Influence of substrate exposure history on biodegradation in a porous medium

This study investigates the influence of fluctuating toluene concentrations on aerobic toluene degradation in a sandy porous medium colonized with Ralstonia pickettii PKO1. Column effluent toluene concentrations were found to increase after a temporary decrease in influent toluene concentration. Subsequent examination of the spatial gradient of toluene degradative activity in the column suggested that the observed increase in effluent toluene concentrations was attributable to an adverse effect of toluene limitation on the biodegradative activity of attached cells. The traditional Michaelis-Menten-type biodegradation equation associated with batch-measured V_max (2.26 mg_toluene/mg_{living cell}/day) and K_S (1.20 mg_toluene/l) of nonstarved cells was unable to predict the observed toluene breakthrough behavior when the column had been previously exposed to no-toluene conditions. An alternative modeling approach was developed based upon the assumptions that (i) degradative activity was completely deactivated within the no-toluene exposure period (53.5 h) and (ii) a lag-phase was present prior to the subsequent reactivation of degradative activity in previously toluene-starved cells. These assumptions were independently verified by batch microbial investigations, and the modified model provided a good fit to the same observed toluene breakthrough curve. Application of single lag-time and threshold concentration values, however, failed to predict observed toluene breakthrough under different toluene exposure conditions. Results of this experimental and modeling investigation suggested that substrate exposure history, including the length of the starvation period and the level of substrate concentration, affected the induction of biodegradation in the porous medium.