Modeling and Control Studies of Wastewater Neutralization Process

In this paper, the study of the dynamics and control of a semibatch wastewater neutralization process in modeling and simulation is presented. The wastewater neutralization process is modeled based on the reaction between a strong basic solution and a strong acidic solution in a semibatch reactor. It is assumed that the wastewater is acidic in nature. The semibatch system chosen here consists of two input streams and a mixing tank which contains an initial amount of acid solution. The initial simulation involves studying the dynamics and control of the model formulated. The objective of the control was to regulate the base flow, while keeping the acid flowrate constant, until the pH in the mixing tank stabilized at pH 7. A digital PI control algorithm was used as the controller, and the control simulation was performed in Matlab’s Simulink environment. The control studies were done to include the effect of changes in process dead time, base concentrations and base flowrates to the controllability of the semibatch system. From the simulation study, it was found that an increase in process dead time would result in process instability. While, an increase in base concentration and flowrate would result in faster neutralization time. For a given set of condition, the process dead time gave no effect to the volume of wastewater accumulated in the tank. INTRODUCTION Wastewater neutralization plays an important part in a wastewater treatment process. It provides the optimum environment for microorganism activity i.e. between pH 6.5 and 7.5, and the right water discharge to the public sewage as mandated by the Department of Environment of between pH 5 and 9 (Environmental Quality Act, 1974). Wastewater of pH below 4.5 and above 9 may greatly reduce the activity of the microorganisms which treat the water and may not support their life at all. __________________________________________________________________ Dept. of Chemical Engineering, Faculty of Mechanical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia e-mail: [email protected] Assoc. Prof., Dept. of Chemical Engineering, Faculty of Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia Prof., Dept. of Chemical Engineering, Faculty of Engineering, University of Bahrain, Isa Town, Bahrain Proceeding to Brunei International Conference on Engineering & Technology, Bandar Seri Begawan, 9 – 11 Oct. 2001, pp. 221-231 222 In the neutralization process, acidic wastewater is treated with alkaline solution, while alkaline wastewater is treated with acidic solution. In either case, the wastewater is treated until it neutralized to pH 7 or any other desired pH. The process can be either operated as a continuous stirred tank (CSTR) or in batch operation. In contrast to CSTR, a semibatch operation ensures complete neutralization of wastewater present in the reactor. The control system implemented in the neutralization process depends upon the desirable ‘quality’ of the treated wastewater. An on-off control system is suitable for a broad range of pH control, such as the final stage of the neutralization process, whereby the controlled range is between 5 and 9. For a narrower range of pH control, a proportional-integral (PI) control system is recommended. Controlling the wastewater neutralization process has been known to be intrinsically difficult, due to various factors such as nonlinearity of pH system, varying process parameters (Horwitz, 1993), vessel design and process equipment used (Hoyle and McMillan, 1996), control valve characteristics (Shinskey, 2000) and type of controllers used (Shinskey, 1998). In this paper, we intend to show the effect of varying process parameters such as reagent flowrates and concentrations and process dead time to the controllability of the wastewater neutralization using a PI controller in a semibatch reactor. MODELING OF STRONG ACID – STRONG BASE NEUTRALIZATION PROCESS In this study, the semibatch process model (Figure 1), consists of a mixing tank with an initial amount of acidic solution (wastewater) and of two input streams of strong acidic (wastewater) and strong basic (reagent) solutions. The aim of this process is to simultaneously neutralize the wastewater already in the mixing tank and the wastewater from the input stream by regulating the flowrate of the reagent solution Fb until the mixed solution stabilizes at pH 7. This model assumes perfect mixing and an ideal linear control valve. Figure 1: A semibatch pH neutralization process The model equations for the neutralization of sulfuric acid and sodium hydroxide in a semibatch system can be written as: Fb, BOH (reagent) V0 Fa, HA (wastewater) initial acidic solution (wastewater) pHT