Dynamic models and control strategies for wastewater treatment processes

Abstract

Consideration of dynamic behavior and the incorporation of modern control systems can lead to substantial improvements in the performance of wastewater treatment plants. Other potential benefits include increased productivity, greater reliability, lower operational costs, more stable operation and faster start-ups. Several powerful tools are available for the study of dynamic behavior. Included among these are dynamic mathematical models, computer simulation, transient response analysis and process stability evaluation. Simple examples illustrating the theory and use of these tools are presented. The development of a control strategy is intimately related to the dynamic behavior of a process. Some of the basic questions which must be answered in developing a control strategy are: (1) What information should be collected? (2) How should the information be transmitted? (3) How should the information be processed? (4) What control actions should be taken? A wide variety of control algorithms are available for the processing of information for control. A discussion of several of these, on-off, PID. ratio, cascade, and feedforward, is presented and an example is given for the application of on-off control to a biological CFSTR. Digital computers are also being increasingly used for control of wastewater treatment plants and a discussion of computer control systems is presented. Examples of dynamic models and control strategies for two wastewater treatment processes, the step feed activated sludge process and the anaerobic digester, are presented and discussed, In addition, dynamic modeling and computer simulations are used to indicate those design and operational factors for the anaerobic digester which are effective in improving process stability. An effective control strategy for the activated sludge process is that provided by the step feed process which permits changing the points at which wastewater is added along the length of the reactor. Example control strategies for the anaerobic digester include the addition of base and the recirculation of the digested sludge. The effectiveness of the control strategy is dependent upon the type of forcing to which the digester is subjected. © 1974.