Factors affecting mass transfer limited biodegradation in saturated porousmedia

Microbial degradation rates in the subsurface are not only limited by the p hysiological capacity of the organisms, but also by inefficient supply of n utrients to the microbes. Although mass transfer limitation of biodegradati on in the subsurface has been postulated for years, experimental evidence i s still scarce. In the column experiments described here, diffusive transpo rt of 4-nitroanisole from the bulk solution to cells of Rhodococcus opacus strain AS2 immobilized on glass beads or sand appeared to be responsible fo r the slow transformation rates observed. Assuming steady state, we applied a coupled transformation/transport equation to these data (Best equation) and apparent bead-related mass transfer coefficients were found to increase in proportion to the surface area covered with bacteria. This implies that mass transfer coefficients for individual cells remained constant. In an i dealized oligotrophic environment where cells are only loosely clustered an d do not shield each other, we would therefore expect biodegradation rates to be independent from the longitudinal distribution of the total biomass a long a given flow path. Moreover, apparent mass transfer coefficients incre ased with the grain size of the column fillings, but did not change upon va rying the flow rate. With a limiting external transport step, overall trans formation fluxes do not become saturated at concentrations as low as predic ted for Michaelis-Menten-type kinetics. Mass transfer limitation thus offer s a justification for the common assumption that biodegradation rates in th e subsurface follow first order kinetics in a wide concentration range. (C) 2001 Elsevier Science B.V. All rights reserved.