Effects of thermo-chemical and enzymatic pre-treatment of tropical seaweeds and freshwater macrophytes on biogas and bioethanol production

The use of aquatic biomass as potential sources for bioenergy production has attracted significant attention worldwide. Production of biogas and bioethanol from both marine and freshwater plants using same pre-treatment methods were evaluated and the results indicate that both processes can be potentially enhanced appropriate methods of pre-treatment. In this study, the effects of thermochemical and enzymatic pre-treatment of selected seaweeds and freshwater macrophytes for biogas and bioethanol production were investigated. It was found that methane biogas yield from the anaerobic digestion of selected aquatic plants was highly dependent on the plant species. For example, biomethane yields of 189, 195, 221, 234 mL/g volatile solids were obtained following anaerobic digestion of acid and enzymatic pre-treatment of Laminaria digitata, Sargassum fluitans, Eichhornia crassipies and Pistia stratiotes, respectively. Additionally, alcoholic fermentation by the yeast Saccharomyces cerevisiae (distiller's strain) was carried out on aquatic plant hydrolysates and the highest ethanol yields (of over 4 g/L) were obtained from Eichhornia crassipies and Pistia stratiotes. Poor fermentation yields from Laminaria digitata, and Sargassum fluitans hydrolysates were attributed to the predominance of un-fermented rhamnose sugars in these plants. The findings demonstrate the importance of reliance on empirical data for each substrate when designing and operating anaerobic digestion and alcohol fermentation systems. The results show that the same pre-treatment methods can be used for both types of bioenergy production, i.e., biogas and bioethanol, from marine and freshwater plants, thereby enhancing the economic viability of both processes in industry-scale applications.