An Examination of Calculations: From Anaerobic Mono-digestion to Co-digestion

Anaerobic digestion is a biochemical process and form of sustainable technology. In comparison to mono-digestion, co-digestion is more effective in the utilization of diverse resources, supply of nutrient balance, and overall digestibility and stability. The co-digestion biodegradability index, co-digestion performance index, and synergistic effect index are generally used as anaerobic metrics to evaluate the co-digestion performance. Although they are numerical parameters, little information on their working procedures has been revealed. There is also a lack of supporting calculations for some published data hence their values cannot be verified. The aim of this article is to reconsider and examine the mathematical calculations from anaerobic mono-digestion to co-digestion. The method of study is a pen-and-paper analysis that processes both experimental and theoretical data from past studies. The article serves four purposes. First, it identifies processing parameters. Second, it shows step-by-step procedures of a series of calculations. Third, it identifies the core parameter in the calculation of anaerobic co-digestion. Fourth, it establishes the relationships between co-digestion biodegradability index, co-digestion performance index, and synergistic effect index. Critically, the mass fraction of volatile solid is identified as the core parameter that determines the valid calculations of co-digestion.