MODELING OF THE DISSOLVED-OXYGEN PROBE RESPONSE IN THE IMPROVEMENT OF THE PERFORMANCE OF A CONTINUOUS RESPIRATION METER

In the present technique for the measurement of the respiration rate, the DO-probe is repeatedly subjected to a step change of the oxygen concentration with a time interval of typically 20-30 s. The end value of each response is used to calculate the respiration rate of activated sludge. However, those end values can be inaccurate because of probe dynamics or probe response time. This paper describes an improved method to measure the dissolved oxygen (DO) concentration with one and the same probe at the inlet and outlet of a respiration chamber. The real DO concentration at the end of each response can be estimated by fitting the DO measurements to a first-order response model of the probe. Furthermore, at each step response the time constant of the probe response can be estimated which provides a continuous diagnosis of the probe condition. The proposed method provides a reliable estimate of the real DO concentration and, as a result, the reliability of the calculated respiration rate is improved. The first-order probe response constant is a useful indicator for fouling of the probe membrane.