Anaerobic digestion of nut and coffee wastes: different pretreatment techniques to enhance methane production
Oliva, Armando
Oliva, Armando
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Publication Date
2022-07-25
Keywords
nut waste, coffee wastes, pretreatment techniques, methane production, Science and Engineering, Science, Natural Sciences, Microbiology, Lignocellulosic materials, Anaerobic digestion, Methane, Fed-batch reactor, Pretreatment, Organosolv, NMMO, Ultrasounds, Hazelnut skin, Almond shells, Spent coffee grounds, Polyphenols, Sugars
Type
Thesis
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Abstract
Lignocellulosic materials (LMs) are the most abundant residues on the planet and have a great potential for methane production. Nevertheless, the energy potential of LMs for biofuel production is limited by their complex structure. LMs are composed of cellulose, hemicellulose, lignin and non-bound matter, which include free sugars, polyphenols, protein and lipids. This PhD thesis investigated the impact of organosolv, N-methylmorpholine N-oxide (NMMO)- driven, and ultrasounds pretreatment on the methane production potential of hazelnut skin (HS), almond shell (AS), and spent coffee grounds (SCG). The first experimental phase (Chapter 3) investigated a methanol-organosolv pretreatment performed at 130, 160, and 200 °C with and without catalyst addition. The biochemical methane potential (BMP) of HS increased up to 18-folds, and the catalyst addition allowed lowering of the pretreatment temperature. On the contrary, all pretreatment conditions failed to enhance the BMP of SCG and AS. In Chapter 4, a swelling mode NMMO pretreatment was performed for 1, 3, and 5 h. The NMMO pretreatment enhanced the BMP of AS up to 58%. The pretreated SCG showed increased porosity (up to 63%) and a higher sugar percentage (up to 27%) despite failing to increase the methane production. All pretreatment conditions were effective on HS, achieving the highest methane production of 400.4 mL CH4/g VS after increasing the sugar (up to 112%) and reducing the lignin (up to 29%) content. Chapter 5 focused on ultrasound pretreatment. The liquid fraction of ultrasound pretreated HS was particularly rich in polyphenols (up to 11.5 g/L) and sugars (up to 13.2 g/L), showing great potential for biomolecules recovery. The liquid fraction from ultrasound pretreated AS and SCG are suitable for valorisation through anaerobic digestion (AD). The solid residues recovered after ultrasounds were used for methane production and a similar BMP compared to the raw LMs was obtained. Chapter 6 investigated the fed-batch AD of raw, macerated, and methanol-organosolv pretreated HS, focusing on the factors impacting the process in the long term. An efficient reactor configuration was proposed to increase the substrate load while reducing the solid retention time during the fed-batch AD of HS. Maceration and methanol-organosolv pretreatment were used to remove polyphenols from HS (i.e. 82 and 97% removal, respectively) and improve HS biodegradation. Additionally, organosolv pretreatment removed 9% of the lignin. The organosolv-pretreated HS showed an increment in methane production potential of 21%, while macerated HS produced less methane than the raw substrate, probably due to the loss of non-structural sugars during maceration.
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Publisher
NUI Galway