Modeling chromium(VI) reduction in pure and coculture biofilm reactors

Biological Cr(VI) reduction was investigated in Bacillus sp. pure culture and Pseudomonas putida DMP-1/Escherichia coli ATCC 33456 coculture bioreactors. Glucose was the sole supplied carbon and energy source in the Bacillus sp. fixed-film bioreactor whereas phenol was the sole carbon and energy source in the coculture system. E. coli utilized metabolites formed from phenol degradation by P. putida for growth and Cr(VI) reduction. Near complete removal of both Cr(VI) and phenol was observed during steady-state operation of the coculture reactor with Cr(VI) removal efficiency ranging from 86.4 to 100% and phenol degradation efficiency ranging from 96.2 to 100%. 2-hydroxymuconic semialdehyde (2-HMSA) and succinate were the detected steady-state organic acid metabolites in the coculture reactor. Resilience of the coculture system to Cr(VI) toxicity was demonstrated by quick recovery from overloaded conditions after decreasing the influent Cr(VI) concentration to values lower than 5 mg/L (10 mg/L for the pure culture).; A diffusion-reaction model was derived for Cr(VI) and substrate removal by attached biomass based on enzymatic reaction rate and diffusion rate laws. A (fourth-order) Runge-Kutta method was used together with finite difference methods (Crank-Nicholson and Backward Euler) to solve the resulting equations. A heuristic approach (genetic algorithm) was used to optimize Cr(VI) reduction and substrate utilization rate kinetic parameters in the fixed-film bioreactors. The model using optimized kinetic parameters fitted well effluent Cr(VI) and glucose concentration in the pure culture reactor as demonstrated by computed confidence levels as high as 98.9% for effluent Cr(VI) and 96.4% for effluent glucose. In the more complex steady-state coculture system, the model simulated effluent Cr(VI) concentration with approximately 98.6% confidence and effluent phenol concentration with approximately 93.4% confidence. Further analysis of kinetic parameters showed that biological activity and Cr(VI) removal rate were consistently higher in biofilm reactors than in suspended-growth batch cultures of the same species. Higher biological activity in the biofilm systems was attributed to mass transport resistance in the biofilm.