ULTRASOUND FOR IMPROVED SOLIDS MANAGEMENT – HOW, WHO, WHEN AND WHY

The application of high intensity ultrasound has significant potential for improving biosolids management at treatment plants across North America. This paper provides a review of how the technology works, the potential applications at wastewater treatment plants, the different approaches being used by various suppliers, the state of development of the technology, and on-going work in North America and around the globe. Ultrasound is sound above the range of human hearing, with frequencies between 20 kHz and 10 MHz. At the lower end of this range, ultrasound generates cavitation bubbles when applied in a fluid, which implode creating high mechanical shear forces. This can be used for disintegrating solids in the fluid.For wastewater applications it has been shown that ultrasound is most beneficial when applied on biological secondary solids, where rapid hydrolysis can be induced. Hydrolysis ofcellular material is usually the rate-limiting step in digestion. This provides advantages in anaerobic digestion, with increased volatile solids destruction and gas production, Improved dewaterability has also been observed at some installations. Improvements through reduction in biosolids volume and increased gas production can provide significant savings. Other applications of ultrasound include the potential for prevention of foaming caused by filamentous organisms.Most of the work on ultrasound in wastewater applications has been done in Europe, In North America, California has been at the forefront of ultrasound development, with demonstrations conducted by Orange County Sanitation District and the Los Angeles County Sanitation District. The California Energy Commission has also supported a demonstration project at the City of Riverside for comparison of performance and cost effectiveness of two ultrasound manufacturers that use different treatment philosophies. Ultrasound technology is also being implemented in Australia and Singapore. This paper provides an overview of ultrasound system suppliers, existing installations, continuing developments in this field and guidance on when ultrasound technology may prove beneficial for improved solids management.