Estudo comparativo dos métodos contínuo e semicontínuo de determinação da taxa de consumo de oxigênio.

Abstract

Respirometry is used in aerobic waste water treatment systems to determine the Oxygen Uptake Rate (OUR) by the bacterial sludges mass. In principle there are two experimental methods available that can be applied to determine the OUR: The semi continuous and the continuous method. The semi continuous method is attractive because of its practicality and its simplicity, but it cannot be applied routinely in full scale systems. The continuous method is not yet established but it can in principle it can furnish a continuous measurement of OUR in full scale systems. In an attempt to contribute to a more effective application of respirometry in full scale activated sludges systems, a comparative study was carried out with the two methods to determine the OUR. The study has as its main objective to establishe the limit of applicability of the continuous method when conditions were imposed of sudden and large variations of the OUR with time. Also experiments were carried out to determine the influence of several factors on the oxygen transfer constant: sludges concentration, surfactants and temperature. Knowledge of the value of the oxygen transfer constant is necessary to apply the continuous method form determination of OUR. The semi continuous method consists of defining of upper and lower set points of dissolved oxygen in the mixed liquor (ODmáx and (ODmin) respectively. When the ODmáx value is reached the aeration of the system is interrupted and the consequential decrease of the OD concentration as a consequence of the consumption by the microorganisms is observed as a function of time. When the lower set point is reached the OUR value is calculated and a new cycle is initiated. OUR is the ratio between the OD decrease and the time required for this decrease to take place. When the continuous method is used the aeration is constant and the OUR is calculated from variations of the DO concentration with time. For the calculation it is necessary that the oxygen transfer constant of the aeration system is known. The obtained experimental results show good precision and accuracy of the data obtained with both methods, since a statistical analysis with (ANOVA) applied at a significance level of 5% did not reveal significant differences in the obtained results with the two methods. Thus both methods may be applied in practice. With the obtained result it is clear that the choice of which method is to be used in a particular situation depends on the user of the respirometer. The semi continuous method is easier to apply and gives results more readily. It is a good alternative for small scale laboratory or pilot plant tests, but it cannot be applied routinely in full scale systems. If monitoring of systems operating at full scale is the aim, the continuous method must be preferred.