Abstract
Purple phototrophic bacteria (PPB) use infrared light to generate microbial energy, with electron,carbon and nitrogen supplied chemically. They have broad applicability for resource recovery fromwastes and wastewater, as they have potential value as microbial product and very high growth yieldson substrate. Although PPB processes are gaining relevance in the aforementioned industries, reactordesign, particularly effective consideration of light in chemical kinetics and hydraulics is a criticalissue. This thesis aims to assess key limitations and define their physical behaviour mathematically.The first limitation was that there was no mixed culture PPB process model describing the behaviourof PPB in the presence of ammonia, organic matter and other nutrients. The mixed population modelsof the International Water Association (IWA) provide a good framework for process modelling andcontrol. As such, a process model for a mixed-culture PPB system was developed based on fivekey processes: photoheterotrophic growth, photoautotrophic growth, chemoheterotrophic growth,hydrolysis/fermentation, and decay. This model was developed as a set of ordinary differentialequations varying in time and lumped in space (as for the IWA models). The second limitationis that the radiative field has not been considered. This can vary spatially and temporally. Theradiative field in a PPB system attenuates sharply, which influences the growth domains. A CFDmodelling framework was therefore developed in OpenFOAM to describe the spatial variations andinteractions between biomass growth, the flow field and the radiative field. It was found that lumpedmodelling approaches and distributed parameter approaches differed in results based on differentreactor domains and behaviours. The deviation from lumped parameter behaviour was greater for acylindrical stirred reactor than for a flat plate reactor. Therefore, spatial considerations are necessaryin the design phase of a photobioreactor. Finally, biofilms are a critical aspect in PPB growth, anda coupled biofilm radiative transfer model has not been previously considered. The model was thusextended to include biofilm formation of a mixed PPB system in the presence of a spatially varyingradiative field. A volume-of-fluid approach was used as a basis for this model, considering threeparticulate species (phototrophic bacteria, biodegradable particulate matter, and inert particulates). Theradiative-solid-liquid coupling overall was found to be critical in operation, with traditional lumpedparameter approaches to design and analysis not well suited to the emerging challenges of mixedculture photo-bio systems. These aspects should be further considered through model based analysisand experimental validation for future system design.
Published Version
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