Zhen Ya Zhang

Kinetic study on fermentation from CO2 and H2 using the acclimated-methanogen in batch culture

Abstract Methane was produced from H 2 and CO 2 using the acclimated-mixed methanogens in a 3.71 fermentor in batch culture at pH 7.2 and 37°C. The Fermentation kinetics parameter for the growth of methanogens, overall mass transfer coefficient of the reactor, and the conversion rate of H 2 and CO 2 to CH 4 by the acclimated-mixed culture were determined using the technique of Vega et al. The maximum specific growth rate ( μ max ) and H 2 specific consumption rate ( q max ) were found to be 0.064(h −1 ) and 104.8 (mmol h −1 g −1 ) respectively. Monod saturation constants for growth ( Kp ) and for inhibition ( K ′ p ) were found to be 3.54 (kPa) and 0.57 (kPa), respectively. These findings indicate that without very low dissolved H 2 levels, the fermentations are carried out under μ max , and the specific uptake rate ( q ) was almost not affected at any dissolved H 2 level in the range studied. The yield of CH 4 (Yp/s) was calculated to be 0.245 (mol CH 4 mol −1 H 2 ), which is near the stoichiometric value of 0.25. DH 2 was also measured using the Teflon tubing method and was in good agreement with those estimated by kinetic calculations.

Effects of sulfur-containing compounds on the growth and methane production of acclimated-mixed methanogens

Abstract A variety of sulfur-containing compounds were investigated for use as medium reductant and sulfur sources for growth of the acclimated-mixed methanogens. In Hungate tube cultures, the optimum concentrations of sulfur compounds for growth and CH 4 formation were sulfide, 0.9 mmol; cysteine, 3.6 mmol; thiosulfate, 0.675 mmol; elemental sulfur (S 0 ), 0.45 gl −1 ; glutathione, 2.5 mmol; SO 4 2− , 3.6 mmol; SO 3 2− , 0.9 mmol; and S 2 O 5 2− , 0.09 g l −1 . In comparison with sulfide as sole sulfur source, CH 4 formation was about the same while higher increments of cell density were obtained by using cysteine, thiosulfate, S 0 and glutathione. Other sulfur-containing compounds tested resulted in decreased CH 4 formation but with a higher increment of cell density. When contaminating sulfidogenic bacteria were inhibited by addition of antibiotics to the medium which are known to affect eubacteria but not methanogens, only a slight amount of hydrogen sulfide was produced by assimilatory reduction of methanogens, and the growth of the methanogens was poor, especially in the case of growing on SO 4 2− , SO 3 2− , thiosulfate or glutathione, indicating that evolved hydrogen sulfide was mainly due to a sulfide-producer that existed symbiotically with predominant methanogens in the cultures. However, in the case of cysteine or S 0 as sulfur sources, in spite of inhibition of contaminating bacteria, sufficient sulfide was reduced by methanogens to support growth and methane formation effectively. These sulfur compounds were also tested for their ability to enhance and restore growth and CH 4 production with sulfur-depleted cells in culture tubes. The results of the present study show that, for the growth of the acclimated-mixed methanogens, volatile and malodorous sulfide can be replaced with the sulfur-containing compounds described above.