Modeling of photosynthesis and respiration rate for microalgae-bacteria consortia

In this article, the influence of culture conditions (irradiance, temperature, pH, and dissolved oxygen) on the photosynthesis and the respiration rates of microalgae-bacteria consortia in wastewater treatment was analyzed. Specifically, some short photo-respirometric experiments, simulating outdoor raceway reactors, were performed to evaluate the response of microalgae, heterotrophic bacteria, and nitrifying bacteria to variations in environmental parameters. Results demonstrate that irradiance is the most dominant variable to determine microalgae photosynthesis rates. However, reduction in microalgae activity was not observed at higher irradiance, ruling out the existence of photoinhibition phenomena. Related to heterotrophic and nitrifying bacteria, their activities were strongly affected by the influence of temperature and pH. Moreover, the effect of dissolved oxygen concentrations on microalgae, and bacteria activities was studied, displaying a reduced photosynthetic rate at dissolved oxygen concentrations above 20 mg/L. Data have been used to develop an integrated model for each population (microalgae, heterotrophic bacteria, and nitrifying bacteria) based on considering the simultaneous influence of irradiance, temperature, pH, and dissolved oxygen. The models fit the experimental results in the range of culture conditions tested, and they were validated using data obtained by the simultaneous modifications of the variables. These individual models serve as a basis for developing a global biologic microalgae-bacteria model for wastewater treatment to improve the optimal design and management of microalgae-based processes, especially outdoors, where the cultures are subject to variable daily culture conditions.