Study of the thermal, hydraulic, and energetic performances of anaerobic digesters with submerged solid waste: An experimental and numerical approach

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Growing concerns over energy security, climate change, and sustainable waste management have intensified interest in anaerobic digestion as an effective technology for renewable energy production and organic waste valorization. The performance of anaerobic digesters depends on complex interactions between biological, chemical, thermal, and hydraulic phenomena, which govern process stability, methane production, and energy efficiency. Improving the understanding of these interactions is therefore essential for developing more efficient and sustainable digestion systems. This thesis investigates the thermal, hydraulic, and energetic performance of anaerobic digesters with submerged solid waste (ADSW). The research combines pilot-scale experiments with computational fluid dynamics (CFD) modelling and energy balance analyses to evaluate the effects of mixing intensity, temperature distribution, operating temperature, and heating and mixing strategies on reactor performance. Experimental investigations were conducted using a 90 L hydraulically mixed batch digester treating food waste and cow dung under wet anaerobic digestion conditions. The results demonstrate that thermo-hydraulic conditions have a decisive influence on both methane production and overall energy performance. Gentle hydraulic mixing reduces thermo-hydraulic losses while increasing methane production compared with both unmixed and intensely mixed operations. Under unmixed conditions, the formation of temperature gradients, influenced by the position of the submerged waste layer, alters the thermal behavior of the reactor and reduces methane yield. The combined effects of mesophilic and thermophilic operation, mixing conditions, and temperature distribution were further elucidated, providing a comprehensive understanding of the relationship between heat transfer, fluid flow, and anaerobic digestion performance. Building upon these findings, a novel heating and mixing strategy was developed that significantly reduced auxiliary energy consumption. The outcomes of this work advance the understanding of the coupled thermo-hydraulic behavior of anaerobic digesters and concretely demonstrate how the integration of experiments and CFD modelling can support reactor optimization. The proposed operating strategies provide practical guidance for improving the energy efficiency, performance, and sustainability of anaerobic digestion systems for organic waste treatment and renewable biogas production.

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