Sulphate reduction and biomass growth rates for Desulfobacterium autotrophicum in yeast extract – Supplemented media at 38 C

Water pollution by heavy metals and sulphates is common in areas with mineral deposits. Sulphate-reducing bacteria (SRB) are microorganisms widely distributed, and can be used as heavy metals removal agents. Furthermore, the use of organic substrates or wastes in SRB systems could reduce the cost and increase the treatment efficiency. The aim of the study was to assess the sulphate reduction capacity and growth rates of the bacterium Desulfobacterium autotrophicum in batch culture media supplemented with different concentrations of yeast extract. At 38 C, in the growth phase, there was a direct relationship between the concentration of yeast extract employed and the biomass growth and sulphate reduction rates. In the presence of low sulphide concentrations, supplementary 0.5 g/L of yeast extract maximized sulphate reduction on a per-cell basis, but at high concentrations of H2S it would be advisable to use 2 g/L to reduce the product inhibition.

Fig. 6. Accumulated H2S during the batch experiments. depend on the maintenance of high concentrations of biomass to high rates of reduction of sulphate per cell. In batch systems, this operational point matches with the exponential cell growing. In this study, the results show that at a yeast extract concentration media of 0.5 g/L it is produced a very satisfactory outcome with respect to the results obtained to lower concentrations (0 and 0.1 g/L), and the similar it was observed that at higher concentrations of yeast extract (1 and 2 g/L). Moreover, in a high scale system, if the organic substrate is inexpensive and effluents generated can be managed properly, the operation should be carried out at high concentrations, which in the case studied it is up to 2 g/L. H2S, or the species in equilibrium (HSÀ), at high concentrations constrain cell size and biomass growth. Concentrations of H2S are shown in Fig. 6 for the three bioreactor sets containing the highest amounts of yeast extract. Despite the evident discontinuities, all three bioreactor systems produced a sustained increase in H2S concentration, which demonstrates need to improve liquid-mass transfer for clearing the system of H2S. Studying the strain Desulfovibrio desulfuricans in heterotrophic media and at a similar temperature (35 C), Okabe et al. [23] found that cell production fell slightly at a total sulphide concentration of 150 mg/L (equivalent to 100 mg H2S/L), and that at 280 mg/L (equivalent to 187 mg H2S/L) biomass growth declined sharply. In the bioreactors containing 0.5, 1 and 2 g/L, the fall in biomass growth from day 3 (Fig. 1) may be due to the effect accumulated sulphide concentrations We were able to determine design parameters from the study to guide the future development of a good, continuous sulphate-reducing system. Use of per-cell sulphate reduction rates should make comparing different systems much easier with a view to paving a way to understanding the mechanisms involved. Autotrophic cultures promoted little growth and surprisingly poor sulphate reduction. At 0.5 g/L and above of yeast extract supplement, biomass was reasonable and high sulphate reduction rates were achieved. Which concentration of yeast extract supplement to use in a continuous system will depend on how accumulated H2S is dealt with. In the presence of low sulphide concentrations, 0.5 g/L would maximize sulphate reduction on a per-cell basis, but at high concentrations of H2S it would be advisable to use 2 g/L supplementary yeast extract.