The importance of substrate formulation on the hydrolysis process in anaerobic digestion: a numerical and experimental study

Substrate formulation has been widely studied to ensure optimum methane production in anaerobic digestion. This work demonstrated a synergistic degradation in the hydrolysis of carbohydrates, proteins, and lipids that enhances CH4 yields through biochemical methane potential (BMP) tests and a co-digestion index (CI), following a D-Optimal experimental design. Additionally, this work proposes and validates a modification to the hydrolysis step in the Anaerobic Digestion Model No.1 (ADM1), capable of representing such synergistic degradation by incorporating the CI as a dynamic variable into the mathematical structure of the model. High CI values were observed at balanced carbohydrate-protein-lipid and protein-lipid ratios, obtaining values of 3.23 and 2.85, respectively. In contrast, low CI values were obtained at higher lipid-carbohydrate ratios (1.21), demonstrating that adding proteins to the substrate mixture promotes synergy. Incorporating the CI as a dynamic variable in the hydrolysis stage of the ADM1 increases its predictive capacity, reducing the root mean square error value by up to 55.7% when modeling the BMP tests compared to the original ADM1 structure. When subjected to mixtures of real substrates, the proposed model precisely adjusts the experimental data. These results prove the validity of the proposed modification to the ADM1 and its functionality with real substrate mixtures. This work allows the numerical representation of the synergistic effects in the degradation of a substrate and the correct generation of feed formulations that increase CH4 yields.

Keywords: Anaerobic digestion; ADM1; hydrolysis; co-digestion index; carbohydrates, proteins, and lipids degradation; synergistic effects.

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