Evaluation of biodegradation kinetic testing methods and longterm variability in biokinetics for BTEX metabolism

Two methods were investigated for measuring the substrate utilization Monod biodegradation kinetics of benzene, toluene, ethylbenzene, o-xylene, and p-xylene (BTEX) by mixed bacterial cultures grown under three different conditions. The mixed cultures were grown in completely mixed reactors that were batch fed every two hours with either BTEX or benzene and operated at a 5-day solids retention time (SRT); a BTEX-fed culture was also grown at a 20-day SRT. Only maximum specific substrate degradation rates (k) could be determined in batch tests where BTEX concentrations were measured at discrete time points, since not enough data could be acquired at low substrate concentrations near the half-saturation concentration (Ks) of the compounds. Both k and Ks could be determined in batch tests using an indirect method where dissolved oxygen depletion was continuously measured. The measured biokinetics of each of the cultures varied over time (coefficient of variation of the k and Ks values of 27–44% and 43–100%, respectively), despite constant growth conditions. This biokinetic variability should be incorporated into design uncertainty to develop safety factors for reliable bioreactor performance. Results also showed that the operating solids retention time of the reactor and the combination of growth substrates affected the biokinetic coefficient values of the individual BTEX compounds.