SIMULATION OF TCE COMETABOLISM IN BIOFILM REACTORS
|R.L. Segar Jr. and P. Kalia , Department of Civil Engineering, University of Missouri-Columbia, Columbia, MO 65211||
Biofilm reactors have been demonstrated as effective for the removal of trace organic contaminants. The sequencing packed-bed biofilm reactor (SPBR) has been shown to sustain 80 to 90% removal of trichloroethene (TCE) when populated by indigenous phenol-utilizing microorganisms. The fluidizedbed bioreactor can obtain 80% removal of 0.1 mg/L TCE at an empty-bed-contact-time of three to four minutes.
Modeling analysis of the SPBR for design optimization is complicated by the non-steady-state conditions resulting from sequencing feeding, which alternates between operational stages of growth/induction with a phenol feed and TCE cometabolism/deactivation when contaminated ground water is fed. Modeling of the fluidized-bed reactor is complicated by the mobile biomass. Predictive models of these biofilm reactors potentially have much usefulness for process scale-up to field applications, comparative design evaluation and treatment cost estimation, selection of operating conditions, and for the further development of biofilm reactor processes. However, functional reactor simulators for cometabolism and biofilm processes are not available or are presently inadequate for these tasks.
This research was conducted to produce a useful simulator of the biofilm reactor. The SPBR was assumed to operate in a single-pass manner with a specified inlet feed flow rate and dissolved substrates composition, all which could vary over time, and an initial biofilm profile, which could be a thin inoculating layer or a thick biofilm resulting from a growth period. Various attachment media types were addressed by specifying their specific surface area, diameter, porosity, and dispersion number correlations. Physical interactions between the biofilm and bulk fluid were reflected in functional relationships involving biofilm thickness, porosity, mass transfer coefficient, and rate of biofilm shearing.
Biokinetic phenomenon addressed in the simulator include growth and endogenous decay of a structured biomass, oxygen and substrate limited saturation kinetics (multiple-Monod type), oxygenase enzyme induction and decay, inhibition effects due to enzyme competition between growth and cometabolized substrates, toxicity effects due to cometabolism, and inhibitory effects due to elevated concentrations of growth or cometabolized substrates. The kinetic model allowed for one growth substrate, an electron acceptor, and one cometabolized substrate. Biomass fraction and substrate concentration profiles within the biofilm and over the length of the reactor were output by the simulator over the time. The modeling parameters were obtained from the literature and/or from previous experimentation results with SPBRs.
Simulation runs with the SPBR model produced results that matched experiment data for TCE removal. It was shown that a sequencing operation was more effective for controlling biofilm build-up and provided higher TCE removal than continuous feeding. The simulator was used to model the FBBR assuming the media was stationary. The simulator was found to be a useful tool for evaluating designs and understanding reactor behavior.
Key words: simulation, cometabolism, biofilm, TCE, phenol
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