Naphthenic acids are a complex mixture of organic compounds which naturally occur in crude oil. Low molecular weight components of the naphthenic acids are known to be toxic in aquatic environments and there is a need to better understand the factors controlling the kinetics of their biodegradation. In this study, a relatively low molecular weight naphthenic acid compound (trans-isomer of 4-methyl-1-cyclohexane carboxylic acid) and a microbial culture developed in our laboratory were used to study the biodegradation of this naphthenic acid and to evaluate the kinetics of the process in batch cultures. The initial concentration of trans-4-methyl-1-cyclohexane carboxylic acid (50-750 mg l(-1)) did not affect the maximum specific growth rate of the bacteria at 23 degrees C (0.52 day(-1)) to the maximum biodegradable concentration (750 mg l(-1)). The maximum yield observed at this temperature and at a neutral pH was 0.21 mg of biomass per milligram of substrate. Batch experiments indicated that biodegradation can be achieved at low temperatures; however, the biodegradation rate at room temperature (23 degrees C) and neutral pH was 5 times faster than that observed at 4 degrees C. Biodegradation at various pH conditions indicated a maximum specific growth rate of 1.69 day(-1) and yield (0.41 mg mg(-1)) at a pH of 10.

BACKGROUND: Naphthenic acids are carboxylic acid compounds of oil sands wastewaters that contribute to aquatic toxicity. Biodegradation kinetics of an individual naphthenic acid compound in two types of continuous-flow bioreactors were investigated as a means of improving remediation strategies for these compounds.

Previously, using maximum feed concentrations of 500 mg/L, an immobilized cell reactor (ICR) with established biofilm was shown to enhance the biodegradation rate of a candidate naphthenic acid up to two orders of magnitude when compared to a freely suspended cell, well mixed tank reactor, reaching biodegradation rates of 22 000 mg/L day with 100% biodegradation efficiencies [Paslawski et al., J. Chem. Technol. Biotechnol. 2009a]. In this work, dispersion in the ICRs was found to be negligible and a plug flow model was developed and shown to represent the immobilized cell biodegradation data well. The effectiveness coefficient (percent of dry biofilm containing active biomass) was predicted to be approximately 55% with an effective thickness of 0.15 mm.