Unravelling the anaerobic digestion 'black box': Biotechnological approaches for process optimization

Increasing global demand for green energy has necessitated more research interest in anaerobic digestion. Due to the dual benefits of anaerobic digestion, in organic waste management and energy generation, it is rated among the best renewable energy generation options. An important challenge that limits acceptability and adoption of this technology is the quality and quantity of anaerobic digestion products, especially biogas and digestate. A practical solution to this challenge is process optimization. However, process optimization is not easily achieved due to the complexity of the microbiome driving the anaerobic digestion process, which has contributed to anaerobic digesters being considered 'black boxes'. Unravelling the 'black box' would aid in fine-tuning operational conditions, process parameters, intra- and inter-phase interactions as well as rate limiting factors during each phase of the production cycle, thereby optimizing the anaerobic digestion process. To achieve this level of technological advancement, cooperation between experts that understand both the microbial processes and technological design of digesters is important. When factors are optimized, implementation of the optimized conditions through automation may prevent process errors and failures, thereby enhancing quality and quantity of anaerobic digestion products. In addition, process optimization should also reflect economic viability vis-`a-vis small-scale and large-scale biogas production systems. This review provides a detailed overview of individual factors that could affect the performance of the anaerobic digestion process with critical interpretations and suggestions. This work could be employed as a good base to build integrative models or ecological indices that interrelate these factors.