Mass transfer and bioremediation of PAHS in a bead mill bioreactor

Abstract

Polycyclic aromatic hydrocarbons (PAH) have been identified as a serious environmental problem. In past research it has been proven that naphthalene, the simplest PAH, could be biodegraded using roller bioreactors and Pseudomonas putida. In this previous work it became apparent that the mass transfer rate of the hydrophobic naphthalene was the rate limiting factor in biodegradation, as the bacteria could degrade the naphthalene as fast as it entered solution. The challenge for the present research was to find a simple, inexpensive method for increasing the mass transfer rates within the framework of the previously successful reactor. After some deliberation, the addition of inert particles (glass beads) was determined to be the preferred option to increase mass transfer. The inert particles visibly increased the turbulence in the reactor and significant increases in both mass transfer and bioremediation rates were achieved. The augmentation of mass transfer rates was found to be dependent on the type, size and relative loading of the particles. Two types of inert particles were investigated to increase mass transfer rates, spherical glass beads and Raschig rings. Glass beads were found to be far superior to Raschig rings for the intended purpose. Three sizes of spherical glass beads were then compared experimentally (1, 3, and 5mm). It was discovered that the 3mm beads were vastly superior to 1mm beads and 5 mm beads were slightly superior to 3mm beads. Different bead loadings (volume of particles / total working volume) were then explored with 10%, 25% and 50% bead loading investigated. Although slight increases in mass transfer were observed at higher bead loadings, the reduction in working volume for biodegradation meant that 50% was accepted as the optimum loading parameter. The optimum conditions for maximum mass transfer occurred using 5 mm spherical glass beads at 50% loading. The increase in mass transfer and biodegradation rates compared to a traditional roller bioreactor were found to be 10 fold and 11 fold, respectively. The optimum mass transfer conditions were then applied to 2-methylnaphthalene with increases in mass transfer and biodegradation equal to 6 fold and 8 fold, respectively. The candidate bacteria used in this study was found incapable of degrading 1,5 dimethylnaphthalene although the mass transfer results demonstrate promise for the developed technology. To determine the effects of scale on the process, two larger reactors were finally studied. They were eight times and twenty-one times the size of the initial bioreactor. The process was shown to speed up at larger scale which shows great promise for future applications. The maximum degradation rate achieved in the larger reactor was 148 mgL-1h-1. This compares very well with the best result found in literature, 119 mgL-1h-1, which was achieved in a much more complex system. Clearly, the bead mill bioreactor designed during the present work is a simple concept that shows superior performance for the bioremediation of PAH’s.