Abstract
Spent coffee grounds (SCG) are one of the lignocellulosic biomasses that have gained much attention due to their high potential both in valorization and biomethane production. Previous studies have reported single processes that extract either fatty acids/lignin or biogas. In this study, an integrated physicochemical and biological process was investigated, which sequentially recovers lignin, fatty acid methyl esters (FAME) and biogas from the residue of SCG. The determination of optimal conditions for sequential separation was based on central composite design (CCD) and response surface methodology (RSM). Independent variables adopted in this study were reaction temperature (86.1–203.9 °C), concentration of sulfuric acid (0.0–6.4%v/v) and methanol to SCG ratio (1.3–4.7 mL/g). Under determined optimal conditions of 161.0 °C, 3.6% and 4.7 mL/g, lignin and FAME yields were estimated to be 55.5% and 62.4%, respectively. FAME extracted from SCG consisted of 41.7% C16 and 48.16% C18, which makes the extractives appropriate materials to convert into biodiesel. Results from Fourier transform infrared spectroscopy (FT-IR) further support that lignin and FAME extracted from SCG have structures similar to previously reported extractives from other lignocellulosic biomasses. The solid residue remaining after lignin and FAME extraction was anaerobically digested under mesophilic conditions, resulting in a methane yield of 36.0 mL-CH4/g-VSadded. This study is the first to introduce an integrated resource recovery platform capable of valorization of a municipal solid waste stream.
Highlights
Coffee is one of the most widely consumed beverages in the world and is second only to petroleum as a globally traded commodity [1]
Considering its lignin content, spent coffee grounds (SCG) were seen as a valuable source of lignin, which could be extracted with lipids by subsequent organosolv treatment
The results revealed that the developed model could closely estimate the actual amounts of extracting both lignin and fatty acid methyl esters (FAME) from SCG by organosolv treatment
Summary
Coffee is one of the most widely consumed beverages in the world and is second only to petroleum as a globally traded commodity [1]. Energies 2019, 12, 2360 environmental concerns of the bio-toxicity of caffeine, tannins and polyphenols in SCG [3]. This biowaste SCG are rich in a variety of organic compounds in the form of cellulose, hemicellulose, lignin, lipids and antioxidant compounds [4], representing a valuable energy source. Due to the organic richness of SCG, they have been subjects of anaerobic digestion (AD) in the quest to produce biogas using a sustainable biological approach [9]. AD is one of the most favored biological processes that can retrieve energy from solid wastes under anaerobic conditions. Since AD processes are mostly operated under the mesophilic temperature region (30–40 ◦ C), the resource recovery platform needs to be established primarily under mesophilic conditions
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