Biofuel and volatile fatty acids production from raw and pretreated spent coffee grounds in semi-continuous bioreactors

This study shows biofuels and biocommodities, i.e., hydrogen, volatile fatty acids (VFAs), and methane, can be produced in semi-continuous bioreactors from spent coffee grounds (SCG). The semi-continuous reactor operated first on untreated SCG (Reactor 1) followed by feeding ultrasound-pretreated SCG (Reactor 2) to improve the hydrolysis of the substrate. Hydrogen production was unstable in both phases, peaking at 2.8 mL H2/g VS/d for raw SCG, while the ultrasound-pretreated SCG reached 11.1 mL H2/g VS/d. Increments in the relative abundance of Clostridia species correlated with the peaks in hydrogen production in reactors 1 and 2. In terms of VFAs, the raw SCG gave a peak concentration of 3051.7 mg HAceq/L, while the ultrasound-pretreated SCG returned up to 4202.2 mg HAceq/L. In both cases, caproic acid accumulated (1727.5 and 2518.9 mg HAceq/L for, respectively Reactor 1 and 2), with peak concentrations corresponding to increases in the relative abundance of the bacterial genus Caproiciproducens. To further assess the potential of SCG, the VFAs-rich effluent from reactor 2 was fed into an upflow granular sludge reactor (T – 37 °C, HRT – 96 h, OLR – 14 g VS/L/d). The granular sludge was rich in Methanosaeta (Halobacterota phylum), which gave a mean methane production of 9.0 mL CH4/g VS/d at a pH ranging between 4 and 5.