LABORATORY EVALUATION OF A 2-STAGE TREATMENT SYSTEM FOR TCE COMETABOLISM BY A METHANE-OXIDIZING MIXED CULTURE

The objective of this research was to evaluate several factors affecti ng the performance of a two-stage treatment system employing methane-o xidizing bacteria for trichloroethylene (TCE) biodegradation. The syst em consists of a completely mixed growth reactor and a plug-flow trans formation reactor in which the TCE is cometabolized. Laboratory studie s were conducted with continuous growth reactors and batch experiments simulating transformation reactor conditions. Performance was charact erized in terms of TCE transformation capacity (T-C, g TCE/g cells), t ransformation yield (T-Y, g TCE/g CH4), and the rate coefficient ratio k(TCE)/K-S,K-TCE (L/mg-d). The growth reactor variables studied were solids retention time (SRT) and nutrient nitrogen (N) concentration. F ormate and methane were evaluated as potential transformation reactor amendments. Comparison of cultures from 2- and 8-day SRT (nitrogen-lim ited) growth reactors indicated that there was no significant effect o f growth reactor SRT or nitrogen availability on T-C or T-Y, but N-lim ited conditions yielded higher k(TCE)/K-S,K-TCE. The TCE cometabolic a ctivity of the 8-day SRT, N-limited growth reactor culture varied sign ificantly during a 7-year period of operation. The T-C and T-Y the res ting cells increased gradually to levels a factor of 2 higher than the initial values. The reasons for this increase are unknown. Formate ad dition to the transformation reactor gave higher T-C and T-Y for 2-day SRT growth reactor conditions and significantly lower T-C, T-Y, and k (TCE)/K-S,K-TCE for 8-day SRT N-limited conditions. Methane addition t o the transformation reactor inhibited TCE cometabolism at low TCE con centrations and enhanced TCE cometabolism at high TCE concentrations, indicating that the TCE cometabolism in the presence of methane does n ot follow simple competitive inhibition kinetics. (C) 1997 John Wiley & Sons, Inc.