Performance of an active disturbance rejection control on a simulated continuous microalgae photobioreactor

Microalgae are used for the industrial production of high value compounds. The aim in continuous bioreactors is to obtain the highest biomass production. It is necessary to guarantee that the bioprocesses attain and maintain the optimal reference biomass CX*(t), despite endogenous and exogenous disturbances. This paper describes the numerical simulation of the application of active disturbance rejection control (ADRC) to control the dilution rate (D(t)) in a continuous culture of the microalga Chlorella vulgaris. To reduce the bioprocess to a “SISO” system, the authors chose the dilution rate, D(t), to be the only control signal. The control proposal was illustrated and evaluated through a numerical simulation using MATLAB/Simulink™ environment. The performance of the ADRC was tested by the application of external perturbations and variation of parameters over a nominal case. At nominal conditions, D(t) was always maintained within the physical limits imposed by the bioprocess. Step and smooth type signals, at 96.4%·|Dmax(t)|, were imposed as external perturbation on the control signal input, D(t). The controller response kept the output signal CX(t) within an insignificant 0.0043%·|CXmax(t)|. The algal culture had a strongly asymmetric response to variations of the ideal maximum growth rate, μmax(t) ± 30%·|μmax(t)|, and of the nominal light intensity, Iin(t) ± 30%·|Iin(t)|. Nonetheless, the controller promptly returned the output signal to its reference value, CX(t)*. The numerical test of the control proposal, in summary, showed that the ADRC strategy ensures excellent reference tracking capability and robustness towards parametric uncertainties, un-modeled dynamics, and external disturbances.