3.2. Level 1: primary control loops

information and takes the required decisions for the coordination of the entire technological process. The acquired data, the decisions, and the events occurred in the process are communicated to the next level (SCADA) in order to be used and displayed on the graphical monitoring interface. The information flow is bidirectional; that is, the PLCs receive information about global decisions or optimization, such as set points for the control loops, switches between operational regimes, etc. The decisions at this level are taken in real time and such that the operation of the overall process is managed. The primary control loops are implemented at the PLC level, but the set point of each loop is provided by the superior hierarchic level (SCADA). The PLCs also achieve the implementation of the direct commands delivered by the control

Level 2 contains the equipment and the devices from the control room, which receive the information from level 1 (PLC level) and supervise the global operation of the WTP. This level is represented by the SCADA/HMI (human-machine interface) system. The main functions achieved at this level consist in operation optimization, implementation of adaptive and robust control algorithms (proposed in Section 4), operation monitoring via graphical interfaces, remote operation mechanisms, data/event storage, achievement of a data historian, etc. Also, the SCADA ensures the communication between the local dispatcher room (WTP Calafat) and the regional dispatcher of Water Company Oltenia, by using a GSM/GPRS system. The SCADA/HMI runs on two redundant servers. The SCADA supervises the direct control system (real time). If the SCADA system stops, the process will be automatically operated by using the PLCs. Several protection and backup procedures are incorporated in the operation and supervisor programs. The most of control and data acquisition devices used for levels 1 and 2 are provided by Siemens, Telemecanique, etc.

Level 3 is the regional dispatcher of the WCO, and it will coordinate the activity of the WTP

The operation regimes allowed by the DCS are as follows: automatic, manual via the computer, and locally manual. Automatic: The WTP control is achieved exclusively through the command/decision provided by the DCS (PLCs + SCADA/HMI). Manual via the computer: The DCS works only as a data acquisition system, but the decisions are taken by the human operator and are transmitted via PLCs and SCADA to the actuators. The system offers all the information and keeps the inter-blockings at the software and hardware levels. This regime can be achieved for the entire WTP or only for some components. Locally manual: This regime implies the local operation, no matter what regime is set at the PLCs or SCADA levels. This regime is of high priority, but the SCADA will signalize at the dispatcher level in this situation, and the event will be stored. This regime is useful when failures occur or in the case of network

The integration of local SCADA in regional SCADA. The regional SCADA system is a regional centralized structure, which implies the organization of a regional dispatcher, equipped with reliable industrial devices, disposed in a redundant topology in order to ensure a continuous operation. The regional dispatcher role is to coordinate all the subnetworks from the urban areas. The local dispatchers are in fact local process networks from each urban area, which acquire and handle the primary information from the process (levels, flows, concentrations, telemetry, diagnostic signals, etc.). This information is available for the local operator but also

with respect of the performance and of extended monitoring of the geographical area.

algorithms and function of various operational regimes.

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communication problems, startups, and maintenance.

at the regional level.

In this section, a few numbers of primary control loops that are implemented at level 1 of DCS-WTP Calafat will be described.

The control loop 1 ensures the level control in the admission room. The level is measured with the sensor L1, and the control action is achieved via the wastewater pumps P1.A, P1.B, and P1.S (two active pumps (A and B) and one for backup (S)). The block diagram of this control loop is presented in Figure 4. The level set point is preestablished, since the influent flow in the WTP fluctuates. If the level decreases under a limit, the pumps are shut down (decision at the PLC level). This critical situation is transmitted to the SCADA system, where a warning signal will be displayed/stored. The control loop is a classical feedback loop, with a possible PID control law plus a switching mechanism. The control loop is implemented at level 1 of DCS, but the set point (the reference) is provided by level 2. The actuators are the three pumps P1.A, P1.B, and P1.S that have the motors controlled with static frequency converters in order to provide a variable flow. The switching mechanism (switching logic) of the pumps is designed to ensure a rotation in the operation of the pumps. This fact is done to avoid the unevenly wear of the pumps but also for the failure situations. The influent wastewater flows are disturbances for the loop and will be rejected by the control law.

The control loop 2 is designed for the control of dissolved oxygen (DO) concentrations, which are measured with the transducers Q1 and Q2, and the control is provided by the air blowers A1.A, A1.B, and A1.S. The control loops are presented in Figure 5 and are dedicated to the DO concentration control in the biological tanks (aerators).

The control laws are PIDs (with self-tuning facilities), but also some advanced control laws can be implemented. As in the previous case, the control loop is implemented at level 1 of DCS, but the set point is provided by level 2. The actuator for the technological line A is the air blower A1.A and for the line B is A1.B (A1.S is a backup air blower).

The control loop 3 is dedicated to the regulation of the recirculated flow percentage calculated from the aerated water flow (from the input of the distribution room). This process variable (percent) is processed by using an algorithm with several input arguments such as dissolved

Figure 4. Level control loop: pumps room from the WTP admission.

Figure 5. Control loops for the dissolved oxygen concentration.

oxygen, suspended solids, nitrogen, and phosphorus provided by the automated extraction probe system. The set point is given by the operator via the SCADA system, and the control action is based on the emulsion sludge pumps P2.A and P2.B from the aerators. These two control loops (block diagrams in Figure 6) are independent because we have two biological treatment tanks. The control loop is implemented at level 1 of DCS, and the set point is provided by level 2. The actuator for the technological line A is the recirculation pump P2.A, and for the line B is the pump P2.B.

As in the previous cases, the control loop is implemented at level 1 of DCS, and the set point is provided by level 2. The actuators are the submersible pumps P3.A, P3.B, and P3.S. The switching mechanism (switching logic) of the pumps is designed in order to cover the above-described scenarios. The influent wastewater flows and their parameters are disturbances for the control

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In the following sections, some advanced control solutions are proposed in order to be implemented at level 2 of the DCS-WTP Calafat. More precisely, multivariable adaptive and robust control algorithms are proposed for the activated sludge process that takes place at WTP Calafat. The main control objective at this level is to maintain the pollution level at a desired low value despite the load and concentration variations of the pollutant. The controlled variables are the concentrations of pollutant and dissolved oxygen inside the aerator. Therefore, some of the control loops described in the previous section will be used, and other loops will be modified. The simulations performed in realistic conditions and using an adapted model of the activated sludge process showed that the performance of the overall control system can be increased. The implementation of the proposed control algorithms at

4. Advanced control solutions for the activated sludge bioprocess

WTP Calafat will be ensured within the research project TISIPRO [19].

4.1. Dynamical model of the activated sludge bioprocess and control objective

The activated sludge process which works at WTP Calafat is an aerobic process of biological wastewater treatment. As it was mentioned above, this process is operated in at least two interconnected tanks: a bioreactor (aerator) in which the biodegradation of the pollutants takes place and a sedimentation tank (settler) in which the liquid is clarified (the biomass is separated from the treated wastewater) (Figure 8). This bioprocess is very complex, highly nonlinear, and characterized by parametric uncertainties. In the literature there are many models that try to describe the activated sludge processes. The best-known model is ASM1 (Activated Sludge Model No. 1) [3, 10–12]. The main drawback of ASM1 is its complexity, such that it becomes unusable in control issues. Thus, in this chapter a simplified model of a process

loop and will be rejected by the control law.

Figure 7. Control loop for the ratio: activated sludge flow/wastewater flow.

The control loop 4 is designed to control the ratio (flow F1)/(flow F2), which is the activated sludge flow introduced in the influent wastewater flow, by using the submersible pumps P3.A, P3.B, and P3.S. This control loop is presented in Figure 7. The backup pump P3.S will act:


Figure 6. Control loops for the recirculation flows.

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Figure 7. Control loop for the ratio: activated sludge flow/wastewater flow.

oxygen, suspended solids, nitrogen, and phosphorus provided by the automated extraction probe system. The set point is given by the operator via the SCADA system, and the control action is based on the emulsion sludge pumps P2.A and P2.B from the aerators. These two control loops (block diagrams in Figure 6) are independent because we have two biological treatment tanks. The control loop is implemented at level 1 of DCS, and the set point is provided by level 2. The actuator for the technological line A is the recirculation pump P2.A,

The control loop 4 is designed to control the ratio (flow F1)/(flow F2), which is the activated sludge flow introduced in the influent wastewater flow, by using the submersible pumps P3.A, P3.B, and P3.S. This control loop is presented in Figure 7. The backup pump P3.S will act:

• When additional flows of wastewater occur and the active pumps cannot provide the

• Periodically (scheduled by the operator), to ensure a uniform usage of the pumps

and for the line B is the pump P2.B.

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required activated sludge flow

Figure 6. Control loops for the recirculation flows.

• When some failures occur at the active pumps

Figure 5. Control loops for the dissolved oxygen concentration.

As in the previous cases, the control loop is implemented at level 1 of DCS, and the set point is provided by level 2. The actuators are the submersible pumps P3.A, P3.B, and P3.S. The switching mechanism (switching logic) of the pumps is designed in order to cover the above-described scenarios. The influent wastewater flows and their parameters are disturbances for the control loop and will be rejected by the control law.
