Enhanced methane production from bloom algal biomass using hydrothermal and hydrothermal-alkaline pretreatment with...

Coalbeds have the potential as geobioreactors for producing renewable natural gas from biomass derived from photosynthesis. This brings about a number of benefits, including support for sustainable energy and the sequestration of carbon dioxide in coal. In this study, freshwater bloom algae were employed as the substrate to examine the influence of hydrothermal and hydrothermal-alkaline pretreatment on methane production using an inoculum from an anaerobic digester. The morphology and chemical structures of the biomass, as well as the volatile fatty acids (VFAs) in the liquid fraction of the post-treatment and gas production, were analyzed to understand their relationship with the efficacy of methane yields and changes in microorganisms. The results revealed that both hydrothermal and hydrothermal-alkaline pretreatment, under the right conditions, can lead to an increase in methane production. Particularly, a pretreatment condition of 0.2 mol/L NaOH at 150 °C for 30 min resulted in a significant increase in methane yield by up to 303.9%. The addition of NaOH facilitated the hydrothermal-alkaline pretreatment, effectively destroying the cell structure of the bloom algae, promoting the dissolution of intracellular sugars and other substances, and reducing the loss of VFAs caused by heating. Moreover, hydrothermal-alkaline pretreatment was found to support the growth of acetoclastic methanogens and enhance methane production by mitigating pH drops. Overall, the results of this study suggest that hydrothermal-alkaline pretreatment offers significant advantages in methane production compared to hydrothermal pretreatment. These findings have important implications for harnessing bloom algae as a viable source for generating renewable natural gas.