Biogas yields and composition from oil-extracted halophilic algae residues in conventional biogas plants operated at high salinities

Abstract

CO₂-induced climate change drives the development of renewable processes for industrial products. Algae processes can actively fix and convert CO₂ into value adding products, such as oils. Algae lipids hence counteract climate change and provide access to renewable commodities. In this context, valorization of algal residues remaining after oil extraction is a challenge for the emerging cyclic bioeconomy. This study focuses on the valorization of oil-extracted algae residues derived from the halophilic strain Scenedesmus obliquus via anaerobic digestion. We examined the effect of prior oil extraction on microbial digestibility and increasing salt content in the substrate with regard to biogas yield and composition. Our cumulative data demonstrate that the supercritical CO₂ oil extraction acts as a physical pretreatment that facilitates enhanced hydrolysis of both polymeric call wall carbohydrates and cellular proteins, providing methane yields of 213.2 L_N kg⁻¹ VS day⁻¹. Methane yields were 20% higher than literature values obtained with the same algae strain in the absence of prior oil extraction. We obtained these superior results albeit all lipids and nonpolar proteins had been extracted from the biogas substrate. Our data indicate that continuous anaerobic digestion without loss of fermentation efficiency is feasible up to a salt concentration of 2% w/v, if conventional, agricultural biogas plants are gradually adapted to the salt content of the substrate. Monofermentation of the investigated oil-extracted algae residue is technically feasible at loading rates of 1.5 kg VS m⁻³ day⁻¹, but a supplementation with carbohydrate rich biomass would prove beneficial to alleviate ammonia inhibition.