Ionizing radiation applied to the pre-treatment of macroalgae biomass for biogas production

• Autor(a) principal: Batista, Joana Bastos
• Data de Publicação: 2023
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10362/161278
• topic: Anaerobic digestion; Anaerobic co-digestion; Brown algae biomass; Food waste biomass; Biogas; Ionizing radiation; Domínio/Área Científica::Engenharia e Tecnologia::Outras Engenharias e Tecnologias

Anaerobic digestion (AD) has shown to be one of the best approaches to food waste (FW) valor- ization, due to the added value of the final products (biogas and digestate) and less requirement for operational space and input energy. Macroalgae are widely available biomass due to their natural common occurrence in marine and freshwater ecosystems (such as algal blooms) and surplus from the cosmetic and pharmaceutical in- dustries. It can be an excellent co-substrate to co-digest with FW since it is expected that by performing this Anaerobic Co-Digestion (AcoD), an increase of the C/N ratio, avoidance of ammonia accumulation, increase in microbial diversity, and availability of macro- and micro-nutrients essential for microbial ac- tivity can occur. Even though macroalgae are rich in carbohydrates (especially in the cell wall), some of these are extremely complex polysaccharides that are not easily bio-hydrolyzed, making the enzymatic activity extremely slow and AcoD most likely inefficient. The use of pre-treatments to break down the polysaccharides appears to be the best approach to overthrow this problem. Ionizing radiation has many advantages to treating biomass when compared to the more traditional methods since it is simple to operate, doesn’t require the use of any chemical reagents, can deal with large amounts of product, has a high penetrating capacity with a high capacity for breaking long organic molecular chains, and can be carried out under atmospheric temperature and pressure conditions. This makes it an extremely energy-efficient pre-treatment. If the proper radiation dose is applied, it is expected a decrease in inter- and intra-molecular order of the macroalgae’s poly- saccharides in the cell, facilitating AcoD. Two hydraulic retention times (HRTs) were tested for the mono-AD of food waste (FW), and AcoD of FW and irradiated or non-irradiated brown algae (Saccharina latissima): When using an HRT of 16 days, mono-digesting FW had the highest biomethane (CH4) production rate and yield, followed by co- digesting FW and non-irradiated algae. The gamma radiation doses of 22 kGy at a dose rate (DR) of 0.6 kGy.h-1 (on dried and wet algae) and 28 kGy at a DR of 9 kGy.h-1 (on dried algae) appeared not to be effective in the reduction of the algae's polysaccharides’ complexity: Lower biogas and methane yields were observed in these assays. An increase in polysaccharides’ complexity (e.g., cross-linking) induced by gamma radiation occurred. Using an HRT of 25 days increased both biogas and CH4 yields for AcoD with irradiated and non-irradi- ated algae occurred, possibly by allowing a more stable digestion of the substrates. The use of a lower OLR and better microbial growth of methanogenic archaea might have impacted the observed rise in methane content since the time for them to double inside the bioreactor and to consume the provided feedstock increases. An increase in CO2 and a decrease in CH4 content happened on the co-digestion assays using irradiated and non-irradiated algae in both HRTs of 16 and 25 days. The addition of algae biomass might be increasing the quantity of easily digestible polysaccharides, enhancing the acidogenic and acetogenic stages of AD, leading to greater CO2 production and VFA content. Nonetheless, the addition of non-irradiated algae appears to have the most positive impact in AcoD at an HRT of 25 days, where the higher biogas and CH4 yields are obtained. Ionizing radiation as a pre-treatment of brown algae biomass in the tested conditions did not benefit the AcoD process.