Full Length Research Paper

Substrate inhibition kinetics of phenol degradation by binary mixed culture of Pseudomonas aeruginosa and Pseudomonas fluorescence from steady state and wash- out data

S. E. Agarry¹*, B. O. Solomon^2,3 and S. K. Layokun²

¹Biochemical Engineering Research Unit, Department of Chemical Engineering, Ladoke Akintola University of Technology, Ogbomoso, Nigeria.

²Biochemical Engineering Research Laboratory, Obafemi Awolowo University, Ile-Ife, Nigeria.

³National Biotechnology Development Agency, Abuja, Nigeria.

*Corresponding author. E-mail: sam_agarry@yahoo.com.

Nomenclature: CTR, Carbon dioxide transfer rate (mg/L/h); D, dilution rate (h^-1);, half-saturation constant (mg/L); , inhibition constant (mg/L); OTR, oxygen transfer rate (mg/L/h); , specific phenol (substrate) consumption rate (mg/mg/h); , maximum specific phenol (substrate) consumption rate (mg/mg/h); S, C_s, substrate concentration (mg/L); µ_max    maximum specific growth rate (h^-1); X, biomass concentration (mg/L).

Accepted 19 September, 2008

Steady states of a continuous culture with an inhibitory substrate were used to estimate kinetic parameters under substrate limitation (chemo stat operation). Mixed cultures of an indigenous Pseudomonas fluorescence and Pseudomonas aeruginosa were grown in continuous culture on phenol as the sole source of carbon and energy at dilution rates of 0.01 – 0.20 h^-1. Using different dilution rates several steady states were investigated and the specific phenol consumption rates were calculated. In addition, phenol degradation was investigated by increasing the dilution rate above the critical dilution rate (washout cultivation). The results showed that the phenol degradation by mixed culture of P. fluorescence and P. aeruginosa can be described by simple substrate inhibition kinetics under substrate limitation but cannot be described by simple substrate inhibition kinetics under washout cultivation. The phenol consumption rate (degradation rate) increased with increase in dilution rate. Fitting of the steady state data from continuous cultivation to six inhibition models resulted in the best fit for Haldane, Yano and Koga, Aiba et al. and Teissier models, respectively. The r_smax value of 0.322 mg/mg/h obtained from these model equations was comparable to the experimentally calculated r_smax value of 0.342 mg/mg/h obtained under washout cultivation.

Key words: Continuous cultivation, washout cultivation, steady state, substrate inhibition, bioreactor, primary culture, secondary culture, kinetic parameters, mixed culture.