Enhancing biomethane production from anaerobic digestion of Sargassum muticum

Higher methane yields from the anaerobic digestion (AD) of Sargassum muticum could improve the process energy balance and make its use during AD more economical and energetically favourable. Previous research showed that after 28 days of AD, methane yields from S. muticum were low (17% of the theoretical yield). This thesis aims to identify the causes of the low methane yield. Biochemical methane potential tests of freshly harvested, rinsed, and freeze-dried (FD) spring and summer S. muticum sampled over three years delivered yields of 27–39% and 24–32% of the theoretical, respectively. FD samples were extracted with water or with 70% (v/v) aqueous methanol (MeOH); methane yields per gram volatile solids of the extracted samples were higher than the untreated FD samples by up to 19.1% and 26.6%, respectively. Proximate, ultimate, and biochemical analyses showed that untreated FD biomass contained ash contents of 24.2–28.1% dry weight, with total dietary fibre representing 49.8– 67.4% of the organic fraction. Water- or MeOH-extracted spring and summer biomass were higher in total dietary fibre content (75.3–82.8% of the organic fraction) and lower in soluble dietary fibre (SDF) and phenolic content than the FD samples. Indices calculated for bioconversion of the biomass to methane were negatively correlated with the SDF and phenolic contents. Water-extracted spring biomass had lower SDF content than the FD and water- extracted summer samples and produced higher methane yields. The aqueous MeOH extract of S. muticum was examined after repeated extraction (×9) with 1% polyvinylpolypyrrolidone (PVPP). The PVPP-treated extract was 93.7% lower in phenolic content (Folin-Ciocalteu assay) and 24.4% higher in protein content than the untreated extract, with no significant difference in the lipid contents. MeOH-extracted biomass combined with the PVPP-treated extract produced 85.7% higher methane yields than when combined with untreated MeOH extracts. Membrane filtration of the untreated MeOH extract yielded a high molecular weight (MW) (≥ 5 kDa) fraction, which contained 90.7% of the total phenolic content of the extract. It inhibited methane yields by 41.9% when combined with MeOH-extracted biomass; however, since the methane yields of the extracted biomass remained low (≤ 32% of the theoretical, an increase from ≤ 27%), recalcitrance of the total dietary fibre also represents a limiting factor. Improving methane conversion of the residual fibre fraction after phenolics’ extraction is required to utilise this biomass in a biorefinery approach efficiently.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Greenwich

Year dc:date.issued

2022

Author dc:creator

• Maneein, Supattra

Advisors dc:contributor.advisor

• Nielsen, Birthe
• Milledge, John

• SB Plant culture

Language dc:language

en