The Calibration and Evaluation of Dissolved Oxygen Sensors in an Activated Sludge Plant

Abstract

Abstract The performance of three types of dissolved oxygen (DO) sensors was evaluated in a pilot scale activated sludge plant. As the instruments were interfaced to a minicomputer, a curve fitting technique was developed to compensate for instrument non-linearity. Weighted regression was performed to update initial calibrations. Fouling due to the microbial growth on the surface of the probe was the principal cause of decline in sensor performance with time. The rate of fouling depended on the type of sensor used, the mode of operation, the frequency of maintenance and the velocity of fluid past the probe. A number of alternatives were evaluated for reducing or compensating for the effects of fouling and other causes of inaccuracy. To determine the suitability of sensors for tracking DO concentration during non-steady state tests for determining overall mass transfer coefficients (KLa), the probes were subjected to repeated step changes in DO concentration. First-order time constants (τ) for the sensors were estimated based on the results of each step.