Augmentation of biochar in anaerobic digestion

• Main Author: Quintana Najera, Jessica
• Format: Dissertation
• Language: English
• Published: University of Leeds 2022

The augmentation of carbonaceous adsorbent materials, such as biochar and hydrochar, in anaerobic digestion (AD), provides several advantages. The benefits of these materials are attributed to specific material properties, such as porosity, surface area, alkalinity, and the presence of surface functional groups. Biochar can thus adsorb metabolites, provide a buffering effect, act as a support for the immobilisation of cells, and work as an intermediary for direct interspecies electron transfer (DIET) mechanisms. A primary advantage of AD technology is the possibility to use a large variety of biomass substrates, however, the complexity of the biomass often compromises the effectiveness of the microbial community, and in consequence biomethane production. Biochar augmentation is a prospective option for overcoming some of the inherent limitations observed in AD while improving the efficiency and stability of the digester. This area of research is gaining significant interest since the implementation of a low cost sustainable material for stabilising and improving AD is considered to have huge economic benefits. The efforts in the scientific community have focussed on using biochars generated from different feedstocks, produced under a range of conditions, and augmentation with AD of different substrates operating under different conditions, thereby exhibiting diverse outcomes. Even though the general trend is for biochar to improve AD performance, there are contradictory results that create a certain ambiguity. Therefore, it is crucial to determine the most important variables and mechanisms involved in biochar augmentation. In this work, biochar and hydrochar materials were produced and thoroughly characterised to correlate their final properties with feedstock composition, and the pyrolysis and hydrothermal carbonisation (HTC) conditions employed for their production. This research sets out to investigate the effect of biochar and hydrochar during the AD of both model and complex substrate, and to develop correlations between their performance and properties. Oak wood and water hyacinth biochars produced at intermediate temperatures improved biomethane production from cellulose, resulting in a doubling of methane production rate, while higher-temperature biochars have no significant effect on AD. The presence of oxygenated functional groups (OFGs) proved to be the most important feature for biochar amendment in AD. Conversely, seaweed biochars and hydrochars from all feedstocks exhibited an unfavourable effect on methane production, attributed to the presence of toxic or inhibitory by-products. Design of experiment (DOE) and optimisation was employed for investigating complex feedstocks during the digestion of water hyacinth and co-digestion of Chlorella vulgaris and cellulose. The variables of the study included biochar load, inoculum to substrate ratio (ISR), and C/N ratio. The findings indicate that biochar amendment is more beneficial under stressful conditions, such as low ISR, and unfavourable C/N ratios. Small amounts of biochar (up to 0.6 % of a volume basis) are sufficient for providing the mentioned benefits, whereas higher doses are shown to be detrimental to the process. Furthermore, the effect of biochar augmentation in AD resulting from this work in combination with a vast compilation of literature reports was evaluated by principal component analysis (PCA). The PCA agreed with the experimental results generated in this work and established that the biochar must possess a large availability of OFGs, and low ash content. This could be achieved by selecting low ash feedstock, most likely derived from woody biomass, and treated by slow pyrolysis at intermediate temperatures (450-550 °C). The PCA indicated that BC augmentation was more favourable for complex substrates and stressful conditions, such as low ISR. The results also suggest that all the mentioned benefits were subject to an appropriate biochar dosage ranging from 0.4 to 0.6 % (w/v). The integration of biochar and AD is shown to be an important approach for improving the biotransformation efficiency of biomass into biomethane. To help bridge the gap between these heterogeneous and complex technologies, the outcome of this work is to provide an understanding of the most important factors influencing biochar augmentation. This is of great relevance since such knowledge could guide the application and further development of biochar augmentation in AD in the future.