Anaerobic degradation of enzymatically pretreated Chlorella vulgaris was aimed in an upflow anaerobic sludge blanket reactor (UASB) to evaluate the organic loading rate (OLR) effect on biomass valorization. Low OLRs resulted in high methane yields (171 mL CH4/g CODin) at low hydraulic retention time (HRT of 6 days). Firmicutes (35-43%), Bacteroidetes (17-18%) and Euryarchaeota (11%) dominated at low OLRs, promoting methanogenic activity. On the contrary, the highest OLRs resulted in low methane yield (86 mL CH4/gCODin) with a concomitant short-chain fatty acids (SCFAs) accumulation of 37% SCFAs-COD/CODin. The highest OLR decreased UASB reactor biodiversity, hampering Euryarchaeota population development (2.5%) and boosting Firmicutes (55%) and Proteobacteria (14%). These results demonstrated the suitability of UASB reactor configuration to reach high bioprocess efficiency for both, biogas and SCFAs production, with lower energetic and area requirements than those normally needed in continuous stirred tank reactors.