Towards an Energy-Efficient Advanced Anaerobic Digestion Process with Radio Frequency Thermal Hydrolysis

Invited speaker: Ehssan Koupai

The slow enzymatic hydrolysis rate of complex organics such as waste activated sludge (WAS) is a well-known limitation of the conventional anaerobic digestion process. Previous research has proven that thermal hydrolysis (thermal pre-treatment) via MW heating can accelerate the digestion process by releasing the WAS intracellular compounds (e.g., biopolymers) to the liquid phase. Previous studies on MW pre-treatment have been almost exclusively constrained to a single frequency of 2.45 GHz, due to the availability of commercial equipment. The first drawback of the current MW technologies is the short penetration depth of the electromagnetic waves at 2.45 GHz resulting in non-uniform heating. The second limitation is the low energy efficiency of high-power MW generators (~60%), which has led to negative net energy recovery. Therefore, new multi-disciplinary research has been carried out with a collaboration of researchers from bioprocess engineering, electrical engineering, and physics disciplines to design a highly efficient thermal hydrolysis system that goes beyond the limitations of the conventional equipment. This research has led to the implementation of a novel and highly efficient radio frequency (RF) heating system at a frequency of 13.56 MHz. The RF system was custom-designed based on the dielectric characteristics of WAS to achieve very efficient and uniform heating. Experimental investigations proved that the proposed RF technology relies on the ohmic heating mechanism, which is far more efficient than the dielectric energy loss mechanism used in conventional MW equipment. It was also demonstrated that the application of RF thermal hydrolysis could improve the recovery of biomethane from WAS by 20-35% while consuming 70% less energy compared to the existing technologies. This outcome was a breakthrough, as for the first time, it enabled the Advanced Anaerobic Digestion process utilizing thermal hydrolysis to be energy positive.

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