Treatment of coffee waste by slurry-state anaerobic digestion

Abstract

Slurries containing 20% (w/v) coffee waste solids were treated anaerobically in one- and two-phase thermophilic methane fermentation systems (53°C) with or without pH control. In one-phase methane fermentation using a roller bottle reactor, the maximum gas evolution rate of 0.87 l/l·d was achieved during treatment for 91 d. However, this one-phase methane fermentation did not yield reproducible data. In a two-phase methane fermentation system consisting of a completely stirred tank reactor type (CSTR-type) liquefaction reactor without pH control and an anaerobic fluidized bed type gasification reactor, three-repetitions of treatment were conducted. Each treatment was very stable and the average gas evolution rate per volume of the gasification reactor was about 2.4 l/l·d. Two-repetitions of treatment were then done while controlling pH in the liquefaction at more than 6. The average gas evolution rate per volume of gasification reactor was found to have increased to 10.2 l/l·d, a value which corresponded to 0.84 l/l·d per total volume, including the liquefaction reactor. It was observed that treatment in a two-phase methane fermentation could be repeated in a stable fashion even in the closed system without discharging anything but the coffee waste residues.