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 twel ve and sixteen months, respectively. Extant biodegradation kinetics fo r several compounds were periodically tested using a batch respirometr ic 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, organi c loading, etc.) for the duration of the study. At first, there was a large discrepancy between the kinetic parameters of the two bioreactor s. Phenol and 4-chlorophenol were biodegraded according to Monod kinet ics in the selector system and Andrews (inhibitory) kinetics in the no n-selector system, and the <(mu)over cap> and K-s values were signific antly 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 inh ibitory kinetics in both systems. The lack of inhibition when phenol a nd 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.