Optimization of Anaerobic Digestion Processes of Organic Waste in Chiapas, Mexico, for Biogas Production

The production of biogas by the anaerobic decomposition of biomass guarantees an efficient and safe reduction of environmental pollution resulting from treating organic wastes, positioning it as a promising technology for the future.

The anaerobic digestion process carried out in this research project utilized pumpkin peel, tamarind peel, and corn cob leaves as substrates. Cow, sheep, and poultry fecal wastes were also analyzed as microbial inocula for biogas production. This research focused on evaluating the physical, chemical, and bromatological parameters of the aforementioned organic waste to determine the optimal parameters that define the stages of biogas production. This study is proposed because Mexico is the main generator of this waste, and lacks studies on its proper management.

The anaerobic digestion experiments were carried out in batch biodigesters with a capacity of 1.8 L at 36 ± 2 °C for a retention time of 30 days, using the methods of the Mexican Standards for Environmental Protection for the determination of humidity, ash, pH, Total Volatile Solids (TVS), Total Solids (TS), chemical oxygen demand, carbon, and nitrogen.

Among the obtained results, the carbon/nitrogen ratio of the tamarind husk and corn cob leaves residues indicated a positive effect on biogas production in treatments mixtures 1 and 8, with values of 21.87 and 24.34, respectively, considered as values within the optimal range of 20:1 to 30:1, these values constitute the ideal availability of energy and nitrogen for cell replication of biogas producing methanogenic bacteria. These treatments were influenced by the Specific Methanogenic Activity (SMA) of the cattle manure inoculum, which had an organic matter biodegradability value of 0.58 grams of Chemical Oxygen Demand de Methane entre grams Total Volatile Solids for a day (COD-CH₄/g TVS/day). This value supports the efficient assimilation of the carbon/nitrogen ratio based on the mentioned values. On the other hand, determining the TVS/TS ratio yielded percentages of 60%, 92%, and 95% in pumpkin peel, tamarind husk, and corn cob husks, respectively.

Comparing the results with other studies, the optimal TVS/TS ratio should be greater than 50%; therefore, the results indicate that the substrates contain sufficient assimilable organic matter for anaerobic microorganisms and could produce biogas. Similarly, many scientific studies indicate that calculating the Specific Methanogenic Activity can not only determine the degree of biodegradability of substrates but also estimate possible pH changes in the medium, in addition to determining the maximum load of organic matter applied to any other type of anaerobic inoculum outside of this research work.

Under these conditions, the treatments obtained a cumulative biogas yield of 3645 and 4250 milliliters of methane, respectively. This research contributes to improving the quality of biogas through the management of standardized waste.