Fuzzy Control Strategy for an Anaerobic Wastewater Treatment Process

S. Carlos-Hernandez, E. N. Sanchez and J. F. Beteau

Abstract
In this paper, a fuzzy control strategy (FCS) for an anaerobic wastewater treatment process is proposed in order to reject large disturbances on input substrate allowing a high methane production. This strategy is composed of: i) a state observer, which is based on a principal components analysis (PCA) and Takagi-Sugeno (TS) algorithm; it is designed to estimate variables hard to measure: biomass and substrate, ii) proportional-integral (PI) controllers based on a combination of the L/A(logarithm/antiloga - rithm) and fuzzy approaches; these controllers have variable gains and are designed to regulate bicarbonate in the reactor by two control actions: a base supplying (binc) and dilution rate (D) changes, iii) a TS supervisor which detects the process state, selects and applies the most adequate control action, allowing a smooth switching between open loop and closed loop. Applicability of the proposed structure in a completely stirred tank reactor (CSTR) is illustrated via simulations. The obtained results show that the process works in open loop in presence of small disturbances. For large disturbances, the supervisor allows the control actions to be applied avoiding washout; after that, the process returns to open loop operation. In general, the FCS improves the performances of the anaerobic process and is feasible for application in real processes, since the control scheme shows a good compromise between efficiency and complexity.

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Keywords
Fuzzy control, state estimation, anaerobic digestion, wastewater treatment