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Technology Ultrasonic Sludge Disintegration: A 'Bubbly' Solution For Anaerobic Digestion Or BNR 'Sonicating' sludge proves to be a sound method for handling biosolids. By Bryen Woo and James Goldhardt O ne of the ongoing challenges in wastewater any energy. At that state, the bubbles implode violently (durtreatment facilities is the handling of biosolids. ing the compression cycle). This implosion is referred to as It has been found that the total biosolids gencavitation and, in liquids, produces an enormous amount of eration continues to increase each year. In fact, energy. It is estimated that these bubbles (from cavitation) the processing, treatment, and have temperatures as hot as disposal of sludge for benefi8,900 degrees F and pressures cial use accounts for approxias high as 7,250 psi. Ultrasonic sludge disintegration mately 40 to 60 percent of the Ultrasonic sludge disintegratotal wastewater treatment plant tion uses acoustic cavitation as is a prime application for (WWTP) expenditures. These a means to provide the necimproving the anaerobic digestion expenditures can certainly add essary physical and chemical process and for biological nutrient up over the life of a wastewater changes in order to rupture the removal (BNR) processes. treatment plant; therefore, it is cell walls of sludge microorimportant when designing a ganisms. Ultrasonic energy is wastewater treatment plant to very effective primarily because have an efficient and effectively the compression of the bubbles operated solids handling system. Ultrasonic sludge disinteduring cavitation is more rapid than thermal transport, allowgration, by way of ultrasound, is a proven method to help ing for a localized "hot spot" to be created. The localized mitigate this very issue. hot spot allows for sludge disintegration to occur easily, as it only takes approximately 100 microseconds for cavitation to Ultrasound And The Ultrasonic Sludge Disintegration occur and less than 400 microseconds for microorganism cell Sound travels in waves and generates acoustic energy. The walls to rupture. human audible frequency range is between 20 Hz and Ultrasonic sludge disintegration is a prime application for 20 kHz; anything above that is referred to as ultrasound. A improving the anaerobic digestion process and for biological sound wave, at any given frequency, has a minimum and nutrient removal (BNR) processes. A typical ultrasonic reactor maximum point referred to as rarefaction and compression. consists of a 5-kW control power supply producing ultrasonic During the rarefaction stage, particles move away from each energy and an oscillating unit consisting of a transducer, other into lower-density areas to create a lower pressure. In booster, and horn that is positioned within the contacting the compression stage, particles move together into highreactor, applying ultrasonic energy to the feed sludge. density areas to create a higher pressure (or the opposite of Certain criteria are needed for optimizing the ultrasonic rarefaction). reactor, one of which is the feed sludge type. Since primary By way of ultrasound, acous- Figure 1: Acoustic cavitation tic cavitation can be created. When sonicating liquids (applying acoustic energy to agitate particles), the sound waves alternate between compression and rarefaction cycles, where small voids are created in the liquid, creating microbubbles. As shown in Figure 1, the bubbles will grow until they attain a volume at which they can no longer absorb 32 wateronline.com ■ Water Online The Magazine