Modeling and Control of pH in Pulp and Paper Wastewater Treatment Process

Zhongjun XIAO,Jiayu KANG,Mengxiao WANG

doi:10.4236/jwarp.2009.12016

Abstract

Pulp and paper industry is responsible for large discharge of highly polluted effluents, which often be treated by biological treatment process. For biological treatment system, pH is an important environmental factor that can influence the activity of microorganisms. In general, the optimal pH for aerobic processes is around neutral pH (7_7.8) and for the anaerobic process is between 6.8_7.2. The control of pH is a difficult link in the biological treatment system due to its nonlinearity and large time-delay. Aiming at the difficult point in the pH control of the biological wastewater treatment system, a mathematical model of pH control is estab-lished in the essay. On this basis, a traditional PID control and a cascade control are adopted to carry out simulation and comparison with MATLAB. The results show that the cascade control has better comprehen-sive effect in terms of response speed, stability and disturbance resistance.

Highlights

Pulp and paper industry is the greatest industrial polluter in terms of wastewater volumes and organic discharges
As shown in the acid-base titration curve [6], the changes at the two ends of the curve are gentle, that is to say, the pHSV ) -T ( (pH) varies very little with the addition of strong acid; the sensitivity is very high around the neutral point, and the pH is to change dramatically even when a little strong acid is added in. the sensitivity is very high near the neutral point, while is extremely low at the place far from the neutral point, and it has been proven by experiments that the nonlinearity of a strong acid-strong base neutralization process is more obvious than that of a weak acid-strong base process
As for the pH control system in biological wastewater treatment, the target range of pH is required to be controlled within 7_7.8 for aerobic processes and 6.8-7.2 for the anaerobic process, which are just located in the vicinity of the neutral point of the titration curve, the sensitivity of change is high; on the one hand, a controller is required to have relatively high control precision, on the other hand, control difficulty is raised as the controller becomes more sensitive to lagging

Summary

Introduction

Pulp and paper industry is the greatest industrial polluter in terms of wastewater volumes and organic discharges. The general characteristics of the pulp and paper industry effluent can be listed as: 1) high lignin content, 2) high absorbable organic halide concentration, 3) color, 4) low biodegradability which is indicated by their high chemical oxygen demand to biochemical oxygen demand ratios, and 5) potential toxicity problems. Until now aerobic treatment systems are the most widely used method to treat effluents from pulp plants. Reductive dechlorination may occur in anaerobic stage and produce less chlorinated organics These organics may be further biotransformed in conventional aerobic stage. Biological treatment such as aerobic and anaerobic process is indisputably the most effective, economical way used for treating pulp and paper effluents [2]. For control of the pH in the system, an automatic dosing system is installed and strong acid (HCL) and strong base (NaOH) are adopted as neutralizing liquid in the system

Basis and the Difficult Point of pH Control

Modelling of pH Control System

Building a Simulation Model

Setting the Parameters of the Adjusters

Conclusions