3. A distributed control solution for the wastewater treatment process: WTP Calafat

#### 3.1. The control architecture

The proposed DCS-SCADA solution for the WTP Calafat is presented in Figure 3. The levels of the DCS and their functionality are described in the next paragraphs.

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Figure 3. The structure of the DCS with four levels (WTP Calafat).

by using polymeric membranes. Two technological variables are very important for the aerobic process: the dissolved oxygen (DO) and the pH of the mixture. These are measured and indicated by using the transducers Q1 and Q2 (for DO concentration) and Q3 and Q4, respectively, (for pH). The air is blown by the air blowers A1.A, A1.B, and A1.S (two active and one for backup) through galvanized steel pipes. The air pressure and temperature can be monitored by using the local devices. The airflows at the two tanks are measured by using the flow meters F1 and F2. The flow control is necessary in order to maintain the DO concentration between the specified values. The DO is measured by using the sensors Q1 and Q2, and the

The mixture from the aerobic zone arrives at the partition wall where one-third from the flow is taken by the recirculation pumps P2.A and P2.B and delivered to the anoxic zones and twothirds from the flow is passed through a pipeline (via the communicating vessel principle) into the central pipe of the settler. From here the mixture exits in a radial and uniform way at the superior part. The effluent is separated from the sludge and after that is gravitationally removed through a circular drain. Finally, the effluent is flushed in the Danube through a channel. The settlers are equipped with radial scrapper bridges, which have a double goal:

• The superior scrapper collects the foam and directs it to a foam-collecting chamber.

• The decomposition of the organic matters by using enzymes (enzymatic reactions)

3. A distributed control solution for the wastewater treatment process:

The proposed DCS-SCADA solution for the WTP Calafat is presented in Figure 3. The levels of

• Oxygen consumption for endogenic respiration and biochemical oxidation

As a conclusion, in the biological tanks, the following processes occur:

• Assimilation of some components by the microorganisms

• Nitrification and denitrification • The removal of the excess sludge

WTP Calafat

158 Wastewater and Water Quality

3.1. The control architecture

• Microorganism growth (increase of the activated sludge mass)

the DCS and their functionality are described in the next paragraphs.

• The inferior scrapper cleans the sediments and directs the sludge to a discharge whirl.

The activated sludge is transported to a collector from which is exhausted through three ramifications by the pumps RAS/SAS P3.A, P3.B, P3.S (RAS is the recirculated activated sludge at the anoxic zones; SAS is the surplus activated sludge, which is carried to a special tank).

information is used to control the air blower speed by using frequency converters.

Level 0 contains all the field devices placed at the technological process level. Classically, at this level we have the measurements of various variables and the final control elements. The components of this level are:


Level 1 comprises the data acquisition devices and the controllers, including the real-time data processing. The analogic signals from sensors and also the control inputs to the actuators are unified signals (e.g., currents in the range 4–20 mA). Due to the geographical distribution, the WTP control system is implemented with several PLCs (programmable logic controllers). These PLCs are connected into a master-slave network with extensions, which handles the 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 algorithms and function of various operational regimes.

3.2. Level 1: primary control loops

concentration control in the biological tanks (aerators).

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

A1.A and for the line B is A1.B (A1.S is a backup air blower).

WTP Calafat will be described.

In this section, a few numbers of primary control loops that are implemented at level 1 of DCS-

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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

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

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

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

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 communication problems, startups, and maintenance.

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 at the regional level.
