Abstract
The extraction of lipids from microalgae cells of Botryococcus braunii and Chlorella vulgaris after ultrasonic and microwave pretreatment was evaluated. Cell disruption increased the lipid extraction efficiency, and microwave pretreatment was more effective compared with ultrasonic pretreatment. The maximum lipid yield from B. braunii was 56.42% using microwave radiation and 39.61% for ultrasonication, while from C. vulgaris, it was respectively 41.31% and 35.28%. The fatty acid composition in the lipid extracts was also analyzed. The methane yield from the residual extracted biomass pretreated by microwaves ranged from 148 to 185 NmL CH4/g VS for C. vulgaris and from 128 to 142 NmL CH4/g VS for B. braunii. In the case of ultrasonic pretreatment, the methane production was between 168 and 208 NmL CH4/g VS for C. vulgaris, while for B. braunii ranging from 150 to 174 NmL CH4/g VS. Anaerobic digestion showed that lipid-extracted biomass presented lower methane yield than non-lipid-extracted feedstock, and higher amount of lipid obtained in the extraction contributed less methane production. Anyway, anaerobic digestion of the residual extracted biomass can be a suitable method to increase economic viability of energy recovery from microalgae.
Highlights
Nowadays, there is an increasing demand for energy carriers obtained from renewable sources arising in harmony and respect for the natural environment
The lipid yield from B. braunii biomass without pretreatment was 34.04%, while from C. vulgaris was 26.26% (p < 0.05) (Fig. 1). Both pretreatment methods significantly enhanced the amount of lipid obtained from both microalgae biomasses
Microwave disintegration was more effective in relation to B. braunii and the maximum lipid yield was over 56%
Summary
There is an increasing demand for energy carriers obtained from renewable sources arising in harmony and respect for the natural environment. Physical pretreatment was found to be a high-energy and cost-intensive process; according to the literature, the most promising method for cell disintegration is the use of microwaves and ultrasounds [17]. The heat treatment causes a deep penetration of microwaves through the cell wall structure of microalgae, enhancing the lipid extraction efficiency [25]. In many cases, there is no economic justification for the pretreatment due to the high costs of feedstock preparation In this way, the use of initially disintegrated microalgal biomass after the lipid extraction may increase economic viability of the process because the potential of energy gain may be relatively higher than that obtained in unit operations. The first research objective of the study was to compare the efficiency of ultrasonic and microwave pretreatment of Chlorella vulgaris and Botryococcus braunii microalgae on the lipid yield and composition of the fatty acids in the lipid extracts.
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