Sequent production of proteins and biogas from Chlorella sp. via CO2 assisted hydrothermal treatment and anaerobic digestion

Abstract

Abstracts Chlorella sp., as a kind of microalgae with high proteins content and rigid cell walls, is a potential material for proteins recovery as poultry feed supplement source. This study proposed an algal biorefinery paradigm for sequent production of proteins and biogas from microalgae Chlorella sp. via CO2 assisted hydrothermal treatment and anaerobic digestion. The CO2 could be sustainably refilled as being a bioproduct of biogas. As a widely-used environmentally benign hydrothermal catalyst, the addition of CO2 could reduce treatment temperature and CO2 is easily removed by depressurizing. Microalgal cells were more disrupted with rising temperature from 70 °C to 150 °C, which facilitated the recovery of proteins. The recovery of proteins increased from 6.32% to 69.81%. After proteins recovery, the C/N ratio of microalgal residues was significantly higher than the untreated Chlorella sp. The process of proteins recovery could relieve the hydrolysis limitation of cell walls but reduce organics remained in microalgal residues. Biomethane production from microalgal residues exhibited a similar rising trend to proteins recovery. The highest cumulative biomethane production of 194.63 mL/gVS was achieved for the group at 130 °C, and the lowest value of 104.42 mL/gVS was obtained for the group at 70 °C. Excess microalgal cells disruption and side reactions led to the poor biomethane performance for the group at 150 °C, consistent with the decreased proteins recovery. During anaerobic digestion, the concentration of acetic acid distributed in a range of 0.05–0.44 g/L, which was dominant among total volatile fatty acids. The proteins as poultry feed supplement would be preferable to be considered as the main product, and the biomethane was considered as the side product. Carbon balance analysis revealed that most groups could basically meet the CO2 demand for HTT except the group at 150 °C. 130 °C was considered as the favorable treatment temperature level, leading to a proteins yield of 0.41 g/gVS and biomethane yield of 91.47 mL/gVS by Chlorella sp.