UBC Theses and Dissertations
Radio frequency sludge hydrolysis as an energy efficient alternative to microwave and conductive heating for advanced anaerobic digestion Hosseini Koupaie, Ehssan Ollah
The slow degradation of complex organics such as waste activated sludge (WAS) is a well-known limitation that impacts the process rate of conventional anaerobic digestion (AD). Thermal pretreatment can accelerate the digestion process by disrupting the structure of WAS before AD. The present research was initiated by comparing the two commonly used thermal pretreatment methods, conductive (conventional) heating (CH) and microwave (MW) hydrolysis, for enhanced sludge disintegration and AD performance. A bench-scale programmable MW oven operated at a frequency of 2.45 GHz was used for MW pretreatment. The CH was performed using a custom-built pressure sealed vessel which could simulate the MW pretreatment under any arbitrary heating profiles. After comparing the CH and MW pretreatments, a novel and highly efficient radio frequency (RF) pretreatment system at a frequency of 13.56 MHz was designed, manufactured, and tested for the first time. The RF system was custom-designed based on the dielectric characteristics of thickened WAS (TWAS) to achieve very efficient as well as uniform heating. The effects of the novel RF pretreatment system on sludge solubilization and AD performance were compared with those of the commercially available MW ovens. Considering the obtained results and analyses, under identical thermal profiles, the thermal pretreatment methods (CH, MW at 2.45 GHz, and RF at 13.56 MHz) achieved similar sludge disintegration as well as AD performance (p-value>0.05). However, the pretreatment temperature, heating rate, and holding time were significant factors in determining the sludge solubilization ratio and AD performance. Ohmic heating was found as the primary heating mechanism at a frequency of 13.56 MHz. It causes the ionic conduction flow to dominate the heating mechanism in the custom-designed RF pretreatment system by contributing to more than 99% of the total dissipated power. Considering the impedance measurement results, the power transfer efficiency of the RF heating system was above 88% throughout the operation. The overall energy efficiency of the RF pretreatment system was measured between 67.3 to 95.5% for the temperature range of 25 to 120°C which was significantly higher than the MW system efficiency which varied from 37 to 43%.
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