Measurement and validation of kinetic parameter values for prediction of biodegradation rates in sewage treatment

Analysis of about 140 oxygen uptake curves from standard biodegradation tests with the electrolytic respirometer yielded growth kinetic parameter values. The maximum observed growth rate, mumax, on the 52 different substances varied between 1 and 6/day. The measurement of the half-saturation constant Ks at the depletion phase of the oxygen uptake curves appeared problematic. For 24 substances the values were below 1 mg ThOD/liter. Apparently high Ks values of several tenth mg ThOD/liter for the other 28 substances are explained by retarded mineralization of intermediates due to the relatively high initial test concentrations of up to 300 mg ThOD/liter. At lower test concentrations (down to 15 mg ThOD/liter) the measured Ks values for these substances were also typically below or around 1 mg ThOD/liter. Based on the above-mentioned maximum growth rates and the low values for Ks, the predicted steady-state residual concentration of these substances in the effluent of completely mixed sewage treatment plants is between 10 and 100 micrograms/liter. This level is in accordance with the range of measured values taken from literature. Based on this research and additional data from literature, the separation between readily and inherently degradable can be made at the critical values of mumax approximately 1.5/day and Ks approximately 1 mg ThOD/liter.