The feasibility of using brown seaweed, Laminaria digitata, as feedstock for generating bioenergy and biomaterials

Abstract

The societal need to develop sustainable renewable energy sources has seen a recent increase in the amount of research on anaerobic digestion technologies. Biofuels from algae, known as third generation biofuels, are taking a lead interest in this regard. The characteristics of the biopolymer components of seaweed, particularly brown algae, make it suitable for methanogenic digestion, and brings advantages over other biofuel feedstocks which displace terrestrial food crops from agricultural production. This thesis investigates the feasibility of using brown seaweed, Laminaria digitata (LD), as a viable feedstock for continuous generation of bioenergy (methane) via the anaerobic digestion process, and biomaterial production from thermochemical processes. Results of methane yield from an initial bio-methane potential (BMP) assessment, using a modified BMP method, on pre-treated and dried samples gave yields of between 141 ± 5.77 mL CH4 gVS-1 and 207 ± 0.07 mL CH4 gVS-1. Analysis of the thermochemical properties of the seaweed by pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) identified sixty-four compounds present in all samples, twenty which have been previously reported as major pyrolysis products of Laminaria digitata. Proton Nuclear Magnetic Resonance (1H NMR) analysis of extracted sodium alginate (biomaterial) fraction, gave results in agreement with reported literature on mono and diad frequencies, homopolymeric mannuronic FM (0.36 - 0.46) (FMM = 0.33 - 0.47 ), guluronic (0.54 - 0.64) (FGG =0.19 - 0.25) blocks with alternating block fractions of (FGM =0.17 - 0.21) and (FMG = 0.17 - 0.21). The M/G ratio obtained (1.18 - 1.79) is an indication that the alginate extracted from L. digitata can be used to produce soft and elastic gels rather than brittle ones. Alginate is a major polysaccharide component of brown seaweeds which degrades to glyceraldehyde-3-phosphate and pyruvate as final products during anaerobic digestion. The triad frequencies (FGGG = 0.14 - 0.17, FMGM = 0.11 - 0.126, FGGM = FMGG = 0.05 - 0.09) and the average block lengths are (NG = 2.15 - 2.22 and NM = 2.61 - 3.85) were also evaluated. BMP studies on the effect of temperature on biogas production from L. digitata feedstock showed the trend 35 °C > 25 °C > 45 °C > 55 °C, similar results being found in continuous fermentations, with mesophilic (35 °C) reactors giving better 4 cumulative methane yield than thermophilic (55 °C) reactors. Optimisation of the process using a multivariate technique, fit and multiple regression model, showed the interaction terms for mesophilic and thermophilic reactors were the best indicators of optimal methane production compared to other terms. Research into the potential of mixed co-digestion of the L. digitata feedstock is important as it helps to overcome the limitations of using a mono-digestion feedstock of L. digitata, such as high hydrogen sulphide production, limited availability of L. digitata biomass, and seasonal variation in algal composition. Mono- and co-digestion of L. digitata (LD) with a stimulated food waste (SFW) were assessed using various mix ratios LD100:0%, LD90:10%, LD75:25%, LD50:50%. BMP results showed the co-digested mix ratios exhibited both antagonist (LD90:10%) and synergetic (LD75:25%) effects. In the continuous study, the mono-digestion of LD100:0% was characterized by an accumulation of high total volatile fatty acids (tVFA) concentrations, reduced pH, and an increased FOS: TAC ratio, when the organic loading rate (OLR) was increased, leading to reactor failure. It was proposed that co-digestion brought about the dilution of inhibitory compounds, faster acclimatization of microorganisms to high salinity (chloride) levels in the presence of low ammonia concentrations at high loading rate. Trace element supplementation (TES) during anaerobic digestion of the macroalgae feedstock in various mix ratios: control (TES 0), TES 1 (0.1 mg/l Se, 0.1 mg/l W), TES 2 (0.1 mg/l Se, 0.1 mg/l W, 0.5 mg/l Co, 0.1 mg/l Mo), TES 3 (0.1 mg/l Se, 0.1 mg/l W, 0.5 mg/l Co, 0.1 mg/l Mo, 0.5 mg/l Ni, 0.05 mg/l Cu) and TES 4 (0.1 mg/l Se, 0.1 mg/l W, 0.5 mg/l Co, 0.1 mg/l Mo, 0.5 mg/l Ni, 0.05 mg/l Cu, 0.5 mg/l Fe, 0.1 mg/l Zn) in batch reactors improved methane yield by 17% - 50%, and stimulated a steady digestion process in a continuous reactor when added weekly with increase in OLR compared to a reactor without trace element which led to reactor instability, and eventually failure.