The degradation behaviour of a phenolic wastewater from coking containing easily utilizable carbon sources (volatile fatty acids), substances with toxic effects (phenols and phenolic compounds), and non-biodegradable organic substances (humic-like substances) by an adapted activated sludge is investigated depending on dissolved oxygen concentration (DO) and temperature. In addition, the degradation of an acetate model water with the same population was investigated. The investigations were carried out in a special respiration fermenter system by recording oxygen consumption in DO-static experiments. The oxygen consumption curves were subjected to linear regression analysis, and the maximum oxygen consumption rates were used for further calculations. Evaluation of the degradation kinetics was carried out with non-linear regression analysis via commercial software utilizing the Marquardt-Levenberg algorithm and which tested various kinetic models to obtain the best curve fit. It is shown that the dependence of growth on DO in acetate model water is well represented by Monod kinetics but by contrast can only be described in phenolic wastewater with acceptable goodness by Hill kinetics, modified by a maintenance term. The kinetic constants calculated for the mesophilic range (37 degreesC) are for the model water as follows: maximum specific growth rate = 0.218 h(-1), half saturation constant with respect to oxygen = 0.297 g L-1, and for the wastewater: maximum specific growth rate = 0.387 h(-1), half saturation constant with respect to oxygen = 0.013 g L-1 and maintenance coefficient for oxygen = 0.107 g g(-1) h(-1). For the thermophilic range (55 degreesC), a higher growth rate but a significant lower yield coefficient are to be observed. The kinetic constants calculated are: maximum specific growth rate = 0.8 h(-1), half saturation constant with respect to oxygen = 0.14 g L-1 and maintenance coefficient for oxygen = 0.3 g g(-1) h(-1). Therefore, it can be concluded that the toxic effect of the biodegradable phenols causes a maintenance metabolism which leads to higher oxygen demand. The diminution of the limiting DO and the increased maintenance metabolism confirm the assumption that a higher maintenance metabolism leads to increased sensitivity with respect to the cosubstrate oxygen. It can be concluded that the experimental and mathematical methods used permit the maintenance metabolism with respect to oxygen caused by toxic substrates and milieu influences to be accurately determined.
