The levels of oxygen consumption indicated that the oxidation of thiosulfate was higher with biomass suspended from biofilms formed by T. thioparus growing over polyethylene rings (Figure 3). Volcanic rocks (TZ) always exhibited the highest biomass production, a fact that probably due to the irregular surface and its highly porous structure, but the accumulation of biomass and the elementary sulfur generated as a product of the biological oxidation of thiosulfate obstructed the column and it causes the canalization of the flow through it. These conditions might have generated areas in the supporting material lacking in oxygen and nutrients, a fact that might explain the low activity showed, as a consequence of diffusional restrictions (Cox et al. 1997).
The higher rates of thiosulfate oxidation showed by cells detached from polyethylene rings may be attributed to the adsorbing properties of the surface of the material that leads to the development of a homogeneous biofilm, as shown by electron microscopy (Figure 4). This fact might result in a higher availability of oxygen and nutrients for the immobilized cells thus maintaining them metabolically active.
Figure 4 shows a microscopy photograph of the biofilm formed by T. thioparus. In Figure 4a considerable development of bacteria associated to the rings is observed, whereas in Figure 4b and Figure 4c abundant elementary sulfur generated on the surface of the polyethylene ring was observed. This material was produced by oxidation of thiosulfate, producing crystals with their typical octahedral structure. The analysis of the secondary emission revealed that the elementary sulfur was present in an significant proportion (34,72% p/p). Similar results were obtained for A. thiooxidans.
Figure 5 and Figure 6 show the results obtained in the removal of H2S when using the biotrickling filter with a biofilm formed by T. thioparus, operated at a range of pH between 5,5 and 7,0 as to provide the optimal conditions for growth. The maximal removal capacity attained in this bioreactor was 14 gS m-3 h-1 at 30 gS m-3 h-1 of inlet load, 47% of removal efficiency at a residence time of 26 sec.
Better results were obtained in the biotrickling filter inoculated with A. thiooxidans, and operated without pH control at high inlet concentrations of H2S. The results are shown in Figure 7 and Figure 8.
Removal efficiency of 100% were achieved for higher inlet concentration of H2S (4600 and 982 ppmv) for 120 sec and 45 sec of residence time, respectively, therefore, better removal capacities were obtained as compared with T. thioparus. Also,complete oxidation of H2S (100%) was achieved with an inlet load of 240 gS m-3 h-1 of inlet load. The highest removal capacity was 370 gS m-3 h-1 at 45 sec of residence time, and 405 gS m-3 h-1 of inlet load (91% of efficiency), which represents eliminations capacities of a similar level that previously reported by Cho et al. (2000), who obtained the best results using biotrickling filter packed with porous lava inoculated with A. thiooxidans (428 gS m-3 h-1).
Both values obtained in biotrickling filters inoculated with A. thiooxidans are considerably higher than the capacities reported in biofiltration systems packed with natural carriers (Cho et al. 1992; Yang and Allen, 1994; Wani et al. 1999; Elias et. al. 2002; Oyarzún et al. 2003), possibly due to the possibility to drain sulfur and sulfate.