Improving the anaerobic co-digestion of pelagic Sargassum and food waste into biogas and a bio-fertiliser using hydrothermal pre-treatment: Concept to application in the Caribbean

Abstract

In recent years pelagic Sargassum has invaded the coastlines of the Caribbean region, Gulf of Mexico, Florida and West Africa. With intense expansion since 2011 due to environmental and oceanic anthropogenic changes, these brown seaweeds have negatively impacted aquaculture and the overall productivity of several major sectors in the Caribbean region. Sargassaceae are nutrient-dense and currently utilized as bio-fertiliser and food, while extracts of their phytochemicals exhibit unique biosorption and medicinal properties. This macroalgae has also been studied as potential feedstock for biofuel production and a favourable alternative for conventional fossil fuels, given its low lignin and high polysaccharide content. However, the restricted bioavailability of structurally complex carbohydrates for microbial bioconversion, carbon to nitrogen ratio below 20:1 and the presence of recalcitrant compounds such high insoluble fibre, salt, polyphenol and sulfur content result in a low biomethane potential. Hydrothermal pre-treatment and co-digestion are viable processes which can be applied to these brown seaweeds for biogas enhancement. Hydrothermal pretreatment is green technology which utilizes pressurised liquid “hot water” to enhance biomass hydrolysis and solubilisation, thereby increasing the surface area for microbial attack and improving the efficiency of bioconversion downstream. Consequently, the energy yield from hydrothermally pretreated feedstock is superior to that achieved from the unprocessed substrate. Contrariwise, co-digestion of two organic substrates can amend the carbon to nitrogen ratio, optimise the micro- and macronutrient balance and dilute the effect of anaerobic digestion inhibitory constituents, for higher biomethanation. Of added value, the effluent of biogas production, a solid-liquid digestate, is nutrient-dense and pathogen free, exhibiting application in agricultural practice as an organic bio-fertiliser or soil conditioner. Presently, research on the hydrothermal pretreatment of pelagic Sargassum and its co-digestion with food waste is outstanding in literature. This research project aims to fill the knowledge gap by investigating the effect of hydrothermal pre-treatment on the mono-digestion of pelagic Sargassum. Different pretreatment operation conditions of temperature and processing time were tested to validate the hypothesis. The physicochemical properties of the raw and hydrothermally pre-treated substrates were analysed using a variety of established elementary and spectroscopic techniques. Post fermentation, the biogas composition was evaluated and the optimum processing condition for biogas production identified. Pelagic Sargassum is unsustainable feedstock for energy extraction owing to its low bioconversion and unpredictable influx volume. As such, the next phase of this study explored the co-digestion performance of hydrothermally pretreated Sargassum (optimum processing condition) and food waste. Food waste is viable feedstock for microbial degradation given its rich organic content which promotes higher energy production. This biomass is also sustainable, annually increasing in volume with population growth. In small and industry-scale operations, food waste would be suitable co-substrate for Sargassum to enhance anaerobic digestion efficiency and process performance. Additionally, given the unpredictability of Sargassum influx, food waste can be utilised as the primary substrate for biogas production, with Sargassum incorporated into the input feed during periods of massive beaching. The agronomic potential of the digestate from biogas production was assessed utilising physicochemical and spectroscopic analytical techniques. Following the success of the lab-scale study, a comprehensive techno-economic assessment was performed to evaluate the economic feasibility of technology optimisation and process upscale in Barbados. The findings of this research show that the application of hydrothermal pre-treatment to pelagic Sargassum at 140 ºC for 30 min or severity factor 2.65 accelerated the hydrolysis of organic matter, thereby increasing biomethanation by 237 % when compared to that of the unpretreated feedstock. Moreover, this technology proved advantageous in diminishing the hydrogen sulfide content in the biogas generated from 3 % to 1 %. Co-digestion of hydrothermally pretreated pelagic Sargassum and food waste redistributed metal elements and raised the buffering capacity of the digester, facilitating high organic loadings without pH control. Food waste also supplied lipids to the seaweed feed which augmented the digestion performance. Using a blend of co-pretreated pelagic Sargassum and food waste at the dry weight ratio 25:75, methane production increased by approximately 1.5- and 2.5-fold, when compared to that yielded from mono-digestion of hydrothermally pretreated food waste and hydrothermally pretreated Sargassum, respectively. Screening of the solid-liquid digestate recovered from both mono- and co-digestion studies indicate bio-fertiliser potential. However, the solid fraction of this effluent necessitates remediation of the toxic heavy metals arsenic and mercury to satisfy international soil standard guidelines for utilisation in agricultural practice. From the techno-economic assessment conducted, the implementation of an integrated Sargassum-based biorefinery equipped with hydrothermal pretreatment and anaerobic digestion technologies would offer many socio-economic and environmental advantages to Barbados through the supply of electricity to the national energy grid and production of a bio-fertiliser for local and international utilisation. Details of these results are presented within this thesis. The Caribbean region is primarily impacted by drifting Sargassum blooms and the information disseminated in this thesis would prove crucial in providing regional governments with a viable management approach of repurposing of these brown invasive seaweeds from typical landfill disposal to a feedstock for energy production and bio-fertiliser recovery.