Energy-positive sequential “thermal hydrolysis/anaerobic digestion” process
The slow biodegradation rate of complex organics such as waste activated sludge (WAS) is a well-known limitation of conventional anaerobic digestion (AD) process. Previous research has proven that thermal hydrolysis (pretreatment) via MW heating can accelerate the digestion process by increasing the soluble fraction of organic matter before AD. However, almost all of the MW pretreatment studies ha ve been exclusively constrained to a frequency of 2.45 GHz, due to the availability of the commercial equipment. The short penetration depth of the electromagnetic wave that results in non-uniform heating along with low energy-efficiency (~60%) of high power MW generators are the major disadvantages of MW pretreatment at 2.45 GHz. Studies using MW heating have reported negative net energy balance (output energy as biomethane minus input electrical energy). Therefore, in this research, a novel and highly efficient radio frequency (RF) pretreatment system at a frequency of 13.56 MHz was designed and implemented for the first time. The system was custom-designed based on the dielectric characteristics of WAS to achieve very efficient and uniform heating. The effectiveness of the RF system was assessed in terms of chemical oxygen demand (COD) and biopolymers solubilization, biomethane production rate and yield, solids reduction, input electrical energy, and overall net energy re covery. The application of the newly developed RF system improved the biomethane recovery from WAS by 25%, while consuming 70% less energy compared to the commercially available MW equipment. The overall energy efficiency of the RF system was measured between 67 to 96% for the temperature range of 25 to 160°C, which was significantly higher than that of the MW system (37-43%). The energy assessment results revealed that while the application of MW pretreatment resulted in overall negative net energy production, positive net energy recovery was achieved through the sequential pretreatment/AD process utilizing RF pretreatment.
Dr. Ehssan Hosseini Koupaie is an NSERC Postdoctoral Research Fellow at the Department of Chemical Engineering & Applied Chemistry at the University of Toronto and a Teaching Lecturer at the Civil Engineering Department at Ryerson University (Toronto, Canada). He is also affiliated with Biozone, the Centre for Applied Bioscience & Bioengineering as well as the Pulp & Paper Centre at the University of Toronto. Over the last 10 years, his research activities have significantly evolved, comprising fund amental and applied research on the application of biological processes for organic waste treatment, bioenergy production, and resource recovery. His primary areas of expertise include anaerobic digestion, dark fermentation, emerging pretreatment technologies, and attached-growth biofilm processes. Dr. Koupaie received his Ph.D. in Civil-Environmental Engineering from the University of British Columbia, Canada in 2017 where he collaborated with an interdisciplinary team of researchers from environmental engineering, electrical engineering, and physics disciplines to develop an energy-efficient radio frequency (RF) thermal hydrolysis technique for enhanced anaerobic digestion of municipal sludge. His research activities have resulted in the publication of 21 peer-reviewed journal articles and more than 35 conference presentations. Because of his significant research contribution to the field of “Waste to Bioenergy”, he was awarded a two-year Postdoctoral Fellowship from N atural Sciences and Engineering Research Council of Canada (NSERC) in 2018.