Utilização da respirometria no controle operacional de sistemas aeróbios de tratamento de águas residuárias.

Abstract

In the activated sludge system one of the most important operational parameters is the oxygen uptake rate (OUR) . which represents the respiration rate speed of micro organisms present in the treatment system. Respirometry is used for measurement and interpretation of OUR in treatment aerobic systems, with the aid of some type of respirometer. OUR is the most indicated parameter to evaluate the toxicity of a influent, because the release of a toxic discharge will result in a decrease of the oxygen consumption rate by the intoxicated micro organisms and therefore, of OUR. In this dissertation, the experience obtained with an automated respirometer installed in the Effluent Treatment Plant (ETE) of CETREL at the Petrochemical Complex of Camacari in Bahia is described. The ETE of CETREL treats all the industrial effluents of the Complex and, for this reason, there are frequent discharges of material that potentially are toxic and these can affect the operational stability of the treatment system. Therefore, at CETREL the respirometer operated mainly as a toxicity meter. The toxicity meter used during the experimental investigation, of this dissertation, measured OUR from the variation of the Dissolved Oxygen (DO) concentration in the mixed liquor of an activated sludge systems at laboratory scale. The instrument was composed of two units that worked independently, so that two wastewaters could be tested simultaneously. Such configuration was chosen to increase operational flexibility and augment the reliability of the obtained information. In the case of the experimental investigation, the raw and equalised industrial effluent arriving ate the ETE of CETREL were chosen to determine OUR, Along with the variation of the DO concentration and the subsequent calculation of OUR, the toxicity meter also acquired the value of the temperature as a function of time. The software Toxim-D, developed for the toxicity meter allowed the control of aeration and the on-line determination of the three variables, that were stored in the memory of the computer for posterior analysis. The use of the toxicity meter at CETREL showed the following important points: (1) under normal conditions the raw industrial effluent has a OUR that varies significantly and in a unpredictable way with the time, indicating that there is frequent introduction (a few times a day) of organic over loads and toxic material in the treatment system; (2) OUR of the equalised effluent exhibit very gradual and predicabie variations. Under normal operational conditions, the oscillations of OUR can be attributed, mainly, to variations of the temperature and of the concentration of organic material; (3) at some occasions, the reactors with raw and with equalised effluents indicated the presence of toxic material on moments that coincided with periods of operational problems in the station, where a high concentration of priority pollutants was detected, lethal for the bacteria.