Determining short-term biochemical oxygen demand and respiration rate in an aeration tank by using respirometry and estimation

Abstract

For good control of the activated sludge process, accurate measurement of the short-term biochemical oxygen demand (BOD st) in the aeration tank ( S a) and the actual respiration rate of the activated sludge ( r act) is required. A previously described measurement technique for these variables requires the measurement of three types of respiration rate of which one rate, the actual respiration rate, involves continuous addition of influent sample to a respiration chamber. The technique is only applicable to a plant with one single aeration tank or to the first compartment of a plug flow reactor. In this paper, a method is proposed for the estimation of S a and r act in a completely mixed aeration tank, no matter whether this is a single reactor or a compartment of a plug flow reactor. In contrast to the other technique, this method does not involve addition of influent sample to the respiration chamber. Instead, the transient respiration rate is measured during two modes of operation which are alternately executed. In one mode, sludge from the aeration tank flows through the respiration chamber, whereas in the other mode, sludge having the endogenous rate is directed through the chamber. The S a is obtained by integrating the mass balance equations for each cycle of two different modes. The measured respiration rate and the calculated BOD st during each cycle of two modes are used to find the kinetic relationship in order to calculate r act. The proposed method has been verified using simulated and experimental data. The estimated S a and r act from experimental data are in agreement with the diurnal variation typical for the wastewater used. The pattern of r act is in agreement with the one obtained by the previously described method. There is, however, a significant difference between the average values of the two methods. A possible explanation for this bias is given. Compared to the previously described method, the proposed method is simpler to use in practice.