Pelagic Sargassum for energy and fertiliser production in the Caribbean: A case study on Barbados

In recent years pelagic Sargassum has invaded the coastlines of the Caribbean region, Gulf of Mexico, Florida and West Africa, triggering human health concerns and negatively impacting environmental and economic productivity. Sargassaceae are nutrient-dense and currently utilized as fertiliser and food, while extracts of their phytochemicals exhibit unique biosorption and medicinal properties. This macroalgae also shows biofuel potential but hitherto, methane recovery is low due to a carbon to nitrogen ratio below 20:1, the restricted bioavailability of structurally complex carbohydrates for degradation and high insoluble fibre, salt, polyphenol and sulfur content. To optimise the microbial bioconversion of this biomass, pre-treatment and co-fermentation with other substrates have been explored. This paper reviews the challenges associated with, and potential solutions for, Sargassum inundation, and provides a critical evaluation of its bioconversion to biogas and fertiliser using anaerobic digestion technology. As the Caribbean region is primarily impacted by drifting Sargassum blooms, the paper concludes with a case study on Barbados and investigates the feasibility of repurposing this brown macroalgae from landfill disposal to a feedstock for electricity and fertiliser production. The results of this study indicate that Sargassum mono-digestion is unsustainable for energy extraction given its low bioconversion efficiency and unpredictable influx volume. Alternatively, the co-digestion of these seaweeds with organic municipal solid waste is economically and energetically advantageous, potentially enhancing energy recovery by 5-fold. Notably, the hydrogen sulfide fraction of the biogas generated must be controlled given its corrosive properties and potential to effect co-generation engine damage and failure. Additional income can also be derived through the agricultural application of the digestate generated both locally and externally, following ammonia treatment and heavy metal stripping. Further research and pilot-scale studies are therefore necessary to support the utilisation of this marine biomass in commercial energy and fertiliser production.