Evaluating the relationship between the sorption of PAHs to bacterial biomass and biodegradation

Petroleum refinery wastewater containing polynuclear aromatic hydrocarbons (PAHs) are typically treated by biological processes in the United States. PAHs are recalcitrant, hydrophobic compounds and sorption to biological solids may be a significant mechanism for the removal of PAHs from refinery wastewater. The goal of this research was to investigate PAH sorption by bacterial biomass and examine the relationship between PAH biosorption and biodegradation. In this study, phenanthrene was used as a model PAH for biosorption studies and pyrene and fluoranthene were used as model compounds in biodegradation studies. It was found that phenanthrene biosorption varied with bacterial genus and species. Bacteria with the highest sorption capacity (Kp) belong to the Nocardioforms, organisms that often cause solids separation problems in activated sludge plants. Consequently, blooms of these difficult to settle organisms in refinery treatment plants could exasperate PAH releases to the environment. The measured sorption capacities were reproducible and appeared to represent surface sorption, based on the apparent competition between naphthalene and phenanthrene for sorption sites. Based on a comparison of Kp values, pure bacterial cultures can serve as valid models of biosorption by activated sludge MLSS. Finally it was found that PAH sequestration by high Kp, non-degraders has a significant impact on PAH biodegradation. The results of this study suggest that although biosorption can decrease the rate of PAH biodegradation in the short term, it can also result in the removal of PAHs from the wastewater and PAH retention in the treatment system where it may be ultimately biodegraded. This research improves our understanding of processes contributing to PAH degradation in petroleum refinery wastewater treatment plants.