Integrated microalgae-bacteria modelling: application to an outdoor membrane photobioreactor (MPBR)

A mechanistic model describing the key interactions occurring in microalgae-bacteria consortia systems was developed and validated. The proposed model includes the most relevant features of microalgae, such as light dependence, endogenous respiration, growth, and nutrient consumption for different nutrient sources. The model is coupled to the plant-wide model BNRM2, including heterotrophic and nitrifying bacteria, and chemical precipitation processes, among others. A major novelty of the model is microalgae growth inhibition by nitrite. Validation was conducted using experimental data from a pilot-scale membrane photobioreactor (MPBR) fed with permeate from an anaerobic membrane bioreactor (AnMBR). Three experimental periods dealing with different interactions between nitrifying bacteria and microalgae were validated. The model was able to accurately represent the dynamics occurring in the MPBR, predicting the relative abundance of microalgae and bacteria over time. Specifically, >500 pairs of experimental and modeled data were evaluated, giving an average R² coefficient of 0.9902. The validated model was also used to evaluate different offline control strategies for enhancing process performance. For example, partial-nitrification resulting in NO₂-N accumulation (i.e., microalgae growth inhibition) could be avoided by increasing biomass retention time from 2.0 to 4.5 days. It has been also concluded that microalgae biomass growth rate could be also enhanced by punctually increasing the dilution rate, allowing to outcompete nitrifying bacteria.