivakumar et al., 2000 , hybrid methods such as sonica-� � Žtion followed by wet air oxidation SONIWO Ingale

.and Mahajani, 1995 , sono-photochemical destructionŽ .Shirgaonkar and Pandit, 1998; Gogate et al., 2001b ,

Žsono-electrochemical methods Trabelsi et al., 1996;.Compton et al., 1997 , etc. developed. It should be

noted that these methods have to be used in associa-tion with the conventional basic methods and form partof the so-called secondary or tertiary treatment meth-ods.

Cavitation is one such technique which can be ef-fectively used for the destruction of complex organicchemicals, bio-refractory materials, etc. Cavitation canbe in general defined as the phenomena of the forma-tion, growth and subsequent collapse of microbubblesor cavities occurring in extremely small intervals of

Ž .time milliseconds releasing large magnitudes of en-ergy over a very small location. The resultant effectsare really spectacular and such events occur at millionsof places in the reactor simultaneously. The magni-tudes of the pressures and temperatures are a strongfunction of the operating and geometric conditionsexisting in the reactor. Violent collapse of the cavitiesin the cavitating systems results in the formation ofreactive hydrogen atoms and hydroxyl radicals whichrecombine to form hydrogen peroxide. Numerous illus-

Žtrations Suslick et al., 1997; Naidu et al., 1994; Shir-gaonkar and Pandit, 1997; Senthilkumar and Pandit,

.1999; Senthilkumar et al., 2000; Vichare et al., 2000acan be obtained from the literature which indicate theformation of OH� radicals during cavitation using amodel reaction of the oxidation of aqueous KI solutionat different scales of operation and in different equip-ment.

Generally, cavitation is classified into four typesbased on the mode of its generation:

1. Acoustic cavitation: Here the pressure variations inthe liquid are effected using sound waves, usually

Ž .ultrasound 16 kHz�100 MHz . The chemicalchanges taking place due to the cavitation inducedby the passage of sound waves are commonly knownas sonochemistry.

2. Hydrodynamic cavitation: Cavitation is producedby pressure variations which are obtained by usingthe geometry of the system to create velocity varia-tions. For example, the interchange of pressure andkinetic energy can be achieved using a constrictedorifice, venturi flow in the pipe, etc.

3. Optic cavitation: Cavitation is produced by photonsŽ .of high intensity laser light rupturing the liquid

continuum.

4. Particle cavitation: Cavitation is produced by anyother type of beam of elementary particles, e.g.protons, rupturing a liquid, as in a bubble chamber.

Among the various modes of generating cavitationgiven above, acoustic and hydrodynamic cavitation havebeen of academic and industrial interest due to theease of operation and the ease of the generation of therequired intensities of cavitational conditions. Theacoustic cavitation or sonochemical processes have beenwidely studied over the past few decades. Excellentreviews on the scope and the application of ultrasoundand the processes based on the same are available in

Žthe literature Mason, 1986; Henglein, 1995; Lindleyand Mason, 1987; Moholkar and Pandit, 1996; Mason,1999; Suslick et al., 1999; Von Sonntag et al., 1999; Keiland Swamy, 1999; Shah et al., 1999; Thompson and

.Doraiswamy, 1999 . Modelling of sonochemical reac-tors and the bubble dynamics under a variety of condi-tions have also been extensively studied in the pastŽNaidu et al., 1994; Yan et al., 1988; Yan and Thorpe,1990; Kamath et al., 1993; Moholkar and Pandit, 1997;Senthilkumar, 1997; Sochard et al., 1998; Moss et al.,1999; Storey and Szeri, 1999; Gogate and Pandit, 2000a;

.Storey and Szeri, 2000 . However, it should be notedthat, in spite of extensive research, there is hardly anychemical processing carried out on an industrial scaleowing to lack of expertise required in diverse fields asmaterial science, acoustics, chemical engineering, etc.,for scaling up successful lab scale processes. Someattempts have been made to effectively scale up these

Žreactors Berlan and Mason, 1992; Martin and Ward,.1993 .

Similar cavitation phenomena can also be generatedrelatively easily in hydraulic systems. Engineers havegenerally been looking with caution at cavitation inhydraulic devices due to the problems of mechanicalerosion. In fact all the initial efforts to understand itwere towards the objective of suppressing it, to avoidthe erosion of exposed surfaces. In the last decade,concentrated efforts were made by a few groups aroundthe world to harness the spectacular effects of hydrody-namic cavitation for chemical�physical transformationŽSuslick et al., 1997; Senthilkumar et al., 2000; Vichareet al., 2000a; Chivate and Pandit, 1993; Pandit and

.Joshi, 1993; Save et al., 1994, 1997 .The present work aims at highlighting the use of

cavitation technology for the potential application as asupplementary method in wastewater treatmentschemes and giving the optimum operating and geo-metric parameters of the cavitational reactors for maxi-mum efficiency and yields. Some experimental resultshave also been illustrated to show the capability ofcavitation using sonochemical reactors where cavitationis generated by ultrasonic irradiation.

( )P.R. Gogate � Ad�ances in En�ironmental Research 6 2002 335�358 337

2. Acoustic cavitation

In the case of acoustic cavitation, the pressure varia-tions in the liquid are effected by using high frequencysound waves, usually ultrasound, with frequencies inthe range of 16 kHz�100 MHz. If a sufficiently largepressure is applied to the liquid so that the averagedistance between the molecules exceeds the criticalmolecular distance required to hold the liquid intact,cavities or voids will be created. Subsequent compres-sion and rarefaction cycles of the sound waves causesthe bubble�cavity formed to expand, reach a maximumbubble size depending on the operating conditions andthen collapse releasing a large amount of energy whichproduces spectacular effects. A typical variation of theradius of the cavity during the cavitation phenomenonis shown in Fig. 1, which is obtained by solving theRayleigh�Plesset equation for bubble dynamics using

Ž .numerical simulations Gogate and Pandit, 2000a . Itshould be noted at this stage that the radius profile is astrong function of the operating conditions. In the

Ž .earlier work Gogate and Pandit, 2000a , the effects ofthe operating parameters on the radius profiles andhence, on the magnitudes of pressure pulses generatedat the collapse of cavities have been discussed in detail.Thus, by manipulating the operating conditions, theintensity of collapse can be controlled depending onthe required applications and components of thewastewater stream.

2.1. Optimum design parameters for maximumeffects of acoustic ca�itation

The important parameters affecting the intensitiesof collapse and hence the yields from the reactors are:

1. intensity of irradiation;2. frequency of irradiation;3. initial radius of the nuclei.

2.1.1. Intensity of irradiationIntensity of ultrasonic equipment is defined as the

ratio of the power input to the system to the transmit-ting area. Hence, intensity of irradiation can be variedeither by changing the power input to the system or bychanging the transmittance area of the ultrasonictransducers in the equipment. It has been shown in the

Ž .earlier work Gogate and Pandit, 2000a that the col-lapse pressures generated due to the collapse of asingle cavity decrease with an increase in the intensityof irradiation and thus lower intensities should be usedfor maximum benefits from the cavitation reactors.However, there are some crucial points that must beconsidered before actually finalizing the above conclu-sion.

There exists a critical intensity at which the benefi-cial effects of cavitation start to occur. This is due tothe fact that the chemical reactions due to cavitationoccur as a result of formation of a certain minimumnumber of free radicals and this number of free radi-cals depends on the intensity of collapse and number ofcavitating events which in turn depends on the operat-ing intensity of irradiation. Thus, the intensity of irradi-ation should not be decreased below a certain mini-mum cavitation inception intensity.

Moreover, as said earlier, the two methods by whichintensity can be changed also play an important role inthe actual experimental results. If the intensity is in-

Fig. 1. Variation of radius of cavity with time during cavitation phenomena at initial radius of 0.1 mm, frequency of 50 kHz andintensity of 120 W�cm2.


creased by increasing the power input in the systemŽ .P�V of the system , there will be an increase in thenumber of cavitation events and hence the cumulative

Žpressure pulse number of cavities generated multiplied.by the collapse pressure due to single cavity will in-

crease. In such a case, the degradation rates will behigher due to higher overall magnitude of the pressureenergy released. It should be also noted that the in-crease in yields at higher power inputs is not much andreaches saturation. This can be attributed to the factthat the increase in the number of cavities at higherpower inputs is not much and moreover at higherconcentration of cavities there are chances of coales-cence of the bubbles which increases the size of bubbleleading to lower pressure pulse at the collapse condi-

Ž .tions. Pandit et al. 2000 have clearly illustrated thisfact with experiments on degradation of 2,4,6trichlorophenol using a ultrasonic horn in which theintensity was increased by changing the percentageamplitude of the horn in the range of 10�30%. Similar

Žresults were obtained in the earlier work Gogate et al.,.2001c where the Weissler reaction was studied in an

Ž .ultrasonic horn type of reactor. Hua et al. 1995 havealso shown that the enhanced sonochemical effectswith increasing power dissipation in the system are onlyobtained until an optimum intensity value of 1.2 W�cm2

beyond which the rates of degradation of p-nitrophenoldecrease with increased power input.

However, if the intensity is changed by changing thetransmittance area of ultrasonic equipment, at lowerintensities, the same power dissipation is taking placeover a larger area resulting in uniform dissipation andlarger active area of cavitation resulting in higher cavi-tational yields. Studies on emulsification of an oil�watersystem using an ultrasonic horn and bath showed bet-ter emulsion characteristics for the bath, though thepower input to the horn and the bath were nearly the

Ž .same Mujumdar et al., 1997 . The same magnitude ofpower in the case of an ultrasonic bath is dissipatedover a larger area of transmittance as compared to the

Ž .ultrasonic horn. Entezari and Kruus 1994, 1996 , En-Ž .tezari et al. 1997 have also shown that the rate of

iodine liberation at constant power input for equip-ments of different area of irradiation decreases withthe increase in the intensity of irradiation, i.e. decreas-ing the area of irradiation for constant power input.These illustrations clearly indicate that the use of in-creased transmittance area is favorable.

Thus, it can be said that maximum benefits of thecavitation phenomena can be obtained at lower intensi-ties but just above a critical inception intensity and alsowith the same power dissipation over a larger transmit-tance area. It should also be noted that the criticalinception intensity depends on the type of effluent to

be degraded and should be established with laboratorycharacterization of the effluent.

2.1.2. Frequency of irradiationIt has already been shown, with the help of theoreti-

cal simulations of the Rayleigh�Plesset equations gov-erning bubble dynamics, that the collapse pressuregenerated increases with an increase in the frequency

Ž .of irradiation Gogate and Pandit, 2000a in the rangeŽof frequency studied 20�200 kHz which is commonly

.observed in the ultrasonic applications . Similar resultshave also been obtained using energy analysis of acous-tic cavitation processes considering different collapsing

Ž .conditions and pathways Vichare et al., 2000b . Thushigher frequencies of irradiation will be favored for thedegradation of the chemicals in the wastewater treat-ment schemes. The enhanced sonochemical effects athigher frequencies can be attributed to lower cavity

Žsizes which also results in higher values of collapse. Žpressures and hence higher area to volume ratio en-

hanced diffusion and mass transfer of reactants fromliquid phase to vapor phase where pyrolytic degrada-

.tion will take place . In addition, smaller bubbles pro-duced at higher frequencies require fewer acousticcycles before they reach the requisite resonant size.Given the greater number of acoustic cycles per unittime at higher frequencies, rectified diffusion occursmore rapidly before transient bubble collapse. Thus agreater number of gas nuclei can reach the resonancesize more quickly than at lower frequencies. However,it should be noted that the frequency of an ultrasonicequipment cannot be varied over a wide range as themaximum transfer efficiency is obtained only when thetransducer is driven at its resonating frequency. Therange of frequencies typically used in the application ofpower ultrasound is 20�200 kHz though there are someexamples where higher frequencies have also been

Žused Petrier and Luche, 1987; Petrier et al., 1994,.1992b .

Interesting results have been obtained recently byŽ .Hung and Hoffmann 1999 who have studied the ef-

fect of frequency on the degradation rates over afrequency range of 20�1078 kHz. The rate constant forthe degradation of an aqueous solution of CCl at 6184and 1078 kHz was lower as compared to that at afrequency value of 500 kHz. However, the rate ofdegradation at 500 kHz was more as compared to 20and 218 kHz frequencies indicating that there exists anoptimum frequency at which the rates of degradationare maximum. It should be also noted that the optimaof the frequency does depend on the type of reactants

Ž .and the system geometry. Hua and Hoffmann 1997have also reported that enhanced effects forsonochemical reactions are obtained up to frequencies


of 700 kHz. Thus, care should be taken about theselection of the frequency considering decreased ef-fects at very high frequencies of operation.

Ž .Petrier et al. 1992b have also shown that the oxida-tion of aqueous KI liberating iodine is six times fasterwhile the generation of H O in water is 12 times2 2

faster when the frequency of ultrasound is increasedŽ .from 20 to 514 kHz. Weavers et al. 1998 have shown

that the rates of degradation of aromatic contaminantsare higher at 500 kHz as compared to 20 kHz fre-

Ž .quency. Sivakumar and Pandit 2000b have also shownthat the rates of degradation of Rhodamine B arehigher at 40 kHz as compared to 20 kHz operating

Ž .frequency. Petrier et al. 1994 have shown that phenoldegrades faster at higher frequencies of operation.

Ž .Petrier and Luche 1987 have shown that in the pres-ence of Ar or O , oxidative processes in water occur at2

enhanced yields when a high frequency of 514 kHz isused in comparison with the more commonly used 20kHz frequency.

Usually the active volume of the reactor is justaround the transmitting surface of the irradiator, thusquite localized and the intensity decreases as we moveaway from the surface. Thus, there is a need to in-crease the active volume for cavitation which can beeasily done by using multiple transducers of either thesame or different frequencies of irradiation. Moreoverin the case of sonic reactors, there is a formation ofstanding waves due to reflection from the reactor wallor the medium interface. Due to the standing waves,the large bubbles tend to coalesce resulting in acousticshielding and hence the power is not dissipated uni-formly leading to lower efficiencies of the reactor.Hence for increasing cavitationally active volume, in-creased efficiency of the reactor and for utilizing maxi-

Ž .mum power for the given volume P�V , formation ofstanding waves should be avoided. For this purpose,use of a dual frequency processor or a parallel plate

Ž .ultrasonic processor is a viable alternative. Luche 1999has reported that the parallel plate processor is thebest option available for online use. When the platesare placed close together, the attenuation of sound isminimal and the two waves give rise to an interferencepattern, minimizing the formation of standing wavesand increasing the energy associated per cavity.

The results from use of these types of reactors arespectacular and substantially increased yields are ob-

Žtained Sivakumar et al., 2000; Luche, 1999; Thoma et.al., 1997; Hua et al., 1995 . It is also necessary to use

multiple transducers for ultrasound generation in casesof large scale of operation. Usually the volumes ofchemicals treated so far are of the order of a fewmilliliters which is very small if use of cavitation is tobe considered in the case of wastewater treatment.

Fig. 2. Triple frequency ultrasonic flow cell with a capacity of7.5 l and provision for simultaneous irradiation with UV lightat the center.

Hence, it is essential to develop reactors with muchhigher volumes of operation and use of multiple trans-ducers is a must for higher yields from the reactors.

Use of multiple frequencies has also resulted inincreased cavitational yields and also at higher volumes.

Ž .Sivakumar et al. 2000 have used a dual frequency flowcell with a capacity of 1.5 l and reported that destruc-tion of p-nitrophenol can be successfully done andmore importantly at higher cavitational yields per unitpower density of equipment as compared to the ultra-sonic horn. In a further attempt to scale up the ultra-sonic equipment, a triple frequency hexagonal flow cellof capacity 7.5 l was designed successfully and experi-ments with destruction of formic acid have been per-

Ž .formed in the department Gogate et al., 2001a . AŽschematic sketch of the triple frequency flow cell the

three frequencies used are 20, 30 and 50 kHz with a.power input of 150 W to each face of the hexagon is

depicted in Fig. 2 to give an overall idea and to helpdesigners in scale-up of the ultrasonic equipment. Thesetup has an arrangement for simultaneous irradiationwith a UV lamp as the combination of UV and ultra-

Žsound has been found to be beneficial Shirgaonkar.and Pandit, 1998 .

Theoretical analysis based on the Rayleigh�Plessetequation governing the bubble dynamics has also shownthat the dual frequency combinations result in highercollapse pressures and for half the energy input as

Žcompared to the single frequency wave Sivakumar et.al., 2000 . Moreover, the phase and frequency differ-

ence between the two waves has also a strong effect onthe magnitude of the pressure pulse generated. If twosound waves with the same frequency are introduced inthe sonochemical reactor, the pressure pulse generatedwill be more as compared to the case where two wavesof different frequencies are introduced. Similarly if the


two waves are in phase, the collapse pressures gener-ated will be higher as compared to out of phase soundwaves. Thus it is clear that use of multiple transducersand multiple frequencies offer much flexibility in con-trolling the bubble�cavity behavior in the reactor andalso enhances the reaction rates due to enhancedbubble growth.

Thus it can be said that for successful application ofultrasound induced cavitation for wastewater treatmentit is necessary to develop new designs using multipletransducers and multiple frequencies operable at largerscales to increase the destruction efficiency as it isalmost impossible to treat large volumes of waste witha single high frequency unit.

2.1.3. Initial size of the nucleiŽ .Gogate and Pandit 2000a , with the help of theoret-

ical simulations, have shown that the collapse intensi-ties are higher at lower sizes of the nuclei. It is difficultto give the exact nuclei size in the reactor but sometrends can be obtained depending on the physico-chemical properties of the medium, most importantlythe vapor pressure and the presence of the dissolvedgases in the medium.

As the vapor pressure of the medium increases, thesize of the bubble or cavity which will be formedincreases at the same power and ultrasound frequency,resulting in a decrease in the magnitude of the pres-sure pulse generated. Also due to the increased amountof vapor present in the bubble�cavity at higher vaporpressures, the contribution of evaporation�con-densation increases in the overall energy released re-sulting in lower values of temperature�pressure gener-

Ž .ated at the collapse. Mujumdar and Pandit 1998 haveindicated a decrease in the yield of fumaric acid withan increase in the medium vapor pressure obtained by

Ž .using ethanol�water mixtures. Suslick et al. 1997 havealso reported similar observations with experiments onthe Weissler reaction in a microfluidiser.

The presence of the dissolved gases also affects theinitial cavity size generated but the effect on thesonochemical yield cannot be generalized due to thesimultaneous effect of all the physico-chemical proper-ties of the gases, most importantly the polytropic index,thermal conductivity and the solubility of the gas. Thepolytropic ratio is a measure of the heat releasedduring the adiabatic compression of the gas and isresponsible for raising the temperature. The dramaticnature of the polytropic effect may be illustrated byconsidering cavitation in the presence of a monotonicgas and a typical polytropic gas, Freon. The polytropicindex of the two are 1.66 and 1.1, respectively, and themaximum temperature for the monotonic gas is ap-

Žproximately seven times that for Freon Gogate and.Pandit, 2001 . Sometimes, however, the effect of solu-

bility outweighs that of the polytropic ratio as in thecase of carbon disulfide decomposition. The reactionrate in the presence of He is higher than in thepresence of Ar even though � �� because of theAr He

Žgreater solubility of helium than that of argon En-.tezari et al., 1997 . When hydroxyl radicals are gener-

ated in the reaction, oxygen accelerates the reactionŽmore than a polytropic gas such as argon Hart and

.Henglein, 1985; Berlan et al., 1994 . There are a vastŽnumber of other illustrations Petrier and Luche, 1987;

.Petrier et al., 1982, 1984; Gogate et al., 2001a in theliterature, which also indicate that the rates of thereaction are different depending on the type of the gaspresent in the medium and also the amount of the gasdissolved in the medium.

Ž .In the earlier work Gogate and Pandit, 2001 , thedetailed discussion about effects of various physico-chemical properties of liquid on the collapse pressuresgenerated, and hence the cavitational yields from thereactor, have been presented with guidelines to selectoptimum properties of the medium for achieving thelowest possible sizes of nuclei and hence getting maxi-mum benefits. Some of the experimental observationssuggesting that these properties indeed affect the cavi-tating conditions have been discussed above and for

Ž .more details, the work of Gogate and Pandit 2001 isrecommended.

2.2. Model experiments with destruction of formicacid

It was decided to perform some experiments so as toillustrate the use of cavitating equipment for wastewa-ter treatment schemes and also to study the effect ofsome of the operating parameters on the extent ofsonochemical effects. In the case of destruction of the

Ž .heavier longer chain aliphatic acids and aromaticcompounds, formic acid and other lower acids areformed which are also pollutants. Hence, it is necessaryto characterize the destruction of formic acid so as toproduce a clean effluent.

The extent of degradation of formic acid in aqueousŽsolution was studied in a dual frequency flow cell 1.5 l

.of aqueous solution was used in a batch mode . Fig. 3shows the schematic representation of the flow cell.The flow cell consists of a rectangular vessel with adiameter of 9.5 cm and a height of 20 cm with two sets

Ž .of transducers 3 in number in each set mounted onŽthe two opposite faces. Transducers operating inde-

.pendently or simultaneously at different frequencies,i.e. 25 and 40 kHz, and having equal power rating of120 W per set were provided. The power was adjusted


Fig. 3. Schematic representation of dual frequency flow cell.

in such a way that the dual frequency and singlefrequency operation had the same operating intensity,i.e. power dissipated per unit irradiation area, so thatthe effect of frequency can be studied independent ofthe intensity of irradiation. The initial concentration offormic acid in the aqueous solutions used was 500 and1000 ppm. The amount of formic acid remaining in thesolution was determined using titration with the stan-

Ž .dard alkali NaOH . The concentration of the standardŽ .alkali NaOH was adjusted so that the readings of

titration were in the range of 20�25 ml with the leastcount of the burette used as 0.1 ml. Moreover, thetitrations were repeated three to four times to getaccurate readings and an attempt was made to keep

the errors below 0.1%. The experiments were contin-ued up to 2 h of irradiation time and analysis was doneat intervals of 30 min.

The results of the experiments are shown in Fig. 4. Itcan be easily seen that the rates of degradation in-creased with an increase in the frequency of irradiationand also use of multiple frequencies simultaneouslygave better results as compared to a single operatingfrequency. This also confirms the earlier statementabout use of multiple transducers and frequencies forobtaining better efficiencies. However, it can be seenthat there is only 7% degradation after approximately 2h of irradiating time and it is necessary to increase thesame before any practical applications can materialize.

Fig. 4. Effect of frequency of irradiation on the percentage degradation of formic acid in a dual frequency flow cell for 500 ppminitial concentration.


One of the methods of increasing the degradation issimultaneous use of aeration and irradiation. Prelimi-nary results with aeration indicate that the extent ofdestruction of formic acid is almost doubled with use ofcontinuous aeration. When the formic acid solution

Žwas only aerated without any irradiation aeration rate.of 1.02 cc�s , there was no decrease in the amount and

concentration of formic acid which confirms that therewas no evaporation or destruction of formic acid due toaeration alone. More work is being done to examinethe effect of aeration rate on the destruction efficiencyin different ultrasonic reactors. Also use of catalyst forthe destruction and simultaneous irradiation with UVlight can increase the extent of degradation. Shir-

Ž .gaonkar and Pandit 1998 have shown that the combi-nation of ultrasonic irradiation and UV light is indeedbeneficial for the destruction of 2,4,6 trichlorophenolwith TiO catalyst.2

Fig. 5 shows the effect of initial concentration offormic acid on the extent of degradation and it can beeasily seen from the figure that dilute solutions can bedegraded to a larger extent. More work is being carriedout with different concentrations of formic acid andunder different ultrasonic irradiation conditionsŽ .Gogate et al., 2001a . Similar results have been ob-

Ž . Ž .tained by Petrier et al. 1994 , Serpone et al. 1994 ,Ž . Ž .Hua et al. 1995 , Hua and Hoffmann 1996 , De Viss-Ž .cher et al. 1996 and hence dilution of the effluent can

be done as a polishing step before subjecting to degra-dation using cavitation. It should be also noted at thisstage that there is a downside of dilution. Too muchdilution of the sample will lead to increased powerdemands for the treatment procedure. Sivakumar and

Ž .Pandit 2000b have shown that the rate of degradation�increases with increasing power density percentage

Ž .0.824 �degradation�94.36 P�V . It should also be noted

that the equation is only valid up to a certain optimumpower dissipation beyond which there is a decrease inthe rate of degradation. Thus, for treating largervolumes, increased power input must be supplied to getthe same degree of degradation. This indeed has to betaken into consideration while designing the process.Optimization needs to be carried out considering thefavorable effects obtained by using a lower concentra-tion and the increased power demands due to dilution.If the sole objective is to degrade the effluent withoutany time restrictions and with power savings, dilutionwill not be a good alternative.

Ž .Comparison with earlier work Gogate et al., 2001ain which the destruction of formic acid was studied inan ultrasonic horn and bath shows that the extent ofdestruction in the case of the flow cell is maximum andthe flow cell has the capacity for treating 1.5 l ofsolution as compared to 50 ml for the ultrasonic horn

Ž .and 750 ml for the ultrasonic bath, respectively Fig. 6 .At a first glance, if one compares the percentage degra-dation alone obtained in the equipment, the ultrasonichorn looks like an attractive option as it gives approxi-mately two to three times more degradation as com-pared to the ultrasonic bath and dual frequency flowcell. But this is not the true picture. The flow cell istreating 1.5 l of the pollutant as compared to just 50 mlfor the ultrasonic horn and hence the concept of netdegradation defined as actual amount of formic aciddegraded per unit power density has been used for thecomparison of various equipment. A sample experi-ment with 750 ml of formic acid solution using anultrasonic horn indicated absolutely no destructionwhich indicates that the ultrasonic horn has a limita-tion in terms of scale of operation. Thus, it has beenalso shown on the basis of experiments with formic acidthat use of multiple frequencies is indeed beneficial in

Fig. 5. Effect of initial concentration on destruction rate of formic acid at 40 kHz in dual frequency flow cell.


Fig. 6. Comparison of net degradation of formic acid per unit power density for different equipments.

treating large quantities in an energy efficient way. Theenergy efficiency of the dual frequency flow cell hasalso been found to be much larger as compared to theultrasonic bath and ultrasonic horn.

Thus it can be said that ultrasonic equipment can beused for the destruction of complex, bio-refractorycompounds, although some modifications have to bedone for large scale applications. Detailed discussionsabout the different types of ultrasonic equipment have

Žalready been reported in earlier work Gogate et al.,. Ž . Ž .2001c . Moholkar and Pandit 1996 , Shah et al. 1999

have also described the various types of ultrasonicreactors which can be used for the destruction purpose.

2.3. Applications of acoustic ca�itation to wastewatertreatment: o�er�iew

As said earlier, the application of acoustic cavitationfor the degradation of complex and refractory materi-als is not new to the researchers. It is worthwhile atthis stage to take an overview of the work done in thisarea to assess the level and degree of confidence in theapplication of acoustic cavitation to wastewater treat-ment. Table 1 summarizes the work with discussionabout the operating conditions of the equipment andcompounds used in the study and important resultsobtained in the work. It should be also noted that theliterature presented in the table considers some of therepresentative work to highlight important design con-siderations and there is a vast number of other illustra-tions where ultrasound has been applied for destruc-tion of a particular compound. Based on the detailedanalysis of the work done in this area, the followingconclusions�recommendations for energy efficientoperation of sonochemical reactors can be made:

1. The rate of sonochemical degradation is higher atlower initial concentration of the pollutant and

hence pre-treatment of the waste stream may bedone in terms of diluting the stream for enhancedcavitation effects. However, as discussed earlier, ananalysis must be done comparing the positive ef-fects due to decreased concentration and the nega-tive effects associated with lower power density totreat large quantities of pollutants.

2. Aeration and addition of catalyst such as TiO ,2NiSO , CCl , CuSO and also salts such as NaCl4 4 4

significantly enhances the extent of degradation.The presence of gases or gaseous mixtures such asAr�O mixture also increases the efficiency of3

acoustic cavitation in some cases. It may happen,however, that not all sonochemical reactions areenhanced by the presence of catalyst and hence fora particular application laboratory scale experi-ments are required unless data are available in theliterature.

3. Higher frequencies of operation are suited for ef-fective destruction of pollutants, however, it is dif-ficult to operate at frequencies greater than 200kHz. Again the majority of studies given in thetable are at lower frequencies, typically 20 kHz.More work in terms of designing of efficient trans-ducers for operating at higher frequencies is re-quired. Also existence of optima of frequency be-yond which the rates of degradation are loweredneeds to be checked for the effluent under con-sideration.

4. Use of multiple frequencies produces more intensecavitation and hence results in higher yields oftransformation and hence dual or triple frequencyreactors should be used which will also give similarresults to a single very high frequency transducerwhich is difficult for designing and operation. Alsolarger volumes of effluent can be effectively treateddue to increased cavitationally active volume.

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Table 1Work done in the area of application of ultrasound to wastewater treatment

Sr. No. Reference Type of equipment and experimental details Highlights of work

Ž .1 Toy et al. 1990 Combination of ultrasonic bath with frequency Extent of decomposition is greater for combination as compared toof 20 kHz and 200 W high pressure Xenon lamp, sonochemical and UV alone for both miscible and immiscible pairs.capacity of 3�4 ml, chemical used: 1,1,1 Extent of cavitation is higher for lower vapour of the mixtures.trichloroethane

Ž .2 Kotronarou et al. 1992 Branson 200 sonifier with driving frequency of Degradation was complete in less than 2 h; major hydrolysis220 kHz and intensity of 75 w�cm , 50 ml capacity, product is p-nitrophenol which is also degraded by sonication.

chemical studied: parathion Greater energy efficiencies are obtained in reactors with largerradiating surfaces.

Ž .3 Petrier et al. 1992a Ultrasonic transducer with frequency of 20 kHz, Complete destruction is achieved within 100�200 min. Due to20 W power input and capacity of 100 ml. aeration there is formation of nitrous and nitrogen oxide and alsoCompound studied: pentachlorophenate carbon dioxide which reduces the sonochemical reaction. Pure oxygen

Žwill be more beneficial as compared to aeration complete destruction.within 75 min . Passage of Argon is even more beneficial

Ž .complete destruction within 50 min . Possible explanation on thebasis of difference in thermal conductivity and polytropic index.

Ž .4 Petrier et al. 1994 Ultrasonic converters of 20 and 487 kHz Degradation rates at higher frequencies are much higher, H O2 2driving frequencies, Power input�30 W and production increases linearly with time. Rate of formation of hydrogencapacity�200 ml, Chemicals studied: phenol peroxide decreases with increase in initial concentration of phenol and

saturates after a particular concentrationŽ . Ž5 Serpone et al. 1994 Ultrasonic horn Driving frequency �20 kHz, Induction period of 90 min was observed, Degradation follows first-

.power input�50 W, reaction volume�100 ml , order kinetics with rate constant inversely proportional to initialchemicals studied: 2,3,4 chlorophenol concentration of monophenol, Reaction sites have been identified.

Ž .6 Cheung and Kurup 1994 Ultrasonicator with driving frequency of 20 kHz Degradation of CFC 11 and CFC 113 is fairly rapid and reaction timeand maximum power of 475 w. Two types of is in the range of 10�40 min depending on the system. Volatilization ofreactor configuration; batch mode with capacity CFC was not observed and hence it do not contribute to removalof 35 ml and circulating system with 250 ml process. Rate constant for degradation declines slightly at highervolume. Chemicals studied: CFC 11 and temperatures.CFC113

Ž .7 Ingale and Mahajani Combination of sonication using Ultrasonic Sonication in presence of oxygen is beneficial, Catalyst used NiSO4Ž . Ž1995 cleaning bath capacity�500 ml, operating also increases rates of destruction, Hybrid system of sonication and

.frequency �40 kHz, Power input�150 W wet oxidation is better alternative as compared to sonication or wetfollowed by wet oxidation, refractory waste oxidation alone.containing organic acids, tributylphosphate,sodium sulfate, etc.

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Ž . Ž8 Hua et al. 1995 Near field acoustic processor two opposite Degradation rates higher at lower initial concentrations, kinetic rateplates with driving frequencies of 16 and 20 kHz, constants increases with decrease in the irradiated volume at constantPower input�0�1775 W, Continuous recirculation power, optimum intensity at which the rates of degradation is

. Žat flow rate of 3.2 l�min , Chemical used: p- maximum As power is increased, rate increases up to a point and then.nitrophenol decreases , Rate is maximum in presence of 80:20% v�v Ar�O2

mixture followed by pure Ar and least in pure O .2Ž . Ž9 Trabelsi et al. 1996 Combination of acoustic cavitation Ultrasonic Degradation rates and extent of degradation are higher at 540 than

Žhorn with driving frequency of 20 and 540 20 kHz. Sonoelectrochemical destruction 97% reduction in phenol. . Ž .kHz, capacity�200 ml, power input�50 W and concentration is much higher than sonochemical alone only 5% . At

electrolysis, Chemical studied: phenol high frequency, 95% destruction is complete within 10 min asagainst 60 min for low frequency.

10 Hua and Hoffmann Ultrasonic probe with frequency of 20 kHz, Kinetic rate constant is higher at lower initial concentration of CCl ,4Ž .1996 power input of 135 W and capacity of 110 ml. presence of ozone or argon�ozone mixture does not affect rate of

Chemical studied: carbon tetrachloride and p- degradation significantly. Presence of CCl enhances the destruction of4nitrophenol p-nitrophenol in an air saturated aqueous solution.

Ž . Ž11 Thoma et al. 1997 Near field acoustic processor two opposite Significant sorption taking place at the gaskets used for sealing,plates with driving frequencies of 16 and 20 kHz, Sparging of oxygen results in slight increase of rate constants,Power input�200�500 W�plate, Capacity of Destruction efficiency comparable to other oxidation technologies,362 ml, Chemicals studied: o-dichlorobenzene Mathematical model is presented to analyze the overall destructionand Dichloromethane process

Ž12 Shirgaonkar and Pandit Ultrasonic horn Driving frequency �22.7 kHz, Degradation rate is inversely proportional to operating temperatureŽ .1997 power input�240 W, Capacity�50 ml, and also to initial concentration of KI, Catalyst CCl significantly4

Chemicals studied: potassium iodide and sodium enhances the rates of degradation, however, the amount of CCl does4.cyanide not affect the rate, degradation of NaCN is weakly dependent on

intensity of irradiation and decreases at higher intensity, AgainCatalyst CCl significantly enhances the rates of degradation of NaCN4

13 Seymore and Gupta A 600-W, 20 kHz ultrasound generator with Sonochemical degradation rates are significantly enhanced by presenceŽ .1997 amplitude of 20%, capacity of reactor �400 ml, of NaCl and quantity of NaCl also affects the rate. At 1.38 mol�l of

Chemical studied: chlorobenzene, p-ethylphenol NaCl, a threefold increase in the oxidation rate for phenol is observedŽand phenol whereas for other two pollutants, the increase is still larger six to seven

.fold . Additional desalination unit is required prior to discharge ofclean effluent.

Ž14 Shirgaonkar and Pandit Combination of ultrasound Ultrasonic horn and Degradation rates are higher for the combination at higher temperaturesŽ .1998 bath with frequency of 22 kHz and power rating and also in presence of photocatalyst. At lower temperature the rates

of 600 and 120 W, respectively, capacity� for the combination are comparable with that for sonication alone..1000 ml and Ultraviolet light, Chemical Degradation is benefitted by higher power input into the system.

studied:2,4,6 trichlorophenolŽ .15 Weavers et al. 1998 Ultrasonic horn system with driving frequency of Degradation rates are higher with aeration and also on passage of

20 and 500 kHz, Power input of 48�56 W ozone. Moreover for combination of sonication and aeration, rates areand capacity of 235 and 640 ml, respectively, higher at 500 as compared to 20 kHzCompounds studied: organic contaminants, namelynitrobenzene, 4-nitrophenol and 4-chlorophenol

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Ž .Table 1 Continued


Ž .16 Hung and Hoffmann 1999 Ultrasonic generator with frequencies in the Rate of degradation increases with increase in frequency for samerange 20�1078 kHz and power input of reactor configuration, kinetics of degradation have been explained35�62 W, Capacity of reactor �95�605 ml,Compounds studied: chlorinated hydrocarbonsŽ .CHCl , CH Cl and CCl3 2 2 4

Ž .17 Destaillats et al. 2000 Ultrasonic irradiation at 358 khz with power Very low rates of sonochemical destruction at concentration �CMCinput of 50 W; Capacity of reactor �600 ml, indicating that micelles can be considered as a good shelter fromChemical studied: alkylphenol ethoxylate direct sonochemical effects. Again degradation kinetic rate constantssurfactants are inversely proportional to initial concentration of pollutant.

Ž .18 Pandit et al. 2000 Ultrasonic horn with driving frequency of 22.7 Rates of degradation are higher at higher power inputs due to moreŽKHz and power rating of 600 W operated at 10� cavitation events and also in the presence of photo-catalyst TiO .2

.30% amplitude , capacity of 100 ml, Chemical However, TiO also partially adsorbs TCP which means physical2studied: 2,4,6, trichlorophenol removal of the catalyst, not total degradation and adsorption

desorption characteristics must be considered for correct estimation ofdegradation rates.

Ž .19 Gogate et al. 2001a Ultrasonic horn with driving frequency of 22.7 Formic acid can be degraded using ultrasonic irradiation though theŽkHz and power rating of 600 W, capacity of 100 rates are quite less �10% decrease in concentration is observed in 2

Ž . Ž .ml and Ultrasonic bath frequency of 22 kHz and h . Ultrasonic bath more transmitting area has more energy.power rating of 120 W , capacity of 750 ml, efficiency as compared to horn and treats larger volumes.

Chemical used: formic acid Simultaneous aeration with irradiation increases the rates ofdegradation.

20 Sivakumar and Pandit Ultrasonic horn with driving frequency of 22.7 Degradation rates show an optima with respect to power input whichŽ .2000b kHz and power rating of 600 W, capacity of 100 Means that there is no use of spending indefinite power into the system.

Žml and Ultrasonic bath frequency of 22 kHz and Optimum values are also function of the type of equipment. Better.power rating of 120 W , capacity of 750 ml, Dual degradation rates have been obtained for larger irradiating surface

Žfrequency flow cell frequency of 25 and 40 kHz, Areas. In dual frequency flow cell, degradation is efficient with dual. Ž . Žpower input of 120 W per side , capacity: 1500 frequency 25�40 kHz as compared to individual frequencies 25

.ml, Chemical studied: rhodamine B and 40 kHz operated individually .Ž . Ž Ž21 Sivakumar et al. 2000 Dual frequency flow cell frequency of 25 and Rate of degradation is higher with dual frequency operation 25�40

. . Ž40 kHz, power input of 120 W per side , kHz as compared to individual frequencies 25 and 40 kHz operated.capacity: 1500 ml, Chemical studied: p- individually . Kinetics have been studied and the Rate constant is

nitrophenol inversely proportional to the operating temperature. Modelling of dualfrequency acoustic flowfields have been presented based on the bubbledynamics.


5. Greater energy efficiency has been observed forultrasonic probes with higher irradiating surfaceŽ .lower operating intensity of irradiation which re-sults in uniform dissipation of energy. Also therates of destruction will be higher for lower inten-sity of irradiation as the collapse pressure for asingle cavity is inversely proportional to intensity of

Ž .irradiation Gogate and Pandit, 2000a . It shouldalso be remembered that a minimum cavitationinception intensity also exists where degradationstarts.

6. Rate of destruction is inversely proportional to theoperating temperatures which also affects the va-por pressure of the medium and hence lower tem-

Ž .peratures typically of the order of 10�15�C will bepreferred. However, if the dominant mechanism ofdestruction is pyrolysis as in the case of destructionof trichloroethylene, this will not be true.

7. Sonochemical destruction processes can be effec-tively coupled with photocatalytic oxidation, elec-trolysis, wet oxidation, etc., which show higher ratesof degradation as compared to sonication alone.

However, it should be noted that a majority ofŽstudies are on laboratory scale typical range of a few

.milliliters to maximum of 1000 ml volumes and hencescale-up of sonochemical reactors is difficult and moreinformation is needed from diverse fields such as mate-rial science, acoustics, chemical engineering, etc. Somerecommendations have been made at a later stage foreffectively transforming the current lab scale techniqueof acoustic cavitation to an industrial scale operation.

3. Hydrodynamic cavitation

Hydrodynamic cavitation can simply be generated bythe passage of the liquid through a constriction such asan orifice plate. When the liquid passes through theorifice, the kinetic energy�velocity of the liquid in-creases at the expense of the pressure. If the throttlingis sufficient to cause the pressure around the point ofvena contracta to fall below the threshold pressure for

Žcavitation usually vapor pressure of the medium at the.operating temperature , millions of cavities are gener-

ated. Subsequently as the liquid jet expands, the pres-sure recovers and this results in the collapse of thecavities. Due to the passage of the liquid through theconstriction, boundary layer separation occurs and asubstantial amount of energy is lost in the form of apermanent pressure drop. Very high intensity turbu-lence occurs on the downstream side of the constric-tion; its intensity depends on the magnitude of thepressure drop, which, in turn, depends on the geometryof the constriction and the flow conditions of theliquid. The intensity of turbulence has a profound

effect on the cavitation intensity as shown by MoholkarŽ .and Pandit 1997 . Thus, by controlling the geometric

and operating conditions of the reactor, one can pro-duce the required intensity of the cavitation so as tobring about the desired change with maximum effi-ciency. Also the collapse temperatures and pressuresgenerated during the cavitation phenomena are a strongfunction of the operating and geometric parametersŽ .Gogate and Pandit, 2000b .

A dimensionless number known as cavitation num-Ž .ber C is used to relate the flow conditions with the�

cavitation intensity. Cavitation number is given by thefollowing equation:

P � P2 �C �� 1 2��th2

where P is the fully recovered downstream pressure,2P is the vapor pressure of the liquid�medium and �� this the velocity of the liquid at the throat of the constric-tion. The cavitation number at which the inception ofcavitation occurs is known as the cavitation inceptionnumber C . Ideally speaking, the cavitation inceptionvioccurs at C �1 and there are significant cavitationalvieffects at C values of less than 1. In the earlier work�Ž .Gogate and Pandit, 2000b it has been clearly shownthat the cavities oscillate under the influence of fluc-tuating pressure field and the magnitudes of pressurepulses generated are much less, insignificant to bringabout a desired chemical change for the case where C�values are greater than 1. However, cavitation has beenfound to occur at a higher cavitation number also,possibly due to the presence of dissolved gases or some

Žimpurities in the liquid medium Harrison and Pandit,. Ž .1992 . Yan and Thorpe 1990 have shown that C is avi

function of the flow geometry and usually increaseswith an increase in the size of the constriction. More-over, comparison of experimental data of Yan and

Ž .Thorpe 1990 for pipe diameter of 3.78 cm with dataŽ .of Tullis and Govindrajan 1973 for pipe diameters of

7.80 and 15.4 cm indicates that the cavitation inceptionnumber is a strong function of the pipe diameter alsoand it increases with an increase in the pipe diameter.It is difficult at this stage to give a physical explanationfor these observed variations with the orifice and pipediameter. Still for maximum benefits from the set-up,the flow conditions and the geometry should be ad-justed in such a way that the cavitation number liesbelow 1.

The major advantages of hydrodynamic cavitationare:

1. Reactions that require moderately rigorous condi-tions can be carried out easily under ambient con-ditions.


2. It is one of the cheapest and most energy efficientmethod of generating cavitation.

3. The equipment used for generating cavitation issimple.

4. Maintenance of such reactors is very low.5. The scale-up of the above process is relatively easy.

It should be noted at this stage that the hydrody-namic cavitation phenomenon provides substantiallylower intensity of collapse of the individual cavities interms of temperature and pressure than the acoustic

Ž .cavitation Senthilkumar, 1997; Moholkar et al., 1999though the degree of cavitational intensity and thenumber of cavitation events can also be controlled bymanipulating the operating and geometric conditions

Ž .existing in the reactor Gogate and Pandit, 2000b .Another shortcoming of the hydrodynamic cavitationreactors is the poor pressure recovery downstream of

Žthe constriction typically for an orifice to pipe diame-ter ratio of 0.5, the total permanent pressure head loss

.is approx. 73% of the orifice pressure differential .Therefore, to produce higher intensities of cavitation,the discharge pressure of the pump should be higher.However, it should be again noted that the conditionscan be well improved by adjusting the geometry andoperating conditions of the reactor altering the fluidturbulent structure as discussed in detail later. Alsosimilar to the acoustic cavitation reactors, the perfor-mance of hydrodynamic cavitation reactors is low inthe case of viscous mediums. In such a case cavitationis difficult to produce and the cost of pumping suchviscous materials is very high, affecting the overalleconomy of the process.

3.1. Optimum design parameters for hydrodynamicca�itation

The important parameters which decide the effi-ciency and the overall cavitational yield in the case ofhydrodynamic cavitation reactors are:

1. Inlet pressure into the system�rotor speed depend-ing on the type of experiment;

2. Physicochemical properties of liquid and initial ra-dius of the nuclei;

3. Diameter of the constriction used for the genera-tion of cavities, e.g. hole on the orifice plate; and

4. Percentage free area offered for the flow.

The effect of the various design parameters men-tioned above has been studied extensively in terms ofthe collapse pressures on the basis of the numericalsimulations using bubble dynamics equationsŽSenthilkumar and Pandit, 1999; Yan et al., 1988;Moholkar and Pandit, 1997; Gogate and Pandit, 2000b;

.Tatake et al., 1999 and also on the basis of experi-Žments done in different reactors Suslick et al., 1997;

Senthilkumar et al., 2000; Vichare et al., 2000a; Shir-.gaonkar et al., 1998 .

It should be noted that a detailed discussion aboutthe effects of various design parameters on the perfor-mance of hydrodynamic cavitation reactors has been

Ž .reported in the earlier work Gogate and Pandit, 2001and only guidelines about selecting a particularparameter with some basic discussion in order to in-duce a degree of confidence in the reader have beenpresented here. The readers are requested to refer tothe earlier work for the details along with experimentalillustrations and results based on theoretical simula-tions of bubble dynamics confirming the effects.

3.1.1. Inlet pressure into the system� rotor speeddepending on the type of equipment

It is observed that the spectacular effects of cavita-tion are predominantly observed only after a particularpressure or operating speed which is defined as the

Žcavitational inception threshold value Senthilkumar.and Pandit, 1999; Shirgaonkar et al., 1998 . Further

increase in the inlet pressure increases the collapseŽpressure due to collapse of single cavities Gogate and

.Pandit, 2000b where as the number of cavities goes onŽ .decreasing Oba et al., 1986 thereby resulting in an

optimum inlet pressure in the hydrodynamic cavitationset-up. There are few experimental illustrationsŽVichare et al., 2000a; Save et al., 1997; Shirgaonkar et

.al., 1998; Gopalkrishnan, 1997 which confirm the exis-tence of the optimum pressure.

Thus it can be said that higher operating pressuresin the system are preferred up to an optimum valuewhich is a strong function of the geometry of thesystem. An analysis of the system for the critical cavita-tion is a must, based on the available literature andalso some experiments at laboratory scale.

A similar effect is observed for the rotor speed inthe case of high speed homogenizers. Thus the opera-tion should be within a predecided range of speeds,well above the critical speed but below a certain maxi-mum speed beyond which there is no appreciable in-

Žcrease in the cavitation effects Senthilkumar and Pan-.dit, 1999; Gogate and Pandit, 2000b .

3.1.2. Physico-chemical properties of liquid andinitial size of the nuclei

By far the liquid properties is one of the very impor-tant aspects that affect the cavitational processes,though the magnitude of the effect of all the liquidvariables may not be the same. Most of the liquidproperties affect cavitation in more than one way. For


example, while an increase in the surface tension of theliquid increases the threshold pressure for cavitationmaking generation of cavitation more difficult, the col-lapse of cavities is more violent. The opposing effectsof liquid properties gives ample scope for optimization.

It should also be noted that the physico-chemicalproperties of the liquid also decide the initial size ofthe nuclei and the effect of initial radius must also beconsidered while choosing a particular liquid medium

Ž .and the process conditions. Gogate and Pandit 2000b ,Ž .Moholkar and Pandit 1997 have clearly shown that

the collapse pressure is inversely proportional to theinitial size of the nuclei and hence conditions resultingin smaller cavities will be preferred.

As said earlier, the effects of various physico-chem-ical properties on the performance of the cavitationreactors have been discussed in detail in the earlier

Ž .work Gogate and Pandit, 2001 and important resultshave been summarized in Table 2.

3.1.3. Effect of the geometry of the constrictionThe geometry of the constriction has a crucial effect

on both the number of the cavitation events and alsothe pressure pulse generated due to the collapse of asingle cavity. Moreover, the geometry affects the pres-sure distribution and pressure recovery profile down-stream of the constriction and hence the active cavita-tion volume, which is very important considering globaleffects of the hydrodynamic cavitation reactor. In thecase of orifice plate set-up, the free area offered forthe flow and also the distribution of the same free areain terms of number and diameter of the holes is impor-tant.

3.1.3.1. Diameter of the constriction used for the gener-ation of ca�ities, e.g. hole on the orifice plate. The diame-ter of the constriction used affects the inception ofcavitation and cavitation inception number increases

Žwith increase in the diameter of the hole Yan and.Thorpe, 1990 . Thus for larger diameter holes, the

cavitation starts at a higher cavitation number; there-fore the extent and intensity of cavitation also in-creases for the same cavitation number in the systemŽ .as long as it is below the cavitation inception number .

Ž .Gogate and Pandit 2000b have also confirmed that asthe diameter of the hole increases, the collapse pres-

Table 2aEffect of liquid phase properties on performance of hydrodynamic cavitation reactors

S.No. Liquid property Affects Favorableconditions

1. Dissolved gas Gas content, nucleation, Low solubilityA. Solubility collapse phaseB. Polytropic constant and Intensity of cavitation Gases with higher polytropicthermal conductivity events constant and lower thermal

Ž .conductivity monatomic gases

2. Liquid vapour pressure Cavitation threshold, Liquids with low vapourIntensity of cavitation, rate Pressuresof chemical reaction.

3. Viscosity Transient threshold Low viscosity4. Surface tension Size of the nuclei Low surface tension

Ž .Cavitation threshold5. Bulk liquid temperature Intensity of collapse, rate of Optimum value exits,

the reaction, threshold� generally lower temperaturesnucleation, almost all are preferablephysical properties.

6. Surfactants and electrolytes Cavitation threshold, Case study of each system isreaction kinetics necessary to ascertain the exact

nature of the effect.7. Solid constituents Cavitation threshold� Low concentrations

nucleation, attenuationOf sound intensity.

Immiscible liquid phase Interfacial cavitation, Depends on the nature of thenumber of bubbles per systemunit volume, reactionKinetics.

a Ž .Summary Gogate and Pandit 2001 .


Ž .Fig. 7. Effect of hole diameter in the orifice plate on the iodine liberation for same free area Vichare et al., 2000a .

sure generated for a single cavity increases. However,there is a downside associated with an increase in thediameter of the hole at constant free area. For thesame free area, as one increases the diameter of thehole, the number of holes decrease thereby decreasingthe number of cavities generated.

Ž .Vichare et al. 2000a have shown that for the samefree area, if you increase the diameter of the hole, therate of iodine liberation in the Weissler reaction de-

Ž . Ž .creases Fig. 7 . However, Sivakumar and Pandit 2000ahave observed that the rate of destruction of rho-damine B effluent increases with an increase in the

Ž .diameter of the hole Fig. 8 . This contradiction in theobserved results may be attributed to the fact thatdegradation of iodine requires comparatively lower cav-itation intensities and hence as the number of cavita-tion events increases with an increase in the number ofholes, cavitation yield also increases though the col-lapse pressure due to single cavity decreases. However,degradation of rhodamine B complex requires highercollapse intensities and hence as diameter decreases,the collapse intensities also decrease thereby decreas-ing the overall cavitational yields.

Thus again an optimization in terms of the size and

Ž .Fig. 8. Effect of hole diameter on the degradation of Rhodamine B for same free area Sivakumar and Pandit, 2000a .


number of the constrictions is needed on the basis ofdesired applications and degree of cavitation intensitysuited for the required transformation.

3.1.3.2. Percentage free area offered for the flow. Theeffect of free area on the cavitation intensity and hencethe yield has been well studied both on the basis oftheoretical analysis of the bubble dynamics behaviorŽ .Gogate and Pandit, 2000b and also experimentallyŽ .Senthilkumar et al., 2000; Vichare et al., 2000a .Gogate et al., 2001c have shown that the collapsepressure generated by the collapse of cavities decreaseswith an increase in the percentage free area offered bythe holes in the orifice plates. Senthilkumar et al.Ž . Ž .2000 , Vichare et al. 2000a have confirmed the en-hanced cavitational effects at lower free areas offeredfor the flow experimentally. Thus lower free areasoffered for the flow are preferred when designing hy-drodynamic cavitation reactors.

3.2. Applications of hydrodynamic ca�itation towastewater treatment: o�er�iew

There are not many reports indicating the applica-tions of the hydrodynamic cavitation reactors inwastewater treatment schemes until now. Kalumuck

Ž .and Chahine 1998 have studied the destruction ofp-nitrophenol in recirculating flow loops using a varietyof cavitating jet configurations and operating condi-tions and have shown that, indeed, hydrodynamic cavi-tation degraded p-nitrophenol. Submerged cavitatingliquid jets were found to generate a two orders ofmagnitude increase in energy efficiency compared to

the ultrasonic method. The ultrasonic destruction wasstudied in an ultrasonic horn. Sivakumar and PanditŽ .2000a have reported that orifice plate hydrodynamiccavitation set-up can be used for the destruction of therhodamine B complex in an efficient way as comparedto acoustic cavitation. Acoustic cavitation was studied

Žusing an ultrasonic horn Operating frequency: 22.7.kHz; power input: 240 W; and capacity: 50 ml an

Žultrasonic bath Operating frequency: 22 kHz; power.input: 120 W; and capacity: 0.75 l as well as a dual

frequency flow cell. The energy efficiency of destruc-tion for the hydrodynamic cavitation set-up was approx-imately 150% more as compared to the best in the

Žsonochemical reactors dual frequency flow cell, oper-ating frequency: combination of 25 and 40 kHz; power

.input: 240 W; and capacity: 1.5 l Moreover, the hydro-dynamic cavitation set-up is able to degrade approxi-mately 50 l of effluent under a single operation ascompared to a few milliliters in the case of the ultra-sonic horn and bath and 1.5 l for the ultrasonic flow

Ž .cell. An orifice plate hydrodynamic set-up Fig. 9ŽVichare et al., 2000a; Jyoti and Pandit, 2000; Sivaku-

.mar and Pandit, 2001 has been successfully used fordegradation of potassium iodide, water disinfection,destruction of the Rhodamine B complex and decol-oration of a dye effluent. The intensity and number ofcavitation events can be effectively controlled by using

Ž .different plates Fig. 10 differing in number and di-ameter of holes depending on the suitability to a par-

Ž .ticular type of application. Jyoti and Pandit 2000 havestudied disinfection of water using different techniquesand reported that hydrodynamic cavitation is aneconomically attractive alternative compared to con-

Ž .Fig. 9. Orifice plate pilot plant scale hydrodynamic cavitation set-up.


Fig. 10. Multiple hole orifice plates having different combina-tions of number and diameter of holes.

ventional techniques such as ozonation and steriliza-tion for reducing bacterial counts.

It should be noted that processes based on hydrody-namic cavitation can be supplemented by otherprocesses that could supply the necessary amount ofoxidizing agents to take care of the load in the effluent.These processes can be ozonation, oxidation with hy-drogen peroxide, UV irradiation, ultrasonic irradiation,etc. Thus, a combination of oxidative processes gener-ally increases the destruction efficiency and at the same

Ž .time maintains the cost effectiveness. Botha 1993 ,Ž .Botha and Buckley 1994 studied a combination of

hydrodynamic cavitation and UV irradiation and re-ported higher cavitational yields.

There exists one such commercial process, known as� Ž � .CAV-OX process CAV-OX , 1994 , developed by

Magnum Water Technology Inc., CA, USA. This is ahybrid system involving hydrodynamic cavitation, UVirradiation and oxidation with hydrogen peroxide.Several contaminants of concern such as pen-

Ž .tachlorophenol PCP , benzene, toluene, ethyl ben-zene, xylenes, cyanide, phenol, atrazine have been suc-cessfully degraded to a significant extent. Case studiesat pilot plant scale showed that the process is effectivefor a wide variety of effluents obtained from variouschemical industries.

Thus it appears that hydrodynamic cavitation reac-tors are at an elementary stage as compared to thesono-chemical reactors and not many applications canbe found in the area of wastewater treatment. How-ever, it can be seen in later sections that the hydrody-namic cavitation reactors are much more energy effi-cient as compared to the acoustic cavitation reactorsand hence should not be underestimated. The discus-sion above about optimum operating and geometricparameters can serve as a useful guideline for effectiveoperation using hydrodynamic cavitation reactors for avariety of applications.

4. Comparison of acoustic and hydrodynamiccavitation

Since cavitating conditions identical to acoustic cavi-tation can be generated in the hydrodynamic cavitation

Table 3Ž .Summary of characterization of various equipments used Gogate et al. 2001c

Equipment Electrical Volume Time of Actual energy Energy Iodine Cavitational yieldŽ .consumption used treatment dissipated W efficiency concentration per unit

Ž . Ž . Ž . Ž .W ml min % at the end of power densityŽ . Ž .reaction g g� J�ml

Dakshin horn 240 50 10 7.31 3.04 1.02 E-5 3.53 E-9Dakshin bath 120 500 10 46.63 38.86 8.4E-5 5.83 E-7Ace horn 10% 40 50 10 7.77 15.43 1.00E-6 1.39 E-9

20% 80 50 10 13.42 16.77 7.55E-6 5.25 E-930% 120 50 5 19.08 15.9 5.90E-6 5.48 E-9

Flow Cell 25 kHz 120 1500 15 51.66 43.05 4.47E-5 6.21 E-740 kHz 120 1500 15 32.37 26.97 4.22E-5 5.85 E-725�40 kHz 240 1500 15 71.14 33 1.31E-4 9.12 E-7

High pressure homogenizer 2090 2000 30 1136.97 54.4 6.637E-5 7.38 E-5Ž .5000 psiHigh speed homogenizer 105 1500 30 45.23 43.07 1.83E-4 6.645 E-7Pilot plant scale % free area 5500 50 000 60 3277 59.58 9.82E-2 2.48 E-4Ž .Orifice plates �2.28%

% free area 5500 50 000 60 3344 60.8 7.50 E-2 1.90 E-4�9.14%


reactors in an easier way and hence these can serve asa replacement for the sonochemical reactors, it is nec-essary to compare the hydrodynamic and acoustic cavi-tation reactors to come to some firm conclusions.

Ž .Suslick et al. 1997 have reported that the acousticcavitation provides significantly higher rates for theWeissler reaction for the specific setup used in the

Ž .experimentation Microfluidisers but no quantitativecomparisons in terms of energy efficiency were made.

Ž .In our earlier work Gogate et al., 2001c , the Weisslerreaction has been again used for the comparison ofdifferent cavitational reactors. The important resultsare reproduced in Table 3 to give readers a quantita-tive idea. It can be clearly seen from the table that thehydrodynamic cavitation equipments are relatively farmore energy efficient and the efficiency increases from

Žhigh pressure homogenizer HPH, typically a labora-.tory scale equipment and energy efficiency of 54% to

Žorifice set up of 50 l capacity typically a pilot plant.scale with observed energy efficiency of 60% . Also the

operating scales in the case of hydrodynamic cavitationŽ .capacity of equipment of the order of few liters are

Žmuch larger than those in the acoustic cavitation of.the order of few ml which indicates that the scale up

of hydrodynamic equipment will be much easier ascompared to the acoustic equipment where the scaleup ratio required will be of the order of a few hundred

Ž .to a few thousand for the ultrasonic horn . The valuesŽof cavitational yields defined as iodine liberated per

.unit power density have been found to be higher forthe hydrodynamic cavitation reactors as compared totheir acoustic counterparts. Some earlier studiesŽChivate and Pandit, 1993; Pandit and Joshi, 1993; Save

.et al., 1994, 1997 also reveal that hydrodynamic cavita-tion is far more energy efficient than acoustic cavita-tion.

Thus it can be said from these results that thehydrodynamic cavitation equipment gives better perfor-mance as compared to the acoustic ones at industrialscales of operation. It should be, however, noted thatthe comparison made here is valid only for a model

Ž .reaction decomposition of potassium iodide and theefficiencies of the various equipment may or may notbe the same for the variety of cavitation-based trans-formations and also other applications.

In hydrodynamic cavitation reactors, the cavitation isproduced at the shear layer. The liquid evaporated atthe vena contracta downstream of the orifice is propor-tional to the area of this shear layer. This fact enablesa designer to control the bubble population in the flow.

ŽBy changing the shape of the orifice making it triangu-.lar or hexagonal, etc. , the area of shear layer can be

Ž .varied obviously it is lowest for a circular orifice andhence the rate of vaporization and the bubble�cavitypopulation can be controlled. Bubble behavior similarto acoustic cavitation can be obtained in the hydrody-

namic cavitation reactor by simple modifications in thedesign of the orifice. If a rotating valve is installedinstead of a permanent orifice, bubbles formed in theshear layer will experience a sinusoidally varying pres-sure field than a linear one. Also if two or three orificesare installed one after the other downstream of thepump then the bubbles that are generated experience ahighly fluctuating pressure field and collapse will bemore and more violent giving rise to high temperaturesand pressure pulses of magnitudes comparable to thoseunder acoustic cavitation. Moreover, air or steambubbles of required sizes can be introduced in the flowand the resultant pressure pulse magnitude can bemanipulated.

Now it can be easily concluded that the hydrody-namic reactors are much more versatile, energy effi-cient and also amenable to an efficient scale up tomeet the industrial scale demands. Since hydrodynamiccavitation reactors use principles of fluid mechanics, alarge amount of literature and data are available forthe designer to scale up the reactor. Still it should benoted that more theoretical studies and experimentalwork in designing the hydrodynamic cavitation reactorson an industrial scale is required before one can saywith confidence that cavitation can be an importantstep in wastewater treatment schemes. Some of themissing elements in this field have been given at a laterstage.

5. Suggestions for future work

Although a number of illustrations can be seen inthe literature regarding use of cavitation for wastewa-ter treatment, none of these are on a scale where onecan confidently use the results�data in any of theindustrial applications. The work necessary to translatethis lab technique into technology can be split into twobroad areas, namely, first theoretical analysis to de-velop the fundamentals and second, experimental stud-ies to establish pathways for transferring the funda-mental knowledge into practice. Some guidelines havebeen given for future work so as to enable engineers toeffectively use the technique of cavitation for degrada-tion of many of the complex organic and bio-refractorychemicals.

5.1. Acoustic ca�itation

In the case of acoustic cavitation, the following stud-ies should be made to close the existing R&D gaps:

1. Effects of all the parameters such as frequency andintensity of irradiation should be studied for vari-ous reactions and a generalized correlation relating


the obtained yield with the operating parametersneeds to be developed for a variety of equipment.Such a correlation will also help the designers inthe efficient scale-up of the equipment.

2. In order to minimize energy requirements duringwastewater treatment applications, frequency needsto be optimized according to the properties of theeffluent.

3. Development of high frequency transducers is an-other grey area which needs further work. Alsodetailed analysis of the acoustic flowfield in thecase of dual or multiple frequency units needs tobe done for application on a larger scale.

4. Combination of ultrasonic equipment such as ultra-sonic bath and ultrasonic horn can be done toincrease the active volume of the cavitation result-ing in higher efficiencies and at the same timelarger volumes can be treated. Experimental char-acterization of such combinations needs to be ex-plored.

5. A standardized sonochemical reactor should bedeveloped which enables estimation of yields andselectivities of different reactions and standardizedconditions should be developed for increased en-ergy efficiency.

6. A detailed thermodynamic study of the collapsingconditions in terms of the number of radicals pro-duced and magnitudes of pressures and tempera-tures generated needs to be done for accurateprediction of the sonochemical reactivity.

5.2. Hydrodynamic ca�itation

Based on the issues and aspects of hydrodynamiccavitation discussed earlier, the following grey areasrequire exhaustive further study.

On the theoretical front the issues which need to bestudied are as follows:

1. Realistic modelling of the turbulence phenomenawhich can be then used to model the cavity�bubbledynamics either in isolation or in the form of cavityclusters in high velocity flow. The modern sophisti-cated CFD codes can be employed to get the flowfield information, i.e. mean and fluctuating velocitycomponents, Reynolds stresses, turbulent pressurefluctuations, etc. This can then be used to under-stand the role of these flow field parameters inaltering the cavity dynamics.

2. It is necessary to develop user friendly computerŽ .codes similar to modern CFD codes for the use of

engineers, which will allow them to change thegeometrical and operating parameters of the hy-drodynamic cavitation set-up and, define physico-

chemical properties of the chemical system underconsideration. These codes, with the help ofbubble�cavity dynamics and the equilibrium chem-istry at cavity collapse conditions, will then predictthe expected chemical effects avoiding trial anderror type of experimentation for the engineers.

Efforts required on the experimental front are asfollows:

1. Design and fabrication of different types of hydro-dynamic cavitation set-ups differing in flow field,turbulence characteristics and geometry which willallow efficient large scale operation for effectiveusage in wastewater treatment plants.

2. Laboratory and pilot plant studies with simulatedwaste to understand and address the scale up is-sues, such as alteration of the flow field and turbu-lence characteristics due to the scale of operation.

The studies discussed above should allow the engi-neers to compute the following quantities:

1. The type of cavity�cluster dynamic behavior re-quired to bring about the desired physico-chemicaltransformation.

2. The parameters and their magnitudes required tobring about the desired cavity�cluster dynamicbehavior.

3. The type of equipment including the geometricalconfiguration required to make the cavity�clusterbehave in the required manner most energy effi-ciently; and

4. The energy efficiency of such a cavitational trans-formation process, which then can be comparedwith the conventional transformation processes.

The above four steps are the essential steps requiredin any process selection�optimization. The information

Žand design procedures for the individual steps with.some gaps as outlined before are available in the

literature but an integrated design approach is neces-sary. The development of such an integrated equip-ment design procedure should be the final aim of suchan exercise.

6. Conclusion and recommendations

Acoustic and hydrodynamic cavitation generate con-ditions of high temperature and pressure along withrelease of active radicals which results in many of thechemical transformations under much less severe con-ditions. The magnitudes of pressure and temperatureand number of free radicals can be easily manipulated


by adjusting the operating and geometric parametersdepending on the desired intensity of cavitation pheno-mena suiting a particular application.

In the case of acoustic cavitation, selection of suit-able operating parameters such as the intensity and thefrequency of ultrasound and the vapor pressure of thecavitating media is essential as the bubble behavior andhence the yields of sonochemical transformation aresignificantly altered due to these parameters. For thequantitative relationship of the effects, refer to the

Ž .work of Gogate and Pandit 2000a . They have given acorrelation for the estimation of collapse pressure as afunction of different operating conditions which clearlygives a quantitative idea about the optimum parame-ters. It is necessary that both the frequency and inten-sity of irradiation should not be increased beyond anoptimum value which is also a function of the type ofeffluent to be degraded and the equipment under con-sideration.

The important parameters that affect the collapsepressures generated and hence the overall cavitationalyield in the case of hydrodynamic cavitation are found

Žto be inlet pressure�speed of rotation specific to High.Speed Homogenizer , physico-chemical properties of

the liquid medium, constituents of the liquid, geometryŽ .of the holes number and diameter of the holes on the

orifice plate and initial radius of the nuclei. These mustbe properly selected or manipulated while designingthe reactors based on the guidelines given in the pre-sent work for achieving maximum benefits from thecavitation phenomena. For the detailed analysis of the

Žeffects of operating parameters, earlier work Gogate.and Pandit, 2001 is recommended. The work of Gogate

Ž .and Pandit 2000b is recommended for a quantitativeestimation of pressure generated at the collapse ofcavities and its effect on the overall cavitational yield

Žfrom the reactor has also been studied Gogate et al.,.2001c .

Acoustic cavitation reactors generate much moreintense cavitation and it appears at a first glance thatone should go in for sonochemical reactors. However,it should be noted that these are associated with scale-up problems and information in a variety of fields isrequired for efficient design of large scale equipmentwhich is not available at times. Moreover, hydrody-namic cavitation reactors offer versatility in terms ofthe operation and similar conditions to acoustic cavita-tion can be generated in a much easier way. Thescale-up of hydrodynamic cavitation units is much eas-ier as the knowledge regarding the hydrodynamic con-ditions existing downstream of the orifice is easily avail-able in the literature or can be obtained with the helpof modern CFD codes. Moreover, the centrifugal pumpsoffer higher energy efficiency at larger scales of opera-tion.

Overall it can be said that cavitation can be effec-

tively applied for degradation of chemicals in waste-water treatment schemes where the majority of conven-tional techniques fail to give substantial conversions.However, the majority of studies available in this areaare on a lab scale and hence much more work needs tobe done before this goal is realized. Some of the gapshave been highlighted in the earlier sections.

C : Cavitation numbervC : Inception cavitation numbervi

Ž .P: Power input to the system wŽ 2.P : Pressure at downstream of the orifice N�m2

Ž 2.P : Vapor pressure of the medium N�mvV: Volume of the medium

Ž .� : Velocity at the throat of the orifice m�sth�: Polytropic index of gas

Ž 3.�: Density of liquid medium kg�m

Acknowledgements

I would like to thank Prof. A.B. Pandit for the usefuldiscussions and suggestions offered which has madethis manuscript much more emphatic.

References

Baillod, C.R., Lamparter, R.A., Barna, B.A., 1985. Wet airoxidation for waste treatments. Chem. Eng. Prog. 81, 51.

Berlan, M.J., Mason, T.J., 1992. Sonochemistry from researchlaboratories to industrial plants. Ultrasonics 30, 203.

Berlan, J., Trabelsi, F., Delmas, H., 1994. Oxidative degrada-tion of phenol in liquid media using ultrasound. Ultrason.Sonochem. 1, S97.

Bhatnagar, A., Cheung, H.M., 1994. Sonochemical destructionof chlorinated C1 and C2 volatile compounds in diluteaqueous solutions. Environ. Sci. Technol. 28, 1481.

Botha, C.J., 1993. A Study on Hydrodynamic Cavitation as aMethod of Water Treatment. MS Thesis Univ. of Natal,Durban, South Africa.

Botha, C.J., Buckley, C.A., 1994. Disinfection of Potable Wa-ter: The Role of Hydrodynamic Cavitation. Water Supply,

Ž13 2, IWSA International Specialized Conference on Dis-. Ž .infection of Potable Water 219 .

CAV-OX� , 1994. Cavitation Oxidation Process ApplicationAnalysis Report, Magnum Water Technology, Inc., RiskReduction Engg. Laboratory, Office of Research and De-velopment, USEPA, Cincinnati, Ohio 45268.

Cheung, H.M., Kurup, S., 1994. Sonochemical destruction ofCFC 11 and CFC 113 in dilute aqueous solution. Environ.Sci. Technol. 28, 1619.

Cheung, H.M., Bhatnagar, A., Jansen, G., 1991. Sonochemicaldestruction of chlorinated hydrocarbons in diluted aqueoussolutions. Environ. Sci. Technol. 25, 1510.

Chivate, M.M, Pandit, A.B., 1993. Effect of hydrodynamic andsonic cavitation on aqueous polymeric solutions. Ind. Chem.Engr. 35, 52.


Compton, R.G., Eklund, J.C., Marken, F., Rebbitt, T.O.,Akkermans, R.P., Waller, D.N., 1997. Dual activation: cou-pling ultrasound to electrochemistry � an overview. Elec-trochim. Acta 42, 2919.

De Visscher, A., Eanov, P.V., Drijvers, D., Langenhove, H.V.,1996. Kinetic model for the sonochemical degradation ofmonocyclic aromatic compounds in aqueous solutions. J.Phy. Chem. 100, 11636.

Destaillats, H., Hung, H-M., Hoffmann, M.R., 2000. Degrada-tion of alkylphenol ethoxylate surfactants in water withultrasonic irradiation. Env. Sci. Technol. 34, 311.

Entezari, M.H., Kruus, P., 1994. Effect of frequency onsonochemical reactions I. Oxidation of iodide. Ultrason.Sonochem. 1, 75.

Entezari, M.H., Kruus, P., 1996. Effect of frequency onsonochemical reactions II. Temperature and intensity ef-fects. Ultrason. Sonochem. 3, 19.

Entezari, M.H., Kruus, P., Otson, R., 1997. The effect offrequency on sonochemical reactions III: Dissociation ofcarbon disulfide. Ultrason. Sonochem. 4, 49.

Gogate, P.R., Pandit, A.B., 2000a. Engineering design meth-ods for cavitation reactors I: Sonochemical reactors. AIChEJ. 46, 372.

Gogate, P.R., Pandit, A.B., 2000b. Engineering design meth-ods for cavitation reactors II: Hydrodynamic cavitationreactors. AIChE J. 46, 1641.

Gogate, P.R., Pandit, A.B., 2001. Hydrodynamic cavitation: astate of the art review. Rev. Chem. Engg. 17, 1.

Gogate, P.R., Mujumdar, S., Pandit, A.B., 2001a. Sonochemi-cal reactors for waste water treatment. Adv. Env. Res.Forwarded for publication.

Gogate, P.R., Mujumdar, S., Pandit, A.B., 2001b. Sonopho-tochemical reactors for waste water treatment. Water Res.Forwarded for publication.

Gogate, P.R., Shirgaonkar, I.Z., Sivakumar, M., Senthilkumar,P., Vichare, N.P., Pandit, A.B., 2001c. Cavitation reactors:efficiency analysis using a model reaction. AIChE J. Ac-cepted for publication.

Gopalkrishnan, J., 1997. Cell Disruption and Enzyme Recov-ery. M.Sc. Tech. Thesis University of Mumbai Mumbai,India.

Harrison, S.T.L., Pandit, A.B., 1992. The disruption of mi-crobial cells by hydrodynamic cavitation. Proceedings of 9thInt. Biotech. Symp. Washington USA.

Hart, E.J., Henglein, A., 1985. Free radical and free atomreactions in the sonolysis of aqueous iodide and formatesolution. J. Phys. Chem. 89, 4342.

Henglein, A., 1995. Chemical effects of continuous and pulsedultrasound in aqueous solutions. Ultrason. Sonochem. 2,S115.

Hua, I., Hoffmann, M.R., 1996. Kinetics and mechanism ofthe sonolytic degradation of CCl : intermediates and4byproducts. Environ. Sci. Technol. 30, 864.

Hua, I., Hoffmann, M.R., 1997. Optimisation of ultrasonicirradiation as an advanced oxidation technology. Environ.Sci. Technol. 31, 2237.

Hua, I., Hochemer, R.H., Hoffmann, M.R., 1995. Sonochemi-cal degradation of p-nitrophenol in a parallel plate nearfield acoustical processor. Environ. Sci. Technol. 29, 2790.

Hung, H-M., Hoffmann, M.R., 1999. Kinetics and mechanismof the sonolytic degradation of chlorinated hydrocarbons:frequency effects. J. Phys. Chem. A 103, 2734.

Ingale, M.N., Mahajani, V.V., 1995. A novel way to treatrefractory waste: sonication followed by wet air oxidationŽ .SONIWO . J. Chem. Technol. Biotechnol. 64, 80.

Jyoti, K.K., Pandit, A.B., 2000. Water disinfection by acousticand hydrodynamic cavitation. Biochem. Eng. J. Acceptedfor publication.

Kalumuck, K.M., Chahine, G.L., 1998. The use of cavitatingŽjets to oxidize organic compounds in water. FED Am. Soc.

.Mech. Eng. 245, 3�45.Kamath, V., Prosperetti, A., Egolfopoulos, F.N., 1993. A theo-

retical study of sonoluminescence. J. Acous. Soc. Am. 94,248.

Keil, F.J., Swamy, K.M., 1999. Reactors for sonochemicalengineering � present status. Rev. Chem. Engg. 15, 85.

Kotronarou, A., Mills, G., Hoffmann, M.R., 1991. Ultrasonicirradiation of p-nitrophenol in aqueous solutions. J. Phys.Chem. 95, 3630.

Kotronarou, A., Mills, G., Hoffmann, M.R., 1992. Decomposi-tion of parathion in aqueous solution by ultrasonic irradia-tion. Environ. Sci. Technol. 26, 1460.

Lindley, J., Mason, T.J., 1987. Use of ultrasound in chemicalsynthesis. Chem. Soc. Rev. 16, 275.

Luche, J.L., 1999. Synthetic Organic Sonochemistry. PlenumPress NY USA.

Martin, P.D., Ward, L.D., 1993. Reactor design forsonochemical engineering. Chem. Engg. Res. Des. 70, 203.

Mason, T.J., 1986. Use of ultrasound in chemical synthesis.Ultrasonics 24, 245.

Mason, T.J., 1999. Sonochemistry: current uses and futureprospects in the chemical and processing industries. Phil.Trans. R. Soc. Lond. A 357, 355.

Moholkar, V.S., Pandit, A.B., 1996. Harness cavitation toimprove processing. Chem. Engg. Prog. 96, 57.

Moholkar, V.S., Pandit, A.B., 1997. Bubble behaviour in hy-drodynamic cavitation: effect of turbulence. AIChE J. 43,1641.

Moholkar, V.S., Senthilkumar, P., Pandit, A.B., 1999. Hydro-dynamic cavitation for sonochemical effects. Ultrason.Sonochem. 6, 53.

Moss, W.C., Young, D.A., Harte, J.A., Levatin, J.L., Rozsnyai,B.F., Zimmerman, G.B., Zimmerman, I.H., 1999. Com-puted optical emissions from sonoluminescing bubble. Phys.Rev. E 59, 2986.

Mujumdar, S., Pandit, A.B., 1998. Study of catalytic isomerisa-tion of maleic acid to fumaric acid: effect of ultrasound.Ind. Chem. Engr. 40, 187.

Mujumdar, S., Senthilkumar, P., Pandit, A.B., 1997. Emulsifi-cation by ultrasound: effect of intensity on emulsion qual-ity. Ind. J. Chem. Technol. 4, 277.

Naidu, D.V., Rajan, R., Gandhi, K.S., Kumar, R., Chan-drasekaran, S., Arakeri, V.H., 1994. Modelling of batchsonochemical reactor. Chem. Eng. Sci. 49, 877.

Oba, R., Ikohagi, T., Ito, Y., Miyakura, H., Sato, K., 1986.Ž .Stochastic behaviour randomness of desinent cavitation.

Trans. ASME J. Fluid Engg. 108, 438.Pandit, A.B., Joshi, J.B., 1993. Hydrolysis of fatty oils: effect of

cavitation. Chem. Eng. Sci. 48, 3440.


Pandit, A.B., Gogate, P.R., Mujumdar, S., 2000. UltrasonicDegradation of 2-4-6 Trichlorophenol in Presence of TiO2catalyst. Ultrasonics Sonochemistry Accepted for publica-tion also presented at ESS7 May 14�17 Toulose France.

Petrier, C., Luche, J.L., 1987. Ultrasonically improved reduc-tive properties of an aqueous Zn�NiCl system I: selective2reduction of �-� unsaturated carbonyl compounds. Tetra-hedron Lett. 28, 2347.

Petrier, C., Gemal, A.L., Luche, J.L., 1982. Ultrasound inorganic synthesis 3: a simple high yield modification of theBouveault reaction. Tetrahedron Lett. 23, 3361.

Petrier, C., Luche, J.L., Dupuy, C., 1984. Ultrasound in or-ganic synthesis 6: an easy preparation of organozincreagents and their conjugate addition to �-enones. Tetra-hedron Lett. 25, 3463.

Petrier, C., Jeanet, A., Luche, J.L., Reverdy, G., 1992a. Unex-pected frequency effects on rate of oxidative processesinduced by ultrasound. J. Am. Chem. Soc. 114, 3148.

Petrier, C., Micolle, M., Merlin, G., Luche, J.-L., Reverdy, G.,1992b. Characteristics of pentachlorophenate degradationin aqueous solutions by means of ultrasound. Environ. Sci.Technol. 26, 1639.

Petrier, C., Lamy, M-F., Francony, A., Benahcene, A., David,B., Renaudin, V., Gondrexon, N., 1994. Sonochemicaldegradation of phenol in dilute aqueous solutions: compar-ison of the reaction rates at 20 and 487 kHz. J. Phys. Chem.98, 10514.

Save, S.S., Pandit, A.B., Joshi, J.B., 1994. Microbial cell disrup-tion: role of cavitation. Chem. Engg. J. 55, B67.

Save, S.S., Pandit, A.B., Joshi, J.B., 1997. Use of hydrodynamiccavitation for large scale cell disruption. Chem. Engg. Res.Des. 75, 41.

Senthilkumar, P., 1997. Studies in Cavitation: Modelling andScaleup of Cavitational Reactors. M. Chem. Engg. ThesisUniv. of Mumbai, Mumbai, India.

Senthilkumar, P., Pandit, A.B., 1999. Modelling hydrodynamiccavitation. Chem. Engg. Technol. 22, 1017.

Senthilkumar, P., Sivakumar, M., Pandit, A.B., 2000. Experi-mental quantification of chemical effects of hydrodynamiccavitation. Chem. Engg. Sci. 55, 1633.

Serpone, N., Terzian, R., Hidaka, H., Pelizzetti, E., 1994.Ultrasonic induced dehalogenation and oxidation of 2-, 3-and 4-chlorophenol in air-equilibrated aqueous media:similarities with irradiated semiconductor particulates. J.Phys. Chem. 98, 2634.

Seymore, J., Gupta, R.B., 1997. Oxidation of aqueous pollu-tants using ultrasound: salt induced enhancement. Ind.Eng. Chem. Res. 36, 3453.

Shah, Y.T., Pandit, A.B., Moholkar, V.S., 1999. CavitationReaction Engineering. Plenum Publishers USA.

Shirgaonkar, I.Z., Pandit, A.B., 1997. Degradation of aqueoussolution of potassium iodide and sodium cyanide in pres-ence of carbon tetrachloride. Ultrason. Sonochem. 4, 245.

Shirgaonkar, I.Z., Pandit, A.B., 1998. Sonophotochemical de-struction of aqueous solution of 2,4,6 trichlorophenol. Ul-trason. Sonochem. 5, 53.

Shirgaonkar, I.Z., Lothe, R.R., Pandit, A.B., 1998. Commentson the mechanism of microbial cell disruption in highpressure and high speed devices. Biotechnol. Prog. 14, 657.

Sivakumar, M., Pandit, A.B., 2000a. Hydrodynamic CavitationAssisted Rhodamine B Degradation: A TechnologicallyViable Waste Water Treatment Technique. Presented atthe Int. conf. on Science and Technology October 12�13New Delhi India.

Sivakumar, M., Pandit, A.B., 2000b. Ultrasound enhanceddegradation of rhodamine B: optimization with power den-sity. Ultrasonics Sonochemistry Accepted for publication,also presented at ESS 7 Toulose France.

Sivakumar, M., Tatake, P.A., Pandit, A.B., 2000. Kinetics ofp-nitrophenol degradation: effect of reaction conditionsand cavitational parameters for a multiple frequency sys-tem. Chem. Engg. Tech. Forwarded for publication.

Sivakumar, M., Pandit, A.B., 2001. Wastewater treatment: anovel energy efficient hydrodynamic cavitational technique.Ultrason. Sonochem. Forwarded for publication.

Sochard, S., Wilhelm, A.M., Delmas, H., 1998. Gas vapourbubble dynamics and homogenous sonochemsitry. Chem.Eng. Sci. 53, 239.

Storey, B.D., Szeri, A.J., 1999. Mixture segregation withinsonoluminescence bubbles. J. Fluid Mech. 396, 203.

Storey, B.D., Szeri, A.J., 2000. Water vapour, sonolumines-cence and sonochemistry. Proc. R. Soc. Lond. A 456: inpress.

Suslick, K.S., Mdleeni, M.M., Reis, J.T., 1997. Chemistry in-duced by hydrodynamic cavitation. J. Am. Chem. Soc. 119,9303.

Suslick, K.S., Didenko, Y., Fang, M.F., Hyeon, T., Kolbeck,K.J., McNamara, W.B., Mdleleni, M.M., Wong, M., 1999.Acoustic cavitation and its chemical consequences. Phil.Trans. R. Soc. Lond. A 357, 335.

Tatake, P.A., Vichare, N.P., Pandit, A.B., 1999. Effect ofTurbulence in Hydrodynamic Cavitation. Presented atCHEMCON-99 Chandigarh India.

Thoma, G., Swofford, J., Popov, V., Som, M., 1997.Sonochemical destruction of dichloromethane and o-di-chlorobenzene in aqueous solution using a nearfield acous-tic processor. Adv. Env. Res. 1, 178.

Thompson, L.H., Doraiswamy, L.K., 1999. Sonochemistry: Sci-ence and Engineering. Ind. Eng. Chem. Res. 38, 1215.

Toy, M.S., Carter, M.K., Passell, T.O., 1990. Photosonochemi-cal decomposition of aqueous 1,1,1-trichloroethane. Envi-ron. Technol. 11, 837.

Trabelsi, F., Ait-Lyazidi, H., Ratsimba, B., Wilhelm, A.M.,Delmas, H., Fabre, P.-L., Berlan, J., 1996. Oxidation ofphenol in wastewater by sonoelectrochemistry. Chem. Engg.Sci. 51, 1857.

Tullis, J.P., Govindrajan, R., 1973. Cavitation and size scaleeffect for orifices. J. Hydraul. Div. HY13, 417.

Vichare, N.P., Gogate, P.R., Pandit, A.B., 2000a. Optimizationof hydrodynamic cavitation using a model reaction. Chem.Engg. Technol. 23, 683.

Vichare, N.P., Senthilkumar, P., Moholkar, V.S., Gogate, P.R.,Pandit, A.B., 2000b. Energy analysis in acoustic cavitation.Ind. Eng. Chem. Chem. 39, 1480.

Von Sonntag, C., Mark, G., Tauber, A., Schuchmann, H.P.,1999. OH radicle formation and dosimetry in the sonolysisof aqueous solutions. Adv. Sonochem. 5, 109.

Weavers, L.K., Ling, F.H., Hoffmann, M.R., 1998. Aromatic


compound degradation in water using a combination ofsonolysis and ozonolysis. Environ. Sci. Technol. 32, 2727.

Wilhelm, A.R., Knoop, P.V., 1979. Wet air oxidation � analternative to incineration. Chem. Eng. Prog. 75, 46.

Yan, Y., Thorpe, R.B., 1990. Flow regime transitions due tocavitation in flow through an orifice. Int. J. MultiphaseFlow 16, 1023.

Yan, Y., Thorpe, R.B., Pandit, A.B., 1988. Cavitation Noiseand its Suppression By Air in Orifice Flow Proc. of the Int.Symp. on Flow Induced Vibration and Noise Chicago USAAmerican Soc. of Mechanical Engineers, 25�40.

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