* Corresponding author: john.h.gibson@ec.gc.ca

23

A Literature Review of Ultrasound Technology and Its Application in Wastewater Disinfection

John H.Gibson,1* Darrell Hai Nien Yong,2 Ramin R. Farnood,2 and Peter Seto1

1 Environment Canada, Aquatic Ecosystems Research Divison (AEMRD) 867 Lakeshore Road., P.O. Box 5050, Burlington, ON L7R 4A6

2 University of Toronto, Department of Chemical Engineering and Applied Chemistry, Toronto, ON M5S 3E5

In recent years, there has been an increase in the application of ultraviolet (UV) light as an alternative to chemical disinfection technologies. However, in the case of poor quality effl uents, the practical limit of UV disinfection of wastewater is dictated by disinfection-resistant, particle-associated bacteria. Although these particles may be removed by fi ltration, an alternative method to reduce the impact of suspended particles on disinfection effi ciency is to decrease particle size using ultrasound technology. Mechanical forces exerted on particles due to the collapse of cavitation bubbles created by sonication break suspended particles into small fragments. In this paper, a critical review of ultrasound application for wastewater treatment is presented with emphasis on disinfection. Much of the work in this area remains at the laboratory scale. As a result, there is a need for fundamental information regarding the effect of sonication on the kinetics of disinfection and interaction of ultrasound with suspended particles. Such information is necessary for process engineering, design, and scale-up of ultrasound systems.

Key words: ultrasound, disinfection, wastewater, ultraviolet, cavitation, chlorine

Introduction

The potential for biological contamination of receiving waters due to the discharge of poor quality effl uents remains a concern for public health. To address this issue, strict disinfection requirements have been implemented to reduce the concentration of pathogenic microorganisms in wastewater. Chlorination has the advantages of low cost, relatively easy application and, from the point of view of the plant operations, low power requirement. However, chlorine can also form persistent toxic by-products. These disinfection by-products have been added to the Canadian Environmental Protection Act’s list of toxic substances. Chlorine is particularly unsuitable for wet weather pollution and urban runoff because of the abundance of organic precursor molecules. In order to address the by-product issues related to chemical disinfection, there has been an increase in the application of ultraviolet (UV) light as an alternative disinfection method. UV disinfection has the advantage that no detrimental by-products have been detected in the treated effl uent (U.S. EPA 1986). UV is power intensive, especially for effl uents with low UV transmittance, so that a large number of UV lamps are required to achieve the disinfection targets. The practical limit of UV disinfection of wastewater is often determined by disinfection-resistant, particle-associated bacteria (Qualls and Johnson 1985; Emerick et al. 1999; Loge et al. 2002; Blume and Neis 2005). The presence of suspended particles in effl uent increases the UV dose demand and

restricts the application of UV disinfection in turbid particle-rich wastewaters. Since ultrasound is known to disperse particles, ultrasound may be a chemical-free way to increase the operating range of UV disinfection. This paper provides an overview of ultrasound technology and its application in wastewater disinfection. A brief review of UV disinfection and the fundamentals of ultrasound technology are provided. Following this, there is a detailed review of quantitative methods used to study ultrasound. Opportunities for synergistic interactions are explored by providing a detailed review of the chemical, physical, and biological effects of ultrasound. Finally a review of current literature on the combined effects of ultrasound with chlorine and UV light will be presented.

UV Disinfection

Dose-response curves can be generated by exposing the sample of interest to different amounts of disinfectant and measuring the concentration of surviving organisms. The characteristics of the typical dose-response curves emphasize the importance of particles in wastewater disinfection. Dose-response curves for the UV disinfection of particle-free organisms often follow fi rst order kinetics:

Water Qual. Res. J. Can. 2008 · Volume 43, No. 1, 23-35Copyright © 2008, CAWQ

N/No = exp (-kD) (1a)

where N is the number of surviving viable organisms after exposure to disinfectant, No is the initial number of viable organisms, k is the fi rst order rate constant, and D is the UV dose defi ned by:

D = I × t (1b)

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Here, I is the average UV intensity and t is the exposure time. This expression is similar for chemical disinfectants where the dose, in this case, is the product of the disinfectant concentration and time (Metcalf and Eddy 2003). Based on equation 1.a, an increasing disinfectant dose decreases the microbial population exponentially. The fi rst order rate constant (k) depends on the viability and resistance of the target organism. Table 1 provides a short list of typical UV disinfection rate constants for several organisms. Two commonly observed deviations from fi rst-order kinetics are shouldering and tailing (Farnood 2005). Shouldering is a plateau in the dose-response curve at low doses, while tailing refers to a decrease in the slope of the dose-response curve at higher doses (Fig. 1). Both can result from the presence of wastewater particles. Shouldering can be caused by the existence of small clumps of target organisms. This phenomenon can be described by modifying equation 1.a to account for the clumping of organisms:

N / No = 1 – [1 – exp(-kD)] m (2)

where m is the average number of target organisms per clump. In the tailing region of dose-response curves, increasing UV dose has only a marginal effect on the survival of the remaining microorganisms. In the case of UV disinfection of secondary effl uents, UV resistance has been shown to be closely related to the concentration of particles larger than a critical size (Darby et al. 1993; Emerick et al. 1999; Loge et al. 2002). Emerick et al. (2000) developed a general model that describes tailing based on the number of UV disinfection-resistant particles. In this model, the population of target microorganisms is split into two subpopulations: a disperse population (Nd) and a UV disinfection-resistant population (Np). In most cases, the vast majority of countable bacteria are in the disperse fraction so that it can be approximated simply by the initial bacterial count (No). This gives:

N / No = e-k D + ß / kD (1 – e-kD) (3)

where k is the disinfection rate constant, and ß is the fraction of disinfection-resistant organisms (i.e., Np/No). An example of predictions of this model for different values of ß are shown in Fig. 2. In this model, the same inactivation rate constant (k) is used for both disperse and UV disinfection-resistant organisms. The disperse population includes the individual free-swimming bacteria and those associated with particles smaller than about ten microns in diameter. Emerick et al. (1999, 2000) suggested that particles of this size are small enough for UV light to penetrate and disinfect them at a rate comparable to individual bacteria.

Based on the above discussions, any method that can disperse, increase light penetration, or reduce the size of particles has the potential to correct tailing and shouldering.

Fig. 1. Schematic illustration of tailing and shouldering effects in UV dose-response curves.

Fig. 2. Normalized UV dose-response curves as predicted by Emerick’s model for various values of ß (k = 0.46 cm2/J).

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Ultrasound for Wastewater Disinfection

Ultrasound

Ultrasound is the transmission of sound energy in the form of pressure waves at frequencies above the limit of human hearing, about 16 kHz. The fl ux of ultrasound power (Ps) over a given area (A) is termed “acoustic intensity of ultrasound” (I). For a plane wave that is not diverging, if the intensity is doubled, the amplitude of the pressure variation (p) increases by a factor of four. This relationship is shown by the following equation:

(4)

Here ρ is the liquid density, ε is the ultrasound amplitude, ω is the angular velocity (ω = 2πf where f is the frequency), c is the speed of sound, and Z is the acoustic impedance defi ned as ρ×c. Commonly used equations for the attenuation, refl ection, and scattering of sound by different materials are shown in Table 2. At 20 kHz, the attenuation coeffi cient of water is about 2.0×10-7 cm-1. Only about 2% of the ultrasound energy is attenuated over a path length of 1 km in water. For ultrasound at 20 kHz (λ = 7.4 cm), the Rayleigh scattering cross section of a typical wastewater particle (2 to 200 μm) is extremely small. Scattering due to particles contributes little to the overall ultrasound attenuation in wastewater. Air-water interfaces such as bubbles are good refl ectors of ultrasound. This is due to the 4 orders of magnitude difference in acoustic impedance between air and water. Bubbles attenuate sound transfer. The impedance difference between water and biological tissues is small, approximately 5%. In this case, the water and cell are coupled so that most of the sound energy will be transmitted through the cell. As

a fi rst approximation, the ultrasonic heating of a two-micron spherical cell will produce a temperature rise of only a few thousandths of a degree above the surrounding water (Cavicchi 1985). Overall, ultrasound alone has little direct impact on suspended material in wastewater.

Ultrasonic Cavitation

Since an ultrasound wave itself has little effect on particles in wastewater, the main interactions must be associated with ultrasonic cavitation. Cavitation is the process of formation, growth, and violent collapse of cavities in a liquid. Acoustic cavitation can be thought of a means to concentrate the energy of a uniform sound fi eld at a fi nite number of locations. The energy release during a cavitation can be quite large. Bubble collapse conditions can create localized temperatures from 2,000 to 5,000K, and pressures from 1,800 to 3,000 atm (Brennen 1995). Conditions inside the bubble are suffi cient to split water to produce hydroxyl radicals (sonochemistry) or excite gases inside the bubble to emit light (sonoluminescence). When a bubble collapses near a solid surface, a jet with a velocity of up to 300 m/s is formed (Lauterborn and Bolle 1975). This localized high-speed jet could mechanically damage nearby solid surfaces. At some distance away from the site of bubble collapse, other physical and mechanical effects such as acoustic shockwaves, microstreaming, and emission of sound and light are observed. These effects could contribute to disinfection. By observing the dynamics of collapse of a 30-μm bubble, Didenko and Suslick (2002) estimated the fractional conversion of the potential energy of a collapsing bubble to chemical reactions and light emission. Fractional conversion is the ratio of the energy observed in the stated reaction to the calculated potential energy of the bubble before collapse. As summarized in

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Table 3, their results suggest that the energy diverted to sonoluminescence is two orders of magnitude less than the energy transformed to sonochemical processes. Furthermore, 99.992% of the potential energy of the bubble is associated with neither sonochemistry nor sonoluminescence. Therefore, the main effects of cavitation on the surrounding environment must be dominated by heating and mechanical effects.

Nucleation Theory

Wastewater contains many different types of particles. These particles can impact how the water responds to ultrasound. To initiate cavitation, one must overcome the surface tension of the liquid. The critical negative pressure required to expand a bubble of size R in a liquid with vapour pressure Pv and surface tension S is given by:

Pcrit = Pv - 2S/R (5)

Note that at the same vapour pressure, large bubbles are easier to expand than smaller ones. When a bubble is attached to a solid surface, the magnitude of the surface tension holding the bubble is a function not only of the surface tension, but also of the contact angle. The magnitude of the surface tension preventing bubble growth on a solid is proportional to the sine of the contact angle, θ:

S’ = 2S·sinθ/R (6)

Pcrit = Pv - S’ = Pv – 2S·sinθ/R (7)

Hydrophilic substances are believed to produce a contact angle that prevents bubble growth (i.e., Fig. 3, case A: the angle approaches 180 degrees, and sinθ approaches zero). In contrast, hydrophobic substances are thought to facilitate nucleation, in part due to their relatively shallow angle of attachment (Fig. 3, case B). When a bubble is associated with a sharp crevice, the contact angle remains large and the effective radius is small, almost regardless of the material properties (Fig. 3, case C). Bubbles stabilized in crevices are believed to be the main initiators of cavitation in natural waters (Brennen 1995).

Fig. 3. Effect of contact angle and surface defects on surface nucleation process.

Energy Flow in Ultrasound

Ultrasonic cavitation effi ciency depends on several energy conversion steps. The energy conversion pathway for ultrasound can be described as follows (Löning et al. 2002):

EEL → EHF → ETH → ECAV → EDOS→ EEFF (8)

where EEL is the electrical energy input, EHF is the high frequency energy from the ultrasound generator, ETH is power input into the fl uid, ECAV is energy of the cavitating bubbles, EDOS is the energy determined by chemical dosimetry, and EEFF is the energy spent on the desired effect. Electrical energy (EEL) can be measured using a power meter. High frequency energy (EHF) output of the ultrasound generator can be determined by using an oscilloscope (Löning et al. 2002). Calorimetry (ETH) is commonly used to estimate the energy input to the fl uid (Kimura et al. 1996; Kuijpers 2002; Koda et al. 2003; Margulis and Margulis 2003; Sá ez et al. 2005). According to Faid et al. (1998), the initial rate of temperature increase (dT/dt)0, varies linearly with the temperature difference. The rate of temperature increase over a fi nite interval can be used to approximate the rate of energy transfer:

ETH = mCp(dT/dt)0 ≈ mCp(ΔT/Δt) (9)

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Ultrasound for Wastewater Disinfection

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For an ultrasonic horn with piezoelectric transducers, the overall effi ciency of conversion of electrical energy (EEL) to high frequency vibration (EHF) and then thermal energy (ETH) in the fl uid is typically about 75%, with 85% effi ciency for each of the two stages. This conversion effi ciency remains essentially constant across a wide range of outputs and amplitudes, and over a wide range of operating temperatures (Löning et al. 2002). The measurement of energy converted into cavitation (ECAV), dose (EDOS), and fi nally, the desired result (EEF), can be slightly more complicated. Table 4 shows some methods that can be used. These can be categorized into four main groups:

Measures of primary ultrasound effects (1. ECAV);Measures of secondary chemical effects (2. EDOS, chemical dosimetry);Measures of secondary mechanical effects (3. EDOS or EEF, physical dosimetry); Measures of secondary biological effects (4. EEF , biodosimetry).

Chemical Effects of Ultrasound

Table 5 provides a summary of current research into sonochemistry. In general, the rate of sonochemical reaction is improved by:

Increasing ultrasound frequency;1. Increasing concentration of dissolved gases;2. Addition of salt. 3.

However, the extent of these effects depends on the nature of the sonochemical reaction, reactor geometry, and liquid characteristics. The chemistry of ultrasound has been widely studied.

Mechanical Effects of Ultrasound

A summary of research conducted regarding the mechanical effects of ultrasound are provided in Table 6. These studies show that:

The mechanical effects of ultrasound, such as 1. particle disruption, are more pronounced at lower ultrasound frequencies;Long-chain polymers may be degraded by the 2. mechanical action of ultrasound, and the degradation rate is larger for polymers with higher molecular mass;Sonochemical and sonomechanical effects are not 3. necessarily correlated, that is, higher sonochemical effects may not translate to larger sonomechanical effects. This shows that the mechanical effects of ultrasound may operate through different pathways than the chemical effects;Mechanical effects can include cell lysis and loss of 4. cell viability.

There is comparatively little information about the mechanical effects of ultrasound, especially on wastewater particles. Our lab-scale experiments show

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Ultrasound for Wastewater Disinfection

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that a 50% reduction in 100-μm wastewater particles requires from 400 to 1,500 J/L of ultrasound energy. It has been observed that larger fl ocs tend to break more easily and as such require less power (Jarvis et al. 2005). In comparison, typical medium pressure UV equipment requires from 87 to 200 J/L of water treated, while advanced oxidation treatments used in the destruction of industrial organic substances by UV typically require at least one order-of-magnitude larger power (Stefan and Bolton 2002). Although other methods such as hydrodynamic cavitation, high speed mixing, or emulsifi cation may be used for particle breakage, these methods are based on the bulk liquid movement and hence, compared with sonication, are more energy intensive. Moreover, ultrasound offers a more attractive option since it is compact and does not impose an excessive pressure drop.

Biological Effects of Ultrasound

High intensity ultrasound is known to result in the inactivation of bacteria. However, direct disinfection using ultrasound is prohibitively energy intensive. It has been reported that more than 30 minutes of sonication with a delivered power density of 350 W/L was required for one-log inactivation of Escherichia coli 0157:h7 serotype (Dehghani 2005). At sublethal levels, ultrasound induces stress in bacteria that results in various bacterial expressions and responses. For instance, Vollmer et al. (1998) found that E. coli exhibited several different types of stress response, including membrane damage, ammonia starvation, heat shock, and oxidative damage after fi ve minutes of sonication with 1 MHz ultrasound (Table 7). In this study, promoter sequences were fused to the bioluminescent luxCDABE in order to measure the increase in activity of certain genes before and after ultrasound treatment. The bacterial response seems to be dominated by a heat shock response. This is consistent with the fi ndings of other researchers who saw that, for relatively long treatment times, bacterial inactivation is dominated by heating effects (Madge and Jensen 2002). The results presented in Table 7 are consistent with the known effects of high-frequency ultrasound, that is, membrane damage from microstreaming, oxidative damage from sonochemistry, and UV damage

from sonoluminescence. It may be that the bacterial microenvironment serves to concentrate these effects. Much of the previous research is based on the implicit assumption that, when the same ultrasound power is applied, the same cavitation conditions will be created. However, Turai et al. (1980) suggest that this is not always the case. They argue that a hydrophobic cell surface is more likely to form dislocations at the water interface, increasing cavitation activity. It may be the interaction between biological factors, such as cell hydrophobicity, and physical factors, like power density, that determine the ultimate disinfection effi ciency. Sublethal doses of ultrasound may also decrease virulence of bacterial pathogens. Kochemasova et al. (1983) have shown that pilae, needed for bacterial attachment, were removed by ultrasound. Changes in colony forming units may overestimate the actual virulence of ultrasound-treated cells (Villarino et al. 2003). Ultrasound has long been known to immobilize motile water organisms. This fact has been used at one of the few full-scale applications of ultrasound in water treatment to treat up to 4,800 m3/h of drinking water prior to entering sand fi lters (Hoyer and Clasen 2002). Zooplankton immobilization provides a chemical free means to improve sand fi lter performance.

Combined Ultrasonic Disinfection

The chemical, mechanical, and biological effects of ultrasound can contribute to the disinfection process either by direct inactivation of organisms, or by enhancing the effects of other disinfectants. Much of the ultrasonic energy that is transferred to the liquid has the potential to enhance disinfection. Heating alone, in extreme cases, could lead directly to disinfection of pathogens. Cavitation creates shear forces and microjets which are known to disrupt particles. Smaller particles are more susceptible to UV and chemical treatment. Shear forces may cause cell lysis, resulting in disinfection. Reactive chemical species that are formed during the cavitation process could damage cell walls and assist the disinfection process. Sublethal effects can reduce pathogen virulence or resistance to other disinfectants. Using ultrasound in combination with conventional disinfectants may create opportunities for synergistic interactions to increase the overall disinfection rate.

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Ultrasound and Chemical Disinfection

Ultrasound treatment may be useful in increasing the activity of both traditional (e.g., chlorine) and alternative (e.g., hydrogen peroxide) chemical disinfectants. Research in this area is summarised in Table 8. Based on the data presented in Table 8, ultrasound provides the most synergy if applied simultaneously with the chemical disinfecting agent. Effects are more pronounced when more suspended solids are present. Ultrasound can both accelerate chemical disinfection and slow it by degassing. Observed synergies when combining chemical disinfection with ultrasound are likely due to a combination of several factors. Many chemical reactions are accelerated in the presence of ultrasound, especially at high frequencies. Also, dispersion of bacterial fl ocs into individual cells allows better exposure to the disinfecting agent, reducing both tailing and shouldering effects. Finally, mass transfer of the disinfectants to the cells is enhanced by turbulence caused by ultrasonic cavitation.

Ultrasound and Ultraviolet Disinfection

The performance of UV disinfection systems is adversely affected by the presence of microbial aggregates. The mechanical effects of ultrasound have the potential to disperse these aggregates and therefore improve the disinfection kinetics and decrease the UV dose demand. The combined use of ultrasound prior to UV disinfection was fi rst explored by Oliver and Cosgrove (1975). Neis and Blume (2003a) studied the effects of ultrasound followed by UV disinfection of secondary effl uents. After sonication for 20 seconds at 20 kHz, the volume fraction of particles larger than 50 microns was reduced from more than 60% to less than 5%. This improved subsequent UV treatment effi ciency, resulting in a 60% decrease in the total power requirement when compared with UV alone. The target effl uent concentration was 400 colony forming units (CFU) per mL in this laboratory-scale experiment. Wu et al. (2005) studied the disinfection of E. coli associated with small iron oxide particles (0.2 to 2 μm).

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Iron oxides are often added in wastewater treatment for phosphorous precipitation. UV dose demand was found to be proportional to the degree of association of E. coli with iron oxide particles. Treatment with ultrasound to disperse the iron oxide particles improved the subsequent UV dose-response by 1.5-log units. The relationship between changes in the particle size distribution due to sonication, and the resulting improvement in the disinfection effi ciency of primary effl uent has been studied by Yong (2007). In these experiments, a low intensity ultrasound bath (0.6/cm2) was used to sonicate a fi xed volume of sample for various periods of time (0 to 300 s). It was found that the decrease in the number of large particles (>60 μm) due to sonication was strongly related to the reduction in the tailing level of the UV dose-response curve. In addition, the disinfection rate constant increased with increasing the sonication time, indicating synergy in the combined process. The effect of sonication on the effi ciency of UV disinfection was found to depend on the type of effl uent. At the same level of ultrasound treatment, the improvement in UV disinfection varied in the following order: activated sludge effl uent < primary effl uent < trickling fi lter effl uent. Joyce et al. (2006) investigated the simultaneous use of ultrasound and UV to improve the disinfection of fecal coliform. In their experiments, a UV reactor was located inside an ultrasound bath with an ultrasound power density of 0.05 W/cm3. They observed that with no fl ow through the UV reactor, no appreciable disinfection was achieved even after 1 hour of UV irradiation (Table 9). Applying ultrasound without bulk fl ow through the reactor caused a signifi cant improvement in overall disinfection; total inactivation was achieved in 50 minutes. Applying UV with fl ow and without ultrasound further reduced the inactivation time to 40 minutes. And fi nally, using ultrasound and UV simultaneously with fl ow through the UV reactor reduced the time for inactivation to 30 minutes. This is another example of potential synergy due to increased exposure of bacteria to UV light. An important practical consideration in ultrasonic pretreatment of wastewater is the potential for a decrease in the UV transmittance of effl uent. However, Yong (2007) showed that the decrease in UV transmittance for secondary effl uents is insignifi cant. In contrast, for primary effl uents, the UV transmittance decreased by as much as 30% after sonication. Despite the relatively large energy demand of ultrasound pretreatment, there may be situations where the overall disinfection cost would be lower for a combined ultrasound-UV system. For example, ultrasound is not affected by turbidity or fouling while UV disinfection effi ciency is strongly affected by UV transmittance. However, research into ways to improve the effi ciency of ultrasonic cavitation is required to increase the applicability of this technology.

Summary and Conclusions

The chemical effects of ultrasound with and without chlorine have been widely studied. Increased disinfection rates have been observed when combining ultrasound with chemical disinfectants like chlorine. This improvement likely is the result of improved chemistry, the effect of particle size reduction, as well as increased mixing and rate of diffusion of disinfectant. There is comparatively little information about the mechanical effects of ultrasound on wastewater particles. Laboratory tests have shown that relatively low ultrasound doses can dramatically affect particle size, especially of large particles (Neis and Blume 2003b). Mechanical forces exerted on particles due to the collapse of cavitation bubbles created by sonication could break suspended particles into small fragments. This is particularly important since suspended particles in wastewater harbour pathogens, making disinfection more costly. A reduction in particle size will improve disinfection through reduced shouldering and tailing effects. In addition, the sublethal effects of ultrasound may decrease pathogen virulence and motility. Much of the research into ultrasound remains at the laboratory scale. The effectiveness of ultrasound in improving disinfection will depend not only on the ultrasonic cavitation process, but also on the effl uent quality and the complex and variable nature of fl oc structure. Further research is required to gain a better understanding of these interactions, and to develop the fundamental knowledge that is necessary for process engineering, design, and scale-up of ultrasound systems.

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Ultrasound for Wastewater Disinfection

List of Symbols and Units

f (1/s): Frequency of ultrasoundω (rad/s): Angular velocity (= 2πf)k: Disinfection rate constantc (m/s): Speed of sound in travelling waven: Refractive index of soundf (Hz): Frequencyλ (m): Wave lengthZ (Rayals or Pa·s/m): Acoustic impedancep (Pa): Root mean square (RMS) pressure, pressureα (neper): Attenuation coeffi cientW (J/s or W): Energy fl ow, work

Received: 30 November 2007; accepted: 4 April 2008