Characterization of the growth kinetics of Pseudomonas sp. OX1 on phenol: continuous culture under controlled unstable steady state conditions

Pseudomonas strains have long been proven to be effective in aromatic pollutant conversion. Typically, Pseudomonas strains growth on aromatic species is characterized by substrate inhibition and the kinetic parameters are assessed working out results of batch tests. On the other hand, continuous cultures operated under steady state conditions most closely reflect realistic operational conditions and are, as such, better suited for reliable assessment of kinetic parameters. The peculiar growth kinetics makes the chemostat intrinsically unstable, so that properly tuned control systems are required to counterbalance the inherent process instability. Turbidostats and nutristats have been reported in the literature as possible configurations of controlled chemostat to accomplish continuously operated cultures. However, it may be difficult to continuously measure state variables like biomass or carbon-source concentrations as compared with dissolved oxygen concentration. The present study addresses the characterization of the kinetics of Pseudomonas sp. OX1 growth using phenol as carbon and energy source. Tests were carried out in a chemostat equipped with a dilution rate control unit. The dissolved oxygen concentration was adopted as the controlled variable and an ad-hoc proportional-integral control loop was developed. A test procedure was purposely adopted in which the control system forced the chemostat to operate steadily under conditions that would otherwise be inherently unstable. Tests demonstrated that the control strategy was successful in stabilizing steady state continuous cultures at phenol concentration characterized by unstable behaviour. The unstable branch of the growth kinetics was characterized. Moreover, phenol degradation under starving conditions was investigated. The specific growth rate () as a function of pH are well fitted by the Haldane model, data regression yielding max = 0.26 h-1, KM = 0.005 g/L and KI = 0.2 g/L. Phenol to biomass fractional yield increases with  according to the Pirt model, with a maintenance contribution of 0.017 g/(g ·h) and a theoretical fractional yield of 1.3 g/g.