Biofuels & Bioenergy

The microalgal biomass conversion into methane as a biofuel offers the best energetic balance among the different biomass–to-biofuel scenarios for microalgae containing less than 40% lipids. The anaerobic degradation limitation of algae at around 50% emphasizes the need for pre-treatment to obtain higher methane production from algae. This study was performed using Scenedesmus sp. AMDD, a green microalgae, as a model strain. Over 20 series of different pretreatments were evaluated, alone, or in sequence. The enzymatic pretreatments were performed with pectate-lyase and cellulase at incubation time from 2 to 24 hours. Chemicals pretreatments were done with H2SO4, NaOH or H2O2, at 0.2N and 2N and 2 to 24 h of reaction time. Thermal treatments were completed in an oven or a pressure vessel at 121 – 180°C or using a microwave (175 – 300°C). The enzymatic hydrolysis of Scenedesmus sp. AMDD followed with a three hours incubation in NaOH 0.2N resulted into a 75% solubilization. Similar results were found with incubation in 0.2N NaOH followed with short thermal treatment. Caustic and thermal pretreatments improved the methane production by around 12% compared with the anaerobic digestion of untreated algal biomass, at 335 ± 28 ml CH4 STP/g volatile solid (VS) added. The results from the enzymatic pretreatment were less encouraging with improvement of 2-7% of the methane production only. However., a combination of enzymatic with a thermal treatment successfully solubilized up to 75% of Scenedesmus sp. AMDD biomass. The resulting methane production, although up to 15% higher than for the control biomass, did not fully correlate with the increased dissolved organic matter. In anaerobic digesters continuously fed with solubilized biomass after combined enzymatic and thermal pre-treatment, the CH4 yield was improved by up to 35% in some operational conditions, while the degradation rate was faster, allowing for lower retention time.