Biogas technology represents a mature yet continually evolving renewable energy pathway that integrates waste management with decentralized energy production. Small- and medium-scale biogas systems, particularly those utilizing animal manure and agricultural residues, remain central to sustainable energy strategies in developing and transitional economies. This article presents a comprehensive analytical synthesis of biogas plant design principles, feedstock characteristics, digestion performance, and environmental implications, with particular emphasis on simple and flexible biogas systems. Drawing upon foundational and contemporary literature, the study examines how substrate selection, pretreatment methods, digester configuration, and operational conditions are associated with variations in biogas yield and system stability. The review also explores environmental considerations, including greenhouse gas mitigation, waste valorization, and the role of biogas in flexible power generation contexts. By systematically integrating evidence from experimental studies, pilot-scale systems, and applied engineering designs, this manuscript identifies persistent research gaps related to performance variability, scalability, and contextual adaptation. The discussion highlights methodological limitations in existing studies and outlines directions for future research that emphasize system optimization, lifecycle assessment, and socio-technical integration. Overall, the article contributes a structured, critical perspective on biogas systems that supports informed design, policy development, and interdisciplinary research in renewable energy and sustainable waste management.

Biogas systems, anaerobic digestion, organic waste management, renewable energy, environmental impacts, small-scale digesters

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