Ultrasound refers to sound energy with frequencies above the audible range; that is, starting at about 20 kHz. Using ultrasound for enhanced sludge digestion relies on inducing cavitation in the liquid to produce lysis of the cellular sludge material, thereby increasing the availability
of the organic material to bacteria in anaerobic digesters. This paper presents the results from a fullscale ultrasonic trial at Orange County Sanitation District (OCSD) in 2002. The benefits of enhanced anaerobic digestion include: Increased
gas production Greater solids destruction Improved digester stability Reduced digester foaming Improved biosolids dewaterability The demonstration trial at OCSD ran for 5 months. In order that a direct comparison could be made between sonicated and unsonicated feed sludge, two digesters (test and control) were chosen for study. These were operated under identical conditions, and the proportion of TWAS
fed to both digesters was maintained at 60 – 65% throughout. This enabled the results to be compared both against baseline conditions (longitudinal study) and between digesters (crosssectional study) so that conclusions could be drawn for full installation of ultrasound sludge
treatment at all of OCSD's facilities. At OCSD, the ultrasound unit was located on the thickened waste activated sludge (TWAS) feed line to the test digester. Compared to the control digester, significant enhancements of gas production (50% increase) and solids destruction (15
percentage points increase) were achieved by sonicating the secondary sludge. The increases were comparable to the results from a similar UK trial.
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