Performance Evaluation of Semi-continuous Plug-Flow Baffled Digester and Batch Digester for Biogas Production from Cattle Manure under the Influence of Different Fe3O4 Nanoparticle Concentrations

Introduction
A stable and readily available energy supply is of paramount importance for economic development. This is evidenced by historical global competition for energy resources, which has been driven by their significance for national security and governmental stability. The current global energy consumption trend presents significant resource depletion and environmental degradation challenges. Two principal solutions are put forth: the enhancement of energy efficiency and the transition to renewable energy sources, with the latter being the optimal long-term strategy. Nanoparticles, due to their minute size, diverse shapes, high reactivity, and stability, have garnered considerable research interest. This study assesses the impact of Fe₃O₄ nanoparticles on the anaerobic digestion of cow manure in two types of digesters: discontinuous and semi-continuous.
Materials and Methods
This study utilized four transparent plastic batch-flow digesters, each with a volume of 4 liters, and four semi-continuous horizontal plug-flow digesters, constructed from a combination of PVC and plexiglass pipes. The length of the digesters was approximately 120 centimeters, divided into three sections by two baffles to create a three-stage digester system. The volume of each digester was approximately 12 liters, resulting in a volume of approximately 4 liters per section. Each section was furnished with a gas discharge valve and an inspection and sampling port. The four digesters were situated within an enclosure. The experiments were conducted at temperatures suitable for mesophilic organisms. A thermostat module and two 1000-watt heaters were employed to regulate the temperature. Two fans were positioned behind the heaters to facilitate air circulation. To prevent the accumulation of sediment, clogging, and the formation of foam on the surface of the substrate, and to ensure the uniform dispersion of nanoparticles within the substrate, agitators were installed within the digesters. The experiments were conducted using iron oxide nanoparticles with a diameter of 50-100 nanometers, manufactured by Sigma-Aldrich. Three different concentrations of Fe₃O₄ nanoparticles were utilized: 50, 100, and 200 milligrams per liter, respectively, for the first, second, and third experiments.
Results and Discussion
During the initial four-day period, digesters one, two, and three generated greater quantities of biogas than the control in both batch and plug-flow systems, despite the overall low production levels observed initially. Digester 2 in the batch system demonstrated the highest biogas production, with a volume of approximately 37 liters over 39 days, representing a 41% increase compared to the control. In the plug-flow system, digesters 2 and 3 produced 165.68 and 149.45 liters of biogas, respectively, representing a 36% and 23% increase over the control. The biogas production of Digester 3 was comparable to that of the control. The decomposition of organic matter was found to be accelerated by lower concentrations of Fe₃O₄ nanoparticles, while higher concentrations were observed to inhibit anaerobic digestion. The highest methane production in the batch system was 11.94 liters in digester 2, representing a 51% increase over the control, while digesters 1 and 3 exhibited comparatively smaller increases. It is necessary to allow sufficient time for methanogenic microorganisms to adapt to additions of nanoparticles. In the plug-flow system, digester 2 produced a total of 50.3 liters of methane, representing a 48% increase over the control. This result demonstrates the effectiveness of a 100 mg/L dose. The findings indicate that there is no linear relationship between nanoparticle concentration and methane production. Instead, effective concentrations vary based on nanoparticle size and other factors.
Conclusion
The study revealed that the incorporation of nanoparticles into anaerobic digestion processes enhances biogas and methane production. However, the optimal concentration of nanoparticles varies depending on the specific conditions, feedstock type, and size of the system. The highest biogas and methane production was observed at 100 mg/L in a semi-continuous digester, with a 36% and 48% increase, respectively. The highest biogas and methane production was observed in tank number two (36%), followed by tank number three (34%), and the lowest in tank number one (29%). This indicates that biogas production necessitates an adequate period for microorganisms to effectively engage in methanogenesis.