Analysis of the re-oxigenation process in water streams

The problem of river water quality and the strategies to be adopted in order to limit the impact of pollutants in the receiving water bodies have increasingly become object of interest for the scientific and environmental government communities. Concerning Italy, it is necessary to remember the different environmental legislation, which is based on limit values, allowing defining environmental indicators and determining specific indications and deadlines for the re-qualification of the most degraded streams. The process of re-aeration has been systematically and scientifically studied since 1925, when Streeter and Phelps published the famous article “A study for the pollution and natural purification of the Ohio River, III. Factor concerning the phenomena of oxidation and re-aeration”. Since then, many authors have put an effort in finding out the hydraulic parameters on which the value of the re-aeration coefficient depends. This coefficient indicates the stream capability to reacquire oxygen from the air, following its reduction due to the effect of the bacterial activity developing from the discharging of biodegradable organic matter. There have been many studies in the literature that used different approaches: from the theoretical models, including turbulence, to the semi-empirical ones, which use the experimental data to calibrate some of the coefficients in the formulas, and to the empirical models, for which data can be given by field and/or laboratory experiments. Such models can be interpreted in various ways. In fact, the expressions given by purely empirical models can be taken from the simple correlations between the hydraulic parameters and the re-aeration coefficient, identifying the one which guaranties the maximum value of the correlation coefficient, or from the application of the Dimensional Analysis starting from parameters which are independent from each other. In this paper the continuation is presented of a research project which began many years ago at the University of Calabria. Some of the conceptual and semi-empirical models were analyzed: starting from the interpretation of experimental results through the classical Dimensional Analysis, the literature data from two experimental studies in different shape laboratory flumes was re-elaborated. The analysis made possible the identification of well-correlated dimensionless groups, which show a more than satisfactory correspondence between the re-aeration coefficient and the hydraulic and geometric parameters such as flow, depth and top-width.