An Accurate Measurement Technique for the Biological Oxygen Uptake Rate

Abstract

For any wastewater treatment aeration tank, the paper proposes an accurate technique to deal with the Oxygen Uptake Rate (OUR) measurements. Since it measures the rate at which oxygen is used (in mg O²/L/hour), it is a useful tool to evaluate process performance, aeration equipment, and the biodegradability of the waste. Unfortunately, the literature abounds with examples of inconsistent measurement results. The manuscript observes that if a sample of mixed liquor is withdrawn from an aeration tank operating at low Dissolved Oxygen (DO) (dissolved oxygen), the OUR measured in the sample after shaking (or other means of perturbation) will be higher than the true OUR which is limited by oxygen supply. The composition of a sample of activated sludge being analyzed is continually changing, making it necessary to obtain measurements as quickly as possible at the site of the aeration basin. To alleviate the many problems in measurement, the proposed method using water dilution with saturated DO may give a more accurate measurement than the current standard method as described by the American Public Health Association (APHA). The discrepancy in the new method between the measured and the calculated SOTR_pw is in the assumed mole fraction of the exit gas of 0.19 which is reasonable but still based on guesswork. However, the discrepancies in the conventional method are that, the measured value of R is incorrect because of the inherent shortfall in the APHA (Biological Oxygen Demand (BOD) bottle shaking) technique, and it is more realistically given by the modified Eq. (2-3), which is originally stated as for a batch process provided by the American Society of Civil Engineers (ASCE) Guidelines, ASCE/Environmental and Water Resources Institute (EWRI) 18-18 recently published; and secondly, the incorrect driving force at the steady state, making the OTR at test conditions erroneously high. With a more accurate measurement of the OUR, it may be justified to modify the fundamental equation for oxygen transfer in a respiring system, as applied to the example given by the Guidelines. The specific content of the revised formula is proposed to be for a batch process.