2.2. Process flow: biological stage

equipped with three Flygt submersible centrifugal pumps P1.A, P1.B, and P1.S (two active pumps (A and B) and one for backup (S)). The water level is measured by using an ultrasonic transducer L1 and is kept between two limits (preestablished limits depending on average water flow). By using the level information, the pumps act to keep the level into the limits. Also, the level information is transmitted to the process computer in the central control room. The water exits from the pumps P1.A, P1.B, and P1.S through three vertical pipes, which at the superior part of the tank (in the valve room) pass to a horizontal configuration and after that merge into a collector. From the valve room, a pipeline goes to the preliminary treatment

Figure 1. Block diagram (WTP Calafat).

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Figure 2. WTP Calafat (aerial photo).

The biological tanks consist of two biological reactors (bioreactors) and two settlers (sedimentation tanks). These are circular tanks, positioned in a concentric manner, with the settler in the inner part and the biological reactor in the exterior. The walls of the tanks (5 m height) are from special glassed steel. The walls are embedded into concrete structures plated with Izocor hydro-isolation. The external diameter is d<sup>1</sup> = 35.16 m, the volume is V = 3800 m3 , and the inner diameter is d<sup>2</sup> = 18.86 m. At the biological reactor, the bottom is plane, but the bottom of the settler is in the shape of a truncated cone. The bioreactor is divided in two zones, anoxic and aerobic, by using two steel walls, radially disposed. The ratio of the volumes is 30% anoxic/ 70% aerobic.

In the anoxic zone of each bioreactor, the wastewater from the pretreatment is mixed with the activated sludge which is recirculated by the Flygt pumps RAS/SAS P3.A, P3.B, P3.S. These pumps are controlled with frequency converters, and the flow ratio is 1:1. An equal flow of mixture from the aerobic zone is pumped through a slit from the zone separation wall by the internal recirculation pumps P2.A and P2.B (Flygt type, with frequency converters). The mixers placed in this anoxic zone achieve the homogenization of the three inputs (wastewater, activated sludge, aerobic mixture). In the anoxic zone, the next actions are achieved:


From the anoxic zone, the compound passes into the aerobic zone, where the biochemical oxidation of the organic matter is achieved. The needed oxygen is provided from the air delivered by the BOC Edwards air blowers A1.A, A1.B, and A1.S and bubbled as thin bubbles 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 information is used to control the air blower speed by using frequency converters.

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 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).

> 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

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

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• Sensors (analogic) for flows, levels, pressures, pH, and DO concentrations, such as electromagnetic sensors (Siemens, 0–10 m/s water speed) for flows, ultrasonic sensors (Siemens, 0.3–8 m) for level measurements, pH, and DO concentrations measured via

• Contact sensors that provide data about the state of some equipment and operations

• Dedicated devices for various operations such as dosages, recipes, and technological

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

integrated measurement systems (4–20 mA) for dosing devices, etc.

• Final control elements (actuators) such as control valves, pumps, etc.

• On/off elements for various actions such as pump starting, etc.

processing, which will interact with the DCS at the monitoring level

components of this level are:

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

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

