Economic MPC of Wastewater Treatment Plants: Distributed Computing and Model Reduction

• Author / Creator

  Zhang, An

• Wastewater treatment plays an important role in the sustainable development of our society. A wastewater treatment plant (WWTP) is typically a large-scale nonlin- ear process composed of several interconnected operating units. To meet the strict environmental regulations, to ensure the operation safety and to reduce the operat- ing cost, it is highly desired to monitor and operate WWTPs effectively. However, significant variations in the inlet flow rate and wastewater compositions have made the monitoring and control of WWTPs a very challenging task. Centralized eco- nomic model predictive control (EMPC) approach has been proposed to improve the control performance based on the economic considerations. However, the high com- putational complexity caused by solving the associated EMPC optimization problem can render the online implementation of this method intractable. In recent years, the distributed framework has been considered to be a promising framework to im- prove the applicability of EMPC for large-scale processes. Model linearization and model order reduction are also widely used to reduce the computational complexity of complex control problems.
  This thesis focuses on improving the computational efficiency of EMPC for WWTPs. Two distributed EMPC designs are presented in this thesis. In the first design, the centralized model is used in each subsystem EMPC controller design; and in the second design, a subsystem model is used in each subsystem EMPC design. The performance of these two distributed EMPC designs are compared with a centralized model predictive control (MPC) scheme and a centralized EMPC scheme from dif- ferent aspects including effluent quality, operating cost, and computational efficiency. It is found through vast simulations that the distributed EMPC with subsystem con- troller designed based on the entire system model is more favorable in terms of control performance.
  Model reduction methods are also applied to the WWTP process in this thesis. In particular, the trajectory piecewise linear model and the order reduced trajectory piecewise linear model are used to approximate the original nonlinear system model. Two EMPC designs are proposed based on the two models. The approximated model accuracy are compared with the original nonlinear model. The performance of these two EMPC designs are compared with the EMPC based on the nonlinear model from control performance and computational efficiency points of view. We also investigate how the number of linearization points affect the EMPC control performance and computational efficiency through these applications. It is shown that there is a trade- off between the control performance and computational efficiency. The EMPC design based on the order reduced trajectory piecewise linear (TPWL-POD) model is more favorable since it significantly reduces the computational cost although degrades the control performance slightly.

• Subjects / Keywords

  • wastewater treatment plant
  • distributed control
  • model reduction
  • Economic MPC

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-0g0d-6f90

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.