Microalgae and Cyanobacteria Biomass Pretreatment Methods: A Comparative Analysis of Chemical and Thermochemical Pretreatment Methods Aimed at Methane Production

Abstract

Anaerobic digestion of microalgae and cyanobacteria was first proposed as a destination for algal biomass accumulated on stabilization ponds since it could not be disposed of directly in the environment. Now, the versatility of algal biomass makes them a suitable candidate to produce biofuels and other biomolecules in biorefineries. Anaerobic digestion of biomass is advantageous because it does not require the extraction of specific cellular constituents or drying of the biomass. Nevertheless, challenges remain regarding biomass concentration and their resistant cell walls, which are factors that could hamper methane yield. Many pretreatment methods, including chemical and thermochemical, have been proposed to break down the complex polymers present on the cell wall into smaller molecules. Unfortunately, the relationship between biomass solubilization and methane yield is not well defined. This article intends to review the anaerobic digestion of algal biomass and the role of chemical and thermochemical pretreatments in enhancing methane production. Several pretreatment conditions selected from the scientific literature were compared to verify which conditions actually improve methane yield. The severity of the selected pretreatments was also assessed using the combined severity factor. Results suggest that thermochemical pretreatment in less severe conditions is the most efficient, leading to a greater increase in methane yield. Only enzymatic pretreatments and some thermal pretreatments result in a positive energy balance. The large-scale implementation of pretreatment methods requires technological innovations to reduce energy consumption and its integration with other processes in wastewater treatment plants.

Fermentation of micro- and macroalgae as a way to produce value-added products

“…The use of AD for algae processing has the advantage of eliminating the need for preliminary drying of biomass, which significantly reduces the cost of the process [34] . Also, the AD process does not require high temperatures and pressures [35] . The disadvantages of AD include difficulty in operating the equipment, maintaining a certain temperature, the need for additional treatment of organic sludge (in case the sludge will be used as fertilizer), the sensitivity of methanogenic bacteria to various compounds (in particular, antibiotics contained in wastewater), their low growth rate, as well as an unpleasant odor due to sulfur compounds.…”

“…The disadvantages of AD include difficulty in operating the equipment, maintaining a certain temperature, the need for additional treatment of organic sludge (in case the sludge will be used as fertilizer), the sensitivity of methanogenic bacteria to various compounds (in particular, antibiotics contained in wastewater), their low growth rate, as well as an unpleasant odor due to sulfur compounds. Furthermore, methane loss, process duration, and instability make AD technology economically unattractive [35] . The AD utilizes different types of algae (micro- and macroalgae) as well as different microbial communities ( Table 1 ).…”

A Review of Aquatic Plant Biomass Pretreatment Methods for Biogas Production

“…From numerous studies, it is clear that to improve the efficiency of methane production from microalgae biomass, the optimal pretreatment temperature should range from 55-170 °C. This temperature range is critical for the decomposition of organic components of microalgae, which contributes to increased methane yield, highlighting the importance of fine-tuning pretreatment conditions in bioenergy processes (de Oliveira et al, 2022). Biogas production by anaerobic digestion of algae is significantly increased by thermal pretreatment at 90 °C with a maximum increase of 41% per unit volatile solids added.…”

Algae biogas production focusing on operating conditions and conversion mechanisms – A review

“…Biomass is heated at temperatures below 100 °C during the thermal process, while it requires higher temperatures in hydrothermal pretreatment followed by an increase in pressure. A positive association exists between biomass solubilization affected by temperature and time and methane yield, with temperature sometimes having a greater impact on thermal pretreatment [ 65 ].…”