Oil sand deposits in the Athabasca Basin in Alberta represent one of the largest global oil reserves. The bitumen contents of oil sand shallow deposits are recovered by surface mining using modified version of the Clark hot water process. Extraction of bitumen results in extremely large volumes of process water, which are contaminated with naphthenic acids. Various ex-situ treatment techniques including ozonation, advanced oxidation, adsorption, and bioremediation have been evaluated for the treatment of these waters. Previous studies conducted by Paslawski et al. (2009) investigated aerobic biodegradation of naphthenic acids in properly designed and carefully operated bioreactors. In the current work, anaerobic biodegradation of naphthenic acids under denitrifying condition was examined as a potential approach to eliminate the aeration cost in ex-situ treatment and as an alternative for application of in-situ treatment of oil sand process water in stabilization ponds was examined. Using trans-4-methyl-1-cyclohexane carboxylic acid (trans-4MCHCA), a microbial mixed culture developed in earlier works (Paslawski et al., 2009), and nitrate as an electron acceptor, anaerobic biodegradation of trans-4MCHCA were studied in batch and continuous bioreactors: continuous stirred tank reactor (CSTR) and biofilm system. Effects of naphthenic acid concentration, temperature, and loading rate on biodegradation process were investigated.
The batch studies showed that initial concentration of trans-4MCHCA influenced the biodegradation rate where the increase in initial concentration of trans-4MCHCA from 100 to 250 mg L-1 led to a higher rate but further increase in concentration did not have a marked effect. Moreover, batch experiments at temperatures ranging from 10° to 35°C demonstrated that the optimum temperature was in the range of 20 - 24°C. Continuous anaerobic biodegradation in the CSTR showed that increase in loading rate of trans-4MCHCA caused an increase in removal rate of both trans-4MCHCA and nitrate. Rates were decreased as the system approached the cell washout. The maximum biodegradation rate and nitrate removal rate, achieved at trans- 4MCHCA loading rate of 157.8 mg L-1 h-1, were 105.4 mg L-1 h-1 and 144.5 mg L-1 h-1, respectively. A similar dependency between the loading and removal rates was also observed in the biofilm reactor. The maximum removal rate of trans-4MCHCA and nitrate in the biofilm reactor, operated at room temperature (24 ± 2oC) were 2,028.1 mg L-1 h-1 and 3,164.7 mg L-1 h-1, respectively and obtained at trans-4MCHCA loading rate of 2,607.9 mg L-1 h-1.
Comparison of the results from aerobic batch systems obtained by Paslawski et al. (2009) and the current results showed similar profile where increase in initial concentration of naphthenic acid increased the biodegradation rate of trans-4MCHCA. As far as the effect of temperature is concerned, room temperature (20 - 24oC) was identified as optimum temperature regardless of mode of biodegradation. Under continuous mode of operation (CSTR and biofilm reactors), anaerobic biodegradation was much faster than its aerobic counterpart. For instance the maximum anaerobic removal rate of trans-4MCHCA in the CSTR was 105.4 mg L-1 h-1, while the highest removal rate achieved in the aerobic CSTR was 9.6 mg L-1 h-1. Similarly, anaerobic biofilm reactor achieved a higher maximum removal rate of 2,028.1 mg L-1 h-1 compared to a 924.4 mg L-1 h-1 removal rate in the aerobic biofilm reactor. The overall finding indicated that biodegradation of trans-4MCHCA can be achieved effectively under anaerobic condition with the rates markedly higher than those for aerobic system.