2.1. Process flow: pretreatment

solution envisaged the wastewater treatment plant Calafat but can be adapted and implemented

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

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

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

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

for other similar wastewater treatment plants from the WCO.

based on a DCS structure.

154 Wastewater and Water Quality

proposed for level 2 of the DCS.

on the model of Nejjari et al. [8], adapted for WTP Calafat.

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

(pretreatment: mechanical stage). This plant is placed on a metallic structure at the +6.40 m

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An electromagnetic flow meter is placed on the vertical part of the pipe, and it is used to measure the hourly flow of the wastewater provided by the pumps P1.A, P1.B, and P1.S. By opening two sluice valves located at the entrance of the channels used for thin filtering, the wastewater resulted from the pumps P1.A, P1.B, and P1.S passes through the channels and is filtrated by using some rotational filters. The thin impurities which are separated by the filters

The wastewater enters in a tangential manner in the workers, and, under the mixer action, the water has a descendant spiral movement. After that, the wastewater is lifted through a central pipe, and finally it is evacuated through a radial pipe. This movement, due to the gravitational and centrifugal forces, allows the sedimentation of the solids in the lower part of the workers. The fat and grease floating on the surface are collected by the skimmers. The solid particles are removed by opening the sliding valves, and thus these particles are periodically drained via a spiral conveyer. The grit is cleared into a special tank. After the pretreatment, the water passes via a pipeline which ramifies at the superior part of the biological tanks in the anoxic zones.

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

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/

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,

• An appropriate ratio between the substrate (the organic content of the wastewater +

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

activated sludge, aerobic mixture). In the anoxic zone, the next actions are achieved:

nutrients) and the microorganisms (the active content of the sludge)

• The denitrification process (the nitrogen removal)

, and the inner

hydro-isolation. The external diameter is d<sup>1</sup> = 35.16 m, the volume is V = 3800 m3

are then discharged on a conveyer belt and stored into a special tank.

elevation.

2.2. Process flow: biological stage

70% aerobic.

Figure 1. Block diagram (WTP Calafat).

Figure 2. WTP Calafat (aerial photo).

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

(pretreatment: mechanical stage). This plant is placed on a metallic structure at the +6.40 m elevation.

An electromagnetic flow meter is placed on the vertical part of the pipe, and it is used to measure the hourly flow of the wastewater provided by the pumps P1.A, P1.B, and P1.S. By opening two sluice valves located at the entrance of the channels used for thin filtering, the wastewater resulted from the pumps P1.A, P1.B, and P1.S passes through the channels and is filtrated by using some rotational filters. The thin impurities which are separated by the filters are then discharged on a conveyer belt and stored into a special tank.

The wastewater enters in a tangential manner in the workers, and, under the mixer action, the water has a descendant spiral movement. After that, the wastewater is lifted through a central pipe, and finally it is evacuated through a radial pipe. This movement, due to the gravitational and centrifugal forces, allows the sedimentation of the solids in the lower part of the workers. The fat and grease floating on the surface are collected by the skimmers. The solid particles are removed by opening the sliding valves, and thus these particles are periodically drained via a spiral conveyer. The grit is cleared into a special tank. After the pretreatment, the water passes via a pipeline which ramifies at the superior part of the biological tanks in the anoxic zones.
