Scaled-Up Ultrasonic Systems

Full-scale ultrasonic treatment is desirable for in situ soil and sediment remediation. Though sonochemistry is a “clean remediation method," several barriers preclude full-scale operation for remediation, including ultrasonic and environmental conditions. Ultrasonic systems have technical inefficiencies related to transducers and horns. Electrical and thermal energy losses occur from multiple energy-conversion steps (electrical energy → mechanical sound energy → cavitational energy → physical/chemical effects). Energy also attenuates in irradiated media with distance. Although ultrasonic horns with small cross-sectional areas allow for high acoustic intensities, cavitation is localize below the horn tip. This limits cavitation volumes, uniform mixing, and •OH mass transfer.

The energy efficiency of ultrasonic probes can be measured by calorimetry, a technique relating temperature increases from ultrasound to acoustic power dissipated into solution (see Further Reading section). Energy efficiency is defined as the ratio of calorimetric acoustic power to total supplied electrical energy.

Traditional horns are <30% efficient. Another challenge associated with scale-up is pitting of transducer tips that require routine replacement. Environmental factors also limit scale-up. Spatial and compositional heterogeneities in organic matter and contaminants are consistent challenges in soil and sediment remediation. Heterogeneities impede optimization of in situ ultrasound by complicating the interpretation of physical/chemical effects and remediation effectiveness.

While optimization of reactor geometry and operating conditions have been scaled-up in applications such as wastewater treatment, different approaches are needed for in situ applications. Scale-up should strive to maximize physical/chemical effects for contaminant degradation and minimize treatment time. Novel ultrasonic horns with higher energy efficiencies and larger cavitation volumes are imperative for large-scale, in situ applications. Some studies have designed horns (e.g. multistepped and barbell) with higher energy output surfaces and improved sonochemical performance.

Economic feasibility of ultrasonic treatment and coupled technologies must also be considered (see Further Reading section). Ultrasound, when coupled with other remediation technologies, increases percent degradation, cost effectiveness, and efficiency. Nevertheless, challenges remain for the in situ, full-scale application of ultrasound for contaminated sediments and soils.

References:
1. NRC (2005). Contaminants in the Subsurface: Source Zone Assessment and Remediation. National Academies Press.
2. USEPA (2005). Cleaning Up the Nation’s Waste Sites: Markets and Technology Trends, Office of Solid Waste and Emergency Response, EPA 542-R-04-015.
3. USEPA (1998). EPA’s Contaminated Sediment Management Strategy, Office of Water, EPA-823-R-98-001.

4. Siegrist, R.L., Crimi, M., and Brown, R.A. (2011). In: In Situ Chemical Oxidation for Groundwater Remediation (ed. R.L. Siegrist, M. Crimi and T.J. Simpkin), 1-32. New York, NY: Springer New York.
5. ITRC (2015). Integrated DNAPL Site Characterization and Tools Selection. Washington, DC: Interstate Technology & Regulatory Council.
6. NRC (2013). Alternatives for Managing the Nation’s Complex Contaminated Groundwater Sites. National Academies Press.

7. USEPA (2005). Contaminated Sediment Remediation Guidance for Hazardous Waste Sites, EPA-540-R-05-012.
8. NRC (1994). Alternatives for Ground Water Cleanup. Washington, DC: National Academies Press.
9. Luthy, R.G., Aiken, G.R., Brusseau, M.L. et al. (1997). Environ. Sci. Technol. 31: 3341-3347. doi: 10.1021/es970512m.