Recent advances and challenges in mechanistic modelling of photosynthetic processes for wastewater treatment

Phototrophy-based wastewater treatment has the potential to reduce wastewater bioremediation costs, improving environmental impacts and allowing for enhanced resource recovery. Microbial interactions occurring in phototrophic-chemotrophic consortia treating wastewater are particularly complex, and with varying impact on each clade by different chemical, biological and physical, including light—related factors. For this reason, mechanistic mathematical modelling of these systems is challenging, and the resulting models are especially complex. In this work, we focus particularly on the extension of microalgae-focused models to simulation of phototrophic-chemotrophic systems, especially as for (i) microalgae-bacteria and (ii) purple bacteria-enriched communities. The review identifies model structures and typical modelling choices, as well as the potential applications and limitations of available experimental protocols for model calibration, identifying relevant research needs and requirements. Simplified models have been proposed, which allows assessment of dominating mechanisms, but may not represent more complex behaviour, including nutrient removal and response to light cycling, and have been largely applied to simple (oxygen and carbon dioxide) exchange between algae and aerobic heterotrophs. More comprehensive models including all relevant microbial clades have been recently published, which consider nutrient cycling, competitive uptake, and other features, including temperature, pH, and gas transfer. These models have comparable structures but can be difficult to compare in a quantitative manner, often due to different fundamental stoichiometry (e.g., in the assumed algae composition), or in differing approaches to storage compounds. Particularly for models with a high complexity, it is often difficult to properly estimate biokinetic species-specific parameters for the different phototrophic and chemotrophic populations involved. Several methods have been proposed for model calibration, among which photo-respirometry has shown high potential. Photo-respirometric methods do not follow a standardised approach, which has limited their application and comparability between studies. Finally, the validation of models on long term data sets, demonstrating the impact of seasonality, as well as long term population adaptation is rare.