Temperature Correction for Clean-Water Tests with Reference to ASCE/EWRI Standard 2-22

Abstract

In wastewater treatment design, it is common practice to test aeration equipment in clean water first, and then extrapolate the result to wastewater via a correction factor. The most commonly adopted procedure for testing in clean water is the ASCE/EWRI Standard 2-22 that measures the oxygen transfer rate (OTR) as a mass of oxygen per unit time dissolved in a volume of water by an oxygen transfer system operating under a given gas rate and power conditions, based on a simplified mass transfer model. The procedure is applicable to ordinary test conditions, such as overhead pressure (atmospheric pressure), water temperature (between 10°C and 30°C), water depth (between 3 and 6 m), mixing conditions (as produced by ordinary gas flow), and so forth. It may not be suitable for outside these boundary conditions. A weakness of this standard is temperature correction, where it was recommended to use a correction factor named theta (Ɵ) to adjust the test result to a common temperature of 20°C. A recent discovery of the validity of the mass transfer model opens the door to a more precise method of estimating correction factors, the findings of which have been published in various journals. This article proposes a Rational Method in dealing with the problem of temperature correction that has led to overestimations or underestimations of the mass transfer coefficient (MTC) at standard conditions. Inaccurate determination of the mass transfer coefficient at standard conditions poses many serious problems down the line, from equipment bidding to projection of usage in the field. An overestimation in particular may lead to specifying equipment that can underperform in the field. By eliminating the effects of extensive properties (scale-dependent properties), such as tank height (water depth), the environmental conditions (temperature and pressure), gas flow rate, agitation, mixing, and so forth, we find that a constant coefficient may be obtained from the standard mass transfer model. To do this for an aeration system, the baseline from a single clean water test (or a series of tests) must be determined. The baseline mass transfer coefficient is denoted by the symbol KLa0. Then, one can translate any test result to 20°C using a temperature correction model only suitable for the baseline to give (KLa0)20, from which a scale-up model for depth correction will produce a reasonable (KLa)20. In verification of the Rational Method, based on various experiments cited in the literature, they do all seem to give the same (KLa0)20. Therefore, the results seem promising for evaluating (KLa)20 for any test. These are best explained by way of various examples given in the paper based on extracted data from cited literature.