Spent coffee grounds anaerobic digestion: Investigating substrate to inoculum ratio and dilute acid thermal pretreatment

Spent coffee grounds have the potential of being used in further bioprocesses to produce materials and fuels. In Norway, the relative abundance and ease of collection of this waste substrate make it a candidate for investigation. For this study, the substrate-to-inoculum ratio as well as a combined dilute acid-thermal pretreatment were assessed by a series of biochemical methane potential assays using spent coffee grounds as a substrate. Reactors with substrate-to-inoculum ratio 2 demonstrated a relatively low hydrolysis rate constant (k_h) and comparatively high volatile fatty acids/alkalinity concentrations rendering them inapt to produce bio-CH₄. Pretreatment was conducted over varying contact times (15–45 min), dilute acid concentrations (1.5–2.5 %, v/v), and liquid-to-solid ratios (10–20 %, v/w) and evaluated using response surface methodology. To determine bio-CH₄ yield, pretreatment time and the interaction between acid concentration and liquid-to-solid ratio are considered significant variables, suggesting a shared importance. Chemical oxygen demand_removal is primarily contingent upon changes in liquid-to-solid ratio. Finally, Fourier-transform infrared spectroscopy of the discarded solid phase showed that the major functional groups are still widely present in the coffee grounds even after pretreatment was applied. A better understanding of the biodegradability profile of spent coffee grounds as a function of substrate-to-inoculum ratio is achieved.