Importance of the BOD plateau

Abstract

ThE BIOChEMICAL oxygen demand test has been a subject of continual discussion and controversy. MONTGOMERY (1967) has pointed out that the standard test is slow, that there is little relation between laboratory and field conditions, and that the test possesses no theoretical significance and lacks reproducibility. Yet the five day 20°C BOD is the standard measure of the organic pollution potential of a waste water. All other methods of measurement of oxygen demand have been compared or correlated with the five day BOD. Thus the use of the Warburg Respirometer to measure BOD was only tentatively accepted by Standard Methods in the l lth edition and was eliminated from the 12th edition (1966). Biochemical oxygen demand data have been utilized for two purposes: to predict the total oxygen demand of a waste water and to predict the rate of deoxygenation. SWILLEY et al. (1964), I~TD/HRBERGER et aL (1964) and GULEVICH et al. (1968) have demonstrated the importance of diffusion on the rate of BOD exertion. Their work indicates that rate data obtained from a laboratory experiment, using quiescent bottles, would not be applicable to streams or waste treatment processes. The only use for BOD rate data would then be in predicting the total oxygen demand. Prediction of the total oxygen demand from five day BOD values is a questionable procedure. An assumption must be made with respect to the mathematical description of the BOD progression with time. A first order model is normally used [(dc/dt) = -kc]. Only two data points are needed to determine the rate constant, k, for which the use of a five day test would seem unreasonable. Any point on the curve should provide the necessary information. PhELPS (1944) states that the adoption of the five day test as the standard was based upon the unreliability of data taken previous to five days. The unreliability was assumed to be due to variation in the lag time before BOD exertion began. This fact contradicts the assumed first order models and is particularly important because both the model and experience indicate that the highest oxygen uptake rates occur early in the test. The model is therefore unreliable for the most sensitive part of the BOD progression and is usable only in the least sensitive region.