Production of Methane Biofuel From Anaerobic Digestion of Brown Seaweeds

Abstract

Decomposing beach cast seaweeds release methane (potent greenhouse gas) and unpleasant odour. Collection and anaerobic digestion (AD) of beach cast seaweeds overcomes this issue. "Inoculum 8" was used in all AD studies and comprised equal proportions of North Ronaldsay sheep (seaweed diet) nimen and faeces, normal sheep (grass diet) rumen and faeces), municipal AD leachates, human sewage AD leachates and marine sediments with rotting seaweed. Phlorotannin (PT) toxicity studies employed Ascophyllum nodosum and Fucus serratus. Both brown seaweeds were greatly inhibitory to inoculum 8. Heating the seaweed to 80°C for 1 hour followed by precipitation with polyvinylpolypyrrolidone (PVPP) or polyethylene glycol (PEG) removed/neutralised PT. Addition of PEG directly into AD fermenters reduced toxicity and resulted in improved biogas production. Also, metagenomic analysis of inoculum 8 and its constituents involved qPCR, DGGE (denaturing gradient gel
electrophoresis) and 454-pyrosequencing. Leachate samples from triplicate AD fermenters of inoculum 8 (taken over 4 weeks), and its constituents, were studied to see how the inoculum 8 microbiota developed over this period, and how its constituent microbiota compared. The qPCR results showed changes in the microbiota at each week during both hydrolytic and methanogenic stages. DGGE showed several distinct and singular bands on the bacterial gels and fewer, but distinct bands on the archaeal gels. Multivariate analysis showed degree of similarity among bacterial and archaeal samples suggesting the dominant bands involved in hydrolysis and methanogenesis respectively. Pyrosequencing analysis confirmed these results providing a detailed description of the various bacterial and archaeal species present and showed that inoculum 8 microbiota developed uniquely compared to its constituents. Finally, during AD, seaweed stored in vacuum bags over a period of six months produced significantly more biogas compared to fresh/frozen seaweed. These results will help optimise the AD of brown seaweed on a larger scale.

Date of Award	2021
Original language	English
Awarding Institution	Glasgow Caledonian University
Sponsors	Marie Curie Senior Researcher Fellowship & Dunbar Marine Resources Ltd.
Supervisor	Alastair Sutherland (Supervisor), Adrian Pierotti (Supervisor) & Rodrigo Costa (Supervisor)