Phenol and p-nitrophenol biodegradations by acclimated activated sludge: Influence of operational conditions on biodegradation kinetics and responding microbial communities

This study aims to evaluate the influence of operational factors (i.e. substrate concentration, activated sludge concentration and acclimation concentration) and the microbial communities of activated sludge on the aerobic biodegradation of phenol and p-nitrophenol (PNP). During the biodegradation process, the phenol or PNP biodegradation and activated sludge growth kinetics were determined and the interaction effects of the operational factors on the kinetics were assessed. The phenol biodegradation kinetics was well described by zero-order model (R2 > 0.813). At low phenol concentration (< 200 mg/L), the biodegradation rate can be enhanced by increasing activated sludge concentration and acclimation concentration. From the specific oxygen uptake rate (SOUR) analysis, lower SOUR values for PNP relative to phenol suggests higher toxicity of PNP, with inhibition detected above 50 mg/L for both phenol and PNP. In view of the inadequacy of zero-order equation for PNP biodegradation, the kinetic data was fitted into Haldane equation that incorporate the inhibitory effect. From the results, the value of critical PNP concentration was observed to increase at higher acclimation concentration, indicating the attenuation of inhibitory effects on PNP-acclimated activated sludge. For microbial growth, only the factor of acclimation concentration greatly affected the growth kinetics, in which the Ks/Ki ratio was greatly reduced when the acclimation concentration was doubled. Lastly, potential degrading strains of Acinetobacter sp. USM, Rhodococcus sp. USM1 and Caballeronia sp. USM2, were successfully isolated from the acclimated activated sludge. From the results of dioxygenase genes amplification, the phenol degradation mechanism was identified following ortho fission pathway.