Development of a two-phase bioreactor for the biological removal of hydrogen sulfide from biogas

Abstract

In this study, a two-phase bioreactor, consisting of an anaerobic absorption column and an aerobic biofilter, was constructed and utilized to determine the removal efficiency of high strength hydrogen sulfide from biogas. A microbial strain of sulfur oxidizing bacteria (SOB), Acidithiobacillus thiooxidans, isolated from earthworm casts was inoculated in the aerobic biofilter. Initially, an inlet concentration of hydrogen sulfide supplied to the two-phase bioreactor was 180ppm, and overall removal efficiencies of hydrogen sulfide were 30 to 60% due to low cell density and activity of the SOB in the bioreactor. As bioreactor operation continued, microbial activity and bioreactor performance increased with sharply decreasing pH levels from 6.3 to 1.5. During the same operational period, the optical density (OD600) increased from 0.05 to 0.4, indicating that the SOB was in an experiential growth stage at the low pH condition. The overall removal efficiency of hydrogen sulfide was found to be greater than 97% from day 8, and the high removal efficiency maintained. On day 30, the inlet concentration of hydrogen sulfide was increased to 400ppm, and the removal efficiency did not decline, showing that the activity of the SOB was high enough to handle the high loading rate of hydrogen sulfide. An interesting finding of the study was that the activity of the SOB strain used in this study was not affected at the extremely low pH values, and no chemical additives were required to maintain the pH. Dissolved oxygen concentrations in the anaerobic column and the aerobic biofilter were maintained at 2 and 8mg/L, respectively. Consequently, the two-phase bioreactor showed advantages over conventional biofilters for the treatment of high strength hydrogen sulfide from biogas.