1. Introduction

In this chapter, a control architecture developed at the wastewater treatment plant (WTP) Calafat (located in Oltenia region, Romania) is presented. This control structure was developed in the frame of research project ADCOSBIO (no. 211/2014, UEFISCDI) [1] and contract no. 168/ 2017, University of Craiova-Water Company Oltenia (WCO). More precisely, a distributed control system (DCS)-supervisory control and data acquisition (SCADA) architecture was proposed, which is organized as a distributed and hierarchized control system. This control

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

solution envisaged the wastewater treatment plant Calafat but can be adapted and implemented for other similar wastewater treatment plants from the WCO.

coping with the problems posed by both the uncertainties in the inputs and the incomplete

Distributed Control Systems for a Wastewater Treatment Plant: Architectures and Advanced Control Solutions

http://dx.doi.org/10.5772/intechopen.74827

155

In this chapter, some of our previous results [12] are extended for WTP Calafat in order to design multivariable adaptive and robust control algorithms. The proposed control strategies are able to handle the model uncertainties of an activated sludge process used for removal of two pollutants carried out in a continuous recycle reactor. The main control objective is to maintain the pollution level at a desired low value despite the load and concentration variations of the pollutant. The adaptive control scheme is designed by combining a linearizing control law with a state asymptotic observer and with an estimator used for online estimation of unknown kinetics. The robust control structure is designed as a linearizing control law plus an interval observer able to estimate lower and upper bounds in which the unmeasured states are guaranteed to lie. Moreover, the uncertain process parameters are replaced by their lower

The chapter is organized as follows. In Section 2, the general characteristics of the WTP Calafat and the process flow are presented. Section 3 proposes a distributed control solution for the wastewater treatment process from WTP Calafat. The control architecture and the primary control loops are analyzed. In Section 4, the design of multivariable adaptive and robust control schemes for the activated sludge process is provided. The behavior of the proposed control algorithms is analyzed by performing realistic simulation experiments. The final con-

The general characteristics of the wastewater treatment plant (WTP) Calafat and the process flow will be presented. The process flow comprises two stages: the pretreatment (which is the so-called mechanical stage) and the biological stage. Figure 1 presents the general block

of maximum 29,000 inhabitants of Calafat town, predicted for 2020. The treatment method is a classical one, with a mechanical stage for the impurity removal and a biological stage based on activated sludge. The wastewater enters in a tank; it is lifted with special pumps to the pretreatment area and after that is gravitationally discharged in the biological tanks, where the water is aerated and mixed with the activated sludge. Thus, the biodegradation of the

The process technological lines will be succinctly described. After the entering in the WTP, the influent wastewater passes through a bar screen (the gross filter) to remove all large objects, and after that it flows in a gravitationally way through a slit in the pump room. This unit is

/h. The WTP size was chosen in order to solve the needs of a number

/day and of a

diagram of the WTP, and Figure 2 shows an aerial photography of the WTP. The WTP was designed for the treatment of a daily average flow of 8366 m3

water occurs. Finally, the effluent is discarded through decantation.

knowledge of process kinetics [15–18].

and upper bounds assumed known.

clusions are presented in Section 5.

maximum flow of 530 m3

2.1. Process flow: pretreatment

2. Description of the technological process flow

The wastewater treatment is a process operated to convert wastewater into an effluent that can be returned to the water cycle with minimal impact on the environment. This process takes place in a wastewater treatment plant (WTP) [2]. In a WTP, the treatment usually comprises three stages, called primary, secondary, and tertiary treatments [3]. Primary treatment consists in the mechanical removing of settled and floating materials, and the remaining liquid can be discharged or directed to secondary treatment. Secondary treatment removes dissolved and suspended biological matter, and it is typically performed by microorganisms in a special habitat. The goal of tertiary treatment is to provide a final treatment stage to improve the effluent quality before it is released to the environment. The treatment method used at the WTP Calafat is a classical one, with a mechanical stage for the impurity removal and a biological stage based on activated sludge. The proposed control solution for this WTP is based on a DCS structure.

The paradigm of DCSs is related to the control of medium and high complexity processes, and it consists in the implementation of distributed and hierarchized systems in a number of four to five levels [4]. The two main attributes of the DCS should be mentioned here: the horizontal functionality in each level is managed by a real-time operating system, and the communication between levels is characterized by the network used in the DCS. Currently, some modern technologies from the networks and processing devices are incorporated into the DCSs [5, 6]. The DCS-SCADA solution for the WTP Calafat consists in four levels: the field level (level 0), the direct control level (level 1), the plant supervisory level (level 2), and the production control/regional coordination level (level 3). In this chapter, the structure of the first three levels and their functionality are presented. The primary control loops implemented at level 1 of DCS-WTP Calafat are described. Also, due to the fact that the performance improvement of the WTP control system is possible only by managing the activated sludge bioprocess, some advanced control solutions based on nonlinear adaptive and robust control algorithms are proposed for level 2 of the DCS.

The activated sludge process implemented at WTP Calafat is an aerobic process, highly nonlinear and characterized by parametric uncertainties [3, 7–11]. The best-known model that tries to describe the activated sludge processes is ASM1 (Activated Sludge Model No. 1) [3, 10–12]. The main drawback of ASM1 is its complexity, such that it becomes unfeasible for control. Thus, in this chapter a simplified model of the activated sludge process will be used. The model is based on the model of Nejjari et al. [8], adapted for WTP Calafat.

Several control strategies were developed for bioprocesses, such as linearizing strategy, adaptive approach [3, 7–9], robust and optimal control, sliding mode control [13], model predictive control [14], etc. Yet, in all these schemes, the knowledge of all inputs is required. Unfortunately, for wastewater treatment processes, usually, the complete knowledge of inputs is not available. For these cases, interval observers (or set observers) were developed in the last period, allowing the reconstruction of a guaranteed interval for the unmeasured states instead of estimating their precise numerical values. The only requirement is to know an interval in which the unmeasured inputs of the process evolve. These robust observers are capable of coping with the problems posed by both the uncertainties in the inputs and the incomplete knowledge of process kinetics [15–18].

In this chapter, some of our previous results [12] are extended for WTP Calafat in order to design multivariable adaptive and robust control algorithms. The proposed control strategies are able to handle the model uncertainties of an activated sludge process used for removal of two pollutants carried out in a continuous recycle reactor. The main control objective is to maintain the pollution level at a desired low value despite the load and concentration variations of the pollutant. The adaptive control scheme is designed by combining a linearizing control law with a state asymptotic observer and with an estimator used for online estimation of unknown kinetics. The robust control structure is designed as a linearizing control law plus an interval observer able to estimate lower and upper bounds in which the unmeasured states are guaranteed to lie. Moreover, the uncertain process parameters are replaced by their lower and upper bounds assumed known.

The chapter is organized as follows. In Section 2, the general characteristics of the WTP Calafat and the process flow are presented. Section 3 proposes a distributed control solution for the wastewater treatment process from WTP Calafat. The control architecture and the primary control loops are analyzed. In Section 4, the design of multivariable adaptive and robust control schemes for the activated sludge process is provided. The behavior of the proposed control algorithms is analyzed by performing realistic simulation experiments. The final conclusions are presented in Section 5.
