Effect of Solidified Carbon Dioxide Pretreatment on Chlorella vulgaris Biomass Prior to Anaerobic Digestion

Abstract

The aim of this study was to evaluate the effect of low-temperature disintegration of Chlorella vulgaris using solidified carbon dioxide (SCO2) on the efficiency of anaerobic digestion of microalgae biomass. The novelty of this study resides in the pioneering application of SCO2 for the pretreatment of C. vulgaris biomass to enhance methane fermentation. This approach integrates mechanical disruption of cell walls with improved solubilization of organic fractions at low temperatures, providing an innovative and energy-efficient strategy to boost biomethanogenesis performance. This study was carried out in four stages, including characterisation of substrate properties, evaluation of organic compound solubilization following SCO2 pretreatment, and fermentation under both batch and continuous conditions. Analysis of dissolved COD and TOC fractions revealed a significant increase in the bioavailability of organic matter as a result of SCO2 application, with the highest degree of solubilization observed at a SCO2/C. vulgaris biomass volume ratio of 1:3. In batch reactors, CH4 yield increased significantly to 369 ± 16 mL CH4/g VS, methane content in biogas reached 65.9 ± 1.0%, and kinetic process parameters were improved. Comparable enhancements were observed in continuous fermentation, with the best scenario yielding 243.4 ± 9.5 mL CH4/g VS. Digestate analysis confirmed more efficient degradation of organic fractions, and the stability of methanogenic consortia was maintained, with only moderate changes in the relative abundance of the main groups (Methanosarcinaceae, Methanosaeta). Energy balance calculations indicated a positive net effect of the process. This study represents a pioneering application of SCO2 pretreatment in the context of microalgal biomass and highlights its high potential for intensifying anaerobic digestion.