Model simulations in support of field scale design and operation of bioremediation based on cometabolic degradation

This paper addresses questions fundamental to the design and operation of aquifer bioremediation based on cometabolic degradation. A model of a full-scale, in situ system for bioremediation of chlorinated ethenes relying on cometabolic degradation was developed and applied to a hypothetical aquifer being considered for a large-scale field demonstration of in situ bioremediation with recirculation. The model was used to identify feasible substrate (electron donor and electron acceptor) delivery schedules. Trichloroethylene (TCE) was the target contaminant. Methane and phenol were considered as electron donors. The delivery of the electron donors and the electron acceptor, oxygen, was varied to evaluate the rate and extent of bioremediation under different substrate delivery schedules. Maximum removal of TCE was predicted when substrates are delivered at ratios near the stoichiometric requirement of electron donor and acceptor for net microbial growth. Additionally, the decrease in TCE removal that results from using substrate delivery schedules other than those achieving the maximum removal of TCE was quantified. This decrease was greater for the methane-oxygen system because the two gaseous substrates compete for transfer into the recirculated ground water. If one substrate is introduced in excess of the amount required for net microbial growth, it accumulates, thus limiting the ability to introduce the second substrate. This imbalance both limits the introduction of the second substrate and accelerates the accumulation of the substrate added in excess. The phenol-oxygen system is less sensitive to deviation away from the best observed substrate delivery schedule because phenol is a relatively soluble liquid and its introduction does not compete with the mass transfer of oxygen