Simulation of bioaugmentation involving exogenous bacteria injection

Abstract

The degradation of organic hydrocarbons through microbial reactions in subsurface environments has been widely studied in the literature. Most studies, however, have focused on biostimulation, which aims to enhance biodegradation by indigenous bacteria. This study presents a mathematical model to simulate the fate and transport of a reactive contaminant degraded through cometabolism during the in situ bioaugmentations involving the injection of nutrients and/or exogenous bacteria. We incorporate hydrogeologic factors affecting the transport of contaminant as well as microbial metabolic reactions into the model. Modified Monod kinetics and a microcolony concept are used to investigate the effects of mobile bacteria in the aqueous phase and bacteria attached on solid surfaces on the transport and biodegradation of an organic contaminant. Permeability reduction due to microbial accumulation in pore spaces and its effect on the biodegradation processes are examined by performing a numerical experiment involving in situ biodegradations by exogenous bacteria. The effect of bacteria as biosorbents is also considered in model formulations to investigate the biocolloid-facilitated transport of a contaminant. The two-dimensional governing equations are solved numerically using a fully implicit finite difference method with an alternating direction implicit scheme. For model evaluation purposes, a model comparison is performed against an independently developed biodegradation model and showed remarkably close matches in the concentration profiles. The model is applied to a case study to demonstrate its behavior. The results of simulations show significant effects of bioaugmentative operations on the fate and distribution of all the chemical species and biomass and their interactions. In addition to the enhanced efficiency caused by the operation, operation-induced limitations such as permeability reduction is also demonstrated. Factors determining the overall biodegradation rate include the bioavailability of the contaminant and the distribution of biomass and relevant chemical species. The overall results implied that the success of in situ bioaugmentation depends on how to control the contact of biodegrading microbes with contaminants and how to supply enough nutrients into the contaminated zone and stimulate microbial activities.