Biological Co-treatment of H2S and reduction of CO2 to methane in an anoxic biological trickling filter upgrading biogas.

This study investigated the feasibility of co-treating H2S and CO2 in a biological trickling filter (BTF) inoculated with hydrogenotrophic methanogens (HMs) and nitrate-reducing, sulfur-oxidizing bacteria. This was accomplished by introducing a pure culture of Thiobacillus denitrificans in a BTF that was successfully upgrading a biogas mimic (60:40 CH4:CO2) to >97% methane using an enriched HM consortium. Nitrate was fed as the electron acceptor to oxidize H2S. The results revealed that a severe competition for hydrogen's electrons occurred between carbon dioxide and nitrate. Due to this competition, N:S loading rates of 16:1 were required to achieve >98% H2S removal, a ratio which is four times greater than the theoretical N:S ratio for complete sulfur oxidation. However, such high nitrate loading rates (>50 g N-NO3- m-3 h-1) had a negative impact on the BTF's biogas upgrading performance. An electron balance illustrated the increasing diversion of H2 electrons towards nitrate reduction as nitrate loading increased. Overall, this study showed that simultaneous biogas upgrading and H2S removal in a single bioreactor is possible, but that achieving high yields for both reactions requires further research in process and culture optimization.