Inhibition Kinetics

In general, the growth of bacteria in liquid culture can be described by the first-order equation.

Cx dt dCx 1

μ = (Eq. 4-2) Where, μ is the specific growth rate (1/d), and Cx is the biomass concentration (mg/L).

The specific growth rates for treatments with different initial fluorene concentrations are illustrated in Fig. 4-22. It can be observed that the specific growth rate increased with an increase in initial concentration of fluorene. A maximum specific growth rate of 0.35/d was reached at 5 mg/L fluorene concentration. When the fluorene concentration was higher than 5 mg/L the rate declined gradually. This observation indicates that the inhibition of SRB may occur at fluorene concentrations higher than 5 mg/L.

Initial concentration of fluorene (mg/L)

Initial concentration of fluorene (mg/L)

0 5 10 15 20 25

Fig. 4-22 Specific growth rate as a function of the different initial fluorene concentration

Because of the inhibition of high fluorene concentration on the biomass growth, the Haldane’s equation (Eq. 3-1) was selected for assessing the dynamic behavior of SRB. The values of Haldane’s parameters were derived using a nonlinear least-squares regression analysis of FORTRAN. The calculated values of Haldane’s parameters were as follows: μmax of 0.35/d, Ks of 0.07 mg/L and Ki of 601.1 mg/L (R² = 0.80).

The presence of other substrates, i.e. DMF and yeast extract, in the bioreactor might be utilized by SRB without an inhibition of toxicity in the present study (experimental range of fluorene concentration from 0 to 100 mg/L). The inhibition can be expected at 600 mg/L of fluorene by using the FORTRAN as per Haldane’s equation.

Chapter 5

Summary and conclusions

1. The SRB enrichment culture was successful in the biodegradation of fluorene. The relationship between biomass concentrations and PAHs removals indicates that the rate of fluorene biodegradation increased with an increase in the biomass concentration. The relationship between the sulfate consumption and PAHs removals reveals that sulfate consumption was coupled to the biodegradation of fluorene.

2. Concentrations of sulfate and biomass required for fluorene biodegradation by SRB can be predicted by the empirical model developed in this study. The optimum concentrations of sulfate and biomass determined by the RSM and desirability function method were estimated as 14.4 mM and 37.8 mg/L, respectively. The maximum fluorene removal percentage observed in this study was 90% at optimum conditions.

3. Results of FISH analysis demonstrate that SRB was the major species (around 85%) in the enriched bacterial culture. A higher degradation rate of fluorene was observed in the present study than that described by previous reports. This result indicates that the efficiency of fluorene biodegradation was enhanced by the SRB enrichment culture.

4. According to the results of inhibition study, the fluorene biodegradation by SRB can be inhibited when the fluorene concentration is higher than 600 mg/L.

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