Evaluation of two control strategies for a sequencing batch reactor degrading high concentration peaks of 4-chlorophenol

The operation of a sequencing batch reactor (SBR) exposed to high concentration peaks (shock loads) of a toxic compound (4-chlorophenol, 4CP) was evaluated. Two control strategies based on on-line measurements of the dissolved oxygen concentration were tested. The first strategy, called variable timing control (VTC), detects the end of the reaction period to stop it. In the second control strategy, called observer-based time optimal control (OB-TOC), the automated system tries to maintain the critical specific growth rate by controlling the feed rate, i.e. the maximum growth rate when the substrate is toxic. The system operating under the VTC strategy presented a stable and efficient operation when the acclimated microorganisms (to an initial concentration of 350 mg 4CP/L) were exposed to punctual concentration peaks of 700 mg 4CP/L. A 4CP concentration peak higher than or equal to 1050 mg/L disturbed the system (1 month to recover). A 1400 mg/L peak caused strong inhibition that shut down the metabolic activity of the microorganisms, leading to reactor failure. With the OB-TOC strategy, the system was stable and worked efficiently when punctual concentration peaks of 700, 1050 and 1400 mg 4CP/L were fed. The system controlled by the OB-TOC strategy treated 1400 mg 4CP/L in less than 8 h without affecting the operation of the reactor. The conclusion is that the OB-TOC strategy is more efficient than the VTC strategy to control a bioreactor when there are variations of concentrations of toxic organic compounds.