Enhancing the energetic performance of cattle manure anaerobic digestion: a combined CFD and experimental study on mixing, HRT, and feeding strategies

Anaerobic digestion (AD) can convert agricultural waste into renewable energy, but the process itself is energy-intensive. This study combines computational fluid dynamics (CFD) simulations and experiments to investigate the energy performance of a pilot-scale digester operating under mesophilic conditions (35 °C). Using cow manure as the substrate, we systematically evaluated the effects of mixing intensity, feeding strategy, and Hydraulic Retention Time (HRT). The optimal biogas production was observed under the conditions of 60 rpm stirring speed and a 30-day HRT. A non-Newtonian numerical model was developed, informed by experimental rheological data of the biogas slurry at various total solids (TS) contents. Analysis of entropy generation and energy transfer revealed that impeller-induced turbulence is the predominant mechanism for kinetic energy distribution and the principal source of energy dissipation. Proper Orthogonal Decomposition (POD) of the velocity field showed that the dominant flow structures are collectively governed by rotational speed, system geometry, and fluid rheology. Notably, implementing a mid-level feeding strategy, compared to top feeding, reduced mixing time and energy consumption by approximately 30 %, enhancing both mixing efficiency and operational sustainability. These findings not only provide critical insights into the hydrodynamics of the mixing process but also offer valuable theoretical and practical guidance for the design, optimization, and operation of industrial-scale biogas digesters.