Evaluation of different strategies to produce biofuels from Nannochloropsis oculata and Chlorella vulgaris

•Organic solvents influence lipid and biodiesel yields, and waste biodegradability.•Lipid-extracted microalgae reach similar or higher CH4 yields than raw microalgae.•The SIR hardly affects the CH4 production from N. oculata and C. vulgaris.•Microalgae co-digestion with sewage sludge has no antagonistic effects.

The lipid extraction using hexane and methanol:hexane increased the biodegradability of Nannochloropsis oculata by 36% and 24% respectively. Moreover, hexane increased the methane production from raw microalgae, from 253 ± 11 to 313 ± 9 mLCH4/gVS. Methanol:hexane did not affect the methane production, which yielded 254 ± 10 mLCH4/gVS, mainly due to the significant changes in the biomass composition.On the other hand, the lipid extraction failed to increase the biodegradability of Chlorella vulgaris, which resulted around 44% for raw and lipid-extracted microalgae. The methane productions were 219 ± 6, 202 ± 1 and 200 ± 4 mLCH4/gVS from raw and pre-treated microalgae using hexane and methanol:hexane respectively.Regarding the lipid extraction yields, using methanol:hexane the yields were 4.7 and 3.7 times higher for N. oculata and C. vulgaris than using hexane. The biodiesel yields were also higher using methanol:hexane, 2.4 and 1.9 times than using hexane. However, the biodiesel composition was unaffected by the solvent.The substrate to inoculum ratio influenced raw N. oculata digestion. At 1:1 VSSubstrate:VSInoculum, the methane production throughout the first days decreased but not the ultimate methane production. C. vulgaris digestion was unaffected, probably due to the biomass characteristics.Finally, the co-digestion of microalgae and sewage sludge showed no synergy, nor inhibition.

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