Production of microalgae biomass in a continuous stirred bioreactor: Analysis of microalgae-bacteria competition mediated by nitrogen and organic carbon

•Continuous heterotrophic production of microalgae in presence of competing bacteria.•Microalgae-bacteria competition can be controlled by the feed nitrogen to carbon ratio.•Analytical expressions derived for stability limits of bioreactor steady states.•The stable microalgae solution always coexists with the stable bacteria solution.•Global stability of the microalgae-bacteria consortia solution can be attained. A model-based analysis of the competition between microalgae and bacteria mediated by nitrogen and organic carbon in a continuous bioreactor with recycle is presented. The analysis is aimed to evaluate if the ability of microalgae to uncouple nitrogen and carbon uptake can allow to prevent the contamination of bacteria with negligible carbon accumulation or to sustain the coexistence of microalgae and bacteria. To this purpose, a Droop-like model and a multiplicative Monod model are used to describe microalgae and bacteria growth kinetics, respectively. The regions of the bioreactor parameter space ensuring steady state stability of microalgae monocultures, microalgae-bacteria consortia and bacteria monocultures are identified by analytical bifurcation analysis of the mass balance equations governing bioreactor dynamics. Expressions in closed form are derived for the stability boundaries of the steady states as bioreactor parameters vary, providing guidelines for plant design and control.