Anaerobic digestion of nut and coffee wastes: different pretreatment techniques to enhance methane production
Date
2022-07-25Author
Oliva, Armando
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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.