Chloroform aerobic cometabolic biodegardation in a continuous-flow reactor

This study, focused on chloroform (CF) cometabolism by butane-grown bacteria, was conducted in a 2-m continuous-flow column reactor simulating a portion of saturated aquifer. The main goals were: a) to investigate the pulsed injection of growth substrate and oxygen as a tool to control clogging of the porous medium and to attain a wide bioreactive zone; b) to determine the minimum substrate/CAH ratio required to sustain the cometabolic process; and c) to determine the most suitable kinetic and fluid-dynamic model to fit the experimental data of butane utilization and CF cometabolism. The complete fluid-dynamic/kinetic model was utilized to run a series of simulations solved with Comsol Multiphysics, with the goal to design three types of injection of alternated pulses of growth substrate (butane) and oxygen, characterized by different values of the ratio of butane utilized to CF degraded (B/CF ratio). The column reactor was then run for three consecutive periods, characterized by the three above-mentioned pulsed injections. In these tests, the interstitial velocity was set to 0.5 m/d (corresponding to a 2-day hydraulic retention time), and the CF inlet concentration to about 0.4 mg/L. In conclusion, the experimental and modeling results show that: a) the pulsed injection of growth substrate and oxygen is an effective tool to prevent aquifer clogging as a result of an excessive biomass growth, and to attain a long bioreactive zone; b) in the specific case of our process of CF cometabolism with butane, the minimum ratio of substrate utilized / CAH degraded ranges between 1.6 and 2, with a good agreement between the results obtained in batch slurry assays and those deriving from the column tests; the corresponding ratio of substrate supplied / CAH supplied depends on several factors, among which the number of substrate and oxygen pulses in each cycle plays an important role; c) the kinetic parameters previously estimated in batch, single-strain assays, combined with the fluid-dynamic parameters evaluated in the first part of this work, allowed the development of an effective modeling tool for the design of the pulsed injection and for the interpretation of the experimental data. Overall, this work provides encouraging indications on the successful application of aerobic cometabolism for the in-situ remediation of sites contaminated by a wide range of CAHs.