Experimentation and CFD modeling of continuous degradation of formaldehyde by immobilized Ralstonia eutropha in a semi-pilot-scale plug flow bioreactor.

This study focuses on continuous formaldehyde (FA) biodegradation by Ralstonia eutropha immobilized on polyurethane foam in a semi-pilot-scale plug flow packed-bed bioreactor. The stepwise increasing of the influent FA concentration from 43.9 to 1325.1 mg L-1 was studied in the bioreactor during 70 days of operation. A complete removal of FA was achieved for inlet concentration up to 425.5 mg L-1 and the initial specific biodegradation rate reached to its maximum value about 44.3 mg gcell -1  h-1 at 425.5 mg L-1 . However, further increase of inlet concentration resulted in decrease of the biodegradation performance of the immobilized cells due to the inhibitory effect of FA on the enzymatic system involved in the biodegradation process. Based on kinetic modeling results, the Luong equation with the following constants could best describe the behavior of the bio-system: maximum specific FA biodegradation rate (qmax ) of 124 mg gcell -1  h-1 , half-saturation constant (KS ) of 337.2 mg L-1 , maximum degradable FA concentration (Smax ) of 1582 mg L-1 , and shape factor (n) of 1.49. Also, three-dimensional simulation of the bioreactor was performed using an integrated computational fluid dynamics (CFD) approach that takes into account both the biokinetic constants of the immobilized system as well as the fluid properties under steady-state condition. Eulerian computations successfully anticipated the concentration gradients through the reactor for different inlet FA concentrations, and uniform vertical velocity pathlines and non-dispersed plug flow inside the reactor were verified by the presented velocity distribution and flow streamlines.