Radio frequency sludge hydrolysis as an energy efficient alternative to microwave and conductive heating for advanced anaerobic digestion

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%.Applied Science, Faculty ofEngineering, School of (Okanagan)Graduat

Comparison of Different Electricity-Based Thermal Pretreatment Methods for Enhanced Bioenergy Production from Municipal Sludge

This paper presents results for a comprehensive study that compares the performance of three electricity-based thermal pretreatment methods for improving the effectiveness of anaerobic digestion (AD) to process municipal wastewater sludge. The study compares thermal pretreatment using conventional heating (CH), microwave (MW), and radio frequency (RF) heating techniques. The effectiveness of the pretreatment methods was assessed in terms of chemical oxygen demand (COD) and biopolymers solubilization, AD bioenergy production, input electrical energy, and overall net energy production of the sequential pretreatment/AD process. The heating applicators for the bench-scale testing consisted of a custom-built pressure-sealed heating vessel for CH experiments, an off-the-shelf programmable MW oven operating at a frequency of 2.45 GHz for MW heating experiments, and a newly developed 1 kW RF heating system operating at a frequency of 13.56 MHz for RF heating experiments. Under identical thermal profiles, all three thermal pretreatment methods achieved similar sludge disintegration in terms of COD and biopolymer solubilization as well as AD bioenergy production (p-value > 0.05). According to the energy assessment results, the application of CH and MW pretreatments resulted in overall negative energy production, while positive net energy production was obtained through the sequential pretreatment/AD process utilizing RF pretreatment.Applied Science, Faculty ofEngineering, School of (Okanagan)ReviewedFacult

Effect of Hydrothermal Pretreatment on Volatile Fatty Acids Production from Source-Separated Organics

The current study investigates the effect of hydrothermal pretreatment (HTP) on acidification of source-separated organics (SSO) in terms of volatile fatty acids (VFAs) production and solubilization. Temperature and retention time for HTP of SSO ranged from 150 to 240 °C and 5 to 30 min, respectively. The soluble substance after hydrothermal pretreatment initially increased, reaching its peak at 210 °C and then declined gradually. The highest overall chemical oxygen demand (COD) solubilization of 63% was observed at “210 °C-20 min” compared to 17% for raw SSO. The highest VFAs yield of 1536 mg VFAs/g VSS added was observed at “210 °C-20 min” compared to 768 mg VFAs/g VSS for raw SSO. Intensification of hydrothermal pretreatment temperature beyond 210 °C resulted in the mineralization of the organics and adversely affected the process

Bioenergy production data from anaerobic digestion of thermally hydrolyzed organic fraction of municipal solid waste

The presented dataset in this data article provides quantitative data on the production of bioenergy (biogas and biomethane) from mesophilic batch anaerobic digestion (AD) of thermally hydrolyzed organic fraction of municipal solid waste (OFMSW). The discussion and interpretation of the data are provided in another publication entitled “Hydrothermal Pretreatment of Source Separated Organics for Enhanced Solubilization and Biomethane Recovery” (Razavi et al., 2019). The data and information presented in the current data article include (1) the ratio of soluble to particulate chemical oxygen demand (COD) under different thermal hydrolysis condition, (2) the daily measured biogas and biomethane data, (3) the cumulative methane yield data in terms of mL CH4 produced per gram of volatile suspended solids (VSS) as well as feedstock added, (4) the ultimate methane yield data as well as the relative improvement in methane recovery compared to the control (non-hydrolyzed) digester, (5) the data of first-order organics biodegradation rate constants, (6) the procedure of measuring biogas composition via gas chromatography, (7) the procedure of converting the biogas/methane volume data acquired under the actual experimental condition (mesophilic temperature of 38 °C and atmospheric pressure) to the standard temperature (0 °C) and pressure (1 atm) condition, and (8) the procedure of determining the first-order kinetic rate constants