Sustainable biogas production via anaerobic digestion with focus on CSTR technology: A review

Background: Waste pollution can negatively impact the ecosystem directly or indirectly. Utilizing waste as a resource to produce value-added products, like biofuels, is one option to address this problem. This would create a circular economy, which is in line with the Sustainable Development Goals (SDGs) 5, 7, 8, and 9 of the United Nations (UN). Anaerobic digestion (AD) is a technology that humans have extensively used to generate renewable energy. However, a great deal of research is required to optimize biogas production under diverse conditions. Several factors have been considered pertaining to the design and operation of digesters to ensure cost-effective and efficient digestion. Method: This article summarizes the underlying AD processes and the operating parameters used to enhance process efficiency. Considering older research and recent developments, a comprehensive summary of the theory of continuous stirred tank reactor (CSTR) technology for AD has been presented. The novelty of this review article lies in the emphasis on advanced intensification strategies to stabilize the AD process in CSTR for enhanced biogas/biomethane production. Furthermore, energy balance calculations of CSTR for biogas production are reviewed to promote the practical application of the technology on a global scale. Significant Findings: While pretreatment enhances the substrate's surface area, eliminates lignin, and decreases the crystallinity of cellulose to stimulate microbial uptake, co -digestion provides feedstock accessibility to the microorganisms and improves CH4 yield. Energy balance studies of CSTR revealed that the majority of the energy consumed is used during heating, mixing, and feedstock transportation. In addition, the use of digestate enhances the energy recovery efficiency of the CSTR system. With proper process optimizations, AD for biogas production can help the country overcome current energy issues.