AbstractThe use of alternative feedstock sources to enhance the energy production of anaerobic systems, and thus their economic value, is one of the current research areas in the field of bioenergy production. Marine biomass represents a unique source of organic matter for the optimisation of anaerobic digestion systems and can be regarded as a sustainable alternative to purposely grown energy crops requiring significant amounts of water, fertiliser and land for their cultivation. Seaweeds are of particular interest as they are characterised by high biomass yields and interesting conversion rates. In temperate seas, brown seaweed species generally dominate the flora and their relative abundance on the sublittoral zone of the British coastline make them a substrate of choice for anaerobic digestion. However, little information is available on commercial-scale anaerobic digestion of seaweed for biogas production and the potential factors that could impair its successful conversion. This work was proposed in order to establish the potential and optimise the use of seaweed as an additional source of organic matter for anaerobic digesters. The study also investigated the use of the Anaerobic Digestion Model No.1 (ADM1) as a platform for process simulation. The model original structure is inadequate to accurately represent the anaerobic co-digestion of seaweed and was therefore updated with the addition of specific processes. The study was carried out in three main experimental stages. In a first stage, the effect of seaweed salinity (represented by sodium ions) on anaerobic digestion was investigated using a mesophilic laboratory-scale anaerobic digester. It was found that a rapid increase in sodium ion levels can negatively impact on biogas production and result in the accumulation of volatile fatty acids. The ADM1 does not originally take into account the inhibitory effect of sodium and was therefore modified to include a function representing the effect of sodium ions on the rate of acetate uptake. The extended model was able to reproduce experimental observations and was used to predict the effect of sodium ions in the presence of other process inhibitors. Microbial adaptation to salinity was also investigated during batch assays. It was found that a suitable period of adaptation can significantly reduce the adverse effect of salinity on methanogens. The phenomenon was successfully implemented in the model through the addition of a specific inhibition function and the calibration of kinetic parameters. The second stage of this research focused on the effect and mode of action of phlorotannin (a phenolic compound found exclusively in brown seaweed) on mixed microbial cultures through the monitoring of intracellular material leakage and transmission electron microscopy observations. Results suggested that phlorotannin induces strong extra- and intra-cellular effects on cells exposed to
the compound, thus adversely impacting on energy requirements and final methane yields. The effect of phlorotannin was found to be dependent on both the degree of polymerisation of the compound and the morphology of microorganisms. Furthermore, the effect of phlorotannin during the anaerobic co-digestion of brown seaweed (Laminaria digitata) and vegetable residues was also investigated. Experimental results were successfully modelled using an extensively modified version of the ADM1, which introduces an uncompetitive function to the rate of acetate uptake in order to represent the inhibition of methanogenesis by phlorotannin. The model was also updated with a combination module for the simulation of co-digestion processes. The third stage focused on establishing operational guidelines for the anaerobic co-digestion of brown seaweed and non-saline feedstocks. Results suggested that although seaweed can be an alternative organic substrate in anaerobic digestion systems, phlorotannin content might limit its use for commercial-scale application. Whilst this study identified salinity and phlorotannin as key barriers to the use of brown seaweed as a substrate for anaerobic systems, the adaptation of operating conditions to favour microbial adaptation could lead to its effective use in large-scale applications.
|Date of Award||2013|
|Supervisor||Joseph Akunna (Supervisor) & Phillip Collier (Supervisor)|
- Anaerobic digestion
- Brown seaweed