In this paper, the practical implementation and validation of advanced control strategies, designed using model based techniques, at an industrial wastewater treatment plant is demonstrated. The plant under study is treating the wastewater of a large pharmaceutical production facility. The process characteristics of the wastewater treatment were quantified by means of tracer tests, intensive measurement campaigns and the use of on-line sensors. In parallel, a dynamical model of the complete wastewater plant was developed according to the specific kinetic characteristics of the sludge and the highly varying composition of the industrial wastewater. Based on real-time data and dynamic models, control strategies for the equalisation system, the polymer dosing and phosphorus addition were established. The control strategies are being integrated in the existing SCADA system combining traditional PLC technology with robust PC based control calculations. The use of intelligent control in wastewater treatment offers a wide spectrum of possibilities to upgrade existing plants, to increase the capacity of the plant and to eliminate peaks. This can result in a more stable and secure overall performance and, finally, in cost savings. The use of on-line sensors has a potential not only for monitoring concentrations, but also for manipulating flows and concentrations. This way the performance of the plant can be secured.
This paper considers the question of which is better: the batch or the continuous activated sludge processes? It is an important question because dissension still exists in the wastewater industry as to the relative merits of each of the processes. A review of perceived differences in the processes from the point of view of two related disciplines, process engineering and biotechnology, is presented together with the results of previous comparative studies. These reviews highlight possible areas where more understanding is required. This is provided in the paper by application of the flexibility index to two case studies. The flexibility index is a useful process design tool that measures the ability of the process to cope with long term changes in operation.
An increasing focus on the design of biological nutrient removal processes and the development of new and varied designs (propelled by increasing needs for higher efficiencies and lower costs) has introduced the need for improved methods of design for wastewater treatment plant (WWTP) processes. These design methods may include studies of process operability (i.e., flexibility, controllability, and resilience). This paper focuses on flexibility and the tools available to facilitate the study of flexibility. A simple example is used to demonstrate a method of flexibility analysis and a more complex example is used to illustrate the significance of the results to WWTP process design. The most significant result is that the flexibility of a process can sometimes be significantly increased with negligible increases in capital costs. Water Environ. Res., 73, 486 (2001). IntroductionA dramatic growth in biological nutrient removal technology for wastewater treatment is occurring worldwide. As a result, there is growing debate about the best designs and mode of operation. Design of biological nutrient removal plants, particularly those operated in the continuous mode, has received considerable attention. Until now, the focus of design methods has been on minimizing capital costs. However, another factor that is just as important is the operating cost of the plant. This factor is usually not considered at the design stage. Consequently, wastewater treatment plants (WWTPs) are often difficult to operate.A design engineer should attempt to design a process with optimal trade-off between capital and operating costs. The field of operability analysis attempts to incorporate the two at the process design stage. The operability of a process refers to its ability to perform satisfactorily under conditions that are different from nominal design conditions (Grossmann and Morari, 1983). The operability of a process is composed of the facets shown in Figure 1. The most operable process is highly flexible, highly resilient, safe and reliable, controllable, simple to operate, and easy to start up and shut down.This paper investigates the possibilities of studying the flexibility of a WWTP at the design stage and of using this study to increase the flexibility of the process. The investigation uses two case studies that vary in complexity to assess the application of a flexibility study to WWTP design.Various tools have been developed to facilitate the study of flexibility in WWTP design. These tools will be reviewed first, and the most promising, the flexibility index, will be defined and illustrated via a simple example. Calculation of the flexibility index requires solution of the model equations and solution of an optimization problem. These two aspects also are addressed before
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