Pretreatment of spent coffee grounds with alkaline soju bottle-washing wastewater for enhanced biomethanation

Abstract

The production of spent coffee grounds (SCG) is steadily increasing, and its proper treatment has become an increasingly challenging task. This study explored the potential use of alkaline bottle-washing wastewater (BWW) from a soju (a Korean hard liquor) manufacturer as an alternative pretreatment agent to enhance the biodegradability and thus the biomethanation of SCG through anaerobic digestion. Thermo-alkaline pretreatment with BWW was effective in increasing the soluble organic fraction of SCG (i.e., improved bioavailability) at all temperatures tested (23-70 degrees C). The disintegration degree of the particulate organic matter in SCG ranged from 8.6 to 13.1% and increased with increasing pretreatment temperature. The 30-day methane yield (per g chemical oxygen demand (COD) of SCG fed) was significantly higher for the BWW-pretreated SCG (342.4-361.2 mL/g COD) than for the raw SCG (284.7 mL/g COD) and even for the NaOH-pretreated SCG (at the same pH as BWW, pH 12.3) tested for comparison (299.7-320.9 mL/g COD). This significant increase in methane production can be attributed to the presence of readily biodegradable ethanol derived from waste soju in BWW and the alkaline solubilization of SCG. Our results suggest a straightforward way to manage SCG and BWW together in a sustainable and economical manner.