Changes in measured biodegradation kinetics during the long-term operation of completely mixed activated sludge (CMAS) bioreactors

Two completely mixed activated sludge (CMAS) bioreactors, one with an aerobic selector and one without, were operated for approximately twelve and sixteen months, respectively. Extant biodegradation kinetics for several compounds were periodically tested using a batch respirometric procedure. Kinetic parameters from the CMAS unit without a selector showed considerable variability (standard deviation of +/- 50%) even though it was operated at steady state (i.e. constant HRT, SRT, organic loading, etc.) for the duration of the study. At first, there was a large discrepancy between the kinetic parameters of the two bioreactors. Phenol and 4-chlorophenol were biodegraded according to Monod kinetics in the selector system and Andrews (inhibitory) kinetics in the non-selector system, and the <(mu)over cap> and K-s values were significantly greater in the selector system. The kinetic parameter values of the two systems converged, however, when the xenobiotic compounds were no longer fed to the selector in that system but were fed to the main bioreactor. After this switch, phenol and 4-chlorophenol followed inhibitory kinetics in both systems. The lack of inhibition when phenol and 4-chlorophenol were fed to the selector suggests that, contrary to conventional wisdom, bioreactors which have a concentration gradient (e.g. plug flow, sequencing batch, and tanks in series bioreactors) may be more resistant to inhibition. Copyright (C) 1996 IAWQ.