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Ultrasonics Sonochemistry 15 (2008) 903–908

A parametrical study of disinfection with hydrodynamic cavitation

S. Arrojo *, Y. Benito, A. Martınez Tarifa

Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, Avda Complutense 22, 28040 Madrid, Spain

Received 2 February 2007; received in revised form 26 October 2007; accepted 3 November 2007Available online 9 November 2007

Abstract

The physical and chemical conditions generated by cavitation bubbles can be used to destroy microorganisms and disinfect wastewa-ter. The effect of different cavitation chamber designs and diverse operational parameters on the inactivation rate of Escherichia coli havebeen studied and used to understand the mechanisms involved in cell disruption.� 2007 Elsevier B.V. All rights reserved.

Keywords: Escherichia coli; Hydrodynamic cavitation; Ultrasonic cavitation; Disinfection; Parametrical; Venturi; Orifice plates

1. Introduction

Disinfection constitutes an essential step in water treat-ment for public water supplies. After Pasteur and Kochformulated the germ theory of disease, in 1881 Koch him-self discovered the bactericidal properties of chlorination.It was the beginning of the disinfection technologies.

The use of alternative technologies for disinfectionintends to overcome the weak points of the conventionalmethods. Chemical biocides are usually effective and com-paratively cheap, but can generate dangerous or inconve-nient organic by-products (especially in chlorination). Onthe other hand, physical technologies tend to be moreexpensive, and UV-based techniques are inefficient wheneither turbidity or colorants are present due to a blockingeffect which inactivates or reduces the efficiency of theirradiation.

Cavitation acts as a biocide through chemical (genera-tion of OH� radicals [1]) and through physical mechanisms(shock waves, pressure gradients, shear forces, etc. [2]). Thepredominant mechanism depends on the cavitation pro-cess. Low frequencies tend to generate more violent col-lapses, producing strong shock waves and gas phasereactions, but the low number of collapses per unit time

1350-4177/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ultsonch.2007.11.001

* Corresponding author.E-mail address: sarrojo@gmail.com (S. Arrojo).

reduces the speed of chemical reactions and the diffusionof �OH radicals, particularly in the liquid phase [3]. Onthe other hand, high frequencies generate small and lessenergetic bubbles, but also promote OH� radical diffusionand produce a higher number of collapses per unit time.

High intensity sonicators are efficient at both low [4] andmedium frequencies [5] but low intensity ultrasonic cavita-tion (UC) reactors (i.e. cavitation baths) usually workpoorly [6]. Although cavitation has been demonstrated tobe neither cheaper nor more efficient than conventionaltechnologies [7], there are some interesting applicationswhich ought to be studied.

Many authors have proposed using cavitation for disin-fection as a pre-treatment rather than as a treatment itself.Phull et al. found that the effectiveness of chlorination sig-nificantly improves in combination with ultrasound [7].Ozonization, often ruled out due to the comparatively highprice of ozone, also improves its performance (reducing therequired quantities of ozone) when applied in combinationwith either ultrasonic [8] or hydrodynamic cavitation [9,10].The synergic effects only take place when ultrasound isapplied either during or before chlorination [7]. The useof UC also improves the biocidal performance of UV irra-diation [11] as well as that of hypochlorite [12].

Bacteria tend to form agglomerates in which the exter-nal microorganisms act as a protective barrier against bio-cides. Cavitation produces a declumping effect, breaking

904 S. Arrojo et al. / Ultrasonics Sonochemistry 15 (2008) 903–908

the agglomerates through the shock waves and isolating theindividual bacteria. Initially, the declumped agglomeratesgive rise to an apparent increase in the bacteria populationduring the first stages of cavitation. Once the bacteria clus-ters are broken the efficiency of biocides increases [13].

1.1. Hydrodynamic cavitation for disinfection

Hydrodynamic cavitation (HC) has also been studiedfor disinfection purposes. Save et al. first observed the dis-ruption of yeast cells using HC. According to some authorsthe energy efficiency of cell disruption in HC is at least oneorder of magnitude above some established physical tech-niques such as mixer–blender methods, high pressurehomogenizers and even UC [14,15]. Nevertheless, the com-parative costs of chemical methods such as chlorination oreven ozonization are orders of magnitude lower [16].

In previous works by the author, �OH radicals weredetected under similar experimental conditions (designsand flow-rate) as those used in the present work [17]. Theseresults were obtained using radical traps of salicylic acid,and demonstrated that HC works as a very low frequencyultrasonic reactor, generating big bubbles, large pressureshock waves and �OH radicals. Since gas phase reactionscan be ruled out as a possible cause of disinfection, it isexpected that the inactivation of microorganisms in HCtakes place mainly through the mechanical effects of theshock waves. However, other authors have observed liquidphase reactions in HC [18], and therefore the effect of OH�

radicals can not be entirely neglected.Whilst in UC the mechanisms can be easily studied by

changing the applied frequency and intensity, a completeparametrical study in HC requires changing both the oper-ating conditions and the cavitation chamber design. In thepresent work, various designs and operating conditions arestudied in order to gain some understanding of the maindisruption mechanisms which take place around the cavi-tating bubbles.

1.2. Physical and chemical effects of HC

The physical and chemical effects caused by HC are usu-ally difficult to characterize. The physical effects of cavita-tion (i.e. cavitation intensity) depend critically on theinertial forces during the collapse of the bubbles. Theoret-ical studies reveal that the inertial effects and therefore thecavitation intensity increase dramatically with the parame-ter Rmax/R0 (maximum radius reached by the bubble overinitial bubble radius) [19] and with the speed of pressurerecovery [20]. Thus, promoting bubble growth and fastpressure recovery gives rise to higher cavitation intensity.On the other hand, in order to evaluate the chemical effectsgenerated by cavitation in the liquid phase the diffusion of�OH radicals has to be taken into account. Diffusion isfavored by smaller bubbles and faster collapses (i.e. orificeplates), so the optimum configurations for chemical andphysical effects in HC are not necessarily similar.

By studying the degradation of substances with differentvolatility in HC [21], it was observed that the diffusion of�OH radicals to the liquid phase has a strong dependenceon the time scales of the pressure pulse (i.e. the pressurerecovery rate in HC). Faster pressure recovery rates leadto faster collapses, allowing some �OH radicals to bereleased in the liquid phase before recombining or reactingwith other scavengers. In fact, whilst volatile substancesdegrade faster with those designs which generated big bub-bles and dense cavitation clouds (e.g. Venturi configura-tions) the degradation of non-volatile substances isfavored by designs which also promote fast pressure recov-ery rates (e.g. multiorifice configurations) [21]. This is con-sistent with the aforementioned frequency studies in UC[3].

The physical effects of cavitation can be characterizedthrough the instantaneous pressure oscillations caused bythe bubble cloud implosion or analogously through thesound generated by the phenomenon [22]. On the otherhand, as aforementioned, the �OH generated in the processcan be estimated using radical traps such as salicylic acidsolutions [17].

1.3. HC versus UC for disinfection purposes

Disinfection processes using UC have shown similartrends to those of liquid phase sonochemical reactions,obtaining maximum degradation rates at similar frequen-cies (around 200 kHz) and being affected by chemical fac-tors such as the gas content of the bubbles [5].Nevertheless, unlike in liquid phase sonochemical reactionsthere seems to be little difference between low and high fre-quency experiments. This is consistent with the combinedeffect of �OH radicals and shock waves in disinfection, aslower frequencies promote more violent shock waves andhigher frequencies favor the diffusion of �OH radicals tothe liquid phase.

The few references addressing a direct comparisonbetween HC and UC indicate that whilst UC-baths andsonicators give rise to faster disinfection rates, HC showsbetter energy efficiency and works at larger scales [23,24].In any case, most of the referenced papers conclude thatthe best alternatives in terms of efficiency and energy con-sumption are those based on hybrid methods, using combi-nations of HC or UC with chemical biocides.

2. Experimental procedure

As shown in Fig. 1, HC is generated by circulating theliquid in a cavitation loop with the following characteris-tics: a 60 L tank, a 9 kW centrifugal multistage pump madeof stainless steel, PVC pipes and a cavitation chamber withflexible design. The cavitation chamber, where HC takesplace, has a rectangular cross-section and consists of twostainless steel beams, 2 methacrylate windows and PVCprofiles which act as either Venturi tubes or orifice plates.The instantaneous pressure measurements were performed

Pump

Flow-meter

Cavitation chamber

Tank

Fig. 1. Scheme of the cavitation loop.

0 20 40 60 80 100 120

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

E.C

oli c

once

ntra

tion

Time [min]

Fig. 2. Normalized E. coli concentration versus cavitation time for a25 · 1 orifice plate (triangles) and Venturi #3 (circles).

S. Arrojo et al. / Ultrasonics Sonochemistry 15 (2008) 903–908 905

using pressure transducers at a rate of 1 · 104 data readingsper second.

Escherichia coli (E. coli) were grown in 250 mL of anutritive medium (5 g/L peptone + 1 g/L glucose + 2.5 g/L yeast) at 37 �C during a period of 48 h, reaching a finalconcentration of around 1 · 107 CFU/mL (colony formingunit per mL of wastewater). The bacteria were diluted in50 L of saline solution (1 g/L of NaCl in deionized waterto prevent cytolysis) to attain the desired CFU/mL valuefor each experiment. All the experiments were run with apower input (pump) of 5 kW (power density of 0.1 W/mL). The CFU counting was performed in agar plates after48 h at 37 �C using three different dilution rates (1:10, 1:100and 1:1000), and collecting six samples per experiment (0,5, 15, 30, 60 and 120 min).

2.1. Cavitation chamber design

The cavitation chamber design has a strong influence onthe cavitation process. Orifice plates, a relatively commonconfiguration in HC devices [25], are probably the simplestway to generate HC, and basically consist of a plate withone or more holes. The three configurations used in thisstudy have approximately the same overall free cross-sec-tion (2.0 · 10�5 m2) distributed in a different number ofholes: one hole with a diameter of 5 mm (1 · 5), six holeswith a diameter of 2 mm (6 · 2) and 25 holes with a diam-eter of 1 mm (25 · 1). On the other hand, the Venturi con-figurations are based on a smooth convergence followed bya throat and a smooth divergence. In general terms, thismeans a more energy efficient process, and larger timescales (bigger bubbles and slower collapses) than the mul-tiorifice plates. The three configurations used in this studyhave a minimum cross-section (gorge) of 4 · 10�5 m2 (Ven-turi 1), 2 · 10�5 m2 (Venturi 2) and 1 · 10�5 m2 (Venturi#3) and a divergence angle of 10�.

3. Results and discussion

The inactivation capacity of HC was studied varyingthree important parameters: the cavitation chamber design,the discharge pressure and the concentration of microor-ganisms. First, the six different designs (three orifice platesand three Venturi tubes) described above were tested. In asecond stage, the best orifice plate and the best Venturi

tube were operated with different discharge pressures.Finally, the best configuration was studied with three dif-ferent concentrations of E. coli.

Fig. 2 shows two examples of the evolution of the E. coli

concentration during a HC process. The declumping effectof bacteria agglomerates was observed in most experimentsthrough a first stage of CFU increase which lasted around30 min for the orifice plates and 5 min for the Venturi con-figurations (unlike the other Venturis, Venturi #3 showedno apparent increase).

Fig. 3 shows the rate constant of bacterial inactivationduring an experiment of 120 min assuming that the disin-fection follows a pseudo-first order reaction (i.e. consider-ing the disruption process as a reaction between the E.coli and the disinfectant agent in which only the formerchanges its concentration with time). The behavior of Ven-turi tubes seems to be much better than that of the orificeplates. The first reason for these observations is that fora given power input the flow-rate in the Venturi tubes ishigher than that of the orifice plates, causing a larger num-ber of cavitation cycles per unit time. Nevertheless, the dif-ference in the flow-rate of each configuration (in the orderof 30% for Venturi 1 and almost negligible for Venturi 3)does not entirely explain the observed differences in disin-fection rates.

The flow-rate in the orifice plates is similar for every case(around 4 m3/h), so the flow speed and the minimum pres-sure are also similar. Nevertheless, as the number of holesincreases and their size decreases, the inertia of the gener-ated jets also decreases. As a result, the jets decelerate morerapidly, and the pressure recovery rate increases. Someauthors have observed a maximum chemical degradationusing smaller holes in multiorifice plates [18]. Arguably,as mentioned above, increasing the pressure recovery rate(analogous to a frequency increase in UC) accelerates thecollapse of the bubbles promoting diffusion of OH� radicalsto the liquid phase. Thus, the improved results in the orifice

OP1x5 OP6x2 OP25x1 Vent.1 Vent.2 Vent.30.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

Rat

e of

E.C

oli i

nact

ivat

ion

[min

-1]

Cavitation chamber design

Fig. 3. Rate of E. coli inactivation for different orifice plates and Venturitubes (initial concentration of 104 CFU/mL and discharge pressure of1.5 bar).

906 S. Arrojo et al. / Ultrasonics Sonochemistry 15 (2008) 903–908

plates with smaller holes could be explained, at least tosome extent, as a result of the improved OH� radicals diffu-sion to the liquid phase.

Apart from increasing the flow-rate for a given powerinput, Venturi tubes tend to give rise to denser cavitationclouds (more cavitation events per unit time and largerbubbles) due to an increase in the available time for bubblegrowth. Under these circumstances, bubble–bubble inter-actions and excessive bubble size might hinder both the for-mation of OH� radicals and their diffusion to the liquidphase [26]. In fact, the few studies made on this subjectindicate that a moderate bubble cloud density combinedwith a fast pressure recovery (i.e. multiorifice plates) opti-mize the chemical reactions outside of the bubble [21,27].Thus, the increase in disinfection rates in Venturi tubesdoes not seem to be caused by �OH radical mechanisms.

OP1x5 OP6x2 OP25x1 Vent.1 Vent.2 Vent.30.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

SD o

f ins

tant

aneo

us p

ress

ure

[Bar

]

Cavitation chamber design

Transducer 1 Transducer 2 Transducer 3

Fig. 4. Mean amplitude of the instantaneous pressure oscillation at threeconsecutive pressure transducers located next to the imploding bubblecloud (5 cm separation between transducers).

Fig. 4 shows the mean amplitude of the instantaneouspressure oscillation measured by three pressure transducerslocated in the flow, next to the imploding bubble cloud. Asaforementioned, the instantaneous oscillation of pressurecan be used to measure the intensity of the imploding bub-bles during cavitation, and so it can be considered as anestimation of the shock waves which give rise to themechanical cell disruption. The transducer #1 (stripes) islocated immediately after the minimum cross-section (i.e.the holes in the orifice plates and the throat in the Venturitubes), the transducer #2 (grey) is located 5 cm down-stream and the transducer #3 (dots) is located 10 cm down-stream of the first transducer. Although multiorifice platesshow larger localized intensities immediately after the ori-fices, the cavitation is clearly concentrated around the firsttransducer. In the Venturi tubes the mechanical effects ofcavitation are extended up to the third transducer (or evenfurther away), indicating a dense cavitation cloud or a largenumber of cavitation events. The chemical reactions gener-ated by cavitation increase exponentially with the cavita-tion intensity, and so an intense localized cavitationwould generally be more efficient than an extended lowerintensity cavitation.

The results obtained in Figs. 3 and 4 reveal someinteresting correlations between cell disruption ratesand instantaneous pressure oscillations. Venturi #3 givesrise to the best results, consistent with the observed largepressure shock waves in transducers 2 and 3 (i.e. cavita-tion intensity). On the other hand the comparison ofVenturi #1 and OP 25 · 1 reveals that E. coli are moreaffected by an extended lower intensity cavitation thanby a more intense localized cavitation. Although thisobservation seems to contradict the general theory ofsonochemical reactions, it is consistent with a processin which the mechanical effects of the shock waves aremore important than the chemical mechanisms with�OH radicals.

3.1. Discharge pressure

Previous studies on disinfection with HC and orificeplates reveal an increasing efficiency with larger dischargepressures [23]. As shown in Fig. 5, multiorifice platesdepend critically on this parameter, and there is even athreshold value under which no disinfection takes place.The behavior of Venturi configurations is quite different.At higher discharge pressures the effect is rather the con-trary. It must be pointed out that increasing the dischargepressure promotes larger and faster pressure recoveries,and therefore increases the violence of bubble collapse.Nevertheless, it also causes a higher minimum pressurehindering bubble growth and decreasing the number ofcavitation events. Moreover, for a given energy inputthe flow-rate decreases and so does the number of hydro-dynamic cycles per unit time. Pressure recovery in orificeplates is very inefficient due to the abrupt divergence afterthe holes. Therefore, it is common to compensate this sit-

OP 25x1 Venturi 30.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

Rat

e of

E.C

oli i

nact

ivat

ion

[min

-1]

Cavitation chamber design

Outlet pressure 1 bar Outlet pressure 1.5 bar Outlet pressure 2 bar

Fig. 5. Rate of E. coli inactivation for the orifice plate 25 · 1 and theVenturi number 3, with a discharge pressure of 1 bar (diagonal stripes),1.5 bar (grey) and 2 bar (horizontal stripes).

OP 25x1 Venturi 30.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

rate

of E

.Col

idis

rupt

ion

Cavitation chamber design

103 CFU/mL 104 CFU/mL 105 CFU/mL

Fig. 6. Normalized rate of E. coli inactivation for the orifice plate 25 · 1and the Venturi number 3, with initial bacteria concentration of 103 CFU/mL (stripes), 104 CFU/mL (grey) and 105 CFU/mL (points).

S. Arrojo et al. / Ultrasonics Sonochemistry 15 (2008) 903–908 907

uation by increasing the discharge pressure. On the otherhand, pressure recovery in Venturi tubes is very efficientand therefore the discharge pressure is just used as ameans for controlling excessive bubble growth. As previ-ously explained, the �OH radical diffusion to the liquidphase is almost negligible in Venturi tubes, but not inmultiorifice plates. The results presented in Fig. 5 are con-sistent with this hypothesis. Higher discharge pressurespromote the disinfection through �OH radicals in multio-rifice plates by increasing the violence of bubble collapse,but it hinders bubble growth and mechanical effects inVenturi configurations due to the lower number of col-lapsing bubbles. In other words, the �OH radical disinfec-tion is promoted by concentrating the energy (even at theexpense of the overall collapsing volume) whilst themechanical effects of cavitation increase with the numberof cavitation events (even at the expense of individualbubble collapse violence).

3.2. Initial CFU concentration

Three different experiments were carried out varying theconcentration of E. coli. Fig. 6 shows the normalized inac-tivation rate constants (one is equivalent to the maximumattained rate constant for each configuration). The rateconstant is moderately reduced in the orifice plate as theconcentration of E. coli increases, whilst it stays approxi-mately the same for the Venturi #3 configuration. Theseresults again are consistent with the previous hypothesis.In orifice plates a portion of disinfection is related to the�OH generation and therefore, as the concentration of E.coli increases, the radical concentration acts as the limitingfactor of the disinfection process. On the other hand, thebehavior of the Venturi configuration remains almost unaf-fected, indicating that there is no limiting reactant and thatthe mechanical disruption of bacteria plays a major role inthe process.

4. Conclusions

Disinfection in cavitation is caused by both chemicaland physical cell disruption mechanisms. Whilst in UCthe chemical processes caused by �OH radicals seem to playa major role, theoretical predictions and experimentalobservations have indicated that in HC, with compara-tively slow pressure oscillations (low frequency), disinfec-tion is mainly caused by mechanical disruption ofbacteria. Thus, the disinfection rates are maximized bythose configurations and operation parameters which pro-mote large bubbles, extended pressure oscillations and alarger number of cavitation events (i.e. those conditionsfound in the Venturi tubes).

The results obtained in HC are competitive againstother physical methods but on the other hand, results aresignificantly worse than those obtained with chemical dis-infectants. Nevertheless, there are some advantages, suchas avoiding the problems associated with the use andmanipulation of chemicals, and the independence of thebacterial concentration and the efficiency of the process.

Thus, although the study of disinfection processes in HCcan be interesting from a scientific point of view, theauthors consider that applied research should concentrateon studying wastewater with very large bacterial concentra-tions or on the synergic effects of this technology withchemical biocides.

Acknowledgements

This project has been funded by the Spanish governmentand the CIEMAT. The authors would also like to thankMiguel Angel Crespo Aguirre for his collaboration duringthis work.

908 S. Arrojo et al. / Ultrasonics Sonochemistry 15 (2008) 903–908

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