Simultaneous methylmercaptan and hydrogen sulfide removal in the desulfurization of biogas in aerobic and anoxic biotrickling filters

Abstract

Hydrogen sulfide (H2S) and methylmercaptan (CH3SH) are the most common sulfur compounds found in biogas. The simultaneous removal of H2S and CH3SH was tested at neutral pH in two biotrickling filters, one operated under aerobic conditions and the other one under anoxic conditions. Both reactors were run for several months treating a H2S concentration of around 2000 ppmv. Then, the effect of CH3SH loading rate (LR) on H2S and CH3SH removal was investigated in both reactors maintaining a constant H2S LR of 53–63 gS-H2S m−3 h−1, depending on the reactor. Initially, CH3SH concentration was stepwise increased from 0 to 75–90 ppmv. Maximum elimination capacities (ECs) of around 1.8 gS-CH3SH m−3 h−1 were found. After that, the CH3SH LR was increased by testing different empty bed residence times (EBRTs) between 180 and 30 s. Significantly lower ECs were found at short EBRTs, indicating that the systems were mostly mass transfer limited. Finally, EBRT was stepwise reduced from 180 to 30 s at variable CH3SH and H2S loads. Maximum H2S ECs found for both reactors were between 100 and 140 gS-H2S m−3 h−1. A negative influence was found in the ECs of CH3SH by the presence of high H2S LR in both biotrickling filters. However, sulfur mass balances in both reactors and batch tests under aerobic and anoxic conditions showed that CH3SH chemically reacts with elemental sulfur at neutral pH, enhancing the overall reactors performance by reducing the impact of sulfur accumulation. Also, both reactors were able to treat CH3SH without prior inoculation because of the already existing sulfide-oxidizing microorganisms grown in the reactors during H2S treatment. Co-treatment of H2S and CH3SH under aerobic and anoxic conditions was considered as a feasible operation for concentrations commonly found in biogas (2000 ppmv of H2S and below 20 ppmv of CH3SH).