A $H_{\infty }$ control for optimizing the advanced oxidation processes—Case of a catalytic ozonation reactor

Highlights

• The closed-loop control for water treatment by AOP gives significant reagent savings.

• We address the problem of disturbance attenuation through the $H_{\infty }$ synthesis.

• The set-point tracking is achieved by PI controllers.

• A closed-loop stability analysis with respect to time delays is developed.

• All developments are applied to a catalytic ozonation pilot.

Abstract

In wastewater treatment, advanced oxidation processes are largely accepted as efficient technologies. Unfortunately, in general, these approaches present a high operating cost due to an open-loop implementation, i.e. the inlet reagent is introduced in excess in order to respect discharge standards. Therefore a mixture between automatic control techniques and chemical engineering expertise should significantly improve and expand their industrial applications via closed-loop implementations. More precisely, the present paper deals with a catalytic ozonation reactor where the main objective is to reduce the oxygen consumption by minimizing the ozone overproduction. For this end, a $H_{\infty }$ control strategy is used in order to obtain a significant abatement of the pollutant, and by the way illustrates the gain one can expect when applying such techniques to this kind of process.

Introduction

An advanced oxidation process (AOP) in wastewater treatment is a technology based on a chemical treatment (Metcalf & Eddy, Inc., 2002). This treatment involves the introduction of reagents, ozone or hydrogen peroxide for instance, which leads to the development of hydroxyl radicals to oxidize organic pollutants (Gogate and Pandit, 2004, Oller et al., 2011). There are a large number of applications such as domestic and industrial wastewater effluents, sludge and membrane concentrates. Nevertheless, the main problem, which hinders a wide AOP industrial development, is the high operating cost due to an open-loop implementation. In such an implementation, the inlet reagent is introduced in excess in order to respect discharge standards. A closed-loop AOP control could easily reduce reagents consumption, and hence the operating costs.

The first problem with the AOP control is the online measurement of contaminant concentration or contamination abatement, the output signal to be controlled. An artificial neural network approach was developed to predict the Chemical Oxygen Demand (COD) removal in Hamzaoui et al. (2011). A biosensor based on gas phase monitoring was designed to measure the hydrogen peroxide concentrations in Modrzejewska, Guwy, Dinsdale, and Hawkes (2007). It is worth to mention also a specific continuous monitoring of hydrogen peroxide formed in situ was proposed by Oh, Kim, Kang, Oh, and Kang (2005), or a model-free learning control to deal with the chemical characteristic variations with time is given by Syafiie, Tadeo, Martinez, and Alvarez (2011).

However, even today, many wastewater treatment companies hesitate to invest in such a measurement equipment to implement AOP in a closed-loop control. They prefer to keep open-loop processes and their excessive consumption.

In this paper, the goal is to consider a single application to quantify the potential reagent savings with a closed-loop implementation (Abouzlam, 2014). Thus, we hope to convince industry to invest and to give AOP technologies a better place in wastewater treatment (Oller et al., 2011).

The considered wastewater treatment pilot is based on the catalytic ozonation, an innovative AOP (Legube and Karpel Vel Leitner, 1999, Luck et al., 1997, Pontlevoy et al., 2003). The pollutant to be removed is the paranitrophenol which presents the advantage to have a concentration measurable via its absorbance near the visible spectrum (around 340 nm). With this application, the goal is to reduce the oxygen consumption by minimizing the ozone overproduction with a closed-loop control.

For the same application, an internal model control application is described in Abouzlam et al. (2012a). In Abouzlam et al. (2013), an optimal control (H₂) strategy was adopted. In the present paper, we address the problem of disturbance attenuation through the $H_{\infty }$ synthesis. This attenuation is achieved by the inclusion of weighting transfer functions. A closed-loop simulation with a nonlinear model of the pilot is developed to test different weighting transfer tunings in order to assist the user for the selection of an efficient controller. The set-point tracking is achieved by PI controllers. And a closed-loop stability analysis is dedicated to this new controller structure.

The paper is organized as follows. Section 2 describes the catalytic ozonation pilot. The problem is formulated in Section 3. Section 4 presents preliminary studies on the catalytic ozonation pilot. Section 5 describes the $H_{\infty }$ control implementation, and Section 6 a stability analysis. The experimental results are given in Section 7.