Design and CFD Analysis of a Compact Anaerobic Digestion Bioreactor Evaluating Agitation Designs and Configurations for Energy Efficiency

Anaerobic digestion (AD) plays a crucial role in renewable energy production and waste management by converting organic waste into biogas and reduces greenhouse gas emissions. Optimized bioreactor performance depends on two main categories of factors: (1) reactor and geometric factors of agitator geometry, blade configuration, rotational speed, torque, power consumption, and the impeller-to-tank ration (d/D), and (2) fluid property factors of viscosity and flow characteristics, which relates turbulence, circulation patters, and stratification. Impeller power strongly influences nutrient distribution, gas exchange, and temperature uniformity within the reactor. While higher power inputs improve turbulence and prevent stratification, they also increase energy demand. This study evaluated fifteen blade configurations to determine the optimal fluid circulation using ANSYS 2024 R1 Fluent simulations. The bioreactor tank, with a diameter of 0.130 m and a height of 0.225 m, was tested at speeds ranging from 40 to 150 RPM. Among the single-blade configurations, the curved blade achieved the highest velocity at 0.521 m/s, generating localized circulations. The Rushton blade produced strong radial flows with a velocity of 0.364 m/s, while the propeller blade reached 0.254 m/s, supporting axial flow. In double-blade arrangements, the curved-propeller combination exhibited velocities between 0.261 and 0.342 m/s, enhancing fluid motion. The three-blade configurations resulted in the highest power consumption, ranging from 1.94 W to 1.99 W, with power increasing at higher RPMs and larger impeller sizes. However, torque values decreased over time. The most efficient mixing was achieved at moderate RPMs (80–120) and an impeller-to-tank diameter ratio (d/D) of approximately 0.75. These findings highlight the significance of blade selection in balancing mixing efficiency and energy consumption for scalable AD systems.