Thermal, alkali and thermo-alkali pretreatments applied to monospecific microalgal biomass to improve anaerobic biogas production

In this study, alkaline, thermal, and thermo-alkaline pretreatments of Scenedesmus obliquus microalgae were investigated with the goal of enhancing anaerobic biodegradability. In recent years microalgae, biomass has been used for several purposes; one is to employ this substrate to produce renewable energy. Biogas/methane production of untreated microalgae is limited by the complex cell-wall structure; neither the organic matter constituent of the cell wall nor the other matter inside the microalgae is available to microorganisms involved in anaerobic digestion (AD). Pretreatments could increase the biogas/methane production. High-temperature pretreatment was conducted at 120°C, and alkali pretreatments were performed using NaOH with a dosage range from 4% to 20% of the selected total solids matrix. Finally, hybrid pretreatment (thermo-alkali treatment) was conducted with the same dosage of NaOH and thermal conditions. All the described treatments had a duration of 1.5 h. Untreated and treated microalgae were subjected to AD, then biogas potential tests were performed in batch mode and mesophilic conditions (38°C). The thermo-alkali pretreatment proved to be the best technique in terms of methane production increase. The specific methane production after thermal and thermo-alkali treatments increased by 37% and 45%, respectively, compared with the raw digested microalgae sample. However, after a preliminary cost/revenue analysis the most promising hydrolysis technique applied to the microalgae tested was found to be the thermal treatment.