Sustainable biogas production through anaerobic co-digestion of Ulva lactuca (Chlorophyta) and cow manure: a kinetic and process optimization study

Energy derived from biomass is increasingly appealing due to escalating energy demand and the urgent need to mitigate greenhouse gas emissions. However, to ensure the sustainability of bioenergy, the diversification of feedstocks, including marine biota, is essential. Among the various marine biota, harnessing U. lactuca for biogas production remains scarcely explored in the literature. This study aims to fill this gap by examining the synergistic effects of U. lactuca and cow manure in anaerobic co-digestion to optimize methane yield. The novelty of this study lies in its comprehensive kinetic analysis of biogas production from U. lactuca, offering valuable insights into the digestion process and providing optimal conditions for maximizing methane yield. Anaerobic co-digestion was conducted in a semi-continuous reactor with varying algae-to-cow manure ratios (1 : 1, 2 : 1, and 3 : 1) under controlled conditions for over 30 days. The results showed that a 2 : 1 ratio resulted in the maximum methane yield of 325.75 mL per g VS. Kinetic evaluation using first-order, logistic, transference, and modified Gompertz models revealed that the modified Gompertz model most accurately represented the experimental data, showing a high coefficient of determination (R² = 0.999). RSM revealed that the fermentation time and substrate ratio significantly influenced methane production. These findings highlight the effectiveness of the anaerobic co-digestion of U. lactuca and cow manure, emphasizing its potential as a viable strategy for sustainable bioenergy generation.