Mieczyslaw Metzger, Witold Nocon

Sedimentation is one of the most widely used techniques of separation in the chemical, mineral and wastewater treatment processes. The process takes place in a settler, where solid particles suspended in a liquid are settling downward, leaving clear water at the top of the settler and concentrated slurry at the bottom. This downward movement of solids is caused by gravitational force. Two types of the sedimentation process can by distinguished: continuous sedimentation (solids are continuously fed into the settler) and batch sedimentation (the settler is filled with liquid containing suspended solids and settling occurs afterwards).

An experimental batch sedimentation pilot-plant has been designed and developed in the Control Systems and Control Equipment Group at the Institute of Automatic Control. This pilot-plant is schematically shown in Figure 1, and its view is shown in Figure 2. The sedimentation process takes place in the settler where the level of liquid is measured by a pressure transducer (y2). The cleared water is removed from the settler by a Masterflex peristaltic pump (u2: on/off or continuous flow control) and the suction nozzle is mounted on the float. A turbidity sensor (y1) is mounted on the same float and is used to indicate the presence of solids in the water being removed form the settler. The sensor itself was developed in the CSCE group is designed to be a low cost indicator of sludge blanket presence. The cleared water is fed into the supply tank from which it can be returned to the settler by a second pump (u3: on/off). A stirrer is provided (u1: on/off) to stir the suspended solids inside the settler. The measurement signals and control variables are accessed in a PC computer using the FieldPoint Modular I/O System.

Batch sedimentation pilot plant

Figure 1. Batch sedimentation pilot plant.

 

An example process sequence is as follows. First, the settler is filled with water form the supply tank and the suspended solids inside the settler are stirred. When stirring is complete the sedimentation process begins and lasts for a fixed period of time. The control algorithm than checks the turbidity sensor whether clear water is present below the float. If so, the peristaltic pump is turned on and removal of cleared water from the settler begins. This pump will be turned off however, if high concentration of suspended solids is sensed below the float. When the sedimentation period is over the whole cycle is complete.

This pilot plant is used to design and test different methods and algorithms for control of the clearing processes in batch settlers (including identification of sedimentation process parameters from the simple measurements, calibration of simple sensors and optimisation of the clearing processes). Moreover, it serves as an excellent educational example of non-trivial real-world processes with highly non-linear dynamics.

View of the batch sedimentation pilot plant

Figure 2. View of the batch sedimentation pilot plant.