Essais d'aerateurs: Enseignements tires de 500 essais en eau claire effectues dans 200 stations d'epuration differentes—I. Methodologie

Abstract

Aeration represents the main part of energy consumption in the activated sludge process and the evaluation of aeration systems efficiency is becoming more important, especially as energy cost increases. Since 1972, CEMAGREF teams have carried out more than 500 non-steady state clean water tests in sewage treatment plants. The first aim of these measurements was to compare the results collected in plants with those predicted by manufacturers. The distribution of the different types of aerators tested in the field by the CEMAGREF is given in Table 1. All tests are conducted using tap water under non-steady state conditions: the initial dissolved oxygen (DO) level is brought down to zero by adding cobalt chloride as catalyst and sodium sulfite. When all the sodium sulfite has been used, the increase in water dissolved oxygen content is monitored vs time in various places in the tank by means of membraned probes. The graphical procedure used for estimating the oxygen transfer coefficient ( K L a) is shown in Fig. 1; this procedure is usually called “log deficit method”. The results are expressed for “standard conditions” ( θ = 10°C; P = 760 mm Hg). The influence of temperature on oxygenation capacity is illustrated in Fig. 2. The water quality parameters that may affect oxygen transfer are investigated: it appears that only the presence of surfactants, flocculated suspended solids, or high salinity (conductivity > 1500 μS cm −1—Table 2) are liable to have any appreciable effect on oxygen transfer. The unflocculated SS, pH and alkalinity have no effect on oxygenation results in the common range of values occurring in the tests (Table 3). Authors differ about the operational procedure in non-steady state clean water test. After 7 years' field-measurements the CEMAGREF teams have developed their own recommendations about test procedures; their main conclusions are the following: Dissolved oxygen analysis: the differences observed between the results ( K La ) obtained simultaneously by Winkler titration of piped samples and those from in-tank probes never exceed 4% (Table 4). Reliable dissolved oxygen probes are suitable for accurate measurements of oxygen transfer. The number of sampling points should be no smaller than three for aeration tanks with a volume below or equal to 500 m 3. It should be recommended to add one sampling point for every additional 500 m 3. Location of sampling points requires attention. Differences may appear according to the locations of probes in the basin (Tables 5, 6 and 7). Sulfite pre-dissolution has no influence on results and should be avoided whenever possible.