Modelling Oxygenic Photogranules: Microbial Ecology and Process Performance

This work addresses the modelling of oxygenic photogranules (OPGs), by introducing a mathematical model which describes both the genesis and growth of photogranules and the related treatment process. The photogranule has been modelled as a free boundary domain with radial symmetry, which evolves over time as a result of microbial growth, attachment, and detachment processes. Hyperbolic and parabolic PDEs have been considered to model at mesoscale the transport and growth of sessile biomass and the diffusion and conversion of soluble substrates. The macroscale behavior of the system has been modelled through first order impulsive ordinary differential equations (IDEs), which reproduce a sequencing batch reactor (SBR) configuration. Phototrophic biomass has been considered for the first time in granular biofilms, and cyanobacteria and microalgae have been accounted separately, to model their metabolic differences. To describe the key role of cyanobacteria in the photogranulation process, the attachment velocity of all suspended microbial species has been modelled as a function of the cyanobacteria concentration in suspended form. The model takes into account the main biological processes involved in OPG-based systems: metabolic activity of cyanobacteria, microalgae, heterotrophic and nitrifying bacteria, microbial decay, extracellular polymeric substances (EPS) secretion, symbiotic and competitive interactions between different species, light-dark cycle, light attenuation across the granule, and photoinhibition phenomena. The model has been integrated numerically, and the results show its consistency in describing the photogranule evolution and ecology, and highlight the advantages of the OPG technology, analyzing the effects of the influent wastewater composition and light conditions on the process.