Development In Ultrasound - Non Medical

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Progress in Biophysics and Molecular Biology 93 (2007) 166–175 www.elsevier.com/locate/pbiomolbio

Review

Developments in ultrasound—Non-medical Timothy J. Mason School of Science and the Environment, Sonochemistry Centre, Faculty of Health and Life Sciences, Coventry University, Coventry, CV1 5FB, UK Available online 4 August 2006

Abstract Ultrasound is defined as sound of a frequency that is too high for the human ear to detect—i.e. it is inaudible. Nevertheless this ‘‘silent sound’’ has a large range of applications in science, medicine and industry. The study of the effects of ultrasound on materials—known as sonochemistry—is one of the broadest and most exciting areas in current research. In this review some recent developments with major potential are identified from the fields environmental protection and materials processing. Environmental protection can refer to methods of preventing pollution or to the removal of existing pollution. Here we will look at examples drawn from the latter in which ultrasound has been used for the purification of water (chemical and biological), the decontamination of the atmosphere and soil remediation i.e. the classic three domains of water, air and land. In terms of materials processing two examples have been chosen, the treatment of sewage sludge and the control of crystallisation. In both of these cases it is predominantly the mechanical effects of acoustic cavitation, which produce the enhanced digestion, and dewatering of sludge and provide for the control in crystallisation processes. r 2006 Elsevier Ltd. All rights reserved. Keywords: Cavitation; Sonochemistry; Environmental protection; Processing

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Ultrasound in environmental protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 2.1. Water remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 2.1.1. Removal of biological contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 2.1.2. Removal of chemical contamination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 2.2. Air cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 2.3. Land remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Some examples of ultrasonic processing on an industrial scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 3.1. Treatment of sewage sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 3.1.1. Anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 3.1.2. Dewatering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 3.2. Crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

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E-mail address: [email protected]. 0079-6107/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pbiomolbio.2006.07.007

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1. Introduction Over the last few years the major advances in ultrasonic technology have followed two pathways. The first of these has been in therapeutic ultrasound which has included the uses of focused ultrasound in cancer treatment and the manipulation of living cells in an acoustic field to facilitate the transfer of drugs or genes across the cell membrane (ISTU, 2001). Cell manipulation is heading rapidly towards successful experiments in gene therapy and requires tiny carefully designed reactors with well-defined static acoustic fields operating in the MHz range. The second pathway involves the scaling up of promising laboratory trials involving environmental protection and process technology to deal with volumes which are industrially viable.In between, of course, there are still legions of scientists working with conventional laboratory systems in attempts to open up new areas of applications for ultrasound. 2. Ultrasound in environmental protection 2.1. Water remediation 2.1.1. Removal of biological contamination Some species of bacteria produce colonies and spores, which agglomerate in spherical clusters (e.g. Bacillus subtilis). The use of a biocide can destroy microorganisms on the surface of such clusters but often leaves the innermost bacteria intact. Flocs of fine particles e.g. clay can entrap bacteria which can also protect them against disinfection (Mir et al., 1997). Due to these problems alternative methods of purifying water are being investigated and amongst these the application of ultrasound is proving to be of considerable interest. Ultrasound is able to inactivate bacteria, make them more susceptible to biocides and/or deagglomerate bacterial clusters or flocs depending upon the power and frequency applied through a number of physical, mechanical and chemical effects arising from acoustic cavitation (Table 1). The effects of a range of ultrasonic frequencies (20, 38, 512, 850 kHz), acoustic power and exposure time on bacterial kill have been reported (Joyce et al., 2003). Results showed a significant increase in percent kill for Bacillus species with increasing duration of exposure and intensity of ultrasound in the low-kilohertz range (20 and 38 kHz). Results obtained at two higher frequencies (512 and 850 kHz) indicated significant bacterial declumping. In assessing the bacterial kill with time under different sonication regimes three types of behaviour were characterized:

  

High power ultrasound (lower frequencies) in low volumes of bacterial suspension results in a continuous reduction in bacterial cell numbers i.e. the kill rate predominates. High power ultrasound (lower frequencies) in larger volumes results in effective declumping of the bacteria giving an initial rise in cell numbers but this initial rise then falls as the declumping finishes and the kill rate becomes more important. Low intensity ultrasound (higher frequencies) gives an initial rise in cell numbers as a result of declumping. The kill rate is low and so there is no significant decrease in bacterial cell numbers.

In conjunction with chlorination (Duckhouse et al., 2004) a remarkable frequency effect has been noted. At the lower frequency the improvement in biocidal activity is greatest when the ultrasound is applied at the same time as the hypochlorite. At the higher frequency of 850 kHz the improvement is best when ultrasound is used Table 1 The effects of ultrasonic irradiation on bacteria Decreasing acoustic energyCell destruction

Production of free radicals in water

Temporary cell wall weakening Increasing frequency-

Increased mass transfer to cell

Disruption of suspended cell clumps

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as a pre-treatment immediately followed by hypochlorite addition under normal (silent) conditions. The kill rate achieved for pre-treatment using 850 kHz and simultaneous treatment using 20 kHz are very similar. However, the former involves less acoustic energy and so is considered to be the more efficient. A study has been made of the potential for the use of ultrasonic irradiation to prevent or control algal blooms in eutrophic water (Inman, 2004). A series of experiments have been performed on a laboratory scale, at pilot scale and at full-scale. Laboratory experiments identified loss of buoyancy as the major cause of growth inhibition in blue-green algae (Cyanophyceae) using ultrasonic irradiation at 40 kHz and 40 W. On the other hand a pilot scale experiment based on the growth of diatoms (Bacillariophyceae), blue-green algae and green algae (Chlorophyceae) found that at low power (20 W) ultrasound with a frequency of 28 kHz reduced growth in diatoms by ca. 60% and green algae by ca. 41% but this caused an increase in blue-green algae by ca. 67%. This increase in blue-green algae concentration may have been caused by a decrease in the formation of colonies due to mild sonication allowing increased nutrient uptake. The results indicate that the mechanisms by which ultrasonic irradiation influence algal growth vary between species and shows that the optimum ultrasonic frequency and intensity for control of algae may be species dependent. A full-scale experiment was carried out using a floating 40 W ultrasound source operating at a frequency of 28 kHz. The device was used to test the effects of ultrasonic irradiation on two (1  control, 1  experimental) parallel sedimentation lagoons (300 m2 each) situated in southeast England that were chosen because of their identical size, throughput, and location. Samples were collected weekly from three areas within each lagoon and combined to represent one ‘‘lagoon’’ sample. A second sample was collected from the outflow of each lagoon after sedimentation to represent one ‘‘final effluent’’ sample. Each sample was analysed for chlorophyll ‘a’, total number of algal cells, and cell counts of individual algal species. The results showed a statistically significant reduction in total algal growth measured by chlorophyll ‘a’ concentration in the experimental lagoon. An alternative method of generating cavitational energy is via the pumping of water through a small orifice. As the pressurised water emerges into a larger volume under lower pressure the expansion produces hydrodynamic cavitation and the type of transducer used is commonly referred to as a liquid whistle (Fig. 1). The efficiency of such a devices was assessed in 1960 when a series of experiments was undertaken to compare four methods then in common usage for the emulsification of mineral oil, peanut oil and safflower oil (Singiser and Beal, 1960). The results proved that a homogeniser, which operated via a liquid whistle, was superior to three other types of apparatus namely a colloidal mill and two types of sonicator, one of which employed a quartz crystal and the other a barium titanate transducer. An obvious benefit of the liquid whistle is that it can be used for flow processing and can be installed ‘‘on-line’’ and the method has several advantages.

  

The system was developed, and is ideal for, such processes as emulsification, homogenisation and dispersion. It can be used for the processing of flow systems and, as such, has immediate possibilities for scale-up. With no moving parts, other than a pump, the system is rugged and durable.

Fig. 1. Schematic diagram of a whistle reactor generating hydrodynamic cavitation.

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In more recent years considerable progress has been made in the design of flow systems in which the hydrodynamic cavitation can be controlled by adjusting the process parameters (flow rate, pressure and orifice size). The high Reynolds flow conditions allow for intense micromixing of the reactants, which can be an advantage in the synthesis of metastable support phases. A very important aspect of this type of processing is that it can be scaled up easily to allow commercial processing. One area that has proved particularly important has been the use of such systems to synthesize advanced catalysts and nanostructured materials (Moser et al., 2001). A device developed at Coventry University has shown promising results when applied to the decontamination of water (Cavisys, 2005). Suspensions of E. coli produced from cultures grown in nutrient broth were incubated overnight at 37 1C, dispersed into 300 l water in a tank and then circulated through the device. The results are shown in Fig. 2 in which the concentration of bacteria is seen to drop significantly over a period of 1 h. 2.1.2. Removal of chemical contamination 2.1.2.1. Using biological techniques. An attractive proposition for water remediation is the use of biological material (whole organisms or enzymes) for the removal of chemical contamination since this would provide a natural and inexpensive catalytic process. An obvious drawback might be that the contaminant could be poisonous to the biological catalyst but nevertheless some successes have been reported. Among these is one concerning the use of horse radish peroxidase (HRP) for chlorophenol destruction (Entezari et al., 2006). A comparison was made between the removal of 2-chlorophenol from aqueous solution using three methodologies: stirring in the presence of an enzyme (HRP), sonication and a combination of both methods under the same conditions (Fig. 3). It would appear that using the enzyme (with a concentration 0.165 unit/ml) removes approximately 70% of pollutant from solution in 60 min. Sonication alone can remove 90% of the pollutant in the same time period. However, in the combination method, the pollutant is almost completely removed in about 30 min. The success of the combined sonication and biological degradation was explained through a combination of effects which include (a) a possible improvement to the efficiency of the enzyme action through the enhancement of the diffusion processes (b) a possible change in the structure of the enzyme, leading to a change in the availability of the active centre and (c) through the production of hydroxyl radicals via cavitation which can then react with the intermediate molecules produced by the enzyme. It is also possible that this last effect might also reduce the enzyme inactivation. The rate of degradation exhibits pseudo-first order behaviour and the combination method was more effective than either sonolysis or enzyme treatments applied separately. 2.2. Air cleaning The inhalation of airborne particles is now recognized as a serious public health concern. Such fine particles originate in the emissions associated with carbon-fired power plants, cement factories, the chemical industry and diesel-powered vehicles have increasingly become the focus of stricter government regulations. The ideal solution to the problem is to stop these emissions at source but current filters and electrostatic precipitators

Fig. 2. Reduction in concentration of E. coli over 60 min using a circulating system involving a liquid whistle.

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Fig. 3. Comparison between stirring, sonication and combined of both methods for the removal of 2-chlorophenol from water. (31 1C; 2chlorophenol ¼ 0.1 mM; H2O2 ¼ 1 mM; Power ¼ 15 W; HRP ¼ 0.165 unit/ml; K, Ultrasound; J, Enzyme; ., ultrasound+enzyme).

have problems in coping with the smallest particles. Therefore, there is a need for a process by which particles can be agglomerated to larger particles before being submitted to conventional separation technologies. Over a period of many years it has been shown that airborne acoustic energy in the ultrasonic frequency range could be used to precipitate suspended particles (aerosol or smoke) (Hoffmann, 2000). In the past, the most common transducers for use in gases were aerodynamic systems of various kinds, such as whistles and sirens, in which the acoustic energy was provided by a gas jet. Despite the fact that it is possible to generate large acoustic powers with these devices, their efficiencies are low and the sounds emitted are over a range of frequencies. In addition to this they have poor directivity and also have some difficulty in reaching ultrasonic frequencies (Greguss, 1964). In order to obtain the efficient generation and transmission of acoustic energy into gases it is necessary to use ultrasonic generators with specifications that include good impedance matching with the gas, large amplitude of vibration, high directional or focused radiation and high power capacity. In recent years, Gallego et al. (1994) have been developing applications for a new type of sonic and ultrasonic power generator for airborne ultrasound that can achieve a much more efficient energy transmission. The structure of the transducer is schematically shown in Fig. 4. It consists essentially of a vibrating circular plate with a stepped profile driven at its centre by a piezoelectrically activated vibrator. The ribbed surface of the plate produces high powers and provides the vibrating system with good impedance matching with the medium. The profile of the radiating plate allows good control of the vibration amplitude and the radiation pattern in such a way that very directional or focused radiation can be obtained. A simple flat plate radiator vibrating in its flexural modes generally provides poor directivity due to phase cancellation. Using a radiating plate of 35 cm in diameter sound pressure levels of about 165 dB (ca. 3 W/cm2) have been recorded at a distance of 33 cm from the centre of the plate, when a maximum power of 150 W is applied to the transducer. Using this type of device airborne ultrasound has been used for both the precipitation of airborne powders and defoaming. In one configuration this stepped plate system operating at 20 kHz with a high-intensity standing wave field (160 dB) was introduced inside a cylindrical chamber of 2 m in length and 0.22 m inside diameter for smoke precipitation (Gallego et al., 1999). Precipitation tests on polydispersed carbon black smoke particles (mean radius ¼ 0.5 mm; mass loading ¼ 4–14 g/m3) gave a mass collection efficiency of 93% under static gas-flow conditions. These collection efficiencies were achieved with an energy consumption of approximately 3 kWh/ 1000 m3. The ultrasonic chamber was then investigated as a pre-conditioning agglomerator/precipitator when placed upstream of a conventional collector system (cyclone, electrostatic filter, scrubber, etc.). The main advantage with this new configuration was that for a given agglomeration efficiency, higher flow rates could be

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Fig. 4. Schematic diagram of a power transducer designed for airborne applications.

used. Dynamic tests were performed to assess the degree of particle agglomeration as a function of acoustic intensity and treatment time. An increase of one order of magnitude in the average size of the particles remaining in the aerosol at the chamber exit was obtained (initial mean radius ¼ 0.3 mm; final mean radius ¼ 4.8 mm). These results confirmed the feasibility of the acoustic system developed as an acoustic preconditioner of fine particles to facilitate their collection by conventional separators at lower energy consumptions. More recently, Riera et al. (2003) reported the influence of humidity on the ultrasonic agglomeration and precipitation of submicron particles in diesel exhaust. The experimental pilot scale plant involved a 97 kW diesel engine, an ultrasonic agglomeration chamber, a dilution system, a nozzle atomizer, and an aerosol sampling and measuring station. The effect of the ultrasonic treatment, generated by a linear array of four high-power stepped-plate transducers (operating frequency 21 kHz) on fumes at flow rates of 900 Nm3/h, was to produce a small reduction in the number concentration of particles at the outlet of the chamber of the order of 25%. However, the agglomeration rate was raised up to 56% by an increase in humidity (0.06 kgwater/kggas). The results showed the benefit of using high-power ultrasound together with an increase in humidity to enhance the agglomeration of particles much smaller than 1 mm. 2.3. Land remediation For contaminated soil wastes the currently available options for management and disposal are principally:

  

Permanent storage in a secure landfill. This will result in a permanent retained liability by the waste generator. Incineration in a permitted waste incinerator. This is costly and entails the risk of atmospheric emissions. Soil washing to produce bulk soil with low-level contamination. However, the washing process itself will produce a volume of solvent that must be treated before disposal.

For many years ultrasound has been considered as a technology to promote the process of soil washing and if subsequent disposal of the washings was considered at all this was perhaps to be a separate treatment. An integrated system has been developed in Canada (by Sonic Environmental Solutions Inc.) for large scale continuous processing using acoustic frequencies in the audible range that incorporates the clean-up of the washings and recycling of the solvent. The equipment itself affords vibrational amplitudes considerably larger than those available using ultrasound and it has proved to be particularly efficient for the removal and destruction of PCB contaminants in soils (Mason et al., 2004). The equipment generates vibrational energy

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through the use of resonant bending modes in a large cylindrical steel bar (SESI, 2004). The bar is driven into a cloverleaf type of motion by firing six powerful magnets (three at each end of the bar) in sequence. The bar is supported by air springs so that the ends and the centre are then caused to rotate at a resonance frequency depending on its size (Fig. 5a). One such unit, operating at a power of 75 kW, drives a bar that is 4.1 m long and 34 cm in diameter at its resonance frequency of 100 Hz. The bar weighs 3 tonnes and produces an amplitude of vibration at each end of 6 mm. For the washing of soils a mixing chamber is rigidly mounted on each end of the bar and these are used in three process areas: PCB extraction, PCB destruction and solvent recovery (Fig. 5b). Table 2 shows some typical results from the clean up of PCB contaminated site in British Columbia, Canada. The 2 ppm PCB level in the soil residue (cleaned soil) is significant since this is emerging as an international standard for unconfined disposal. It is also, for practical purposes, the limit of detection for dirty sample analysis by soxhlet extraction and gas chromatography with an electron capture detector. The PCB extracted into the solvent is destroyed in solution and the solvent is then recycled. The use of the 75 kW generator for pilot testing has proved that this process can be achieved at a commercial scale of around 3–4 tonnes of soil/hour. 3. Some examples of ultrasonic processing on an industrial scale 3.1. Treatment of sewage sludge In biological wastewater treatment large quantities of biomass (sludge) are produced. Treatment of such material requires several processes that include digestion, settling and dewatering. The overall treatment procedure is referred to as stabilisation since the sludge material will continue to degrade with the evolution of noxious gases under ambient conditions and must be stopped (stabilised) before it can be used or disposed of. 3.1.1. Anaerobic digestion This is the standard stabilization technique for sludge and results in a reduced content of organic matter. The slow biological sludge hydrolysis is the rate-limiting step of the anaerobic degradation. One of the major

Fig. 5. (a) Schematic of the vibrating beam and (b) photograph of mixing chamber.

Table 2 Typical product PCB contents (mg/kg) Test conditions

Feed soil (ppm)

Soil residue (ppm)

0.2 kW/L, 105 min 0.2 kW/L, 105 min 2.0 kW/L, 105 min

700 700 550

o2 o0.4 o2

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effects of power ultrasound on micro-organisms is the breakdown of cell walls and subsequent release of cellular material. This makes the dissolved organic compounds more readily available in the anaerobic digestion process and the fermentation rate is greatly improved (Neis, et al., 2001). The same group have also demonstrated sonication is capable of improving the settling of sludge through the mechanical effect of cavitation. The shear forces developed on bubble collapse can remove filamentous material which supports the buoyancy of the sludge. 3.1.2. Dewatering One of the later stages of sewage treatment is dewatering of the sludge. This is generally a slow and difficult process since the water is not only trapped between particulate material but is also held within the cellular material itself. The requirement to isolate the solid from its mother liquor is an important factor in sewage treatment since it results in a reduction in the overall mass and facilitates disposal either in land fill or by incineration (Pirkonen, 2001). The requirement to remove suspensions of solids from liquids is common to many industries. This separation can be either for the production of solids-free liquid or to isolate the solid from its mother liquors. Conventionally, membranes of various sorts have been employed for these processes ranging from the simple filter pad through semi-permeable osmotic type membranes to those that are used on a size-exclusion principle for the purification of polymeric materials. Unfortunately, the conventional methodologies often lead to ‘‘clogged’’ filters and, as a consequence, there will always be the need to either replace filters or stop the operation and clean them on a regular basis. The application of ultrasound enables the filtration system to operate more efficiently and for much longer periods without maintenance (Tarleton and Wakeman, 1997). Different effects seem to play a role in the mechanism of dewatering. First of all, the alternating acoustic stresses produce a kind of ‘‘sponge effect’’ which facilitates the migration of moisture through natural channels created between solid particles. Small liquid droplets retained inside the capillaries can be removed by acoustic stresses if they become greater than the surface stresses. The air bubbles trapped in micropores can grow by rectified diffusion and produce the displacement of the liquid out of these micropores. Finally, cavitation is a powerful effect that can separate the colloidal and chemically attached moisture from the solid phase. 3.2. Crystallization Ultrasound can be used to influence crystallization. In conventional crystallization techniques a solution containing materials to be crystallized is super-saturated either by cooling or by evaporation and is then seeded. The problem with seeding is that it may be initiated non-uniformly and this can result in crystal growth proceeding at different rates at different nuclei sites. The resulting crystals may then show an unwanted broad and uneven crystal size distribution. It is also of considerable practical importance to be able to control the onset of crystallization in a large-scale production process because this often occurs in an uncontrolled manner simply due to a slight change in external factors such as a temperature or pressure fluctuation. Ultrasound has proved to be extremely useful in crystallization processes since it can initiate seeding and control subsequent crystal growth in a saturated or supercooled medium (McCausland and Cains, 2003). This is thought to be due to cavitation bubbles themselves acting as nuclei for crystal growth and to the disruption of seeds/nuclei already present within the medium thus increasing the number of nuclei present in the medium. Through the correct choice of sonication conditions it is possible to produce crystals of a uniform and designated size that is of great importance in pharmaceutical preparations. Power ultrasound also has an additional property which is particularly beneficial in crystallization operations namely that the cleaning action of the cavitation effectively stops the encrustation of crystals on cooling elements in the crystallization vat and thereby ensures continuous efficient heat transfer. A group in the UK (Accentus C3) has produced a flow reactor that is capable of generating high intensities with an electrical power rating of 1500 W (Fig. 6) (Accentus, 2005). This reactor has since been developed to a large scale industrial system which is said to suffer no cavitational damage to the walls while providing high-energy insonation to the contained liquid. In February 2005 this system was installed for continuous sonocrystallization in Europe’s largest alumina production facility at Aughinish Alumina,

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Fig. 6. (a) Accentus C3 Modular flow cell unit showing arrangement of transducers, (b) U shape arrangement of flow cells as used in alumina production.

Askeaton, Ireland. The process operates continuously at 20 kHz and 10 bar at 70 1C using a slurry in 6 M NaOH with a throughput of 70 tons per hour. 4. Conclusions The uses of power ultrasound in chemistry and the processing industries (sonochemistry) is an expanding field of study that continues to thrive on outstanding laboratory results that have even more significance with the availability of the types of scale-up systems used in processing. Compared with the past, there is now far greater contact and co-operation between the scientific disciplines interested in the effects of cavitation and through this we can predict a rosy future for sonochemistry. The research and development will progress both from the point of view of a greater interest in the fundamental principles of cavitation action and in the development of international programmes in applied research and technology.

References Accentus, 2005. Information supplied by Accentus C3 Technology plc, B551, Harwell International Business Centre, Didcot, Oxfordshire, OX11 0QJ, UK. Cavisys, 2005. Information supplied by Cavisys, Coventry University Science Park, Coventry CV1 5FB UK. Duckhouse, H., Mason, T.J., Phull, S.S., Lorimer, J.P., 2004. The effect of sonication on microbial disinfection using hypochlorite. Ultrasonics Sonochem. 11, 173–176. Entezari, M.H., Mostafai, M., Sarafraz-Yazdi, A., 2006. A combination of ultrasound and a Bio-catalyst: removal of 2-chlorophenol from aqueous solution. Ultrasonics Sonochem. 13, 37–41. Gallego, J.A., Rodriguez, G., San Emeterio, J.L., Montoya, F., 1994. Electroacoustic unit for generating high sonic and ultrasonic intensities in gases and interphases, USA, Patent no 5299175. Gallego, J.A., Riera, E., Rodrı´ guez, G., Hoffmann, T.L., Ga´lvez, J.C., Rodrı´ guez, J.J., Go´mez, F.J., Bahillo, A., Martı´ n, M., 1999. Application of acoustic agglomeration to reduce fine particle emissions from coal combustion plants. Environ. Sci. Technol. 33, 3843–3849. Greguss, P., 1964. The applications of air-borne and liquid-borne sounds to industrial technology. Ultrasonics 2, 5–10. Hoffmann, T.L., 2000. Environmental implications of acoustic aerosol agglomeration. Ultrasonics 38, 353–357. Inman, D.A., 2004. Ultrasonic treatment of algae. MSc Thesis, Cranfield University. ISTU, 2001. A new group was founded in 2001 for the promotion of research and development in all areas of therapeutic ultrasound: the ‘‘International Society for Therapeutic Ultrasound’’. Joyce, E., Phull, S.S., Lorimer, J.P., Mason, T.J., 2003. The development and evaluation of ultrasound for the treatment of bacterial suspensions. A study of frequency, power and sonication time on cultured bacillus species. Ultrasonics Sonochem. 10, 315–318.

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Mason, T.J., Collings, A., Sumel, A., 2004. Sonic and ultrasonic removal of chemical contaminants from soil in the laboratory and on a large scale. Ultrasonics Sonochem. 11, 205–210. McCausland, L.J., Cains, P.W., 2003. Sonocrystallisation—using ultrasound to improve crystallisation products and processes. Chem. Ind., 15–17. Mir, J., Morato, J., Ribas, F., 1997. Resistance to chlorine of freshwater bacterial strains. J. Appl. Microbiol. 82, 7–18. Moser, W.R., Find, J., Emerson, S.C., Krausz, I.M., 2001. Engineered synthesis of nanostructured materials and catalysts. Adv. Chem. Eng. 27, 1–48. Neis, U., Nickel, K., Tiehm, A., 2001. Ultrasonic disintegration of sewage sludge for enhanced anaerobic biodegradation. In: Mason, T.J., Tiehm, A. (Eds.), Advances in Sonochemistry, vol. 6. Elsevier, Amsterdam, pp. 59–91. Pirkonen, P., 2001. Ultrasound in filtration and sludge dewatering. In: Mason, T.J., Tiehm, A. (Eds.), Advances in Sonochemistry, vol. 6. Elsevier, Amsterdam, pp. 181–220. Riera, E., Elvira, L., Gonzalez, I., Rodriguez, J.J., Munoz, R., Dorronsoro, J.L., 2003. Investigation of the influence of humidity on the ultrasonic agglomeration of submicron particles in diesel exhausts. Ultrasonics 41, 277–281. SESI, 2004. Information supplied by Sonic Environmental Solutions Inc., Suite 1778 West 2nd Avenue, Vancouver, British Columbia, V6J 1H6, Canada. Singiser, R.E., Beal, H.M., 1960. Emulsification with ultrasonic waves. J. Am. Pharm. Assoc. (Scient. Ed.) 49, 482–488. Tarleton, E.S., Wakeman, R.J., 1997. Ultrasonically assisted separation processes. In: Povey, M.J.W., Mason, T.J. (Eds.), Ultrasound in Food Processing. Thomson Science, London, pp. 193–218.